[1] Fine particles (PM 2.5 ) 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 À3 ) than during fall-winter (25.1 ng m À3 ), owing largely to the higher isoprene emission and reaction rates in summer. Average monoterpene SOA tracers during summer (11.6 ng m À3) and fall-winter (16.4 ng m À3) 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 À3 with an average of 15.1 ng m À3, which was the highest reported in ambient air. The secondary organic carbon (SOC) estimated by the SOA-tracer method averaged 3.07 mg C m À3 in summer and 2.00 mg C m À3 in fall-winter, contributing 38.4% and 8.7% to OC, respectively. During summer, aromatics-SOC and isoprene-SOC reached 2.25 AE 1.5 mg C m À3 and 0.64 AE 0.7 mg C m À3 and accounted for 76% and 18% of the estimated SOC, respectively, while during fall-winter, aromatics-SOC (1.64 AE 1.4 mg C m À3) 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.

Twenty-four hour integrated filter samples of fine particulate matter (PM2.5) were collected from May 2004 to April 2005 at one rural site and three urban sites located in the southeastern United States. Filters were extracted and analyzed for both biogenic secondary organic aerosol (SOA) tracers via gas chromatography-mass spectrometry (GC-MS), and water-soluble organic carbon (WSOC) concentrations. The tracers reported in this study include isoprene-derived 2-methylthreitol and 2-methylerythritol, as well as pinene-derived cis-pinonic acid. The mean ambient concentrations ranged from 21.7 to 94.3 ng/m3, 5.31 to 17.9 ng/m3, and 1.87 to 3.18 microgC/m3 for 2-methyltetrols (sum of 2-methylerythritol and 2-methylthreitol), cispinonic acid and WSOC, respectively. Distinct spatial distributions were observed for all tracers with the highest concentration at the rural site and the lowest level at a coastal site. Although 2-methyltetrols were small fractions of WSOC, varying from 0.35% at an urban site to highest fractions of 1.09% at the rural site, WSOC exhibited significant correlation with 2-methyltetrols during summer, suggesting isoprene SOA makes an important contribution to WSOC. 2-Methyltetrols had the highest concentrations during the summer,when high temperature, intense solar radiation, and high ozone level occurred. However, no obvious seasonal variation was found for cispinonic acid. Between inland sites WSOC was more spatially homogeneous than the 2-methyltetrols, suggesting that WSOC was produced from a variety of mechanisms.

We conducted a source apportionment and investigated the atmospheric behavior of carbonaceous aerosols during hazy and normal days using radiocarbon ( 14 C) and biomass burning/secondary organic aerosol (SOA) tracers during winter in Guangzhou, China. Haze episodes were formed either abruptly by local emissions or through the accumulation of particles transported from other areas. The average contributions of fossil carbon to elemental carbon (EC), water-insoluble organic carbon, and water-soluble organic carbon were 71 ± 10%, 40 ± 6% and 33 ± 3%, respectively. High contributions of fossil carbon to EC (80−90%) were observed for haze samples that were substantially impacted by local emissions, as were the highest (lowest) ratios for NO 3 −

Biogenic organosulfates (OSs) are important markers of secondary organic aerosol (SOA) formation involving cross reactions of biogenic precursors (terpenoids) with anthropogenic pollutants. Until now, there has been rare information about biogenic OSs in the air of highly polluted areas. In this study, fine particle (PM 2.5 ) samples were separately collected in daytime and nighttime from summer to fall 2010 at a site in the central Pearl River Delta (PRD), South China. Pinene-derived nitrooxyorganosulfates (pNOSs) and isoprene-derived OSs (iOSs) were quantified using a liquid chromatograph (LC) coupled with a tandem mass spectrometer (MS/MS) operated in negative electrospray ionization (ESI) mode. The pNOSs with MW 295 exhibited higher levels in fall (151 ± 86.9 ng m −3 ) than summer (52.4 ± 34.0 ng m −3 ), probably owing to the elevated levels of NOx and sulfate in fall when air masses mainly passed through city clusters in the PRD and biomass burning was enhanced. In contrast to observations elsewhere where higher levels occurred at nighttime, pNOS levels in the PRD were higher during the daytime in both seasons, indicating that pNOS formation was likely driven by photochemistry over the PRD. This conclusion is supported by several lines of evidence: the specific pNOS which could be formed through both daytime photochemistry and nighttime NO 3 chemistry exhibited no day− night variation in abundance relative to other pNOS isomers; the production of the hydroxynitrate that is the key precursor for this specific pNOS was found to be significant through photochemistry but negligible through NO 3 chemistry based on the mechanisms in the Master Chemical Mechanism (MCM). For iOSs, 2-methyltetrol sulfate ester which could be formed from isoprene-derived epoxydiols (IEPOX) under low-NOx conditions showed low concentrations (below the detection limit to 2.09 ng m −3 ), largely due to the depression of IEPOX formation by the high NOx levels over the PRD. ■ INTRODUCTIONBiogenic volatile organic compounds (BVOCs) including isoprene and monoterpenes 1 contribute significantly to the global secondary organic aerosol (SOA) budget. 2 Recent studies have shown that the conversion of BVOCs to SOA can be significantly promoted in the presence of high anthropogenic emissions. 3−6 As notable SOA products of BVOCs reacting with anthropogenic pollutants under acidic conditions, organosulfates (OSs) have been detected in both laboratory-generated SOA 7−10 and ambient aerosols. 11−18 Moreover, OSs could contribute a significant fraction of fine particles, accounting for up to 30% of organic matter (OM) 9,19−23 and up to 10% of total sulfate. 24,25 The formation mechanisms of OSs are still unclear. Previous chamber studies have demonstrated that both OH-photooxidation and NO 3 dark reactions can form pinene-derived nitrooxy-organosulfates (pNOSs) on acidic particles. 9 The pNOSs were only detected in nighttime samples in northeastern Bavaria, Germany, suggesting a role for nighttime NO 3 chemistry in pNOS formation. 15 How...

Filter-based particle samples were simultaneously collected at 14 sites across 6 regions of China during the summer of 2012. These filters were analyzed for secondary organic aerosol (SOA) tracers from biogenic precursors (isoprene, monoterpenes, and β-caryophyllene) and anthropogenic aromatics. The sum of all SOA tracers ranged from 29.9 to 371 ng m À3 with the majority from isoprene (123 ± 78.8 ng m À3 ), followed by monoterpenes (10.5 ± 6.64 ng m À3 ), β-caryophyllene (5.07 ± 3.99 ng m À3 ), and aromatics (2.90 ± 1.52 ng m À3 ). The highest levels of biogenic SOA tracers were observed in East China, whereas the highest concentrations of the aromatic SOA tracer, 2,3-dihydroxy-4-oxopentanoic acid (DHOPA), occurred in North China. All biogenic SOA tracers exhibited positive correlations with temperature, most likely resulting from enhanced biogenic volatile organic compounds (BVOCs) emissions and photochemistry in high-temperature regions. Among the isoprene SOA tracers, the low-NO x products 2-methyltetrols were the largest by mass concentration. However, at certain urban sites, the contribution of the high-NO x product 2-methylglyceric acid was significantly higher, implying a greater influence of NO x on isoprene SOA formation in urban areas. For the monoterpene SOA tracers, the ratio of the first-generation products (cis-pinonic acid plus pinic acid) to the high-generation product (3-methyl-1,2,3-butanetricarboxylic acid) exhibited a negative correlation with the amount of high-generation products, indicating that this ratio could serve as an indicator of the aging of monoterpene SOA. The ratio ranged from 0.89 to 21.0, with an average of 7.00 ± 6.02, among the observation sites, suggesting that monoterpene SOA was generally fresh over China during the summer. As a typical anthropogenic SOA tracer, DHOPA exhibited higher levels at urban sites than at remote sites. These SOA tracers were further used to attribute SOA origins via the SOA-tracer method. The total concentrations of secondary organic carbon (SOC) and SOA were estimated to be in the range of 0.37 to 2.47 μgC m À3 and 0.81 to 5.44 μg m À3 , respectively, with the highest levels observed in the eastern regions of China. Isoprene (46 ± 14%) and aromatics (27 ± 8%) were the two major contributors to SOC in every region. In North China, aromatics were the largest SOA contributor. Our ground-based observations suggest that anthropogenic aromatics are important SOA precursors in China.

Atmospheric aerosols exert a substantial influence on climate, ecosystems, visibility, and human health. Although secondary organic aerosols (SOA) dominate fine-particle mass, they comprise myriad compounds with uncertain sources, chemistry, and interactions. SOA formation involves absorption of vapors into particles, either because gas-phase chemistry produces low-volatility or semivolatile products that partition into particles or because morevolatile organics enter particles and react to form lower-volatility products. Thus, SOA formation involves both production of low-volatility compounds and their diffusion into particles. Most chemical transport models assume a single well-mixed phase of condensing organics and an instantaneous equilibrium between bulk gas and particle phases; however, direct observations constraining diffusion of semivolatile organics into particles containing SOA are scarce. Here we perform unique mixing experiments between SOA populations including semivolatile constituents using quantitative, single-particle mass spectrometry to probe any mass-transfer limitations in particles containing SOA. We show that, for several hours, particles containing SOA from toluene oxidation resist exchange of semivolatile constituents at low relative humidity (RH) but start to lose that resistance above 20% RH. Above 40% RH, the exchange of material remains constant up to 90% RH. We also show that dry particles containing SOA from α-pinene ozonolysis do not appear to resist exchange of semivolatile compounds. Our interpretation is that in-particle diffusion is not rate-limiting to mass transfer in these systems above 40% RH. To the extent that these systems are representative of ambient SOA, we conclude that diffusion limitations are likely not common under typical ambient boundary layer conditions. secondary organic aerosols | mixing | relative humidity | single-particle mass spectrometry D ifferent types of organic aerosols can phase-separate, most notably nonpolar primary organic aerosol (POA) and relatively polar secondary organic aerosols (SOA). Asa-Awuku et al.(1) reported limited miscibility between motor oil POA and SOA derived from β-caryophyllene, and Robinson et al. (2) showed that particles of squalane POA and SOA derived from toluene photooxidation persisted as an external mixture for more than 3 h after being combined into a single well-mixed Teflon chamber. Theory supports high activity coefficients for these dissimilar mixtures (3), yet detailed calculations of phase partitioning thermodynamics suggest that, at least within a single more or less similar type (e.g., α-pinene SOA), the constituents are well mixed over a broad relative humidity (RH) range (4).Even if thermodynamics favors a single, relatively ideal organic phase, diffusive limitations within particles may impede mixing driven by absorption and heterogeneous chemistry. So-called "glassy" organic aerosols associated with highly viscous states were first considered in the context of water uptake into particles at low temperatu...

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We have investigated the vapor wall loss of semi-volatile organic compounds (SVOCs) in a Teflon smog chamber. We studied the vapor wall loss of seven SVOCs with known saturation concentrations, including alkanes (hexacosane, pentacosane, docosane, eicosane, and d 62 -squalane), an organic acid (oleic acid), and a polyol (levoglucosan) in single-component and binary-component (organic) systems, using ammonium sulfate (AS) seeds to constrain the particle wall loss. We coated inorganic particles with SVOCs and measured the loss of organics from those particles to constrain the wall losses, observing loss rates proportional to the saturation concentrations of the SVOCs. The loss rate of oleic acid mixed with d 62 -squalane was proportional to its mole fraction in the mixture. Our results show that the vapor wallloss rates of SVOCs are significant, quasi-irreversible, and proportional to the SVOC vapor concentrations. The vapor wall-loss rate constant of the SVOCs that we studied in the CMU chamber is 3.8 § 0.3 h ¡1 ; this is comparable to values in other chambers with similar surface area to volume ratios. Our results are also consistent with a relatively high mass accommodation coefficient for SVOCs, a org > 0.1.