The Sequential Spot Sampler (S3), a newly developed instrument to collect
aerosols for time-resolved chemical composition measurements, was evaluated
in the laboratory and field for the measurement of particulate sulfate and
nitrate. The S3 uses a multi-temperature condensation growth tube to grow
individual aerosols to droplets which are then deposited as a
Atmospheric aerosols have important local, regional, and global impacts. To understand the extent and nature of these effects, time-resolved observations of aerosol chemical composition are needed. Many different semi-continuous and online methods of measurement have been explored to address this need. Various instruments using aerosol mass spectrometry have been developed in recent years (e.g., Jayne et al., 2000; Allan et al., 2003; Drewnick et al., 2003; Jimenez et al., 2003; Takegawa et al., 2005) to make direct online measurements of particle chemical composition. These include single-particle measurements (e.g., Prather et al., 1994; Noble and Prather, 1996) and measurements of submicron, non-refractory aerosols (e.g., Canagaratna et al., 2007; Hings et al., 2007; Ng et al., 2011; Budisulistiorini et al., 2014). Several other semi-continuous methods have also been developed, including the Particle-Into-Liquid Sampler coupled with an Ion Chromatograph (PILS-IC), which can grow and collect aerosol particles into a flowing liquid stream and utilize an IC for the analysis (Orsini et al., 2003; Weber et al., 2001); the Monitor for AeRosols and Gases in ambient Air (MARGA), which uses steam capture and IC analysis of aerosols and trace gases (Schaap et al., 2011; Makkonen et al, 2012); the humidified impaction with flash volatilization method, developed by Stolzenburg et al. (2003) and used by Wittig et al. (2004); the Ambient Ion Monitor–Ion Chromatograph (AIM-IC), which has been characterized and used by Markovic et al. (2012) for the measurement of water-soluble chemical composition of atmospheric fine particulate matter; and the Gas Particle Ion Chromatography (GPIC) system developed by Dionex (Godri et al., 2009). Most of these systems provide very useful information about aerosol composition with time resolutions of several minutes or better. However, because these instruments locate the analytical measurement component (e.g., an IC or a mass spectrometer) in the field, they carry significant capital cost, have a large footprint, and work best with a highly skilled field operator. These factors have so far limited the use of such approaches in large measurement networks, where cost, space, and operator time are critical considerations.
A more common and cost-effective approach for aerosol composition measurement in aerosol networks is to collect the samples in the field and then analyze them at a central laboratory location. This approach has been used for decades to measure aerosol composition in monitoring networks around the world (e.g., Chow et al., 1994). These offline aerosol measurement systems often require the collection of aerosols on filters. Extraction of the filters usually involves sonicating them in a liquid (e.g., ultrapure water), then filtering the liquid, relocating it into a different vessel, and analyzing it. Filter collections in the field are relatively inexpensive, as long as sample frequency is modest, but once in the laboratory they are labor intensive and subject to possible contamination during operator handling. Another disadvantage to this approach is the need for collection of large air volumes to ensure sufficient sensitivity for analyte measurement in filter extracts (usually several milliliters or greater in volume), only a small fraction of which is typically analyzed. As a consequence of the large air sampling volumes required and the inconvenience of making frequent filter changes in the field, filter sampling is typically conducted with sample durations of 24 h or longer. In addition, for network monitoring purposes, samples are usually only collected once a week, every 3 days or every 6th day. A higher time resolution (at least hourly samples) for the measurement of aerosol chemical composition is an important tool in addressing the impact of aerosols on the environment (Schaap et al., 2011). In order to provide convenient and higher time resolution field collection of aerosols in a manner suitable for automated post-collection analysis at a centralized laboratory, the Sequential Spot Sampler (S3) was developed.
The development and function of the S3 have been discussed in detail by
Eiguren-Fernandez et al. (2014a). The S3 uses the water-based condensation
growth technique developed by Hering et al. (2014) to grow fine particles
into micrometer-sized droplets. The droplets are deposited as dry,
This sample collection method facilitates the automation of laboratory
aerosol sample extraction and analysis. The well plate is placed on a
customized tray in a needle-based, commercial autosampler. The autosampler
handles the addition of solution, sample extraction, and injection onto a
chromatograph such that the entire plate may be analyzed without user
intervention. The small volume of a well (
The suitability of the choice of well plate material, sample storage, and extraction processes and the integrity of the collected samples over time were investigated in the laboratory, and the results are presented here. After the completion of these tests, the S3 was deployed at a site in southern California to test its capability for accurate hourly aerosol composition measurements. The field site was chosen to present the S3 with a broad range of aerosol concentrations. During this field measurement two S3 systems were operated side by side for comparison. Measurements using the S3 samplers were also compared with two well-established reference methods: a PILS-IC system and a URG denuder–filter-pack sampler. The overall field performance of the S3 during the field campaign is summarized, including quantification of sampler precision and accuracy.
The S3 consists of a three-stage water condensation growth tube followed by a
single-jet impactor collector, as shown in Fig. 1. It operates at a flow
rate of 1.0–1.5 L min
Diagram of S3 with sample collection steps (step 1: conditioner;
step 2: initiator; step 3: moderator) and a multi-well plate where samples
are deposited. (First published in Aerosol Science and Technology, 48, 656,
2014,
The S3 has been characterized in the laboratory as described by
Eiguren-Fernandez et al. (2014a). Particles as small as 8 nm grow through
water condensation to form 1–3
At the conclusion of a sequence of samples, the well plate is removed from the S3 sampler, placed in a sealed Petri dish, stored cold, and returned to the laboratory for analysis. The laboratory analyses are done in an automated fashion using a needle-based autosampler such as the Dionex AS or the PAL System CTC (both of which have been used in the S3 development and testing). The well plate is simply placed on the autosampler; the autosampler adds liquid (e.g., water, eluent) to the active well and then, after the required extraction period, injects the extracted sample into a multi-port valve, from where it is transferred onto an IC. The autosampler is programmed such that the soaking period overlaps with the analysis of the previously extracted sample. Thus the complete set of samples from the well plate may be analyzed in no more time than required for the chromatography and without operator intervention. During the analyses for this study, the soaking time was 60 min and the IC analysis time was 30 min.
Laboratory studies were conducted to test the efficiency and suitability of well plates for use with the S3 system for the analysis of sulfate and nitrate. Initial well plate material tests were conducted using acrylonitrile–butadiene–styrene (ABS), high-density polyethylene (HDPE), ultra-high-molecular-weight polyethylene (UHMWP), and poly ether ether ketone (PEEK) to test for sample extraction efficiency and sample stability over time. A combined seven-anion standard from Dionex (Sunnyvale, CA) was used as a surrogate sample for testing purposes.
PEEK (McMaster Carr, Elmhurst, IL) was chosen to manufacture well plates for further S3 testing, based on superior results of the sample recovery and stability tests, and because it is inert and does not interact with the types of compounds investigated here (i.e., nitrate and sulfate). PEEK has a low water absorption percentage (0.1–0.5 %), which is important as the deposited aerosols are extracted in aqueous solutions. Also, PEEK has been previously used in other aerosol measurement instruments due to some of its characteristics noted above (e.g., Orsini et al., 2003; Morales-Riffo and Richter, 2004). Initial tests on the system were conducted through a manual setup where the operator would pipette the extracted deposit to vials, and the vials were transferred to an autosampler for IC analysis.
The amount of time required for complete extraction of sulfate and nitrate
from the dry deposits was evaluated using the combined seven-anion standard
from Dionex (the anions in the Dionex standards were prepared from sodium
salts). Multiple samples were prepared by pipetting 10
PEEK well plates were investigated for possible background analyte
interference from plate material and environmental contaminants that may
have been introduced from storing the plate in the S3, on the autosampler, and
in the freezer, and/or from exposure to laboratory air during analysis. Ultrapure
water (> 18.2 M
In order to check the integrity of samples over time, 10
During 13 June–5 July 2012, two S3 systems, a PILS-IC and URG annular
denuder–filter-packs, were used to measure ambient PM
The two S3 systems were set up side by side inside the mobile laboratory,
sharing a common inlet of copper tubing (1
When analysis was planned, plates were removed from the freezer and left
inside the sealed Petri dishes until they were at room temperature
(
A PILS-IC was deployed alongside the pair of S3 samplers and used for the
semi-continuous measurement of ambient PM
Two URG denuder–filter-pack systems were deployed outside of the mobile
laboratory in a field near the IMPROVE site. The two systems were not
operated simultaneously but rather programmed for continuous collection of
samples. These systems included a PM
As part of the sample extraction step, liquid (ultrapure water) was injected
in a well and left for a specified amount of time. The required extraction
time for complete recovery of the deposited nitrate and sulfate in each well
was investigated. Different extraction times, ranging from 15 to 120 min,
were tested in the laboratory. This experiment was repeated for five different
concentrations of sulfate (65–1600
Percent recovery of nitrate and sulfate deposits for different sample extraction times. The error bars shown represent the standard deviations of recovery percentage from all five experiments at each extraction time.
Percent recovery of deposited standard in wells after different plate storage periods (up to 23 months). The error bars represent the standard deviation of data from five well plates.
Time series of PM
After samples are collected on the multi-well plates, it may be desirable to store plates in the freezer before analysis for extended periods of time. Therefore, it is important to quantify the duration of time during which the deposited (or sampled) constituents are not lost or contaminated. Deposits in multi-well plates were stored and tested for 23 months. Figure 3 shows the data from this test. These results demonstrate that the mass of the deposit for each analyte stays consistent during this time. The percentage of deposit recovered fell between 94 and 98 % for all 24 wells. This shows that the methods of storage and re-extraction of samples are suitable for periods up to at least 23 months.
Intercomparison of PM
Background concentrations for sulfate and nitrate for several well plates were checked using deposits of ultrapure water. It was found that the PEEK material of the plate and the process of injecting and extracting the sample did not add any artifact to the background concentrations of nitrate and sulfate. The laboratory tests demonstrate the suitability of the PEEK material for the construction of the well plates, the lack of environmental contaminants or artifacts in the procedures used to store and extract the plates, and the possibility of storing sampled well plates for future analysis without any loss of sample or contamination, all with respect to the analysis of nitrate and sulfate. Background concentrations were mostly below the LOD of the IC used for this analysis. The performance of the PEEK well plates was also examined by using sulfate salts associated with cations other than sodium, with similar results.
Concentrations of PM
The results from the side-by-side comparison of the two colocated S3
instruments for both nitrate and sulfate are presented in Fig. 5. For a
least-squares linear regression with the line forced through the origin, the
The data from two S3s were averaged for comparison of concentrations between
the S3s and other measurements. All the data collected were averaged over the
longer data collection period for comparison (i.e., PILS-IC averaged to S3
time, and S3 averaged to URG time). The comparison results are presented in
Fig. 6. The
Comparison of aerosol nitrate and sulfate during the San Gorgonio,
CA study between the S3, PILS-IC, and URG systems. The top panel
PEEK was shown to be suitable for the construction of well plates for the S3 for the collection and analysis of aerosol sulfate and nitrate. Furthermore, the PEEK well plates can be sampled and stably stored in a freezer for future analysis (at least 23 months after the collection of the samples).
Field evaluation of the S3 systems demonstrated good precision, with relative
standard deviations (RSDs) of 2.4 and 8.7 % (for sulfate and nitrate,
respectively) from co-located samplers. When comparing the S3 instrument
with other well-established methods, the S3 results performed well, with
The S3 is a compact system that can be deployed for field measurements to
characterize the chemical composition of ambient aerosols. It requires low
maintenance in the field as demonstrated during its deployment in the 23-day
field measurement campaign discussed in this paper. Operator intervention
was required for changing the plates, adding DI water to the S3 water
reservoir and discarding wastewater, and in case of instrument malfunction.
The S3 has a small footprint (
This research was supported by the National Institute of Environmental Health Sciences of the National Institutes of Health under award number RC3ES019081. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. Edited by: P. Herckes