High molecular weight SOA formation during limonene ozonolysis: insights from ultrahigh-resolution FT-ICR mass spectrometry characterization

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 \u3c 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 \u3e 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

Molecular characterization of monoterpene ozonolysis products using ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry

A detailed knowledge of the chemical composition of secondary organic aerosols (SOA) is required to better understand their roles in climate change, biogeochemical cycling and public health. The chemical composition of the SOA produced by the ozonolysis of limonene was investigated using electrospray ionization Fourier transform ion cyclotron resonance (ESI FT-ICR) mass spectrometry. SOA was generated in a 1.5 m3 teflon chamber with 500 ppb of limonene and 250 ppb of O3, without the presence of hydroxyl radical scavenger. We have identified approximately 1300-1500 molecular masses from negative-ion spectra in the range of 105 \u3c m/z \u3c 870 in each of two samples. The double bond equivalency (DBE), the number of rings and unsaturated bonds, values range from 1 to 13. This indicates the formation of a wide variety of chemical compounds. Several regions of high peak number density were observed in the mass ranges of 170 \u3c m/z \u3c 280, 350 \u3c m/z \u3c 480, 550 \u3c m/z \u3c 650 and 730 \u3c m/z \u3c 850. These regions are generally referred to as monomers, dimers, trimers, and tetramers of oxidized monoterpene units. The average DBE values for monomer to tetramer regions increase from 3.7 to 9.4 and the oxidation number decreases from -0.4 to -0.7. These results suggest that high MW organic compounds in the SOA samples are more unsaturated and less-oxidized. Both the oxygen to carbon (0.2-2) and hydrogen to carbon (0.5-1.1) ratios for limonene-SOA are different from those for α-pinene-SOA, 0.2-1 and 1.1-1.9 (Putman et al., 2010). We will compare the chemical composition of limonene-SOA with that of α- and β-pinene-SOA. We will also discuss the MSn fragmentation behavior of major ions for the structural elucidation of the oligomers

Ultrahigh-resolution FT-ICR mass spectrometry characterization of α-pinene ozonolysis SOA

Secondary organic aerosol (SOA) of α-pinene ozonolysis with and without hydroxyl radical scavenging hexane was characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Molecular formulas for more than 900 negative ions were identified over the mass range of 100–850 u. Hydroxyl radicals formed during the ozonolysis of α-pinene might be expected to alter the composition of SOA, however a majority of the molecular formulas were identified in all three experiments and with a few exceptions they had similar relative abundances. Thus, the detailed composition of SOA was only slightly influenced by the presence or absence of hydroxyl radical scavenging hexane. The negative-ion mass spectra of the SOA contained four groups of peaks with increasing mass spectral complexity corresponding to increasing molecular weight. The mean values of O:C decreased from 0.55 to 0.42 with increasing molecular weight, but the mean value of H:C, approximately 1.5, did not change with increasing molecular weight. The molecular formulas with the highest relative abundances in Groups I and II contained 5–7 and 7–10 oxygen atoms and 3–4 and 5–7 double bond equivalents, respectively. The molecular formulas with the highest relative abundances in Groups III and IV contained 10–13 and 13–16 oxygen atoms and 7–9 and 9–11 double bond equivalents, respectively. Observations of the oxygen content and the double bond equivalents of the SOA products suggest a complex mixture of accretion reaction mechanisms, without an easily confirmable dominating pathway

Characterizing the secondary organic aerosol products of ozone and α-pinene using ultrahigh-resolution FT-ICR mass spectrometry

Three samples of secondary organic aerosol (SOA) were generated by reacting a-pinene and ozone in the presence of variable concentrations of hydroxyl radical scavenging cyclohexane and were characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS). The reactions were performed in the presence of different concentrations of hydroxyl radical scavenger. This provided an opportunity to examine the molecular level differences of SOA. More than 900 chemical formulas for negative ions were identified over the mass range of 100 to 820 u. The experimental reproducibility of the SOA composition and the technical reproducibility of the mass spectra were evaluated. Similar chemical formulas with similar relative abundances were observed in all three experiments. A few exceptions were particular high relative abundance signals such as m/z 357, 367 and 539, whose production efficiency increased in the presence of cyclohexane, and m/z 185, 199, 215, 231 and 261, whose production efficiency decreased in the presence of cyclohexane. In general, the composition of a-pinene SOA was only slightly influenced by the concentration of the hydroxyl radical scavenger, cyclohexane. The negative ion spectra of the SOA contained four groups of peaks over the following mass ranges: 150 \u3c n \u3c 300, 300 \u3c n \u3c 475, 475 \u3c n \u3c 600, 600 \u3c n \u3c 850. As the molecular weight increased, a variety of changes occurred. The number of individual compounds within one nominal mass increased. The range of oxygen to carbon and hydrogen ratios decreased from group I to IV. Likewise, the mean values of oxygen to carbon decreased from 0.55 to 0.42. The mean value of hydrogen to carbon, approximately 1.5, did not change with respect to molecular weight, although the range of values did decrease. The chemical formulas of groups I and II with the highest relative abundances contained 5-7 and 7-10 oxygen atoms and double bond equivalents (DBE) of 3-4 and 5-7, respectively. The chemical formulas of groups III and IV with the highest relative abundances contained 10-13 and 13-16 oxygen atoms and DBE values of 7-9 and 9-11, respectively. Several SOA accretion mechanisms cause increases of DBE of 2 or 3 and alter the O:C and H:C ratios in different ways. Observations of the oxygen content and the DBE of the SOA products suggest they resulted from a complex mixture of accretions, such as reactions of neutral molecules with hydroperoxy or criegee radicals, hemi-acetal reactions, aldol condensations or esterification reactions. To provide insight into the formation mechanisms, the molecular structures of selected group II compounds (300 \u3c n \u3c 475) were investigated using ultra-high resolution MS2

Seasonal and diurnal characteristics of water soluble inorganic compounds in the gas and aerosol phase in the Zurich area

Gas and aerosol samples were taken using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to ion chromatography (IC) in the city of Zurich, Switzerland from August to September 2002 and in March 2003. Major water soluble inorganic ions; nitrate, sulfate, and nitrite were analyzed online with a time resolution of two hours for the gas and aerosol phase. The fraction of water soluble inorganic anions in PM10 varied from 15% in August to about 38% in March. Seasonal and diurnal variations of nitrate in the gas and aerosol phase were observed with more than 50% of the total nitrate in the gas phase during August and more than 80% of nitrate in the aerosol phase during March exceeding the concentration of sulfate by a factor of 2. Aerosol sulfate, on the other hand, did not show significant variability with season. However, in the gas phase, the SO2 concentration was 6.5 times higher in winter than in summer. Nitrous acid (HONO) also showed a diurnal variation in both the gas and aerosol phase with the lowest concentration (0.2–0.6 µg/m3) in the afternoon. The primary pollutants, NO, CO and SO2 mixing ratios were often at their highest between 04:00–10:00 local time due to the build up of fresh vehicle emission under a nocturnal inversion.ISSN:1680-7375ISSN:1680-736

Molecular formula characterization of biogenic secondary organic aerosol: Descriptive statistical evaluation

The detailed molecular composition of approximately 20 laboratory generated terpene ozonolysis secondary organic aerosol (SOA) samples was studied using ultrahigh resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Individual experiments were conducted with one of four terpene SOA precursors (α-pinene, β-pinene, limonene or β-caryophyllene), varied relative humidity (RH) conditions (0%, 4%, or 30%) and the presence or absence of cyclohexane (serving as a radical scavenger). In this work, we focus on the molecular composition of the SOA experiments conducted at 4% and 30% RH without cyclohexane. In each of the experimental SOA samples, the oxygen number and the DBE values increase with increasing carbon number and three or four distinct groups (aka oligomer groups) were observed in the mass spectra. The overall bulk properties, such as the elemental ratios and the average number of double bond equivalents (DBE), of the SOA were highly similar. Despite the high number of identified species (N ≥ 1000) in each SOA sample, compounds unique to the SOA formed at either 4% or 30% RH conditions were comparatively low (\u3c 200). An exception to this was observed for the D-limonene ozonolysis SOA formed at 4% RH conditions where over 450 unique molecular formulas were observed. Due to the similarity in the bulk properties and composition of the SOA from the experiments, multivariate statistics were used to distinguish the experiments from each other. Hierarchical cluster analysis and principal component analysis was performed using the molecular formulas and their relative abundances for all of the identified species. Slight compositional differences between the experiments showed that experiments with the same terpene SOA precursor were most closely related regardless of the RH or the presence/absence of cyclohexane. Furthermore, SOA experiments with D-limonene and β-caryophyllene as precursors were clearly distinguished from β-pinene and α-pinene. When the experimental SOA composition was compared with ambient samples, we observed a high number of common monoisotopic molecular formulas for summer aerosol [63%; Mazzoleni et. al., Env. Chem. 2012] and winter cloudwater samples [60%; Zhao et. al., ACPD 2013]. However the molecular formulas identified as significant using principal components analysis, were not found consistently in both samples indicating variable SOA contributions to summer and winter ambient samples. Mazzoleni, L.R., P. Saranjampour, M.M. Dalbec, V. Samburova, B. Zielinska, A.G. Hallar, D. Lowenthal, and S. Kohl, Identification of Water-Soluble Organic Carbon in Nonurban Organic Aerosols using Ultrahigh-Resolution FT-ICR Mass Spectrometry: Organic Anions, Environmental Chemistry, Vol. 9(3) 285-297, 2012. Zhao, Y., A.G. Hallar, and L.R. Mazzoleni, Atmospheric Organic Matter in Clouds: Exact Masses and Molecular Formula Identification using Ultrahigh Resolution FT-ICR Mass Spectrometry, Atmospheric Chemistry and Physics Discussion, In Press, 2013

Online gas and aerosol measurement of water soluble carboxylic acids in Zurich

[ 1] We discuss the diurnal and seasonal variability of low molecular weight organic acids in Zurich city on the basis of online quasi-continuous measurement in the gas and aerosol phase using a wet effluent diffusion denuder/aerosol collector(WEDD/AC) coupled to ion chromatography. The measurements were performed during August - September 2002 and March 2003. Acetic acid exhibited the highest concentration in the gas phase during all the measurement periods, followed by formic acid. Oxalic acid was predominantly found in the aerosol phase and often below the detection limit in the gas phase. In addition, filter samples were analyzed using ion chromatography - mass spectrometry (IC-MS) to provide more information on organic acids in the aerosol phase. From the offline IC-MS measurements, 20 monocarboxylic, dicarboxylic, and tricarboxylic acids were determined. In addition, more than 20 different masses were detected with the MS; however, identification of the organic acids was not possible. The sum of the carboxylic acids contributed on average 2% to the water soluble organic carbon (WSOC). The fraction of dicarboxylic acids to the WSOC was higher in summer compared to winter suggesting that dicarboxylic acids are mainly a result of photochemical reactions in summer whereas in winter they mainly result from primary sources