Secondary Organic Aerosol Tracers Research Articles

Aircraft measurements of polar organic tracer compounds in tropospheric particles (PM<sub>10</sub>) over central China

Abstract. Atmospheric aerosol samples were collected by aircraft at low to middle altitudes (0.8–3.5 km a.g.l.) over central East to West China during summer 2003 and spring 2004. The samples were analyzed for polar organic compounds using a technique of solvent extraction/BSTFA derivatization/gas chromatography–mass spectrometry. Biogenic secondary organic aerosol (SOA) tracers from the oxidation of isoprene were found to be more abundant in summer (3.3–138 ng m−3, mean 39 ng m−3) than in spring (3.2–42 ng m−3, 15 ng m−3), while α/β-pinene and β-caryophyllene SOA tracers showed similar abundances between these two seasons. A strong positive correlation (R2 = 0.83) between levoglucosan and β-caryophyllinic acid was found in the spring samples vs. a weak correlation (R2 = 0.17) in the summer samples, implying substantial contributions from biomass burning to the β-caryophyllinic acid production in spring. Two organic nitrogen species (oxamic acid and carbamide) were detected in the aircraft aerosol samples, and their concentrations were comparable to those of biogenic SOA tracers. Most of the primary organic aerosol (POA) and SOA tracers were less abundant at higher altitudes, suggesting they are of ground surface origin, either being directly emitted from anthropogenic/natural sources on the ground surface, or rapidly formed through photooxidation of their precursors emitted from the ground surface and then diluted during uplifting into the troposphere. This study demonstrates that primary biological aerosols, biogenic SOA, and organic nitrogen species are important components of organic aerosols in the troposphere over central China during warm seasons.

Investigating the influences of SO<sub>2</sub> and NH<sub>3</sub> levels on isoprene-derived secondary organic aerosol formation using conditional sampling approaches

Abstract. Filter-based PM2.5 samples were chemically analyzed to investigate secondary organic aerosol (SOA) formation from isoprene in a rural atmosphere of the southeastern US influenced by both anthropogenic sulfur dioxide (SO2) and ammonia (NH3) emissions. Daytime PM2.5 samples were collected during summer 2010 using conditional sampling approaches based on pre-defined high and low SO2 or NH3 thresholds. Known molecular-level tracers for isoprene SOA formation, including 2-methylglyceric acid, 3-methyltetrahydrofuran-3,4-diols, 2-methyltetrols, C5-alkene triols, dimers, and organosulfate derivatives, were identified and quantified by gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS) and ultra performance liquid chromatography coupled to electrospray ionization high-resolution quadrupole time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS). Mass concentrations of six isoprene low-NOx SOA tracers contributed to 12–19% of total organic matter (OM) in PM2.5 samples collected during the sampling period, indicating the importance of the hydroxyl radical (OH)-initiated oxidation (so-called photooxidation) of isoprene under low-NOx conditions that lead to SOA formation through reactive uptake of gaseous isoprene epoxydiols (IEPOX) in this region. The contribution of the IEPOX-derived SOA tracers to total organic matter was enhanced by 1.4% (p = 0.012) under high-SO2 sampling scenarios, although only weak associations between aerosol acidity and mass of IEPOX SOA tracers were observed. This suggests that IEPOX-derived SOA formation might be modulated by other factors simultaneously, rather than only aerosol acidity. No clear associations between isoprene SOA formation and high or low NH3 conditional samples were found. Positive correlations between sulfate aerosol loadings and IEPOX-derived SOA tracers for samples collected under all conditions indicates that sulfate aerosol could be a surrogate for surface accommodation in the uptake of IEPOX onto preexisting aerosols.

Constraining carbonaceous aerosol sources in a receptor model by including 14C data with redox species, organic tracers, and elemental/organic carbon measurements

Sources of carbonaceous PM2.5 were quantified in downtown Cleveland, OH and Chippewa Lake, OH located ∼40 miles southwest of Cleveland during the Cleveland Multiple Air Pollutant Study (CMAPS). PM2.5 filter samples were collected daily during July–August 2009 and February 2010 to establish the seasonal emission patterns from local and regional sources. Radiocarbon (14C), redox species (NOx, SO2 and ozone), 28 primary and secondary organic aerosol tracers, elemental carbon (EC) and organic carbon (OC) measurements were analyzed using the EPA Positive Matrix Factorization (PMF) model to apportion carbonaceous aerosol sources. Five sources were identified at each site: mobile sources, fossil fuel combustion from fuels containing sulfur, local biomass combustion, other combustion (regional biomass, waste, meat, coal), and secondary organic aerosol (SOA). 14C data were incorporated in the PMF analysis as a novel method to obtain the modern carbon fraction (fmod) of each source individually which aided all factor interpretations. SOA was the principal carbon source during summer as shown by the PMF analysis and a separate tracer based mass fraction method while biomass burning and other combustion sources were dominant in winter. Elevated levels of EC and fossilized carbon in downtown Cleveland are primarily attributed to increased mobile source and coal combustion emissions.

Investigation of the sources and seasonal variations of secondary organic aerosols in PM2.5 in Shanghai with organic tracers

One hundred and forty seasonal PM2.5 samples were collected from January 2010 to January 2011 at one urban site and one suburban site simultaneously in a Chinese megacity, Shanghai, to study the concentrations and seasonal variation of secondary organic aerosols (SOA). Concentrations of water-soluble organic carbon (WSOC) were determined together with organic and elemental carbons. Thirteen organic tracers, including the tracer for biomass burning and tracers for SOA from isoprene, α-pinene, β-caryophyllene and toluene, were measured. EC-based method, WSOC-based method, tracer-based method and PMF modeling were used to estimate the seasonal contributions of secondary organic carbon (SOC) in Shanghai, and the results from the different methods were compared and evaluated. Biomass burning was the major contributor to the measured WSOC in the autumn sampling period, while SOA was the major contributor in the other seasons. The concentrations of the SOA tracers in summer were obviously higher than that in other seasons. It was found that SOC estimated with the tracer-based method accounted for only a small part of the SOC from the WSOC-based method in Shanghai, especially for the winter and spring sampling periods. PMF results showed that a large part of the SOC was associated with sulfate and nitrate but not with the SOA tracers.

Source apportionment and organic compound characterization of ambient ultrafine particulate matter (PM) in the Los Angeles Basin

In this study, quasi-UFP (PM0.25, dp < 0.25 μm) were collected for 24 h once per week from April 2008 to March 2009 at 10 different locations in the Los Angeles Basin. Samples were chemically analyzed and organic constituents of PM0.25 were grouped into polycyclic aromatic hydrocarbon (PAHs), hopanes and steranes, n-alkanes, and levoglucosan, with concentration levels ranging from 0.16 to 5.5, 0.09 to 2.2, 9.3 to 48, and 2.2 to 106.2 ng m−3 over all sites and seasons, respectively. A molecular marker-based chemical mass balance (MM-CMB) model was applied to estimate the relative contributions from the following primary sources: mobile sources (combined gasoline and diesel vehicles), wood smoke, natural gas combustion, vegetative detritus, and ship emissions. Secondary organic aerosol (SOA) tracers were not included in the model; however their contributions were estimated from non-biomass burning water soluble organic carbon (WSOCnb) and un-apportioned OC from MM-CMB model (“other OC”). High correlation (R2 = 0.8) between “other OC” and WSOCnb in summer suggests that “other OC” is highly impacted by SOA, however un-apportioned primary sources may contribute to “other OC” as well. Mobile sources were expectedly the major primary contributor to PM0.25, with seasonal average contributions of 31 ± 12% in summer and 57 ± 17% in winter. “Other organic matter” was the second largest contributor to PM0.25 in all seasons, across the basin, with substantially higher contribution during warmer spring and summer seasons (27%), while lowest during cold seasons (13%). Wood smoke was the third major contributor to PM0.25 in winter, whereas its contribution was lowest in summer. As expected, ship emissions displayed the highest contribution at the near-harbor HUD site, and their levels continually decreased as a function of distance from coast. Two other primary sources, vegetative detritus and natural gas combustion, collectively contributed to 1.3 ± 0.8% of PM0.25 on an annual average basis over all sites. Comparison of our results to PM2.5 OC apportionment, conducted at central LA, shows that contribution of mobile sources to PM0.25 OC is more than two times higher than PM2.5 (75% and 30%, respectively), while “other OC” contributed significantly higher to PM2.5 OC compared to PM0.25 (56% and 21%, respectively).

Secondary organic aerosols over oceans via oxidation of isoprene and monoterpenes from Arctic to Antarctic

Isoprene and monoterpenes are important precursors of secondary organic aerosols (SOA) in continents. However, their contributions to aerosols over oceans are still inconclusive. Here we analyzed SOA tracers from isoprene and monoterpenes in aerosol samples collected over oceans during the Chinese Arctic and Antarctic Research Expeditions. Combined with literature reports elsewhere, we found that the dominant tracers are the oxidation products of isoprene. The concentrations of tracers varied considerably. The mean average values were approximately one order of magnitude higher in the Northern Hemisphere than in the Southern Hemisphere. High values were generally observed in coastal regions. This phenomenon was ascribed to the outflow influence from continental sources. High levels of isoprene could emit from oceans and consequently have a significant impact on marine SOA as inferred from isoprene SOA during phytoplankton blooms, which may abruptly increase up to 95 ng/m3 in the boundary layer over remote oceans.

Real-Time Continuous Characterization of Secondary Organic Aerosol Derived from Isoprene Epoxydiols in Downtown Atlanta, Georgia, Using the Aerodyne Aerosol Chemical Speciation Monitor

Real-time continuous chemical measurements of fine aerosol were made using an Aerodyne Aerosol Chemical Speciation Monitor (ACSM) during summer and fall 2011 in downtown Atlanta, Georgia. Organic mass spectra measured by the ACSM were analyzed by positive matrix factorization (PMF), yielding three conventional factors: hydrocarbon-like organic aerosol (HOA), semivolatile oxygenated organic aerosol (SV-OOA), and low-volatility oxygenated organic aerosol (LV-OOA). An additional OOA factor that contributed to 33 ± 10% of the organic mass was resolved in summer. This factor had a mass spectrum that strongly correlated (r(2) = 0.74) to that obtained from laboratory-generated secondary organic aerosol (SOA) derived from synthetic isoprene epoxydiols (IEPOX). Time series of this additional factor is also well correlated (r(2) = 0.59) with IEPOX-derived SOA tracers from filters collected in Atlanta but less correlated (r(2) < 0.3) with a methacrylic acid epoxide (MAE)-derived SOA tracer, α-pinene SOA tracers, and a biomass burning tracer (i.e., levoglucosan), and primary emissions. Our analyses suggest IEPOX as the source of this additional factor, which has some correlation with aerosol acidity (r(2) = 0.3), measured as H(+) (nmol m(-3)), and sulfate mass loading (r(2) = 0.48), consistent with prior work showing that these two parameters promote heterogeneous chemistry of IEPOX to form SOA.

Epoxide as a precursor to secondary organic aerosol formation from isoprene photooxidation in the presence of nitrogen oxides.

Isoprene is a substantial contributor to the global secondary organic aerosol (SOA) burden, with implications for public health and the climate system. The mechanism by which isoprene-derived SOA is formed and the influence of environmental conditions, however, remain unclear. We present evidence from controlled smog chamber experiments and field measurements that in the presence of high levels of nitrogen oxides (NO(x) = NO + NO2) typical of urban atmospheres, 2-methyloxirane-2-carboxylic acid (methacrylic acid epoxide, MAE) is a precursor to known isoprene-derived SOA tracers, and ultimately to SOA. We propose that MAE arises from decomposition of the OH adduct of methacryloylperoxynitrate (MPAN). This hypothesis is supported by the similarity of SOA constituents derived from MAE to those from photooxidation of isoprene, methacrolein, and MPAN under high-NOx conditions. Strong support is further derived from computational chemistry calculations and Community Multiscale Air Quality model simulations, yielding predictions consistent with field observations. Field measurements taken in Chapel Hill, North Carolina, considered along with the modeling results indicate the atmospheric significance and relevance of MAE chemistry across the United States, especially in urban areas heavily impacted by isoprene emissions. Identification of MAE implies a major role of atmospheric epoxides in forming SOA from isoprene photooxidation. Updating current atmospheric modeling frameworks with MAE chemistry could improve the way that SOA has been attributed to isoprene based on ambient tracer measurements, and lead to SOA parameterizations that better capture the dependency of yield on NO(x).

Insights into Secondary Organic Aerosol Formation Mechanisms from Measured Gas/Particle Partitioning of Specific Organic Tracer Compounds

In situ measurements of organic compounds in both gas and particle phases were made with a thermal desorption aerosol gas chromatography (TAG) instrument. The gas/particle partitioning of phthalic acid, pinonaldehyde, and 6,10,14-trimethyl-2-pentadecanone is discussed in detail to explore secondary organic aerosol (SOA) formation mechanisms. Measured fractions in the particle phase (f(part)) of 6,10,14-trimethyl-2-pentadecanone were similar to those expected from the absorptive gas/particle partitioning theory, suggesting that its partitioning is dominated by absorption processes. However, f(part) of phthalic acid and pinonaldehyde were substantially higher than predicted. The formation of low-volatility products from reactions of phthalic acid with ammonia is proposed as one possible mechanism to explain the high f(part) of phthalic acid. The observations of particle-phase pinonaldehyde when inorganic acids were fully neutralized indicate that inorganic acids are not required for the occurrence of reactive uptake of pinonaldehyde on particles. The observed relationship between f(part) of pinonaldehyde and relative humidity suggests that the aerosol water plays a significant role in the formation of particle-phase pinonaldehyde. Our results clearly show it is necessary to include multiple gas/particle partitioning pathways in models to predict SOA and multiple SOA tracers in source apportionment models to reconstruct SOA.

Impacts of Siberian Biomass Burning on Organic Aerosols over the North Pacific Ocean and the Arctic: Primary and Secondary Organic Tracers

During the 2003 Chinese Arctic Research Expedition (CHINARE2003) from the Bohai Sea to the high Arctic (37°N-80°N), filter-based particle samples were collected and analyzed for tracers of primary and secondary organic aerosols (SOA) as well as water-soluble organic carbon (WSOC). Biomass burning (BB) tracer levoglucosan had comparatively much higher summertime average levels (476 ± 367 pg/m(3)) during our cruise due to the influence of intense forest fires then in Siberia. On the basis of 5-day back trajectories, samples with air masses passing through Siberia had organic tracers 1.3-4.4 times of those with air masses transporting only over the oceans, suggesting substantial contribution of continental emissions to organic aerosols in the marine atmosphere. SOA tracers from anthropogenic aromatics were negligible or not detected, while those from biogenic terpenenoids were ubiquitously observed with the sum of SOA tracers from isoprene (623 ± 414 pg/m(3)) 1 order of magnitude higher than that from monoterpenes (63 ± 49 pg/m(3)). 2-Methylglyceric acid as a product of isoprene oxidation under high-NOx conditions was dominant among SOA tracers, implying that these BSOA tracers were not formed over the oceans but mainly transported from the adjacent Siberia where a high-NOx environment could be induced by intense forest fires. The carbon fractions shared by biogenic SOA tracers and levoglucosan in WSOC in our ocean samples were 1-2 orders of magnitude lower than those previously reported in continental samples, BB emissions or chamber simulation samples, largely due to the chemical evolution of organic tracers during transport. As a result of the much faster decline in levels of organic tracers than that of WSOC during transport, the trace-based approach, which could well reconstruct WSOC using biogenic SOA and BB tracers for continental samples, only explained ∼4% of measured WSOC during our expedition if the same tracer-WSOC or tracer-SOC relationships were applied.

Organic molecular composition of marine aerosols over the Arctic Ocean in summer: contributions of primary emission and secondary aerosol formation

Abstract. Organic molecular composition of marine aerosol samples collected during the MALINA cruise in the Arctic Ocean was investigated by gas chromatography/mass spectrometry. More than 110 individual organic compounds were determined in the samples and were grouped into different compound classes based on the functionality and sources. The concentrations of total quantified organics ranged from 7.3 to 185 ng m−3 (mean 47.6 ng m−3), accounting for 1.8–11.0% (4.8%) of organic carbon in the marine aerosols. Primary saccharides were found to be dominant organic compound class, followed by secondary organic aerosol (SOA) tracers formed from the oxidation of biogenic volatile organic compounds (VOCs) such as isoprene, α-pinene and β-caryophyllene. Mannitol, the specific tracer for airborne fungal spores, was detected as the most abundant organic species in the samples with a concentration range of 0.052–53.3 ng m−3 (9.2 ng m−3), followed by glucose, arabitol, and the isoprene oxidation products of 2-methyltetrols. Biomass burning tracers such as levoglucosan are evident in all samples with trace levels. On the basis of the tracer-based method for the estimation of fungal-spore OC and biogenic secondary organic carbon (SOC), we estimate that an average of 10.7% (up to 26.2%) of the OC in the marine aerosols was due to the contribution of fungal spores, followed by the contribution of isoprene SOC (mean 3.8%) and α-pinene SOC (2.9%). In contrast, only 0.19% of the OC was due to the photooxidation of β-caryophyllene. This study indicates that primary organic aerosols from biogenic emissions, both from long-range transport of mid-latitude aerosols and from sea-to-air emission of marine organics, as well as secondary organic aerosols formed from the photooxidation of biogenic VOCs are important factors controlling the organic chemical composition of marine aerosols in the Arctic Ocean.

Determination of isoprene-derived secondary organic aerosol tracers (2-methyltetrols) by HPAEC-PAD: Results from size-resolved aerosols in a tropical rainforest

Secondary organic aerosol (SOA) formed from oxidation of isoprene, the most abundant nonmethane hydrocarbon in the atmosphere, has been estimated to contribute significantly to the global aerosol burden. Measurement of isoprene-derived SOA molecular markers has become an effective method for the investigation of biogenic aerosol contributions in the atmosphere. The primary goals of this work are to present a new method based on high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) to quantify diastereoisomeric 2-methyltetrols (2-methylerythritol and 2-methylthreitol), marker compounds of isoprene-derived SOA, and thus to obtain better understanding regarding their abundance and size distribution specifically in a rainforest area. The 2-methyltetrol data, along with water-soluble inorganic ion concentrations, were obtained from size-segregated samples collected at a tropical rainforest site in South China during the period from May to June, 2010. The concentrations of 2-methyltetrols from selected samples measured by HPAEC-PAD showed good agreement with those measured by GC/MS. Overall, the HPAEC-PAD method provides a simple and fast, yet selective and sensitive, alternative to GC/MS for 2-methyltetrol determination, allowing for more efficient analysis of large sample numbers. The size distributions of 2-methylerythritol and 2-methylthreitol both exhibited a unimodal pattern, peaking in the particle size range of 0.44–1.0 μm, where their average concentrations were 11.7 and 4.2 ng m−3, respectively. A strong correlation between 2-methylerythritol and 2-methylthreitol was observed among the entire set of size-segregated samples, indicating their photochemical origin and similar formation mechanism regardless of particle sizes. Compared to the results obtained from previous chamber studies, the similar isomeric fraction of 2-methyltetrols obtained in this study and other field studies confirms their formation through photooxidation of isoprene.

Secondary organic aerosol tracers and malic acid in Hong Kong: seasonal trends and origins

Environmental context Secondary organic aerosols (SOAs), a major organic component of ambient fine particles, contribute to adverse health effects and visibility degradation. Quantification of SOA tracers allows estimation of contributions from specific precursors, which helps the formulation of effective control strategies. We found that malic acid was present in SOA at high abundance in both winter and summer; its seasonally distinct inter-species relationships offer insights into distinct SOA formation pathways. Abstract Fine particle samples collected at an urban location in Hong Kong during winter were analysed by gas chromatography–ion trap mass spectrometry with prior chemical derivatisation. In total, 15 secondary organic aerosol (SOA) tracers from isoprene, monoterpenes, β-caryophyllene and toluene, and 24 other polar oxygenated compounds, were identified and quantified. Monoterpenes and isoprene SOA tracers showed lower levels on winter long-range transport (LRT) days than summer regional days, the latter being reported in our previous study. Opposite seasonal trends were observed for SOA tracers of β-caryophyllene and toluene. The averaged total secondary organic carbon (SOC) apportioned to these four volatile organic compounds (VOCs) was estimated to be 4.73μgCm–3 on winter LRT days, lower than that on summer regional days (5.21μgCm–3). β-caryophyllene and monoterpenes were found to be the most significant SOC contributors to PM2.5 in Hong Kong in both winter and summer, and their averaged SOC contributions on winter LRT days were 2.24 and 1.59μgCm–3. Toluene and isoprene had relatively minor contributions to SOC in Hong Kong in both seasons, with averaged SOC contributions of 0.81 and 0.08μgCm–3 on winter LRT days. Malic acid was well correlated with biogenic SOA tracers and oxalate in both seasons, whereas correlation between malic and succinic acid was only found in winter. Based on the seasonal characteristic inter-species correlations in the region, we hypothesise that malic acid could be formed mainly by the aqueous-phase photodegradation of SOA products of biogenic VOCs during summer. In winter, emissions of biogenic VOCs are greatly reduced and succinic acid then becomes the predominant contributor to malic acid.

Dicarboxylic acids, ketocarboxylic acids and glyoxal in the marine aerosols collected during a round-the-world cruise

This study investigated spatial distributions of water-soluble dicarboxylic acids and related compounds in the marine aerosols collected at low- to mid-latitudes in the Northern Hemisphere for a better understanding of the photochemical aging of organic aerosols during long-range transport. Their molecular distributions were characterized by the predominance of oxalic acid (C2) followed by malonic (C3) and succinic (C4) acids, except for one sample (QFF114, C2>C4>C3) that was collected in the western North Pacific with a heavy influence of biomass burning. Concentration ranges of diacids, ketoacids (including glyoxylic acid and pyruvic acid), and glyoxal were 17–718ngm−3 (average 218ngm−3), 0.40–72ngm−3 (23ngm−3), and 0.16–19ngm−3 (3.3ngm−3), which account for 3.6–23% (14%), 0.09–2.3% (1.3%), and 0.04–0.45% (0.19%) of organic carbon (OC) in the marine aerosols, respectively. Positive correlations were observed between OC and diacids/ketoacids. Positive correlations were also found between biogenic secondary organic aerosol (SOA) tracers (e.g., 2-methylglyceric acid) and diacids/ketoacids, suggesting a significant contribution of biogenic secondary source. The spatial distributions of diacids exhibited higher loadings over the coastal/tropical regions than the open oceans, which are similar to those of the concentration ratios of malonic/succinic (C3/C4) and adipic/azelaic (C6/C9), indicating a more significant influence of anthropogenic sources over the coastal regions than the remote oceans. However, the concentration ratios of oxalic acid to levoglucosan, a biomass-burning tracer, and to C29n-alkane, a tracer for terrestrial biogenic emission showed higher values over the open oceans than the coastal regions, suggesting a continuous production of oxalic acid during long-range atmospheric transport. This study indicates that the long-range transport of primary and secondary aerosols of continental origin and photo-oxidation/aerosol aging are important factors controlling the organic chemical composition of aerosol particles in the marine atmosphere.

The formation of SOA and chemical tracer compounds from the photooxidation of naphthalene and its methyl analogs in the presence and absence of nitrogen oxides

Abstract. Laboratory smog chamber experiments have been carried out to investigate secondary organic aerosol (SOA) formation from the photooxidation of naphthalene and its methyl analogs, 1- and 2-methylnaphthalene (1-MN and 2-MN, respectively). Laboratory smog chamber irradiations were conducted in a flow mode to ensure adequate collection of the aerosol at reasonably low reactant concentrations and in the presence and absence of nitrogen oxides. Phthalic acid and methyl analogs were identified following BSTFA derivatization of the aerosol extract. These compounds were examined to determine whether they could serve as reasonable molecular tracers to estimate the contributions of these precursors to ambient PM2.5. Measurements were also made to determine aerosol parameters from secondary organic aerosol from naphthalene, 1-MN, and 2-MN. A mass fraction approach was used to establish factors which could be applied to phthalic acid concentrations in ambient aerosols, assuming a negligible contribution from primary sources. Phthalic anhydride uptake (and hydrolysis) was tested and found to represent a moderate filter artifact in filter measurements with and without in-line denuders. This study provided the opportunity to examine differences using authentic standards for phthalic acid compared to surrogate standards. While the mass fraction based on a surrogate compounds was somewhat lower, the differences are largely unimportant. For naphthalene, mass fractions of 0.0199 (recommended for ambient samples) and 0.0206 were determined in the presence and absence of nitrogen oxides, respectively, based on phthalic acid standards. The mass fractions determined from the laboratory data were applied to ambient samples where phthalic acid was found and expressed "as naphthalene" since phthalic acid was found to be produced in the particle phase from other methylnaphthalenes. The mass fraction values were applied to samples taken during the 2005 SOAR Study in Riverside, CA and 2010 CalNex Study in Pasadena. In both studies an undetermined isomer of methylphthalic acid was detected in addition to phthalic acid. Laboratory experiment retention times and mass spectra suggest that the major precursor for this compound is 2-MN. For the CalNex Study, SOC values for the 2-ring precursor PAHs (as naphthalene) were found to range from below the detection limit to 20 ngC m−3 which with the laboratory mass fraction data suggests an upper limit of approximately 1 μg m−3 for SOA due to 2-ring PAHs. Temporal data over the course of the one-month CalNex study suggest that primary sources of phthalic acid were probably negligible during this study period. However, the values must still be considered upper limits given a potential hydrolysis reaction or uptake of phthalic anhydride (subsequently hydrolyzed) onto the collection media.

Diurnal variations of organic molecular tracers and stable carbon isotopic composition in atmospheric aerosols over Mt. Tai in the North China Plain: an influence of biomass burning

Abstract. Organic tracer compounds, as well as organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), and stable carbon isotope ratios (δ13C) of total carbon (TC) have been investigated in aerosol samples collected during early and late periods of the Mount Tai eXperiment 2006 (MTX2006) field campaign in the North China Plain. Total solvent-extractable fractions were investigated by gas chromatography/mass spectrometry. More than 130 organic compounds were detected in the aerosol samples. They were grouped into twelve organic compound classes, including biomass burning tracers, biogenic primary sugars, biogenic secondary organic aerosol (SOA) tracers, and anthropogenic tracers such as phthalates, hopanes and polycyclic aromatic hydrocarbons (PAHs). In early June when the field burning activities of wheat straws in the North China Plain were very active, the total identified organics (2090 ± 1170 ng m−3) were double those in late June (926 ± 574 ng m−3). All the compound classes were more abundant in early June than in late June, except phthalate esters, which were higher in late June. Levoglucosan (88–1210 ng m−3, mean 403 ng m−3) was found as the most abundant single compound in early June, while diisobutyl phthalate was the predominant species in late June. During the biomass-burning period in early June, the diurnal trends of most of the primary and secondary organic aerosol tracers were characterized by the concentration peaks observed at mid-night or in early morning, while in late June most of the organic species peaked in late afternoon. This suggests that smoke plumes from biomass burning can uplift the aerosol particulate matter to a certain altitude, which could be further transported to and encountered the summit of Mt. Tai during nighttime. On the basis of the tracer-based method for the estimation of biomass-burning OC, fungal-spore OC and biogenic secondary organic carbon (SOC), we estimate that an average of 24% (up to 64%) of the OC in the Mt. Tai aerosols was due to biomass burning in early June, followed by the contribution of isoprene SOC (mean 4.3%). In contrast, isoprene SOC was the main contributor (6.6%) to OC, and only 3.0% of the OC was due to biomass burning in late June. In early June, δ13C of TC (−26.6 to −23.2‰, mean −25.0‰) were lower than those (−23.9 to −21.9‰, mean −22.9‰) in late June. In addition, a strong anti-correlation was found between levoglucosan and δ13C values. This study demonstrates that crop-residue burning activities can significantly enhance the organic aerosol loading and alter the organic composition and stable carbon isotopic composition of aerosol particles in the troposphere over the North China Plain.

Organosulfates as tracers for secondary organic aerosol (SOA) formation from 2-methyl-3-buten-2-ol (MBO) in the atmosphere.

2-Methyl-3-buten-2-ol (MBO) is an important biogenic volatile organic compound (BVOC) emitted by pine trees and a potential precursor of atmospheric secondary organic aerosol (SOA) in forested regions. In the present study, hydroxyl radical (OH)-initiated oxidation of MBO was examined in smog chambers under varied initial nitric oxide (NO) and aerosol acidity levels. Results indicate measurable SOA from MBO under low-NO conditions. Moreover, increasing aerosol acidity was found to enhance MBO SOA. Chemical characterization of laboratory-generated MBO SOA reveals that an organosulfate species (C5H12O6S, MW 200) formed and was substantially enhanced with elevated aerosol acidity. Ambient fine aerosol (PM2.5) samples collected from the BEARPEX campaign during 2007 and 2009, as well as from the BEACHON-RoMBAS campaign during 2011, were also analyzed. The MBO-derived organosulfate characterized from laboratory-generated aerosol was observed in PM2.5 collected from these campaigns, demonstrating that it is a molecular tracer for MBO-initiated SOA in the atmosphere. Furthermore, mass concentrations of the MBO-derived organosulfate are well correlated with MBO mixing ratio, temperature, and acidity in the field campaigns. Importantly, this compound accounted for an average of 0.25% and as high as 1% of the total organic aerosol mass during BEARPEX 2009. An epoxide intermediate generated under low-NO conditions is tentatively proposed to produce MBO SOA.

Tracer‐based estimation of secondary organic carbon in the Pearl River Delta, south China

Fine particles (PM2.5) were collected using filter‐based high‐volume samplers during summer‐winter 2008 at a rural site in the central Pearl River Delta (PRD), south China, to determine typical secondary organic aerosol (SOA) tracers from significant biogenic (isoprene, monoterpenes, and sesquiterpenes) and anthropogenic (aromatics) precursors. Average isoprene SOA tracers were significantly higher during summer (126 ng m−3) than during fall‐winter (25.1 ng m−3), owing largely to the higher isoprene emission and reaction rates in summer. Average monoterpene SOA tracers during summer (11.6 ng m−3) and fall‐winter (16.4 ng m−3) showed much less difference compared to isoprene SOA tracers, probably resulting from the counteracting effects of temperature on the precursor emission/tracer formation and on gas/particle partitioning. The concentrations of the aromatics' SOA tracer (2,3‐dihydroxy‐4‐oxopentanoic acid) ranged from 1.70 to 52.0 ng m−3 with an average of 15.1 ng m−3, which was the highest reported in ambient air. The secondary organic carbon (SOC) estimated by the SOA‐tracer method averaged 3.07 μg C m−3 in summer and 2.00 μg C m−3 in fall‐winter, contributing 38.4% and 8.7% to OC, respectively. During summer, aromatics‐SOC and isoprene‐SOC reached 2.25 ± 1.5 μg C m−3 and 0.64 ± 0.7 μg C m−3 and accounted for 76% and 18% of the estimated SOC, respectively, while during fall‐winter, aromatics‐SOC (1.64 ± 1.4 μg C m−3) was dominant with a share of 79% in total estimated SOC. These results indicated that anthropogenic aromatics were dominant SOC precursors in the highly industrialized and urbanized PRD region. During summer, SOC levels estimated by elemental carbon (EC) tracer method were not only consistent with but also correlated well with those by SOA‐tracer method. During fall‐winter, however, SOC by SOA‐tracer method was only about one third of that by EC‐tracer method. Their gaps were significantly correlated with the biomass burning tracer levoglucosan, indicating that input from biomass burning emission with very high ratios of OC/EC during fall‐winter would result in an overestimate of SOC by EC‐tracer method. Therefore cautions should be taken when estimating SOC by EC‐tracer method, especially when biomass burning exhibits significant influences.

SOA formation from the atmospheric oxidation of 2-methyl-3-buten-2-ol and its implications for PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt;

Abstract. The formation of secondary organic aerosol (SOA) generated by irradiating 2-methyl-3-buten-2-ol (MBO) in the presence and/or absence of NOx, H2O2, and/or SO2 was examined. Experiments were conducted in smog chambers operated in either dynamic or static mode. A filter/denuder sampling system was used for simultaneously collecting gas- and particle-phase products. The structural characterization of gas and particulate products was investigated using BSTFA, BSTFA + PFBHA, and DNPH derivatization techniques followed by GC-MS and liquid chromatography analysis. This analysis showed the occurrence of more than 68 oxygenated organic compounds in the gas and particle phases, 28 of which were tentatively identified. The major components observed include 2,3-dihydroxyisopentanol (DHIP), 2-hydroxy-2-oxoisopentanol, 2,3-dihydroxy-3-methylbutanal, 2,3-dihydroxy-2-methylsuccinic acid, 2-hydroxy-2-methylpropanedioic acid, acetone, glyoxal, methylglyoxal, glycolaldehyde, and formaldehyde. Most of these oxygenated compounds were detected for the first time in this study. While measurements of the gas-phase photooxidation products have been made, the focus of this work has been an examination of the particle phase. SOA from some experiments was analyzed for the organic mass to organic carbon ratio (OM/OC), the effective enthalpy of vaporization (ΔHvapeff), and the aerosol yield. Additionally, aerosol size, volume, and number concentrations were measured by a Scanning Mobility Particle Sizer coupled to a Condensation Particle Counter system. The OM/OC ratio was 2.1 in the MBO/H2O2 system. The ΔHvapeff was 41 kJ mol−1, a value similar to that of isoprene SOA. The laboratory SOA yield measured in this study was 0.7% in MBO/H2O2 for an aerosol mass of 33 μg m−3. Secondary organic aerosol was found to be negligible under conditions with oxides of nitrogen (NOx) present. Time profiles and proposed reaction schemes are provided for selected compounds. The contribution of SOA products from MBO oxidation to ambient PM2.5 was investigated by analyzing a series of ambient PM2.5 samples collected in several places around the United States. In addition to the occurrence of several organic compounds in both field and laboratory samples, DHIP was found to originate only from the oxidation of MBO, and therefore this compound could potentially serve as a tracer for MBO SOA. Initial attempts have been made to quantify the concentrations of DHIP and other compounds based on surrogate compound calibrations. The average concentrations of DHIP in ambient PM2.5 samples from Duke Forest in North Carolina ranged from zero during cold seasons to approximately 1 ng m−3 during warm seasons. This appears to be the first time that DHIP has been detected in ambient PM2.5 samples. The occurrence of several other compounds in both laboratory and field samples suggests that SOA originating from MBO can contribute under selected ambient conditions to the ambient aerosol mainly in areas where MBO emissions are high.

Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2-Methyl-3-Buten-2-ol (MBO) in the Atmosphere

Organosulfates as Tracers for Secondary Organic Aerosol (SOA) Formation from 2-Methyl-3-Buten-2-ol (MBO) in the Atmosphere