The detailed molecular composition of laboratory generated limonene ozonolysis secondary organic aerosol (SOA) was studied using ultrahigh-resolution Fourier transform ion cyclotron resonance ...(FT-ICR) mass spectrometry. Approximately 1200 molecular formulas were identified in the SOA over the mass range of 140 to 850 Da. Four characteristic groups of high relative abundance species were observed; they indicate an array of accretion products that retain a large fraction of the limonene skeleton. The identified molecular formulas of each of the groups are related to one another by CH2, O and CH2O homologous series. The CH2 and O homologous series of the low molecular weight (MW) SOA (m/z < 300) are explained with a combination of functionalization and fragmentation of radical intermediates and reactive uptake of gas-phase carbonyls. They include isomerization and elimination reactions of Criegee radicals, reactions between alkyl peroxy radicals, and scission of alkoxy radicals resulting from the Criegee radicals. The presence of compounds with 10–15 carbon atoms in the first group (e.g. C11H18O6) provides evidence for SOA formation by the reactive uptake of gas-phase carbonyls during limonene ozonolysis. The high MW compounds (m/z > 300) were found to constitute a significant number fraction of the identified SOA components. The formation of high MW compounds was evaluated by molecular formula trends, fragmentation analysis of select high MW compounds and a comprehensive reaction matrix including the identified low MW SOA, hydroperoxides and Criegee radicals as building blocks. Although the formation of high MW SOA may occur via a variety of radical and non-radical reaction channels, the combined approach indicates a greater importance of the non-condensation reactions over aldol and ester condensation reaction channels. Among these hemi-acetal reactions appear to be most dominant followed by hydroperoxide and Criegee reaction channels.
Results from photooxidation of aromatic compounds in a reaction chamber show that a substantial fraction of the organic aerosol mass is composed of polymers. This polymerization results from ...reactions of carbonyls and their hydrates. After aging for more than 20 hours, about 50% of the particle mass consists of polymers with a molecular mass up to 1000 daltons. This results in a lower volatility of this secondary organic aerosol and a higher aerosol yield than a model using vapor pressures of individual organic species would predict.
Secondary organic aerosol (SOA) formation from the photooxidation of an anthropogenic (1,3,5-trimethylbenzene) and a biogenic (alpha-pinene) precursor was investigated at the new PSI smog chamber. ...The chemistry of the gas phase was followed by proton transfer reaction mass spectrometry, while the aerosol chemistry was investigated with aerosol mass spectrometry, ion chromatography, laser desorption ionization mass spectrometry, and infrared spectroscopy, along with volatility and hygroscopicity studies. Evidence for oligomer formation for SOA from both precursors was given by an increasing abundance of compounds with a high molecular weight (up to 1000 Da) and by an increasing thermal stability with increasing aging time. The results were compared to data obtained from ambient aerosol samples, revealing a number of similar features.
Organic acids in the gas and aerosol phase from photooxidation of 1,3,5-trimethylbenzene in the presence of 300 ppb propene and 300 ppb NOx in smog chamber experiments were determined using a wet ...effluent diffusion denuder/aerosol collector coupled to ion chromatography (IC) with conductivity detection. Behind the IC, the samples were collected using a fraction collector, for identification of unresolved/unidentified organic acids with IC−mass spectrometry (MS). In total, 20 organic acids were found with MS of which 10 were identified. The organic acids identified offline by IC−MS were then further quantified based on the online IC data. The identification was additionally confirmed with gas chromatography−mass spectrometry. At the maximum aerosol concentration, organic acids comprised 20−45% of the total aerosol mass. The method has a detection limit of 10−100 ng/m3 for the identified carboxylic acids.
Measurements of the fraction of contemporary carbon (
f
C) in organic carbon (OC) of ambient aerosols are presented using radiocarbon (
14C). This value directly represents the biogenic contribution ...to OC, as the biosphere releases organic compounds with the present
14C/
12C level (
f
C=1), whereas
14C has become extinct in anthropogenic emissions of fossil carbon (
f
C=0). This simple model assumes biomass burning to be negligible. Daytime and nighttime samples were collected at an urban location in Zürich (Switzerland) in August 2002. Measured
f
C values ranged from 0.51 to 0.80, which indicates a major biogenic influence. This demonstrates that carbonaceous aerosol at this site is more dominated by the rural vicinity of Zürich rather than direct urban sources. Furthermore, this investigation enabled an insight into the processes of particle formation, accumulation, and deposition for biogenic and anthropogenic emission sources. On the one hand, biogenic OC depends on the activity of plants to emit reactive volatile species like monoterpenes and on atmospheric oxidants that promote secondary organic aerosol formation. On the other hand, anthropogenic OC correlates with black carbon or elemental carbon.
Aerosol samples were collected in Zurich, Switzerland, at an urban background site and were analyzed with size exclusion chromatography (SEC) and laser/desorption ionization mass spectrometry ...(LDI‐MS) for water‐soluble organic compounds with high molecular weight. Daily samples were collected during two campaigns in winter and summer, for 1 month each. The concentration of high‐molecular‐weight compounds (humic‐like substances (HULIS)) was between 0.4 and 4 μg/m3 in winter and summer. The most intense signals in the LDI‐MS mass spectra were measured between m/z150 and 500, comparing well with the mode of the two main high mass peaks determined with SEC corresponding to masses between 200 and 600 Da. For the maximum molecular weight, however, different results were obtained by the two techniques: whereas a maximum molecular weight between 1300 and 3300 Da was found with SEC, hardly any peaks above m/z700 were measured with LDI‐MS. During summer the maximum molecular weight of HULIS (determined with SEC) correlates positively with several parameters such as ozone and increased temperature indicative of enhanced atmospheric photo‐oxidation. The HULIS concentration also correlates positively with the oxalic acid concentration in the particles. This suggests that HULIS are generated by secondary processes in summer. The lack of such correlations during winter suggests that other sources and processes might be important during colder seasons.
Stable carbon isotope ratio (
δ
13C) data can provide important information regarding the sources and the processing of atmospheric organic carbon species. Formic, acetic and oxalic acid were ...collected from Zurich city in August–September 2002 and March 2003 in the gas and aerosol phase, and the corresponding
δ
13C analysis was performed using a wet oxidation method followed by isotope ratio mass spectrometry. In August, the
δ
13C values of gas phase formic acid showed a significant correlation with ozone (coefficient of determination (
r
2)
=
0.63) due to the kinetic isotope effect (KIE). This indicates the presence of secondary sources (i.e. production of organic acids in the atmosphere) in addition to direct emission. In March, both gaseous formic and acetic acid exhibited similar
δ
13C values and did not show any correlation with ozone, indicating a predominantly primary origin. Even though oxalic acid is mainly produced by secondary processes, the
δ
13C value of particulate oxalic acid was not depleted and did not show any correlation with ozone, which may be due to the enrichment of
13C during the gas - aerosol partitioning.
The concentrations and
δ
13C values of the different aerosol fractions (water soluble organic carbon, water insoluble organic carbon, carbonate and black carbon) collected during the same period were also determined. Water soluble organic carbon (WSOC) contributed about 60% to the total carbon and was enriched in
13C compared to other fractions indicating a possible effect of gas - aerosol partitioning on
δ
13C of carbonaceous aerosols. The carbonate fraction in general was very low (3% of the total carbon).
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