Real-time methods for estimating organic component mass concentrations from aerosol mass spectrometer data

We use results from positive matrix factorization (PMF) analysis of 15 urban aerosol mass spectrometer (AMS) data sets to derive simple methods for estimating major organic aerosol (OA) component concentrations in real time. PMF analysis extracts mass spectral (MS) profiles and mass concentrations for key OA components such as hydrocarbon-like OA (HOA), oxygenated OA (OOA), low-volatility OOA (LV-OOA), semivolatile OOA (SV-OOA), and biomass burning OA (BBOA). The variability in the component MS across all sites is characterized and used to derive standard profiles for real-time estimation of component concentrations. Two methods for obtaining first-order estimates of the HOA and OOA mass concentrations are evaluated. The first approach is the tracer m/z method, in which the HOA and OOA concentrations are estimated from m/z 57 and m/z 44 as follows: HOA ～ 13.4 × (C(57) - 0.1 × C(44)) and OOA ～ 6.6 × C(44), where C(i) is the equivalent mass concentration of tracer ion m/z i. The second approach uses a chemical mass balance (CMB) method in which standard HOA and OOA profiles are used as a priori information for calculating their mass concentrations. The HOA and OOA mass concentrations obtained from the first-order estimates are evaluated by comparing with the corresponding PMF results for each site. Both methods reproduce the HOA and OOA concentrations to within ～30% of the results from detailed PMF analysis at most sites, with the CMB method being slightly better. For hybrid CMB methods, we find that fixing the LV-OOA spectrum and not constraining the other spectra produces the best results.     

Source attribution of submicron organic aerosols during wintertime inversions by advanced factor analysis of aerosol mass spectra

Real-time measurements of submicrometer aerosol were performed using an Aerodyne aerosol mass spectrometer (AMS) during three weeks at an urban background site in Zurich (Switzerland) in January 2006. A hybrid receptor model which incorporates a priori known source composition was applied to the AMS highly time-resolved organic aerosol mass spectra. Three sources and components of submicrometer organic aerosols were identified: the major component was oxygenated organic aerosol (OOA), mostly representing secondary organic aerosol and accounting on average for 52-57% of the particulate organic mass. Radiocarbon (14C) measurements of organic carbon (OC) indicated that approximately 31 and approximately 69% of OOA originated from fossil and nonfossil sources, respectively. OOA estimates were strongly correlated with measured particulate ammonium. Particles from wood combustion (35-40%) and 3-13% traffic-related hydrocarbon-like organic aerosol (HOA) accounted for the other half of measured organic matter (OM). Emission ratios of modeled HOA to measured nitrogen oxides (NOx) and OM from wood burning to levoglucosan from filter analyses were found to be consistent with literature values.     

Identification of the mass spectral signature of organic aerosols from wood burning emissions

Throughout the winter months, the village of Roveredo, Switzerland, frequently experiences strong temperature inversions that contribute to elevated levels of particulate matter. Wood is used as fuel for 75% of the domestic heating installations in Roveredo, which makes it a suitable location to study wood burning emissions in the atmosphere in winter. An Aerodyne quadrupole aerosol mass spectrometer (Q-AMS) was used to characterize the composition of the submicrometer, non-refractory aerosol particles at this location during two field campaigns in March and December 2005. Wood burning was found to be a major source of aerosols at this location in winter. Organics dominated the composition of the aerosols from this source, contributing up to 85% of the total AMS measured mass during the afternoon and evening hours. Carbonaceous particle analysis showed that organic carbon composed up to 86% of the total carbon mass collected at evening times. Results from 14C isotope determination revealed that up to 94% of the organic mass came from non-fossil sources, which can be attributed mostlyto wood burning. The unique combination of off-line 14C isotope analysis and on-line aerosol mass spectrometry was used to identify periods during which organic mass was mainly from wood burning emissions and allowed for the identification of the AMS spectral signature of this source in the atmosphere. The identified ambient signature of wood burning was found to be very similar to the mass spectral signature obtained during the burning of chestnut wood samples in a small stove and also to the spectrum of levoglucosan. Particles from wood burning appeared to be composed of highly oxygenated organic compounds, and mass fragments 60, 73, and 137 have been suggested as marker fragments for wood burning aerosols. Mass fragment 44, which is used as a marker for oxygenated organic aerosols (OOA), contributed about 5% to the total organic signal from primary wood burning sources. The ratio of the organic mass emitted from wood burning to m/z 60 in Roveredo is 36. This ratio may be used to provide an estimate of the organic aerosol mass emitted from wood burning in other locations.     

Measurements of secondary organic aerosol from oxidation of cycloalkenes, terpenes, and m-xylene using an Aerodyne aerosol mass spectrometer

The Aerodyne aerosol mass spectrometer (AMS) was used to characterize physical and chemical properties of secondary organic aerosol (SOA) formed during ozonolysis of cycloalkenes and biogenic hydrocarbons and photo-oxidation of m-xylene. Comparison of mass and volume distributions from the AMS and differential mobility analyzers yielded estimates of "effective" density of the SOA in the range of 0.64-1.45 g/cm3, depending on the particular system. Increased contribution of the fragment at m/z 44, C02+ ion fragment of oxygenated organics, and higher "delta" values, based on ion series analysis of the mass spectra, in nucleation experiments of cycloalkenes suggest greater contribution of more oxygenated molecules to the SOA as compared to those formed under seeded experiments. Dominant negative "delta" values of SOA formed during ozonolysis of biogenics indicates the presence of terpene derivative structures or cyclic or unsaturated oxygenated compounds in the SOA. Evidence of acid-catalyzed heterogeneous chemistry, characterized by greater contribution of higher molecular weight fragments to the SOA and corresponding changes in "delta" patterns, is observed in the ozonolysis of alpha-pinene. Mass spectra of SOA formed during photooxidation of m-xylene exhibit features consistent with the presence of furandione compounds and nitro organics. This study demonstrates that mixtures of SOA compounds produced from similar precursors result in broadly similar AMS mass spectra. Thus, fragmentation patterns observed for biogenic versus anthropogenic SOA may be useful in determining the sources of ambient SOA.     

A case study of urban particle acidity and its influence on secondary organic aerosol

Size-resolved indicators of aerosol acidity, including H+ ion concentrations (H+Aer) and the ratio of stoichiometric neutralization are evaluated in submicrometer aerosols using highly time-resolved aerosol mass spectrometer (AMS) data from Pittsburgh. The pH and ionic strength within the aqueous particle phase are also estimated using the Aerosol Inorganics Model (AIM). Different mechanisms that contribute to the presence of acidic particles in Pittsburgh are discussed. The largest H+Aer loadings and lowest levels of stoichiometric neutralization were detected when PM1 loadings were high and dominated by SO4(2-). The average size distribution of H+Aer loading shows an accumulation mode at Dva approximately 600 nm and an enhanced smaller mode that centers at Dva approximately 200 nm and tails into smaller sizes. The acidity in the accumulation mode particles suggests that there is generally not enough gas-phase NH3 available on a regional scale to completely neutralize sulfate in Pittsburgh. The lack of stoichiometric neutralization in the 200 nm mode particles is likely caused by the relatively slow mixing of gas-phase NH3 into SO2-rich plumes containing younger particles. We examined the influence of particle acidity on secondary organic aerosol (SOA) formation by comparing the mass concentrations and size distributions of oxygenated organic aerosol (00A--surrogate for SOA in Pittsburgh) during periods when particles are, on average, acidic to those when particles are bulk neutralized. The average mass concentration of ODA during the acidic periods (3.1 +/- 1.7 microg m(-3)) is higher than that during the neutralized periods (2.5 +/- 1.3 microg m(-3)). Possible reasons for this enhancement include increased condensation of SOA species, acid-catalyzed SOA formation, and/or differences in air mass transport and history. However, even if the entire enhancement (approximately 0.6 microg m(-3)) can be attributed to acid catalysis, the upperbound increase of SOA mass in acidic particles is approximately 25%, an enhancement that is much more moderate than the multifold increases in SOA mass observed during some lab studies and inferred in SO2-rich industrial plumes. In addition, the mass spectra of OOA from these two periods are almost identical with no discernible increase in relative signal intensity at larger m/z's (>200 amu), suggesting that the chemical nature of SOA is similar during acidic and neutralized periods and that there is no significant enhancement of SOA oligomer formation during acidic particle periods in Pittsburgh.     

Isoprene epoxydiols as precursors to secondary organic aerosol formation: acid-catalyzed reactive uptake studies with authentic compounds

Isoprene epoxydiols (IEPOX), formed from the photooxidation of isoprene under low-NO(x) conditions, have recently been proposed as precursors of secondary organic aerosol (SOA) on the basis of mass spectrometric evidence. In the present study, IEPOX isomers were synthesized in high purity (>99%) to investigate their potential to form SOA via reactive uptake in a series of controlled dark chamber studies followed by reaction product analyses. IEPOX-derived SOA was substantially observed only in the presence of acidic aerosols, with conservative lower-bound yields of 4.7-6.4% for β-IEPOX and 3.4-5.5% for δ-IEPOX, providing direct evidence for IEPOX isomers as precursors to isoprene SOA. These chamber studies demonstrate that IEPOX uptake explains the formation of known isoprene SOA tracers found in ambient aerosols, including 2-methyltetrols, C(5)-alkene triols, dimers, and IEPOX-derived organosulfates. Additionally, we show reactive uptake on the acidified sulfate aerosols supports a previously unreported acid-catalyzed intramolecular rearrangement of IEPOX to cis- and trans-3-methyltetrahydrofuran-3,4-diols (3-MeTHF-3,4-diols) in the particle phase. Analysis of these novel tracer compounds by aerosol mass spectrometry (AMS) suggests that they contribute to a unique factor resolved from positive matrix factorization (PMF) of AMS organic aerosol spectra collected from low-NO(x), isoprene-dominated regions influenced by the presence of acidic aerosols.     

Comparative analysis of urban atmospheric aerosol by particle-induced X-ray emission (PIXE), proton elastic scattering analysis (PESA), and aerosol mass spectrometry (AMS)

A multifaceted approach to atmospheric aerosol analysis is often desirable in field studies where an understanding of technical comparability among different measurement techniques is essential. Herein, we report quantitative intercomparisons of particle-induced X-ray emission (PIXE) and proton elastic scattering analysis (PESA), performed of fline under a vacuum, with analysis by aerosol mass spectrometry (AMS) carried out in real-time during the MCMA-2003 Field Campaign in the Mexico City Metropolitan Area. Good agreement was observed for mass concentrations of PIXE-measured sulfur (assuming it was dominated by SO4(2-)) and AMS-measured sulfate during most of the campaign. PESA-measured hydrogen mass was separated into sulfate H and organic H mass fractions, assuming the only major contributions were (NH4)2SO4 and organic compounds. Comparison of the organic H mass with AMS organic aerosol measurements indicates that about 75% of the mass of these species evaporated under a vacuum. However approximately 25% of the organics does remain under a vacuum, which is only possible with low-vapor-pressure compounds, and which supports the presence of high-molecular-weight or highly oxidized organics consistent with atmospheric aging. Approximately 10% of the chloride detected by AMS was measured by PIXE, possibly in the form of metal-chloride complexes, while the majority of Cl was likely present as more volatile species including NH4Cl. This is the first comparison of PIXE/PESA and AMS and, to our knowledge, also the first report of PESA hydrogen measurements for urban organic aerosols.     

Formation of aerosol particles from reactions of secondary and tertiary alkylamines: characterization by aerosol time-of-flight mass spectrometry

In ambient field studies conducted with aerosol time-of-flight mass spectrometry (ATOFMS), individual particle mass spectra commonly contain ion peaks at mass/charge (m/z) 86, 101, 102, and 118. Particles with mass spectra containing these peaks show a strong correlation with high relative humidity and low temperatures. In an effort to identify these peaks, a series of smog chamber studies were conducted probing the chemistry of secondary and tertiary alkylamines. Specifically, in separate studies, trimethylamine, di- and triethylamine, and di- and tripropylamine were reacted in a 1 m3 Teflon outdoor smog chamber with naturally occurring levels of gas phase oxidants in ambient air. The aerodynamic sizes and individual mass spectra of the resulting aerosol particles were acquired continuously using aerosol time-of-flight mass spectrometry (ATOFMS). Both oxidation and acid-base reactions between amines and acids commonly present in the atmosphere (i.e., nitric and sulfuric acid) appear to play roles in the formation and chemistry of organic nitrogen-containing particle phase species. Ion peaks in the individual particle mass spectra indicate the presence of alkyl ammonium salts, as well as other tentatively identified organic N-containing compounds formed by oxidation processes. Also, for the first time, tertiary alkylamine-N-oxides have been identified as alkylamine oxidation products in the aerosol particle phase. Smog chamber reactions involving triethylamine produce ATOFMS mass spectra with similar ion peak combinations as those observed in the spectra of particles commonly detected in ambient and vehicular source characterization studies. The results of this study suggest that amine chemistry involving gas-to-particle conversion and photooxidation processes may play a significant role in particle formation in regions with high amine concentrations.     

Evaporative light scattering: a novel detection method for the quantitative analysis of humic-like substances in aerosols

The chemical composition of organic atmospheric aerosols is only poorly understood. Although a significant fraction of organic aerosols consists of humic-like substances (HULIS), only little is known about this class of compound, and accurate quantification remains difficult, partly due to the lack of appropriate standards. Here, evaporative light-scattering detection (ELSD) was applied for the first time to quantify water-soluble HULIS in aerosol particles smaller than 1 microm. This detection method was shown to be suitable for the quantification of compounds with unknown structures and lacking appropriate quantification standards. As compared to organic carbon determination of isolated HULIS, no organic carbon/organic mass (OC/OM) conversion factor needs to be applied with ELSD and therefore eliminates this significant uncertainty factor of the OC/OM method, which is frequently used to quantify HULIS. Solid-phase extraction and size-exclusion chromatography were applied to separate inorganic ions and low molecular weight compounds from HULIS before ELSD quantification. The ELSD itself provides an additional separation step where low volatility HULIS are separated from high volatility, small compounds. Electrospray ionization mass spectrometry was used to identify the molecular weight range of the compounds quantified with ELSD. The most intensive peaks were in the range of m/z 200-500, with some masses upto m/z800. We showed that UV detection using fulvic acid as surrogate quantification standard underestimates the HULIS concentration by a factor of 1.1 to 2.5, which is in agreement with earlier studies. During a 6 week winter 2005-2006 campaign at a suburban site near Zurich, Switzerland, an average of 1.1 microg/m(3) HULIS was found, which is about4-6% of the total particle mass smaller than 1 microm (PM1) and 10-35% of the organic matter in PM1.     

Identification of lubrication oil in the particulate matter emissions from engine exhaust of in-service commercial aircraft

Lubrication oil was identified in the organic particulate matter (PM) emissions of engine exhaust plumes from in-service commercial aircraft at Chicago Midway Airport (MDW) and O'Hare International Airport (ORD). This is the first field study focused on aircraft lubrication oil emissions, and all of the observed plumes described in this work were due to near-idle engine operations. The identification was carried out with an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF AMS) via a collaborative laboratory and field investigation. A characteristic mass marker of lubrication oil, I(85)/I(71), the ratio of ion fragment intensity between m/z = 85 and 71, was used to distinguish lubrication oil from jet engine combustion products. This AMS marker was based on ion fragmentation patterns measured using electron impact ionization for two brands of widely used lubrication oil in a laboratory study. The AMS measurements of exhaust plumes from commercial aircraft in this airport field study reveal that lubrication oil is commonly present in organic PM emissions that are associated with emitted soot particles, unlike the purely oil droplets observed at the lubrication system vent. The characteristic oil marker, I(85)/I(71), was applied to quantitatively determine the contribution from lubrication oil in measured aircraft plumes, which ranges from 5% to 100%.     

Measurements of isoprene-derived organosulfates in ambient aerosols by aerosol time-of-flight mass spectrometry - part 1: single particle atmospheric observations in Atlanta

Organosulfate species have recently been identified as a potentially significant class of secondary organic aerosol (SOA) species, yet little is known about their behavior in the atmosphere. In this work, organosulfates were observed in individual ambient aerosols using single particle mass spectrometry in Atlanta, GA during the 2002 Aerosol Nucleation and Characterization Experiment (ANARChE) and the 2008 August Mini-Intensive Gas and Aerosol Study (AMIGAS). Organosulfates derived from biogenically produced isoprene were detected as deprotonated molecular ions in negative-ion spectra measured by aerosol time-of-flight mass spectrometry; comparison to high-resolution mass spectrometry data obtained from filter samples corroborated the peak assignments. The size-resolved chemical composition measurements revealed that organosulfate species were mostly detected in submicrometer aerosols and across a range of aerosols from different sources, consistent with secondary reaction products. Detection of organosulfates in a large fraction of negative-ion ambient spectra - ca. 90-95% during ANARChE and ~65% of submicrometer particles in AMIGAS - highlights the ubiquity of organosulfate species in the ambient aerosols of biogenically influenced urban environments.     

Applications of high-resolution electrospray ionization mass spectrometry to measurements of average oxygen to carbon ratios in secondary organic aerosols

The applicability of high-resolution electrospray ionization mass spectrometry (HR ESI-MS) to measurements of the average oxygen to carbon ratio (O/C) in secondary organic aerosols (SOAs) was investigated. Solutions with known average O/C containing up to 10 standard compounds representative of low-molecular-weight SOA constituents were analyzed and the corresponding electrospray ionization efficiencies were quantified. The assumption of equal ionization efficiency commonly used in estimating O/C ratios of SOAs was found to be reasonably accurate. We found that the accuracy of the measured O/C ratios increases by averaging the values obtained from both the posive and negative modes. A correlation was found between the ratio of the ionization efficiencies in the positive (+) and negative (-) ESI modes and the octanol-water partition constant and, more importantly, the compound's O/C. To demonstrate the utility of this correlation for estimating average O/C values of unknown mixtures, we analyzed the ESI (+) and ESI (-) data for SOAs produced by oxidation of limonene and isoprene and compared them online to O/C measurements using an aerosol mass spectrometer (AMS). This work demonstrates that the accuracy of the HR ESI-MS method is comparable to that of the AMS with the added benefit of molecular identification of the aerosol constituents.     

Organic aerosol formation from photochemical oxidation of diesel exhaust in a smog chamber

Diluted exhaust from a diesel engine was photo-oxidized in a smog chamber to investigate secondary organic aerosol (SOA) production. Photochemical aging rapidly produces significant SOA, almost doubling the organic aerosol contribution of primary emissions after several hours of processing at atmospherically relevant hydroxyl radical concentrations. Less than 10% of the SOA mass can be explained using a SOA model and the measured oxidation of known precursors such as light aromatics. However, the ultimate yield of SOA is uncertain because it is sensitive to treatment of particle and vapor losses to the chamber walls. Mass spectra from an aerosol mass spectrometer (AMS) reveal that the organic aerosol becomes progressively more oxidized throughout the experiments, consistent with sustained, multi-generational production. The data provide strong evidence that the oxidation of a wide array of precursors that are currently not accounted for in existing models contributes to ambient SOA formation.     

Semiempirical model for organic aerosol growth by acid-catalyzed heterogeneous reactions of organic carbonyls

Aerosol growth by heterogeneous reactions of diverse carbonyls in the presence and absence of acidified seed aerosols was studied in a 4 m long flow reactor (2.5 cm i.d.) and a 2-m3 indoor Teflon film chamber under darkness. The acid catalytic effects on heterogeneous aerosol production were observed for diverse carbonyls in various ranges of humidities and compositions of seed inorganic aerosols. Particle population data measured by a scanning mobility particle sizer were used to calculate organic aerosol growth. To accountforthe aerosol growth contributed by heterogeneous reactions, the increase in organic aerosol mass was normalized bythe organic mass predicted by partitioning or the square of predicted organic mass. The carbonyl heterogeneous reactions were accelerated in the presence of acid catalysts (H2SO4), leading to higher aerosol yields than in their absence. The experimental data from aerosol yields in the flow reactorwere semiempirically fitted to the model parameters to predict the organic aerosol growth. The model parameters consist of environmental characteristics and molecular structure information of organic carbonyls. Basicity constants of carbonyls were used to describe the proton affinity of carbonyls for the acid catalysts. Particle environmental factors, such as humidity, temperature, and inorganic seed composition, were expressed by excess acidity and the parameters obtained from an inorganic thermodynamic model. A stepwise regression analysis of the aerosol growth model for the experimental data revealed that either the chemical structure information of carbonyls or characteristic environmental parameters are statistically significant in the prediction of organic aerosol growth. It was concluded thatthis model approach is applicable to predict secondary organic aerosol formation by heterogeneous reaction.     

Insights into the chemistry of new particle formation and growth events in Pittsburgh based on aerosol mass spectrometry

New particle formation and growth events have been observed in several urban areas and are of concern due to their potential negative effects on human health. The main purpose of this study was to investigate the chemistry of ultrafine particles during the growth phase of the frequently observed nucleation events in Pittsburgh (approximately 100 events per year) and therefore infer the mechanisms of new particle growth in the urban troposphere. An Aerodyne aerosol mass spectrometer (AMS) and two SMPS systems were deployed at the U.S. EPA Pittsburgh Supersite during September 2002. Significant nucleation events were observed in 3 out of the 16 days of this deployment, including one of the 10 strongest nucleation events observed in Pittsburgh over a period of 15 months. These events appear to be representative of the climatology of new particle formation and growth in the Pittsburgh region. Distinctive growth of sulfate, ammonium, organics, and nitrate in the ultrafine mode (33-60 nm in a vacuum aerodynamic diameter or approximately 18-33 nm in physical diameter) was observed during each of these three events, with sulfate always being the first (and the fastest) species to increase. Ultrafine ammonium usually increased 10-40 min later than sulfate, causing the ultrafine mode particles to be more acidic during the initial stages of the nucleation events. Significant increase of ultrafine organics often happened after 11:00 a.m., when photochemistry is more intense. This observation coupled with a parallel increase of ultrafine m/z 44, a mass fragment generally representative of oxygenated organic compounds, indicates that secondary organic species contribute significantly to the growth of particles at a relatively later time of the event. Among all these four species, nitrate was always a minor component of the ultrafine particles and contributed the least to the new particle growth.     

Molecular size evolution of oligomers in organic aerosols collected in urban atmospheres and generated in a smog chamber

Only a minor fraction of the total organic aerosol mass can be resolved on a molecular level. High molecular weight compounds in organic aerosols have recently gained much attention because this class of compound potentially explains a major fraction of the unexplained organic aerosol mass. These compounds have been identified with different mass spectrometric methods, and compounds with molecular masses up to 1000 Da are found in secondary organic aerosols (SOA) generated from aromatic and terpene precursors in smog chamber experiments. Here, we apply matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to SOA particles from two biogenic precursors, alpha-pinene and isoprene. Similar oligomer patterns are found in these two SOA systems, but also in SOA from trimethylbenzene, an anthropogenic SOA precursor. However, different maxima molecular sizes were measured for these three SOA systems. While oligomers in alpha-pinene and isoprene have sizes mostly below 600-700 Da, they grow up to about 1000 Da in trimethylbenzene-SOA. The final molecular size of the oligomers is reached early during the particle aging process, whereas other particle properties related to aging, such as the overall acid concentration or the oligomer concentration, increase continuously over a much longer time scale. This kinetic behavior of the oligomer molecular size growth can be explained by a chain growth kinetic regime. Similar oligomer mass patterns were measured in aqueous extracts of ambient aerosol samples (measured with the same technique). Distinct differences between summer and winter were observed. In summer a few single mass peaks were measured with much higher intensity than in winter, pointing to a possible difference in the formation processes of these compounds in winter and summer.     

Measurements of secondary organic aerosol formed from OH-initiated photo-oxidation of isoprene using online photoionization aerosol mass spectrometry

Isoprene is a significant source of atmospheric organic aerosol; however, the secondary organic aerosol (SOA) formation and involved chemical reaction pathways have remained to be elucidated. Recent works have shown that the photo-oxidation of isoprene leads to form SOA. In this study, the chemical composition of SOA from the OH-initiated photo-oxidation of isoprene, in the absence of seed aerosols, was investigated through the controlled laboratory chamber experiments. Thermal desorption/tunable vacuum-ultraviolet photoionization time-of-flight aerosol mass spectrometry (TD-VUV-TOF-PIAMS) was used in conjunction with the environmental chamber to study SOA formation. The mass spectra obtained at different photon energies and the photoionization efficiency (PIE) spectra of the SOA products can be obtained in real time. Aided by the ionization energies (IE) either from the ab initio calculations or the literatures, a number of SOA products were proposed. In addition to methacrolein, methyl vinyl ketone, and 3-methyl-furan, carbonyls, hydroxycarbonyls, nitrates, hydroxynitrates, and other oxygenated compounds in SOA formed in laboratory photo-oxiadation experiments were identified, some of them were investigated for the first time. Detailed chemical identification of SOA is crucial for understanding the photo-oxidation mechanisms of VOCs and the eventual formation of SOA. Possible reaction mechanisms will be discussed.     

Sorption of Aldrich humic acid onto hematite: insights into fractionation phenomena by electrospray ionization with quadrupole time-of-flight mass spectrometry

Sorption induced fractionation of purified Aldrich humic acid (PAHA) on hematite is studied through the modification of electrospray ionization (ESI) quadrupole time-of-flight (QToF) mass spectra of supernatants from retention experiments. The ESI mass spectra show an increase of the "mean molecular masses" of the molecules that constitutes humic aggregates. The low molecular weight fraction (LMWF; m/z < or = 600 Da) is preferentially sorbed compared to two other fractions. The resolution provided by ESI-QToF mass spectrometer in the low-mass range provided evidence of further fractionation induced by sorption within the LMWF. Among the two latter fractions, the high molecular weight fraction (HMWF; m/z approximately 1700 Da) seems to be more prone to sorption compared to the intermediate molecular weight fraction (IMWF; m/z approximately 900 Da). The IMWF seems to be more hydrophilic as it should be richer in O, N, and alkyl C from the proportion of even mass, and poorer in aromatic structures from mass defect analysis in ESI mass spectra.     

SOA formation by biogenic and carbonyl compounds: data evaluation and application

The organic fraction of atmospheric aerosols affects the physical and chemical properties of the particles and their role in the climate system. Current models greatly underpredict secondary organic aerosol (SOA) mass. Based on a compilation of literature studies that address SOA formation, we discuss different parameters that affect the SOA formation efficiency of biogenic compounds (alpha-pinene, isoprene) and aliphatic aldehydes (glyoxal, hexanal, octanal, hexadienal). Applying a simple model, we find that the estimated SOA mass after one week of aerosol processing under typical atmospheric conditions is increased by a few microg m(-3) (low NO(x) conditions). Acid-catalyzed reactions can create > 50% more SOA mass than processes under neutral conditions; however, other parameters such as the concentration ratio of organics/NO(x), relative humidity, and absorbing mass are more significant. The assumption of irreversible SOA formation not limited by equilibrium in the particle phase or by depletion of the precursor leads to unrealistically high SOA masses for some of the assumptions we made (surface vs volume controlled processes).     

Measurement of contemporary and fossil carbon contents of PM2.5 aerosols: results from Turtleback Dome, Yosemite National Park

The impact of aerosol particulate matter of mean mass aerodynamic diameter < or = 2.5 microm (PM2.5 aerosols) on health, visibility, and compliance with the U.S. EPA's regional haze regulations is a growing concern. Techniques that can help better characterize particulate matter are required to better understand the constituents, causes, and sources of PM2.5 aerosols. Measurement of the 14C/C ratio of the PM2.5 aerosols, the absence of 14C in fossil carbon materials, and the known 14C/C levels in contemporary carbon materials allow the use of a two-component model to derive contemporary and fossil carbon contents of the particulate matter. Such data can be used to estimate the relative contributions of fossil fuels and biogenic aerosols to the total aerosol loading. Here, the methodology for performing such an assessment using total suspended particulate hi-vol aerosol samplers to collect PM2.5 aerosols on quartz fiber filters and the technique of accelerator mass spectrometry to measure 14C/C ratios is presented and illustrated using PM2.5 aerosols collected at Yosemite National Park.