To understand the very first steps of atmospheric particle formation and growth processes, information on the size where the atmospheric nucleation and cluster activation occurs, is crucially needed. The current understanding of the concentrations and dynamics of charged and neutral clusters and particles is based on theoretical predictions and experimental observations. This paper gives a standard operation procedure (SOP) for Neutral cluster and Air Ion Spectrometer (NAIS) measurements and data processing. With the NAIS data, we have improved the scientific understanding by (1) direct detection of freshly formed atmospheric clusters and particles, (2) linking experimental observations and theoretical framework to understand the formation and growth mechanisms of aerosol particles, and (3) parameterizing formation and growth mechanisms for atmospheric models. The SOP provides tools to harmonize the world-wide measurements of small clusters and nucleation mode particles and to verify consistent results measured by the NAIS users. The work is based on discussions and interactions between the NAIS users and the NAIS manufacturer.
Understanding of the detailed formation mechanisms and the chemical composition of vapours, which participate in the atmospheric particle formation processes, has clearly benefited from direct atmospheric measurements and improvements in measurement techniques (Manninen et al., 2010; Kulmala et al., 2013, 2014; Ehn et al., 2014). Aerosol particles have global effects on Earth's climate and regional effects on air quality. In atmospheric particle formation, we study the phase transition from gas phase precursors to aerosol particles. Atmospheric new particle formation can start via molecular clustering, and it is followed by cluster activation for enhanced growth (Kulmala et al., 2013). The freshly formed particles grow by multicomponent condensation. When aerosol particles grow further to sizes where they can act as cloud condensation nuclei, they start to have effect on the climate. One of the main objective of atmospheric aerosol science is to contribute to the reduction of scientific uncertainties concerning global climate change issues, particularly those related to aerosol–cloud interactions (IPCC, 2013).
Although the Neutral cluster and Air Ion Spectrometer (NAIS, Mirme and Mirme, 2013) is a relatively recently developed instrument, it has already been used widely in many atmospheric particle formation studies. First field observations by Kulmala et al. (2007) showed the capacity of the instrument for direct detection of the newly formed particles, and later the long-term observations in field lead into fundamental understanding of the cluster formation and activation (Manninen et al., 2009). The NAIS has been used in various environments in all continents to study both natural and anthropogenic aerosols, both during short-term campaigns and during long-term field studies. For example, the NAIS has been deployed in the boundary layer (e.g. Manninen et al., 2010), in the middle troposphere (Laakso et al., 2007; Boulon et al., 2011; Rose et al., 2015), in the upper free troposphere (Mirme et al., 2010), and in the tropics (Suni et al., 2008; Martin et al., 2010; Siingh et al., 2013), at the middle and high latitudes (Lihavainen et al., 2007; Manninen et al., 2010) and in the polar regions (Virkkula et al., 2007), and in the remote, rural, and urban areas (Tiitta et al., 2007; Backman et al., 2012; Hirsikko et al., 2007, 2013; Herrmann et al., 2014; Jayaratne et al., 2014). Several laboratory studies have been conducted to investigate connection between the small cluster ions and new particle formation (e.g. Ortega et al., 2012; Franchin et al., 2015; Duplissy et al., 2016; Kirkby et al., 2016).
The instrument measures the number size distribution of atmospheric ions and particles by collecting signal simultaneously with many electrometers. The complete distribution of both polarities is determined rapidly using parallel columns. This is the main advantage of the NAIS, but it also creates convoluted instrument construction, and complex maintenance and calibration procedures.
The sampling and detection of small ions and freshly formed particles is
demanding. Firstly, the charging probability of neutral nanometer-sized
particles is very low and the concentration of growing freshly formed
particles is often less than 10 particles per cm
In this paper we present a method to measure number size distribution of clusters at sub-3 nm and nanoparticles at sub-25 nm (i.e. nucleation mode particles; see Kulmala et al., 2012). This standard operation procedure (SOP) is based on scientific and technical discussions between the NAIS and the Air Ion Spectrometer (AIS) users among the ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure network) partners as well as with the (N)AIS manufacturer Airel Ltd., Estonia. The procedure work is led by the University of Helsinki. The aim is to provide consistent results and unified datasets measured with the NAIS around the world, as the NAIS results improve our understanding of the processes producing atmospheric nanoparticles. These results can be used for developing aerosol process parameterization for the atmospheric models, and validating and constraining the global models. Note that the procedures presented here apply to instrumentation which are currently in use. As the NAIS instrument is under continuous development, the maintenance, calibration and data processing methods need to be updated and modified accordingly.
The SOP (Sect. 3) is written for the NAIS users
with different background. The procedure has three main parts that explain
the required actions: detail how to calibrate and verify the instrument
both in laboratory and in the field. This is essential during long-term
operation, and prior and after short-term campaigns to confirm that the
results are reliable. detail how to install, operate and maintain the
instrument during the field, laboratory or chamber measurements. Various
environmental conditions are considered. detail how to process the collected data including
the data corrections and data quality checks. This step is typically
required before the new particle formation data analysis, which is described
in detail in Kulmala et al. (2012).
Critical topics are highlighted. Section 4 provides a troubleshooting section for the most typical issues during the NAIS operation. In Sect. 5, some typical ion and particle number size distributions measured with the NAIS are presented.
The NAIS is a multichannel aerosol mobility spectrometer capable of
measuring a mobility distribution of charged particles and ions of both
polarities in an electrical mobility range from 3.2 to 0.0013 cm
A schematic of the NAIS with:
Figure 1 illustrates that the NAIS can have a “1-blower”, “3-blower” or “4-blower flow system”. Whereas in the 1-blower system all flows are controlled by one blower, in the 3-blower system there is one sample flow blower for the instrument. In the 4-blower version both columns have a separate sample flow blower. All the flow rates – sheath flow for both polarities and sample flow (either single or separate) – are measured using Venturi tubes and differential pressure sensors. The blowers are automatically controlled to maintain the correct flow rates. The flow sensors are calibrated at Airel Ltd., Estonia.
Recommended measurement modes and measurement cycle of the NAIS.
The mobility analyzers of the NAIS are preceded by a software controlled sample preconditioning unit. Depending on the measurement mode of the instrument, the unit may filter particles out to measure a zero signal, charge the particles to measure neutral aerosol, or leave the sample untouched to measure naturally charged ions.
The four main measurement modes of the instrument are as follows:
(1)
The recommended measurement cycle for the NAIS ground-based measurements is alternating between offset, ions, and particle modes as follows: offset – 30; ions – 90; particles – 90; where the numbers represent measurement times in different modes in seconds. Reliable offset measurements are vital for the accuracy of the instrument itself and for the final number size distribution. The offset signal is estimated using a linear regression on the electric current measurements from the previous and following offset measurement cycles. The variance of the electric current signal during the offset cycle is used to estimate the noise level of individual electrometers. It is recommended that the duration of the offset measurement is between 30 and 60 s, and the total length of the measurement cycle is between 2 and 5 min (Table 1).
This is presented in Fig. 2. Although bipolar radioactive chargers are the most widely used chargers due to their well-defined charge distribution (Wiedensohler, 1988; Reischl et al., 1997), unipolar diffusion chargers can attain much higher charging efficiency levels (Intra and Tippayawong, 2009). Thus, the NAIS uses the unipolar corona discharge ionization. The high voltage (HV) supplies feed the corona charger needles (typically with 2–3 kV). The voltages are controlled by a feedback system to maintain a constant electric current of the corona ions to the outer electrode of the charger volume.
A cross section of the aerosol preconditioning unit and upper part of the mobility analyzer for old generation (left column) and new generation (right column) of the NAIS. Grounding of the different parts are visualized.
Both measurement columns use two corona chargers. The first charger is
called the discharger and it can charge particles in the opposite polarity
to the one, which is measured by the subsequent mobility analyser. The
discharger currents are
The main charger current is set to 25 nA for the positive charger and to
The corona charger ions have
a mobility diameter range of 1.0–1.6 nm (1.3–0.8 cm
The size distributions of four different sizes of neutral silver particles measured with the NAIS (positive charging, automatic post-filtering). The shaded area represents the size range of corona charger ions.
The size distribution of 4.5 nm neutral silver particles measured
with the NAIS using
We encourage all NAIS users to follow this procedure, which is based on earlier scientific work by Mirme et al. (2007, 2010), Asmi et al. (2009), Manninen et al. (2009, 2011), Kulmala et al. (2012), Mirme and Mirme (2013), and Wagner et al. (2016). The procedure has been motivated by a need of reliable long-term field measurements and by the comparability of such long-term field data. We aim to improve the comparability of the results by improving the instrument's verification, maintenance, and data processing procedures.
Prior to the measurements, the NAIS flow sensors and the electrometer background levels should be checked. The NAIS software monitors the flows using Venturi flow metres during the measurements continuously. The NAIS flows should be compared to a reference flow metre. If a discrepancy is detected, the flow sensors need to be recalibrated to update the calibration coefficients in the measurement software. The electrometer background can be checked by passing particle-free air into the instrument or by performing a concentration calibration as a function of particle size. The voltages in the inner electrode of the differential mobility analyser (DMA) should be measured before and after ambient measurements. The background and DMA voltages are continuously recorded by software during the measurements.
It is recommended that ion spectrometer users take part in the calibration and intercomparison workshops organized in co-operation by University of Helsinki and Airel Ltd. The ion spectrometers should be calibrated often enough, preferably at the calibration workshops. The goal is to organize these workshops on a regular basis. During the workshops the ion spectrometer flows are calibrated and their mobility classification and concentration measurements are verified.
The sheath and input flows of the NAIS are critical for a precise determination of the particle mobility and concentration. Thus, prior to the mobility and concentration calibrations the instrument should be cleaned, leak tested and flow checked. The cleaning procedures are essential before the determination of flows. If dirt enters the tubing or nets inside the instrument, the flow resistance will alter the volume flow through the Venturi tubes (see Sect. 3.5.2). When maintenance cleaning is done regularly, the instruments can perform well for extended periods and flows stay stable (Gagné et al., 2011). This is primarily relevant for the instruments with the 1-blower flow system as the correct flow balance is very delicate (see Option A). The newer instruments with three and four blowers will maintain flow stability for much longer periods without the maintenance and will only require recalibration in case a simple check indicates a problem (see Option B).
Leak tests can be done using several methods.
The flow calibration should be done once a year during the long-term operation, or before and after a short-term campaign measurement. However, the flows should be determined always, when a blower is replaced or a large leak is detected and sealed.
On the left: a dirty Venturi tube and its net, and a dirty flow adjustment valve in a close-up from the NAIS operated in Marikana, South Africa. Both were required to be cleaned well before the flow verification. On the right: an example of a set-up for flow checks with external pressure difference sensors, which are connected to the 5 Venturi tubes to record all the values simultaneously, including the pressure difference over the blower.
For the 1-blower system, in Fig. 1a, one blower runs all the flows. Five
flow rates (sheath and output flows of both analysers and the total exhaust
flow) are measured with Venturi tubes. In normal operation only the exhaust
flow rate is measured continuously, as it is the most sensitive to the
changes of all the other flows. Each Venturi tube has an individual
calibration where an exact pressure drop, corresponding to the specified
flow rate, is determined in normal conditions. The Venturi calibration
values are provided by Airel Ltd, when the instrument is manufactured. The
Venturi calibration is not needed to be done by the user, but the pressure
drop should be verified and the flow adjusted, if needed. This procedure is
illustrated in Fig. 5. The differential pressure over the five Venturi tubes should
be measured with a reference differential pressure measurement device (e.g.
TESTO 512 0–2 hPa), and checked against the value obtained from the
calibration to be sure that the flow rate though the Venturi is correct. If
the checked pressure difference does not correspond to the calibrated ones,
the flows should be adjusted to match the calibrated pressure difference
while keeping a constant overpressure (80 mm H
The procedure for determining the flows for the 1-blower system is as
follows: first, the user verifies that the sample and outlet flows are equal
and no leaks exist. Second, the blower power should be adjusted so that the
overpressure after the blower is 80 mm H
A quick and easy sheath flow check for 4-blower system using a TSI flowmeter which is placed directly after the sheath air filter.
For the 3- and 4-blower system, in Fig. 1b–c, the verification involves measuring the volumetric flow rates with an external flowmeter (e.g. TSI 4000 series) at the instrument's exhausts and after the sheath air filters. First, place a reference flowmeter at both exhausts (measure one exhaust flow tube at time) to verify the sample flow at the Venturi tubes, which are located just before the exhaust tubes. The sample flow and exhaust flow should be identical when the instrument is not leaking. Second, disconnect a sheath flow tube and place the flowmeter after the sheath air filter to verify the sheath flow at Venturi tubes, which are located prior to the blower and filter. Figure 6 shows a quick and easy way to check the sheath flow of the negative polarity for the 4-blower system. Repeat this step for the sheath flows for the both polarities. For a simple instrument check it is sufficient to compare the reference flowmeter value to the corresponding NAIS flowmeter value. Note that the NAIS flowmeters show the actual volume flow rate that is not adjusted to standard temperature and pressure conditions. A full flow sensor calibration can be done by the user but it requires assistance from Airel Ltd. to provide a customized calibration software, followed by processing the results and updating the measurement software.
In case of the 3- and 4-blower systems, the instrument includes a barometric pressure sensor that is actively used to determine the correct flow rate. The instrument sensor value should be compared to a reference barometric pressure sensor and the calibration coefficients should be adjusted, if necessary. To change these calibration coefficients, contact to Airel Ltd.
In the case of the 3-blower system, there is only one sample flow Venturi sensor in the instrument that measures the total flow from both analysers. An additional step is required to confirm the sample flow balance of the negative and positive analysers. This involves adjusting the two valves before the Y-connector, where the sample flows join from both of the analysers. The two Venturi tubes next to the valves should be measured simultaneously using two handheld differential pressure sensors. The calibrated values for these pressure differences are either written inside the instrument or available from Airel Ltd. The valves should be adjusted accordingly.
The response of the DMA high voltage (HV) supply should be followed from the
instrument diagnostics. Correct sizing of small ions and particles in the
DMA is highly sensitive to the accuracy of the applied HV. Particular care
is required particularly in the low voltage range, which is used to classify
the smallest ions. The voltages in the inner electrode are
In the size range of the cluster ions and small neutral particles the calibration is a challenging task due to limitations in the capability of reference instruments, generation of proper calibration aerosols, and instrumentation for the size-separation. Possible calibration set-ups are presented in detail in Asmi et al. (2009), Gagné et al. (2011), Kangasluoma et al. (2013) and Wagner et al. (2016). To determine the losses and the sizing accuracy of the full size range of the NAIS requires an extensive suite of instrumentation: (1) in the sub-10 nm size range with a high resolution H-DMA (Herrmann-DMA; Herrmann et al., 2000; Kangasluoma et al., 2016) to determine the transfer functions and losses, (2) monomobile molecular standards (Ude and Fernández de la Mora, 2005) to determine specific mobility calibration, and (3) a Hauke DMA (Winklmayr et al., 1991) to perform the mobility and concentration calibrations in the size range from 4 to 40 nm. Prior to the loss and mobility calibration, the flows need to be verified as described above.
Wagner et al. (2016) studied the accuracy of the NAIS in a supplementary
laboratory calibration. They concluded that in ion mode the sizing of the
NAIS was very accurate, regardless of the version of the data inversion, and
the ion number concentrations were underestimated 15–30 %, depending on
the version of the data inversion. Using a correction introduced by Wagner
et al. (2016), the uncertainty of the ion concentration measurement of the
NAIS can be reduced to
Prior to the field or laboratory measurements, the electrometer background levels, and the balance of number concentration measured with the positive and negative columns should be checked. Between calibrations the NAIS should be regularly compared to a reference instrument for a period of few days per year, especially during the long-term operation. The verification should always be done when the measurement location changes.
It is recommended that different methods are used in parallel to determine the cluster and nucleation mode number concentration in order to avoid misinterpretation of results (Kulmala et al., 2012). Participation in the intercomparison workshop is recommended. Another opinion is to perform a side-by-side intercomparison at the measurement site, if a suitable supporting instrumentation is available (e.g. ion spectrometer, Gerdien counter, condensation particle counter, differential/scanning mobility particle sizer, or air conductivity measurement, e.g. Asmi et al., 2009; Gagné et al., 2011).
To verify instrument operation prior a measurement campaign, the balance of number concentration measured with the positive and negative columns should be checked. The concentration verification can be done by generating population of sample aerosol (in equilibrium charge distribution) and measuring it with both columns. A good agreement (10–20 %) between the polarities gives confidence that the instrument is working properly. In the following sections we describe three options for the verification experiments before instrument deployment.
Peeling a citrus fruit and thus releasing D-limonene, a common monoterpene, into the room air can lead to aerosol particle formation in the indoor environment (Vartiainen et al., 2006). This is a fast, easy and cheap way to generate nucleation mode aerosol particles over a whole size range of the NAIS as the D-limonene oxidation products trigger the particle formation and subsequent growth (Gagné et al., 2011). Finding the right amount of fruits to peel to generate the correct amount of vapour can take a few trials. Note that there should be sufficient ozone concentration and low background aerosol concentration in the room to facilitate new particle formation and growth.
Fast concentration verification between the polarities can be done by sampling from indoor and outdoor. The indoor sample due to efficient air-conditioning is typically dominated by cluster ions and can be used to check the balance between small ions, whereas outdoor sample has a typically abundant Aitken mode and can be used to test larger ions and particles (Hirsikko et al., 2007).
Extensive number concentrations in the range from
A recommended inlet is a 50 cm-long metallic tube (diameter 35 mm) with a
bend (90
Example an aerosol sampling inlet for the NAIS used at K-puszta field station, Hungary. A grid is missing in the top image from the end of the sampling line. Below an instrument set-up on field.
The inlet lines should include a proper rain cover as the rain droplets can interfere the measured spectra (Tammet et al., 2009). In the field conditions, the user should make sure that rain does not get into the instrument enabled by high sampling flow rate. If there is a chance of water dripping into the inlet, the end of the inlet tube should point at least slightly downwards. A metallic grid in the inlet is recommended. The performance of the mobility analyser is sensitive to insects and other material which may settle on the electrodes or electrometers. These impurities can cause corona discharge, noise and parasitic currents. Furthermore, the pressure drop from the inlet to the instruments should be kept in the range of few hPa. This is facilitated by a regular cleaning of the inlet grid.
Due to diffusional losses of small particles the inlet lines needs to be kept as short as possible and as straight as possible, while keeping the flows close to laminar. Enhanced diffusional particle losses may occur in the sampling lines with bends or elbows. The particle losses increase with a decreasing radius of the bend. It is very essential that the inlet lines and connectors should be made from a conductive material (preferably stainless steel) to avoid losses caused by a static electric charge. Experience has shown that non-conductive tubing (e.g. plastics) may remove a considerable fraction of any charged particles by the unwanted electrostatic forces. A rough estimation for particle losses should be done on the measurement site after the installation by measuring with and without inlet set-up by performing a short measurement exercise with and without the inlet construction.
When working in a warm and humid atmospheric environment, dew point
temperature of the sample flow can reached in the measurement cabin or
container (20–25
In a typical field operation, the NAIS should be placed indoors in an
air-conditioned space. The instrument should be operated in a temperature of
5–35
In close to sea level, the NAIS software uses a fixed sheath-flow rate value
of 60 L min
At high-altitude sites, the NAIS (with 3- and 4-blower systems) volumetric sample flow rate is kept constant whereas the sheath-flow rate is varied automatically (Mirme et al., 2010). The automatic adjustment of the sheath-flow compensates changes in the particle mobility in exceptionally low or varying air pressures and temperatures. Thus, the classified particle size range is kept invariant of the pressure and temperature changes. The effect of ambient temperature variations to measured ion and charged particle mobilities is considered small because of warming in the sampling line. The 1-blower system is not recommended for high-altitude or aircraft measurements and we recommend upgrading it into the 3- and 4-blower system. Otherwise, the user needs to keep the blower operating in the right volumetric flow range manually or apply a correction to the number size distributions, and a separate airflow calibration is needed for the 1-blower system in the variable environmental conditions.
Several improvements were made to the airborne NAIS to able to measure the size distribution and concentration of ions as a function of altitude inside a pressurized aircraft (see Mirme et al., 2010). When measuring inside a pressurized aircraft, the instrument leaks have to be particularly well controlled. We recommend using the upgraded 3- and 4-blower system in the aircraft deployments. These versions of the NAISs automatically adjust the aerosol sample- and sheath-flow rates so that the particle sizing and volume sample-flow rate remain constant regardless of ambient pressure. Typical modifications to the NAIS in the airborne measurements are (1) replacing the electricity supply to match the system used in the aircraft, (2) designing a special inlet system to sample air from outside the aircraft, (3) reinforcing the instrument rack and attaching it into the aircraft frame, (4) modifying the instrument for increased air tightness in the case it is used in a pressurized cabin, and (5) setting the length of the measurement cycle to a minimum. If the inlet is in over pressure, the exhaust flow may need to be restricted with valves (e.g. adding some soft tubing and a pinch cock). For the airborne measurements the recommended measurement cycle is as follows: offset – 30; ions – 45; particles – 45.
In chamber measurements, where it is required to minimize the amount of
sample air, the high sample flow rate of the NAIS is a challenge. In such an
application, the it is recommended to operate the NAIS with a recirculation system,
which dilutes the inlet sample flow with filtered air coming from the
exhaust of the instrument. In other words, the sample air from the chamber
is diluted with a portion of the exhaust air of the instrument, which is
filtered with a high-efficiency particulate air (HEPA, e.g. Camfil Megalam,
MD 14-305X305X66-10) filter and mixed with the sample air (in more details;
see Franchin et al., 2015). The pressure drop on the filter and dilution
mixer should be below 10 hPa to ensure that the sample flow blowers of the
NAIS are able to comfortably circulate the air. The use of the dilution
system allows reduction of the fresh sample flow from 54 to 20–30 L min
Otherwise in the laboratory measurements, where the available sample flow
rate is limited and sampled concentrations are small, it is recommended to
use only one polarity (column) of the NAIS (e.g. Manninen, 2011) at a time
depending on the polarity of the user needs. This is possible only with the
4-blower system, where the columns work completely independently. To disable
one column, disconnect and close the corresponding tube at the inlet
Y-connector which divides the flows to the two analysers or alternatively
close the corresponding exhaust outlet. The instrument configuration files
should be modified to switch off the blowers for the disabled column. This
must be done to avoid damage to the blowers. When the NAIS is used in
aerosol exposure studies with extremely high concentrations (
The NAIS measurement software (Spectops.exe or Retrospect.exe) is recommended to be used for checking that the instrument is operating properly by following the steps listed in the figure.
A large number of measurement parameters are automatically monitored by the instrument. They include for example flow rates, blower control signals, charger currents and filter voltages. Most of the parameters are continuously checked by the Spectops measurement software and diagnostic warning flags indicate if a problem is detected. The presence of a warning does not definitely mean that the measurements are invalid. The user should always understand and confirm the reason why the warning is raised and fix the issue when necessary. Similarly, an absence of the warning signal does not guarantee that the measurements are correct.
The corona currents and filter voltages are adjusted by varying the HV supply feed voltage. The discharger and the main charger are automatically controlled with a feedback loop driven by the current measured from the surrounding electrode. The post-filter is controlled according to the current measured by the first electrometer channels in particles measurement mode. The blowers are actively controlled according the measured flow signals from the Venturi tubes.
The automatic adjustment with the feedback works as long as all the feedback
controls are between 0.1 and 4.9 V which the NAIS user should check. A
sensor value starts to deviate from a target value if the control voltage
goes too close to 0.0 or 5.0 V. In more detail, in the Spectops diagnostic a
The instrument operation should be checked daily by the user. Figure 8
summarizes a recommended checklist for instrument's performance monitoring.
Airel Ltd. provides tools for this. There are two programs in the
measurement software package provided with the NAIS: (1) Spectops for running the
measurements and viewing online data and (2) Retrospect for viewing and reprocessing
the recorded data offline. The Airel Ltd. has an extensive diagnostic
checklist in the NAIS manual:
The blowers are automatically adjusted using a software digital feedback loop controller based on the flow sensors and barometric sensor so that the mobility analysis and sampling volumetric flow rate remain constant. When measuring through an inlet with a high pressure drop or in polluted conditions, the blower might be under heavy strain as this requires the blower to be operated with a higher voltage. A similar situation is reached, when the flow resistance increases over time due to settled atmospheric aerosols (instrument getting dirty). This calls upon a frequent cleaning of the instrument in the polluted conditions.
To keep the corona charger efficiency at a constant level independent of environmental conditions, the corona needle voltage is adjusted by varying the HV supply feed voltage according to an active feedback loop. The efficiency of the corona charger is directly determined by the charger ion concentration, which is proportional to the electric current carried by the corona ions to the surrounding electrode inside the charger volume. Thus, the discharger and the main charger are controlled with a feedback loop driven by the current measured from the surrounding electrode. In normal operating conditions of the NAIS, the corona-needle voltage is in the range of 2–3 kV. Over time the electrode (i.e. corona needle/wire) gets worn out because of aerosol particles settling on the needle. This will typically cause the corona voltage fluctuate as the corona ignition voltage increases and a stable discharge can no longer be maintained. For this reason, the NAIS corona needles need to be cleaned regularly.
In the particle mode the post-filters remove the cluster ions generated by the corona chargers. The post-filter settings should be optimized according to environmental conditions. Typically the filter operates with a voltage of 40–100 V. The corona-generated ions are at the same size range as the smallest particles measured by the NAIS. The set point of the post-filtering voltage is a compromise between the removal of corona-generated ions and the penetration of small aerosol particles. The main electrical filter voltages are adjusted by varying the HV supply feed voltage according to an active feedback loop. In the 3- and 4-blower systems, the user can change the target values by modifying the configurations with Spectops software (Mirme and Mirme, 2013). In the 1-blower systems, the post-filter voltage is adjusted by the user manually; see Supplement S2 for details. Figure 9 shows when to change settings or adjust manually the post-filter during the particle measurements. We recommend that in the continuous field measurements the automatic adjustment is used, if possible. In the laboratory experiments with rapidly changing or unusual aerosol distributions, the automatic adjustment should be switched off (Manninen et al., 2011).
As important as the instrument verification, a regular maintenance of the NAIS to maintain the calibration during the operation is critical. The maintenance procedures include instrument cleaning, leak tests, and checks on the condition of corona-needles, proper insulation between the inner and outer electrodes, and proper instrument grounding and inlet operation.
The inlet net and inlet tubing should be cleaned thoroughly in 1–3 week intervals to maintain the optimal aerosol sample flow and reduce the amount of dirt settling on the analyzer. The required interval depends on the local aerosol concentrations.
During long-term operation the NAIS should be cleaned every 1–3 months due to the deposition of particulate matter inside the instruments. Within the cleaning procedures all parts, which are in contact with the sample- and sheath-flow, should be thoroughly wiped using delicate task disposable wipes (e.g. Kimberly-Clark Kimtech Science Kimwipes) and alcohol (e.g. 2-propanol). The wipes which get easily worn out and leave fibers should be avoided. The metallic nets inside the Venturi flow tubes (i.e. tubes with narrow slits for adjusting the volumetric flow) should be cleaned carefully. When dirt settles onto the nets, the flow resistance increases, and consequently the volume flow through the Venturi tubes decreases. This alters the mobility classification. An ultrasonic bath is recommended for cleaning these nets. Instead of nets, the instruments with 3 or 4-blowers have typically honeycomb-shaped pieces to make the flow laminar. These are less likely to become dirty and a careful cleaning with a brush or pressurized air is sufficient. Overall, the 3- and 4-blower instruments are significantly less susceptible to flow deviation issues. The corona-needle chargers should be cleaned (scraped with a sharp knife) regularly (1–3 month intervals) to make sure that the corona-generated ion concentration is maintained at a constant level.
Adjusting post-filter during the particle measurement mode:
Cleaning electrometer rings by lifting away the sample preconditioning unit (the top part of the NAIS) and using a long cleaning rod to wipe the surface of outer electrode.
Use plenty of isopropanol and Kimwipes. Do not scratch the inner surfaces of the NAIS. Clean all the surfaces which are in contact with the sample and sheath air flows. Do not wipe the plastic parts with isopropanol, clean them with de-ionized water to avoid leaving a conductive film on the surface. Always wear gloves when handling the DMAs and sample preconditioning units. After the cleaning procedures, check that the ion and particle number size distributions are similar and form a continuous distribution before and after cleaning.
Cleaning electrometer rings by opening the mobility analyzer and wiping with a clean cloth.
Opening and cleaning the dischargers (upper row) and the chargers (bottom row).
Removing and cleaning the sheath air filters. Some of the newest NAIS's do not have corona needle inside the sheath air filter.
The number concentration of ions and aerosol particles are determined by
measuring a current delivered by the flow of charged particles to an
electrometer rings. The electrometers are extremely sensitive. The
deposition of dirt onto the electrometer ring can deteriorate the
signal-to-noise ratio of the electrometer. Dust or fibers that have settled on the
electrode may start to form an occasional corona discharge in the electric
field of the analyzer. This is the reason why the electrometers facing the
bottom inner electrode, which has the highest voltage, are the most likely
to become noisy (electrometers no. 13–21). We recommend wiping the
electrometer rings clean manually when the average current signal of a
certain electrometer increases above few tens of fA (10
To clean the electrometer rings without removing and opening the mobility analyzer, remove the top part of the instrument (i.e. sampling preconditioning unit; see Fig. 1). Then use a cleaning rod to clean the mobility analyzer by moving the rod from top to bottom. Figure 10 shows the procedure. Before lifting the top part of the NAIS, ensure that you remove all the nuts holding the plates together and disconnect all the cables coming from the main compartment of the instrument, and disconnect sheath air tubing between the top and bottom part of the NAIS (on both polarities). The top part should be lifted up and placed onto a clean surface, while the open analyzer should be covered to avoid dropping more dirt into it while cleaning. Now the inner and outer electrode of the mobility analyzer (i.e. electrometer rings) can be cleaned with the cleaning rod by moving it up and down inside the analyzer. Avoid scratching the metallic surfaces. The numbering of the electrometers starts from the top to bottom. Remember that the electrometers detecting the smallest charged particles are at the top of the analyzer.
To open the mobility analyzer it needs to be lifted away from its position. The outer electrode of the analyzer should be lifted up, and separated from the inner electrode. Supplement S1 shows, in detail, how to clean the analyzers after opening the mobility analyzer. The electrometers are located on the surface of the outer electrode, whereas the inner electrode has the four voltage sectors to generate the electric field. Take care not to scratch the inner electrode against the outer electrode. After the electrodes are separated, it is possible to clean the electrometer rings by wiping with a clean cloth and some strong solvent like alcohol or isopropanol; see Fig. 11. Wiping should be done by starting from the centre of the electrode and moving towards the top. Take particular care to clean the gaps between the electrometer rings as well as their surfaces. Flip the electrode upside down and repeat the operation to clean the bottom electrometers.
Cleaning the corona needle with a sharp knife (left) or by rinsing with a dissolvent (middle), and replacing a corona needle using tweezers (right).
The charging efficiency of the corona charger can change over time as the electrode (i.e. corona needle/wire) gets worn out. For this reason, the NAIS corona needles need to be cleaned regularly or replaced. To clean the corona needles located in the preconditioning unit's charger or discharger, shown in Fig. 12, or in the sheath air filter, shown in Fig. 13, the corresponding parts of the NAIS need to be opened and the needles removed. As shown in Fig. 14, the corona needle can be cleaned from dirt by gently scraping the tip with a sharp knife or by dissolving the dirt. The corona needles break and bend easily so minimum pressure should be applied. When the cleaning does not restore the charging efficiency, the needle needs to be replaced. When replacing the corona needle, be careful and handle the needle with flat tip tweezers. See Supplement S2 for details.
A blower needs to be replaced, when the active feedback loop cannot maintain the right volumetric flows for the sample and sheath flows, which leads to wrong sizing of the aerosol particles. The blower must be sealed properly and a leak test should be done to the instrument before determining the flows. For the 1-blower system, the blower sealing should be done using silicone (e.g. Bostik silicone universal) to seal both the blower itself and to connect it in a leakproof manner to the metal casing, shown in Fig. 15.
Procedure for replacing a blower for a 1-blower instrument.
The particle diameter is not a well-defined concept at very small sizes or
for highly non-spherical particles and agglomerates. We recommend using the
electrical mobility equivalent diameter as it can be converted back to the
particle electrical mobility, which is the measured quantity by the NAIS.
The electrical mobility (
Air viscosity can be written as follows:
The raw signal of the NAIS is calculated from the current
The NAIS measures all electrometer signals approximately 12 times per second. Data is averaged typically over 1 s, 10 s and measurement cycle (block) periods. The instrument measures a large number of secondary parameters that describe the detailed state of the whole system. The average electrometer signals together with the secondary measurement parameters are stored into record files by the Spectops measurement software.
The average electrometer signals are converted into ion mobility or particle size distributions by the Spectops software and stored in spectra files. The distribution is calculated using the generalized least squares method to find the size or mobility distribution that best matches the measured electrometer currents according to the instrument matrix while taking into account the noise level estimates. The instrument matrix is based on a mathematical model of the instrument that considers particle losses, charging probability (in case of particle mode measurements), electric field and air flow inside the mobility analyzer.
It is important that the user always stores the record files together with the spectra files containing the measured distributions. The secondary measurement parameters stored in the record files are vital for confirming the validity of the measurements and for problem diagnostics with the instrument. The spectra files can be recalculated from the electrometer signals stored in the record files as part of pre-processing and data analysis.
The sampled particles are assumed to be in charge equilibrium. The particle
charging probability is predicted by Fuchs' diffusion charging theory (Fuchs
and Sutugin, 1971). At a constant corona-wire current, the aerosol charging
depends mainly on the particle size, on the charger ion concentration and on
the residence time of the aerosol in the charging region. The product of the
latter two is called nt product. The model that performs the NAIS inversion,
takes into account measured aerosol volumetric flow rates, particle charging
probabilities, size dependent loss factors, and the charging parameter (i.e.
nt product). The charging probabilities use a calibrated charging parameter
Although the data inversion assumes that the sampled particles are in a charge equilibrium, the unipolar charger does not neutralize the aerosol sample entering the NAIS (McMurry et al., 2009). Thus, if the sample is highly overcharged, this can lead to an overestimation of the ion and particle concentrations.
In the ion mode, the inversion considers only charged particle mobility. It
does not make assumptions about their charging probability or background
aerosols. Therefore, it produces a mobility distribution which the user will
later convert into a size distribution assuming that all the detected ions
are singly charged. To simplify, in ion mode we make an assumption that all
charged particles are singly charged. In practice, this means that ion
concentrations in the size range from
Moreover, Alguacil and Alonso (2006) reported that when using a corona
discharge, a substantial fraction of doubly charged particles occur in the
particle diameters down to
A detailed description of the mathematical model of the NAIS is presented in
Mirme and Mirme (2013). The instrument response of the NAIS is a set of
electric currents that are generated by the flux of ions precipitating on
the collecting electrodes. An ion mobility distribution
The analyzer transfer function
During particles measurements, the corona charger is active. The instrument
response in the particles mode includes the charging probability function
The analyzer transfer function
The data inversion finds an approximate ion mobility distribution
The ratio of sample flow to total analyzer flow is about 1 : 3 for the NAIS, which is quite large and therefore even perfectly monomobile particles will have a response on several electrometers. For the particles with diameters above 20 nm the probability of acquiring more than one elementary charge in the corona charger is non-negligible. Hence the electrometer response for the larger particles becomes even wider. This also means that the measured electrometer signal may be a combination of by both singly charged smaller particles and multiply charged larger particles with the same mobility.
The multiply charged particles do not require special treatment in the data inversion. They are naturally included in the calculated response of the electrometers, i.e. the instrument matrix. However, the uncertainty of the measurement results gradually increases for particle sizes above 20 nm because the electrometer responses become less distinguishable for the larger particles and because a gradually larger portion of the response will be lost beyond the mobility range of the NAIS.
The electrostatic losses inside the corona charger during charging process lead to underestimation of the particle concentration (Alonso et al., 2006; Huang and Alonso, 2011). The diffusion losses decrease and electrostatic loss increase as the charger voltage is increased, whereas charging efficiency increases with particle size and charger voltage. Electrostatic loss of small particles increases with decreasing particle diameter. The electrostatic losses in the sampling lines prior to the instrument should be taken into account by the user.
To obtain the aerosol size distribution,
Typical faulty ion number size distributions before (left column,
problematic area boxed) and after (right column) cleaning and quality
checks. The bad or missing data was selected and replaced with
After the data collection and prior to the data analysis, the data should be
quality controlled. The bad data should be removed from the final data.
Figure 16 illustrates some examples of typical faulty spectra and the data needed
to be removed. Data quality checks and criteria, which should be fulfilled
for the ion number size distributions, are the following: (1) negative and positive ion number size distribution agree
visually (a similar distinct shape for number size distributions in both
polarities), (2) size distribution has a continuous cluster ion mode visible
in both polarities with a mode peak at
The primary data quality checks for the particle number size distributions are similar as for the ion data. In
addition, check that the corona-charger-generated ions do not dominate the
particle spectra due to inadequate post-filter settings: (1) the 2–3 nm
particle concentrations should remain in a range of
If possible the ion and particle data measured with the NAIS should be
crosschecked with the data from additional instruments. The data quality
check for the offset mode: electrometer currents during offset measurements
should not exceed
In the ion mode, the Spectops inversion algorithm converts signal from 21
electrometer signals into 28 normalized ion mobility distributions,
To do this, the user should follow steps: (1) Open the spectra data files to
get the geometric means of all 28 mobility fractions and calculate the lower
and upper mobility limits for each mobility fraction. (2) Calculate the
Diffusion losses inside the sampling lines prior to the instrument; see Fig. 7 (upper panel), should be taken into account by post-processing of the data. The particle losses by diffusion in a straight inlet line can be described by calculating a size-dependent particle penetration (Hinds, 1982). In laminar flow, these losses depend only on the line length, the flow rate through the line, and the particle size. In cases that bends cannot be avoided in the sampling pipe, the size-depended particle penetration can be calculated according to Wang et al. (2002).
In the particle mode, the raw data is typically converted into 29 particle
size distributions,
During subsequent data processing the corona-generated ions below the lowest
detection limit should be always cut out of the particle number size
distribution. The lowest detection of the NAIS is equal to the upper edge of
the corona ion size distribution which is illustrated with the gray shaded
area in Fig. 3. The determination of the lower detection limit should be
done always when no new particle formation (i.e. natural cluster activation
and growth) is taking place. If possible, after removing the corona ions
from the particle number size distribution, the particle concentrations in
the 2-3 nm range should remain in a concentration range of
Due to the design of the instrument, the particle spectra (i.e. particle
number size distribution) is measured with both positive and negative corona
charging. Ideally the distributions should be identical. A large and a
persistent difference may indicate a problem with the measurements. We
recommend giving only one particle spectra in the final processed data to
avoid misunderstanding. Thus, the user needs to decide on the preferred
polarity on the particle data, which is reported as the final particle
number size distribution. Typically, the preferred polarity is chosen with
two main criteria: (1) the polarity which has lower background level of the
corona-charger ions extending to 2–3 nm size range (which is considered the
lowest detection limit of the NAIS in particle mode); (2) the polarity, which
shows no short-term fluctuation over time (i.e. corona-charger ion
background stays same level over a diurnal cycle). (3) The small
corona-charger ion background (
The troubleshooting, in Table 2 lists how to recognize a faulty spectrum, identify potential problems and their corresponding solutions ranging from the instrumental to software issues. For further NAIS problem solving and identifying symptoms while operation; see Supplement S2.
The list of locations and altitudes, where frequent aerosol particle formation has been observed, is still growing as new measurement campaigns are organized and field sites are established. A recent review is presented in Hirsikko et al. (2011).
An exemplary negative ion number size distribution during a new particle formation measured with the NAIS (upper panel), and concentration of negative cluster ions (0.8–2.0 nm), intermediate ions (2.0–7.0 nm), large ions (7.0–20 nm) and gas phase sulfuric acid (lower panel) on 5 May 2007 in Hyytiälä.
Troubleshooting.
Continued.
Continued.
The ion spectrometer measurements performed within the EUCAARI project (Kerminen et al., 2010; Kulmala et al., 2011) present, so far, the most comprehensive effort to experimentally characterize nucleation and growth of atmospheric molecular clusters and nanoparticles at ground-based observation sites on a continental scale (Manninen et al., 2010). The atmospheric particle formation data analysis routines for the NAIS data, e.g. estimating the contribution of ions to particle formation, calculating the cluster ion and aerosol particle formation and growth rates, and the ion–ion recombination rates, is described in details in separate procedure article (Kulmala et al., 2012).
Example of a particle size distribution measured with the NAIS in the size range 2.5–20 nm and with the DMPS in the size range 20–1000 nm on 5 May 2007 in Hyytiälä (upper panel), and concentrations of 2–3 nm particles measured with the NAIS, 3–6 nm particles with the DMPS, and sulphuric acid (lower panel).
Charged particles are divided into small ions (1.3–0.5 cm
Ionization of air molecules (e.g. N
The lowest detection limit for the NAIS in the particle mode is approximately 2 nm due to overlapping corona-charger ions (Asmi et al., 2009; Manninen et al., 2011). Thus, the NAIS is not able to detect the pool of stable neutral clusters at sub-2 nm (Kulmala et al., 2013). The NAIS can detect only a “shoulder” of this neutral cluster pool. The NAIS in a very capable tool for detecting the newly formed particle already at the 2–3 nm size range depending on the post-filter settings. As an example, the time series of 2–3 and 3–6 nm particle concentrations on new particle formation day are shown in Fig. 18. In the particle mode, the NAIS overestimates the total particle number concentrations by a factor of 2–4 (Manninen et al., 2009; Gagné et al., 2011). The quantitative agreement improves at conditions representing particle formation bursts when higher particle concentrations are typically observed in the overlapping size range (nucleation mode). As seen in Fig. 19, merging particle number size distributions measured with the NAIS (2.5–40 nm) and a differential mobility particle sizer (DMPS, 40–1000 nm) without any additional fitting highlights the problem at 20–40 nm range where the agreement is poor, as the NAIS overestimates particle concentrations. Therefore, we recommend using the particle spectra measured with the NAIS up to 20 nm.
Merging particle size distributions measured with the NAIS in the size range 2.5–40 nm and with the DMPS in the size range 40–1000 nm on 23 April 2007 in Hyytiälä, Finland.
Neutral particle formation seems to dominate over ion-induced and ion-recombined nucleation, at least in the continental boundary layer (Lovejoy et al., 2004; Manninen et al., 2009, 2010; Zhang et al., 2012; Kulmala et al., 2013). The results obtained from the NAIS particle and ion measurements agree well with separate independent measurements performed with other electrical mobility spectrometer (e.g. Gagné et al., 2011) and condensation-based techniques (Lehtipalo et al., 2009, 2010; Kulmala et al., 2013; Rose et al., 2015). The atmospheric ions participate in the initial steps of the new particle formation, although their contribution has been shown to be minor in the boundary layer (e.g. Kulmala et al., 2013). The highest atmospheric particle formation rates are observed at the most polluted sites, where the role of ions was the least pronounced (Manninen et al., 2010). Furthermore, an increase of particle growth rate with size suggests that enhancement of the growth by ions is negligible (Yli-Juuti et al., 2011).
Typical negative ion number size distribution measured with the ion
spectrometer in different environments and conditions:
It can be noted from Fig. 20 that typical atmospheric ion and particle distributions measured with the NAIS varies much from a regional new particle event day to a very clean day, when the cluster ions are the most dominant feature in the ion distribution between 0.8 and 42 nm. A closer look at the particle formation, in Fig. 20f–h, reveals that the nucleation bursts are usually observed during daytime and mostly starting before noon. Based on the visual shape of the time series of the number size distribution, several nucleation event types have been characterized (Hirsikko et al., 2007; Manninen et al., 2010); see Fig. 20 for examples.
By conducting measurements according to the work presented here, it is possible to develop simple yet sufficiently accurate nucleation parameterizations for large-scale atmospheric modelling. The secondary aerosol formation includes the production of nanometer-sized clusters from atmospheric vapours and the growth of these clusters to larger particles. One dynamic process modifying the size distributions of neutral and charged clusters is ion–ion recombination, which was parameterized by Kontkanen et al. (2013). Nieminen et al. (2011) derived a parameterization for the ion-induced nucleation or, more precisely, for the formation rate of charged 2 nm particles. In addition, it is important to predict nanoparticle growth accurately in order to reliably estimate the atmospheric cloud condensation nuclei concentrations. Häkkinen et al. (2013) introduced a semi-empirical parameterization for sub-20 nm particle growth that distributes secondary organics to the nanoparticles according to their size and is therefore able to reproduce particle growth observed in the atmosphere. All semi-empirical parameterizations described here are based on extensive NAIS datasets that enable to test how well the parameterization captures the seasonal cycle of the modelled parameters and to determine the required weighing factors in different environments. Leppä et al. (2009) introduced an aerosol dynamical box model, which includes basic dynamical processes (e.g. condensation, coagulation and losses by deposition) as well as ion–aerosol attachment and ion–ion recombination. This model was validated and constrained against the NAIS data.
Small ions are almost always present in the air and are responsible for the atmospheric electrical conductivity (e.g. Harrison and Carslaw, 2003). The early research of air ions was mainly focused on atmospheric electricity to study, e.g. air quality (Israël, 1970). Tammet et al. (2009) suggested that the air ions and the atmospheric electric field controlling the migration of ions should be considered, when discussing the formation of primary and secondary particles. Air (polar) conductivity can be calculated directly from the ion number size distributions measured by the NAIS, in addition to reporting the concentration of small, intermediate, and large ions, and the average small ion mobility.
We work towards a better understanding the formation and growth mechanisms of aerosol particles using experimental observations. The first steps to understand the role of ions and particles in the global climate are to understand where, when and why the nucleation mode particles are formed. The current level of understanding the aerosol effects leads to large uncertainties in global climate model predictions (IPCC, 2013). Thus, the current aerosol process models need to be improved to capture the dynamics at sub-20 nm size range. To constrain and validate these models, reliable field observations are needed. Here we aim to provide tools to harmonize the measurements performed with the NAIS, leading to comparable results that may be used to increase our understanding of the aerosol and ion dynamics in the atmosphere that can be important to various aerosol process parameterization and to global model validation.
This work is part of a protocol work done within an ACTRIS community. The ACTRIS is an European Research Infrastructure for the observation of Aerosol, Clouds, and Trace gases, and it aims to serve a vast community working on models and forecast systems by offering high quality atmospheric data. Several large-scale modelling studies have demonstrated that more reliable nucleation parameterizations than currently available are needed to evaluate the importance of nucleation in climate (Spracklen et al., 2006; Makkonen et al., 2009; Merikanto et al., 2009; Pierce and Adams, 2009; Yu, 2010). Based on the NAIS results, nucleation parameterizations (e.g. size-dependent atmospheric nanoparticle growth and nucleation favouring ion processes) already exist for large-scale modelling but no global model is using those (see Nieminen et al., 2011; Kontkanen et al., 2013; Häkkinen et al., 2013). Overall, the large goal is to integrate the NAIS to various international research networks as a standard instrument to detect the atmospheric nanoparticles when studying climate and air quality, and to increase the utilization of existing extensive NAIS datasets. By improving the accuracy and comparability of the measurements and instrument laboratory characterization, we also improve the fundamental understanding on the atmospheric ion and aerosol population and the physical processes affecting the population dynamics.
All codes necessary for a reader to understand and evaluate the conclusions of the paper will be archived in an approved database and made available to any user via personal communication to authors.
All data necessary for a reader to understand and
evaluate the conclusions of the paper are included in the paper or its
supplement or will be archived in an approved database and made available to any user via personal communication to authors. The ion number size distribution data presented in the figures
is accessible at the EMEP database (
H. E. Manninen was primarily responsible for the design and interpretation of the reported experiments. S. Mirme and A. Mirme provided technical support, and T. Petäjä and M. Kulmala administrative and supervisory support that made a direct substantial intellectual contribution to this research. H. E. Manninen prepared the manuscript with contributions from all co-authors.
This work is based on the long-term experience by the University of Helsinki (UHEL) performing field, laboratory and chamber measurements with the NAIS, and the SOPs written by the UHEL working group for the ACTRIS community (the European Union's FP7 capacities programme under grant no. 262254, and Horizon 2020 research and innovation programme under grant no. 654109). We acknowledge John Backman, Alessando Franchin, Janne Lampilahti, Katri Leino, and Ville Vakkari for their contribution on providing material for this SOP. UHEL acknowledges the Academy of Finland Centre of Excellence (grant no. 272041). H. E. Manninen acknowledges support by the Finnish Cultural Foundation (grant no. 00121082). S. Mirme and A. Mirme acknowledge the Estonian Research Council Project (grant no. IUT20-11), the European Regional Development Fund through the Environmental Conservation and Environmental Technology R&D Programme project BioAtmos (grant no. 3.2.0802.11-0043), and the “Estonian Research Infrastructures Roadmap” project Estonian Environmental Observatory (grant no. 3.2.0304.11-0395). Edited by: J. Curtius Reviewed by: two anonymous referees