Our understanding of formation processes, physical properties, and
climate/health effects of organic aerosols is still limited in part due to
limited knowledge of organic aerosol composition. We present speciated
measurements of organic aerosol composition by two methods: in situ
thermal-desorption proton-transfer-reaction mass spectrometry (TD-PTR-MS) and
offline two-dimensional gas chromatography with a time-of-flight mass
spectrometer (GC
Aerosol particles are ubiquitous in the atmosphere, and are important for two main reasons. Firstly, they scatter and absorb solar radiation, and change cloud properties affecting climate on Earth (Boucher et al., 2013). Secondly, they penetrate into human lungs, causing increased mortality (e.g. Pope and Dockery, 2006). Atmospheric aerosol has various sources, both natural and anthropogenic (e.g. de Gouw and Jimenez, 2009). Organic aerosol (OA) comprises 20 to 90 % of the total aerosol mass (Kanakidou et al., 2005). OA can be emitted directly (primary OA) but can also be produced in the atmosphere via photochemical oxidation of volatile organic compounds (secondary OA).
Elucidating aerosol chemical composition is key to understanding sources and
formation processes and to effectively controlling aerosol amounts in the
atmosphere (e.g. Ulbrich et al., 2009). For example,
Even though many in situ techniques have been deployed to study OA composition (e.g. Jayne et al., 2000; Holzinger et al., 2010a; Weber et al., 2001), it is still commonly characterized on the bulk level using descriptors such as oxygen-to-carbon (O / C) ratio, volatility distribution, or total organic carbon mass. Only a limited number of in situ studies have researched OA at a molecular level using high time resolution (2-hourly or better) measurements (e.g. Williams et al., 2014; Yatavelli et al., 2014; Zhao et al., 2013). Therefore, more detailed studies from various locations and time periods are needed to better understand chemical composition and sources of OA.
Here we deployed two different techniques allowing for detailed chemical
composition measurements of OA: (1) in situ thermal-desorption
proton-transfer-reaction mass spectrometry (TD-PTR-MS) and (2) offline
filter analysis by comprehensive two-dimensional gas chromatography coupled
to time-of-flight mass spectrometry (GC
In this study we aim to use the GC
The data presented in this paper were obtained during the CalNex field
campaign in Pasadena, California, performed from 15 May until 16 June 2010.
The site is located approximately 18 km northeast of downtown of Los Angeles
on the campus of the California Institute of Technology (34.1408
In situ aerosol measurements were carried out with an aerosol sampling unit
with two identical inlet systems attached to a proton-transfer-reaction
time-of-flight mass spectrometer (PTR-TOF-MS, further referred to as
“PTR-MS”) (Fig. 1a). The setup has been described in detail elsewhere
(Holzinger et al., 2010a, 2013). In short, the air flow
passes through 12 m long copper inlet tubes (ID
The in situ
After the measurements from the first inlet are finished, the valve system is switched automatically to allow aerosol measurements from the second inlet to start. Subsequent to the measurements from the second inlet, gas-phase measurements (not considered in the current paper) are carried out and then the measurement cycle starts over (see Fig. 1 in Holzinger et al., 2013).
Filter samples were analysed offline using comprehensive two-dimensional gas
chromatography coupled to a time-of-flight mass spectrometer
(GC
Among the 1100 resolved peaks, the 153 compounds reported here were
positively identified with the GC
The AMS measurements used in the current study has been described previously
in detail (Hayes et al., 2013). In short, AMS allows for measurements of
nonrefractory submicron aerosol (organic and inorganic) (DeCarlo et al.,
2006). The operational principle of AMS can be presented briefly as follows.
Air is sampled through a critical orifice with a consecutive focusing,
acceleration, and separation of particles by size. Next, particles are
vaporized at 600
In this paper we present measurements of two types of standards: single
compounds and a mixture of compounds. The following single compounds were
measured: decanoic, pentadecanoic, and octadecanoic acids. Known quantities
of each acid were first dissolved in ethanol, and then an aliquot of the
solution containing 10
A mixture containing 77 representative organic compounds and C8–C40 alkanes (this mixture is further called “multicomponent mixture”) was carefully prepared by dissolving respective compounds in deuterated acetone. An aliquot of the solution with 0.062 to 20 ng of the substances was placed on quartz filters. In this paper we focus only on acids contained in this standard (21 acids). Again, three filter replicas and two blank filters were measured with the offline TD-PTR-MS. Blank filters were prepared by adding an aliquot of deuterated acetone without a dissolved standard on a piece of filter.
The filter measuring procedure is described in detail by Timkovsky et al. (2015). In short, the sample is placed in the oven and allowed to stabilize
for 2 min. Next, the temperature of the oven is increased stepwise
from 100 to 350
Data evaluation was done with Interactive Data Language (IDL, version 8.1.0,
ITT Visual Information Solutions) using custom-made routines described by
Holzinger et al. (2010b) and Holzinger (2015). The
initial mass lists consisted of 717 and 748 masses for multicomponent
mixture and CalNex measurements respectively. Ions associated with primary
ions and contaminations from the ion source were removed from the mass lists
by filtering out
For the in situ data analysis, the initial mass spectra were first averaged
to obtain data with a time resolution of 5 s. Second, the data were averaged
over the measured temperature step (3 min each) and the data for all
temperature steps were summed. Third, the resulted mixing ratios were
converted to mass concentrations for individual ions by multiplying by ion
molecular mass, volume of nitrogen used for desorption for one measurement
cycle, and dividing by the volume of air sample from which aerosols were
collected. Fourth, the data from inlet A and B were merged and averaged to
match the filter sampling times. Fifth, the resulted mass concentrations
were averaged over the whole comparison period (30–31 May) for the data
presented in Sect. 3.2.2. The maximum total uncertainty of
The same two initial steps were taken for the analysis of the offline
TD-PTR-MS data. The obtained data with a 3 min resolution were processed
according to the procedure described in Timkovsky et al. (2015). In short,
the instrument background and blank corrected mass at a single temperature
step (
GC
In order to match ions measured by the TD-PTR-MS with compounds spiked on the
filters and those measured with the GC
For example, 6H-Indolo[3,2,1-de][1,5]naphthyridin-6-one
(C
Whenever more than one known compound was measured at the same mass, the most
abundant known compound (based on the GC
In the case where structural isomers were identified with the
GC
Applying these rules we were able to match 123 of the 132 distinguishable
compounds measured with the GC
To calibrate the in situ TD-PTR-MS technique for measurements of
monocarboxylic acids, a series of filters with known quantities of the acids
were prepared and measured with the offline setup. Figure 2 shows the ratio of
the detected amount of substance and the amount of monocarboxylic acids that
was applied on the filter, i.e. fraction of acid recovered. The measurements
of single compounds (pink triangles in Fig. 2) and the multicomponent
mixture (blue triangles and black crosses in Fig. 2) are shown together in
this figure. Only the signal of the protonated ion has been used to
calculate mass concentrations of alkanoic acids measured with the offline
TD-PTR-MS technique. In total, 24 monocarboxylic acids are measured (Fig. 2). The lowest fractions (i.e. lower amounts detected by the TD-PTR-MS) are
observed for the high molecular mass acids (
The ratio of the amount of a substance on the filters measured with the offline TD-PTR-MS technique to the known amount of the substance put on the filters (fraction of acid recovered).
Five out of the 21 monocarboxylic acids (
Molecular formula, masses, and fraction recovered of 24 protonated monocarboxylic acids measured as standards on quartz filters individually (in italic) and in the multicomponent mixture. Acids indicated in bold are not detected with the offline TD-PTR-MS technique.
Alkanoic acids with
Based on the presented measurements, a calibration factor for alkanoic acids
with
In Fig. 3 we present the time series of total OA mass concentrations measured
by the in situ TD-PTR-MS and the AMS instruments (further named “total
OA_PTR” and “total OA_AMS” respectively), the total concentration of
the 123 compounds measured by the GC
The 2-day cycle of total OA mass concentration (in black, total
OA_PTR) and OA mass concentration of 64 masses (in red) measured with the in
situ TD-PTR-MS technique, and total OA mass concentration (in grey, total
OA_AMS) measured by the AMS and OA mass concentration of 123 compounds (in
violet) measured with the GC
In general, the total OA_PTR and the total OA_AMS correlate well with a correlation coefficient (
A reasonable qualitative and quantitative correlation is observed between the
123 compounds_GC
Figure 4 presents mass concentrations of compounds measured with the in situ
TD-PTR-MS technique vs. corresponding mass concentrations measured with
the GC
The PTR / (GC
Comparison of aerosol mass concentrations measured with the in situ
TD-PTR-MS and GC
An accuracy of 54 % for TD-PTR-MS and 40 % for GC
For some compounds (such as hopanes and oxygenated PAHs), GC
At the same time, for alkanes and one amide substantially lower concentrations were detected by TD-PTR-MS (the corresponding points are located at a substantial distance from 1 : 1 line). For alkanes this can be explained by the fact that the main masses at which alkanes are detected (43.055, 57.070, 71.086 amu) were not considered because large contamination from the gas phase did not allow to quantify the condensed fraction. More complicated fragmentation in the PTR-MS can likely explain the lower concentrations found for the amide (N,N-dibutylformamide). For all compounds of the class of alkanoic acids (except for decanoic acid), the concentrations were measured to be lower by the TD-PTR-MS, which is consistent with a positive sampling artifact that is common to quartz filter collection. The latter will be discussed in the following section.
The four alkanoic acids shown in Fig. 4 as black crosses in a red oval are
n-dodecanoic, n-tridecanoic, n-tetradecanoic, and n-hexadecanoic acids. These
four compounds are among the most abundant species measured by the
GC
Calculated partitioning coefficients
The fraction of the amount of a compound in the particle phase (
Since
The full 2-day time series for the four alkanoic acids obtained with the
TD-PTR-MS and the GC
Time profiles for mass concentrations measured with the TD-PTR-MS
and the GC
A comparison of the in situ TD-PTR-MS and offline quartz filter analysis by
the GC
The calibration measurements showed that n-alkanoic acids with molecular
mass (
For the comparison of the in situ TD-PTR-MS and the offline
GC
Most classes of compounds were detected well by the TD-PTR-MS. The positive
filter sampling artifacts, caused by the semivolatile nature of the do-,
tri-,
and tetradecanoic acids, likely resulted in the higher concentrations
observed by the GC
The PTR-TOF-MS has been funded by the Netherlands Organization for Scientific
Research (NWO) under the ALW-Middelgroot program (grant 834.08.002).
Deployment of the PTR-TOF-MS at CalNex and the analysis using
GC