Origins and composition of fine atmospheric carbonaceous aerosol in the Sierra Nevada Mountains, California

Abstract

In this paper we report chemically resolved measurements of organic aerosol (OA) and related tracers during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) at the Blodgett Forest Research Station, California. OA contributed the majority of the mass to the fine atmospheric particles and was predominately oxygenated (OOA). The highest concentrations of OA were during sporadic wildfire influence when aged plumes were impacting the site. In situ measurements of particle phase molecular markers were dominated by secondary compounds and could be categorized into three factors or sources: (1) aged biomass burning emissions and oxidized urban emissions, (2) oxidation products of temperature-driven local biogenic emissions and (3) local light-driven emissions and oxidation products. There were multiple biogenic components that contributed to OA at this site whose contributions varied diurnally, seasonally and in response to changing meteorological conditions, e.g., temperature and precipitation events. Concentrations of isoprene oxidation products were larger when temperatures were higher due to more substantial emissions of isoprene and enhanced photochemistry. Methyl chavicol oxidation contributed similarly to OA during both identified meteorological periods. In contrast, the abundances of monoterpene oxidation products in the particle phase were greater during cooler conditions, even though emissions of the precursors were lower. Following the first precipitation event of the fall the abundances of the monoterpene oxidation products increased dramatically, although the mechanism is not known. OA was correlated with the anthropogenic tracers 2-propyl nitrate and carbon monoxide (CO), consistent with previous observations, while being comprised of mostly non-fossil carbon (>75 %). The correlation between OA and an anthropogenic tracer does not necessarily identify the source of the carbon as being anthropogenic but instead suggests a coupling between the anthropogenic and biogenic components in the air mass that might be related to the source of the oxidant and/or the aerosol sulfate. Observations of organosulfates of isoprene and α-pinene provided evidence for the likely importance of aerosol sulfate in spite of neutralized aerosol. This is in contrast to laboratory studies where strongly acidic seed aerosols were needed in order to form these compounds. These compounds together represented only a minor fraction (< 1 %) of the total OA mass and suggest that other mechanisms, e.g., NO<sub>x</sub> enhancement of oxidant levels, are more likely to be responsible for the majority of the anthropogenic enhancement of biogenic secondary organic aerosol observed at this site

Organosulfates from Pinene and Isoprene over the Pearl River Delta, South China: Seasonal Variation and Implication in Formation Mechanisms

“…Unlike our observation in the PRD, high levels of pNOSs were observed in the nighttime and early morning in Antwerp, Belgium, 12 K-puszta, Hungary, 60 northeastern Bavaria, Germany, 15 the Sierra Nevada Mountains, California, 26 and Shanghai. 37 Nighttime NO 3 chemistry was proposed to account for the high levels of nighttime pNOSs.…”

“…9,34,57 High sulfate levels would promote pNOSs' formation. 26 Since there was no significant difference in aerosol acidity between summer and fall (p > 0.05), aerosol acidity should not be the major factor leading to the seasonal differences in correlations between terpenoic acids and pNOSs. On the other hand, sulfate levels were about twice higher in fall than summer.…”

“…Due to the lack of commercial standard, OSs were quantified using camphor sulfonic acid. 21,26 Seven-point linear calibration of peak area vs concentration (both normalized by the internal standard) were established with the correlation coefficient R 2 > 0.99. Three pNOS isomers with MW 295, iOS with MW 216 and MW 200 were quantified since the intensity of their peaks were high enough for quantifications (signal/noise > 3).…”

“…The detection limit for camphor sulfonic acid was 0. 12,21 Asia, 37 and the U.S. 26,48 were in the range of 0.217−140 ng m −3 during the summer (Table 2). Our measurements were consistent with these reported values, although different instruments and standards were used for pNOSs quantification.…”

“…15 However, if acidic aerosols are required by the NO 3 pathway, 9 what is the explanation for the phenomena that neutralized aerosols coincided with the high overnight concentrations of pNOSs? 26 On the other hand, Kristensen and Glasius suggested that photochemical reactions were the dominant source of pNOSs based on the positive correlations observed between pNOSs and photochemically derived species. 21 For isoprene OSs (iOSs), the major product, 2-methyltetrol sulfate ester, is formed through reactive uptake of low-NOx products, epoxydiols (IEPOX), onto acidic particles.…”