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The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane (MeOH / DCM, 1:1 , v/v), MeOH / DCM (1:2 , v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ∼95 %), followed by MeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts had significantly (p<0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (>280 ∘) are less soluble in MeOH (∼80 %) than in DMF (∼90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed consistent temporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 times greater than the MeOH extracts in late spring and summer. The average light absorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p<0.01) than that of MeOH extracts. Source apportionment results indicated that the MeOH solubility of BrC associated with biomass burning, lubricating oil combustion, and coal combustion is similar to their DMF solubility. The BrC linked with unburned fossil fuels and polymerization processes of aerosol organics was less soluble in MeOH than in DMF, which was likely the main reason for the large difference in time series between MeOH and DMF extract absorption. These results highlight the importance of testing different solvents to investigate the structures and light absorption of BrC, particularly for the low-volatility fraction potentially originating from non-combustion sources. [ABSTRACT FROM AUTHOR]

Gas–particle partitioning of water-soluble organic compounds plays a significant role in influencing the formation, transport, and lifetime of organic aerosols in the atmosphere, but is poorly characterized. In this work, gas- and particle-phase concentrations of isoprene oxidation products (C5-alkene triols and 2-methylterols), levoglucosan, and sugar polyols were measured simultaneously at a suburban site of the western Yangtze River Delta in east China. All target polyols were primarily distributed into the particle phase (85.9 %–99.8 %). Given the uncertainties in measurements and vapor pressure predictions, a dependence of particle-phase fractions on vapor pressures cannot be determined. To explore the impact of aerosol liquid water on gas–particle partitioning of polyol tracers, three partitioning schemes (Cases 1–3) were proposed based on equilibriums of gas vs. organic and aqueous phases in aerosols. If particulate organic matter (OM) is presumed as the only absorbing phase (Case 1), the measurement-based absorptive partitioning coefficients (Kp,OMm) of isoprene oxidation products and levoglucosan were more than 10 times greater than predicted values (Kp,OMt). The agreement between Kp,OMm and Kp,OMt was substantially improved when solubility in a separate aqueous phase was included, whenever water-soluble and water-insoluble OM partitioned into separate (Case 2) or single (Case 3) liquid phases, suggesting that the partitioning of polyol tracers into the aqueous phase in aerosols should not be ignored. The measurement-based effective Henry's law coefficients (KH,em) of polyol tracers were orders of magnitude higher than their predicted values in pure water (KH,wt). Due to the moderate correlations between log⁡(KH,em/KH,wt) and molality of sulfate ions, the gap between KH,em and KH,wt of polyol tracers could not be fully parameterized by the equation defining "salting-in" effects and might be ascribed to mechanisms of reactive uptake, aqueous phase reaction, "like-dissolves-like" principle, etc. These study results also partly reveal the discrepancy between observation and modeling of organic aerosols. [ABSTRACT FROM AUTHOR]

N-containing aromatic compounds (NACs) are an important group of light-absorbing molecules in the atmosphere. They are often observed in combustion emissions, but their chemical formulas and structural characteristics remain uncertain. In this study, red oakwood and charcoal fuels were burned in cookstoves using the standard water-boiling test (WBT) procedure. Submicron aerosol particles in the cookstove emissions were collected using quartz (Qf) and polytetrafluoroethylene (PTFE) filter membranes positioned in parallel. A backup quartz filter (Qb) was also installed downstream of the PTFE filter to evaluate the effect of sampling artifacts on NAC measurements. Liquid chromatography–mass spectroscopy (LC–MS) techniques identified 17 NAC chemical formulas in the cookstove emissions. The average concentrations of total NACs in Qb samples (0.37±0.31 – 1.79±0.77 µgm-3) were greater than 50 % of those observed in the Qf samples (0.51±0.43 – 3.91±2.06 µgm-3), and the Qb -to- Qf mass ratios of individual NACs had a range of 0.02–2.71, indicating that the identified NACs might have substantial fractions remaining in the gas phase. In comparison to other sources, cookstove emissions from red oak or charcoal fuels did not exhibit unique NAC structural features but had distinct NAC composition. However, before identifying NAC sources by combining their structural and compositional information, the gas-particle partitioning behaviors of NACs should be further investigated. The average contributions of total NACs to the light absorption of organic matter at λ=365 nm (1.10 %–2.57 %) in Qf and Qb samples (10.7 %–21.0 %) are up to 10 times larger than their mass contributions (Qf : 0.31 %–1.01 %; Qb : 1.08 %–3.31 %), so the identified NACs are mostly strong light absorbers. To explain more sample extract absorption, future research is needed to understand the chemical and optical properties of high-molecular-weight (e.g., molecular weight, MW>500 Da) entities in particulate matter. [ABSTRACT FROM AUTHOR]

Intense new particle formation (NPF) events were observed in the coastal atmosphere during algae growth and farming season at Xiangshan gulf of the east China coast. High nucleation-mode iodine concentrations measured by ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UPLC/Q-TOF-MS) confirmed that the NPF events were induced by iodine species. Our study provides important information on iodine speciation, size distributions, and its role in NPF in the context of heavy air pollution in China's coastal areas. For the first time, we identified 5 inorganic iodine species, 45 organic iodine compounds (35 molecular formulas), and a group of iodide–organic adducts in aerosols. The concentrations and size distributions of iodine species down to 10 nm were measured during the iodine-induced NPF, continental NPF, and non-NPF days at the coastal site and compared to those at an inland site. The iodine in the above four aerosol sample types were characterized by iodate, aromatic iodine compounds, iodoacetic acid or iodopropenoic acid, and iodide–organic adducts, respectively. Iodide and organic iodine compounds were found in the nucleation-mode particles; however, it is still not clear whether they contributed to nucleation or just new particle growth. Wild algae, as well as farmed algae, could be an important NPF source in China's coastal areas. [ABSTRACT FROM AUTHOR]

This study seeks to understand the compositional details of N-containing aromatic compounds (NACs) emitted during biomass burning (BB) and their contribution to light-absorbing organic carbon (OC), also termed brown carbon (BrC). Three laboratory BB experiments were conducted with two United States pine forest understory fuels typical of those consumed during prescribed fires. During the experiments, submicron aerosol particles were collected on filter media and subsequently extracted with methanol and examined for their optical and chemical properties. Significant correlations (p<0.05) were observed between BrC absorption and elemental carbon (EC)/OC ratios for individual burns data. However, the pooled experimental data indicated that EC/OC alone cannot explain the BB BrC absorption. Fourteen NAC formulas were identified in the BB samples, most of which were also observed in simulated secondary organic aerosol (SOA) from photooxidation of aromatic volatile organic compounds (VOCs) with NOx. However, the molecular structures associated with the identical NAC formula from BB and SOA are different. In this work, the identified NACs from BB are featured by methoxy and cyanate groups and are predominately generated during the flaming phase. The mass concentrations of identified NACs were quantified using authentic and surrogate standards, and their contributions to bulk light absorption of solvent-extractable OC were also calculated. The contributions of identified NACs to organic matter (OM) and BrC absorption were significantly higher in flaming-phase samples than those in smoldering-phase samples, and they correlated with the EC/OC ratio (p<0.05) for both individual burns and pooled experimental data, indicating that the formation of NACs from BB largely depends on burn conditions. The average contributions of identified NACs to overall BrC absorption at 365 nm ranged from 0.087±0.024 % to 1.22±0.54 %, which is 3–10 times higher than their mass contributions to OM (0.023±0.0089 % to 0.18±0.067 %), so the NACs with light absorption identified in this work from BB are likely strong BrC chromophores. Further studies are warranted to identify more light-absorbing compounds to explain the unknown fraction (>98 %) of BB BrC absorption. [ABSTRACT FROM AUTHOR]

Atmospheric trace elements, especially metal species, are an emerging environmental and health concern with insufficient understanding of their levels and sources in Shanghai, the most important industrial megacity in China. Here we continuously performed a 1 year (from March 2016 to February 2017) and hourly resolved measurement of 18 elements in fine particles (PM2:5) at the Shanghai urban center with an Xact multi-metals monitor and several collocated instruments. Mass concentrations (mean±1σ; ngm-3) determined by Xact ranged from detection limits (nominally 0.1 to 20 ngm-3) to 15 µgm-3. Element-related oxidized species comprised an appreciable fraction of PM2:5 during all seasons, accounting for 8.3% on average. As a comparison, the atmospheric elements concentration level in Shanghai was comparable with that in other industrialized cities in East Asia but 1 or 2 orders of magnitude higher than at sites in North America and Europe. Positive matrix factorization (PMF) was applied to identify and apportion the sources of the elements in the PM2:5 mass. Five different factors were resolved (notable elements and relative contribution in parentheses): traffic-related (Ca, Fe, Ba, Si; 46%), shipping (V, Ni; 6%), nonferrous metal smelting (Ag, Cd, Au; 15%), coal combustion (As, Se, Hg, Pb; 18%) and ferrous metal smelting (Cr, Mn, Zn; 15%). The contribution from the exhaust and non-exhaust vehicle emissions, i.e., the traffic-related factor shows a strong bimodal diurnal profile with average concentration over 2 times higher during the rush hour than during nighttime. The shipping factor was firmly identified because V and Ni, two recognized tracers of shipping emissions, are almost exclusively transported from the East China Sea and their ratio (around 3.2) falls within the variation range of V= Ni ratios in particles emitted from heavy oil combustion. Interestingly, nearly half of the K was derived from coal combustion with high mineral affinity (elements associated with aluminosilicates, carbonates and other minerals in coal ash). The contributions of nonferrous metal smelting to the trace elements are consistent with a newly developed emission inventory. Although the precipitation scavenging effect on the mass concentration of the trace elements varied among different species and sources, precipitation could effectively lower the concentration of the trafficand coal combustion-related trace elements. Therefore, water spray to simulate natural types of precipitation could be one of the abatement strategies to facilitate the reduction of ambient PM2:5 trace elements in the urban atmosphere. Collectively, our findings in this study provide baseline levels and sources of trace elements with high detail, which are needed for developing effective control strategies to reduce the high risk of acute exposure to atmospheric trace elements in China's megacities. [ABSTRACT FROM AUTHOR]

This study investigates the composition of organic particulate matter in PM2.5 in a remote montane forest in the southeastern US, focusing on the role of organic nitrogen (N) in sulfur-containing secondary organic aerosol (nitrooxy-organosulfates) and aerosols associated with biomass burning (nitro-aromatics). Bulk water-soluble organic N (WSON) represented  ∼  14 % w∕w of water-soluble total N (WSTN) in PM2.5 on average across seasonal measurement campaigns conducted in the spring, summer, and fall of 2015. The largest contributions of WSON to WSTN were observed in spring ( ∼  18 % w∕w) and the lowest in the fall ( ∼  10 % w∕w). On average, identified nitro-aromatic and nitrooxy-organosulfate compounds accounted for a small fraction of WSON, ranging from  ∼  1 % in spring to  ∼  4 % in fall, though were observed to contribute as much as 28 % w∕w of WSON in individual samples that were impacted by local biomass burning. The highest concentrations of oxidized organic N species occurred during summer (average of 0.65 ng N m−3) along with a greater relative abundance of higher-generation oxygenated terpenoic acids, indicating an association with more aged aerosol. The highest concentrations of nitro-aromatics (e.g., nitrocatechol and methyl-nitrocatechol), levoglucosan, and aged SOA tracers were observed during fall, associated with aged biomass burning plumes. Nighttime nitrate radical chemistry is the most likely formation pathway for nitrooxy-organosulfates observed at this low NOx site (generally < 1 ppb). Isoprene-derived organosulfate (MW216, 2-methyltetrol derived), which is formed from isoprene epoxydiols (IEPOX) under low NOx conditions, was the most abundant individual organosulfate. Concentration-weighted average WSON ∕ WSOC ratios for nitro-aromatics + organosulfates + terpenoic acids were 1 order of magnitude lower than the overall aerosol WSON ∕ WSOC ratio, indicating the presence of other uncharacterized higher-N-content species. Although nitrooxy-organosulfates and nitro-aromatics contributed a small fraction of WSON, our results provide new insight into the atmospheric formation processes and sources of these largely uncharacterized components of atmospheric organic N, which also helps to advance the atmospheric models to better understand the chemistry and deposition of reactive N. [ABSTRACT FROM AUTHOR]