Gas/particle partitioning of water-soluble organic aerosol in Atlanta

C. J. Hennigan,M. H. Bergin,A. G. Russell,A. Nenes,R. J. Weber

doi:10.5194/acp-9-3613-2009

C. J. Hennigan, M. H. Bergin + Show 3 more

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https://doi.org/10.5194/acp-9-3613-2009

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Abstract

Abstract. Gas and particle-phase organic carbon compounds soluble in water (e.g., WSOC) were measured simultaneously in Atlanta throughout the summer of 2007 to investigate gas/particle partitioning of ambient secondary organic aerosol (SOA). Previous studies have established that, in the absence of biomass burning, particulate WSOC (WSOCp) is mainly from secondary organic aerosol (SOA) production. Comparisons between WSOCp, organic carbon (OC) and elemental carbon (EC) indicate that WSOCp was a nearly comprehensive measure of SOA in the Atlanta summertime. WSOCp and gas-phase WSOC (WSOCg) concentrations both exhibited afternoon maxima, indicating that photochemistry was a major route for SOA formation. An additional nighttime maximum in the WSOCg concentration indicated a dark source for oxidized organic gases, but this was not accompanied by detectable increases in WSOCp. To study SOA formation mechanisms, WSOC gas/particle partitioning was investigated as a function of temperature, RH, NOx, O3, and organic aerosol mass concentration. No clear relationship was observed between temperature and partitioning, possibly due to a simultaneous effect from other temperature-dependent processes. For example, positive temperature effects on emissions of biogenic SOA precursors and photochemical SOA formation may have accounted for the observed similar proportional increases of WSOCp and WSOCg with temperature. Relative humidity data indicated a linear dependence between partitioning and predicted fine particle liquid water. Lower NOx concentrations were associated with greater partitioning to particles, but WSOC partitioning had no visible relation to O3 or fine particle OC mass concentration. There was, however, a relationship between WSOC partitioning and the WSOCp concentration, suggesting a compositional dependence between partitioning semi-volatile gases and the absorbing organic aerosol. Combined, the overall results suggest two dominant SOA formation processes in urban Atlanta during summer. One was the photochemical production of SOA from presumably biogenic precursors that increased with the onset of sunrise and peaked in the afternoon. The other, which showed no apparent diurnal pattern, involved the partitioning of semi-volatile gases to liquid water, followed by heterogeneous reactions. The co-emission of water vapor and biogenic VOCs from vegetation may link these processes.

Highlights

The gas/particle partitioning of oxidized semi-volatile organic compounds (SVOCs) is an integral process in the formation of secondary organic aerosol (SOA), making it one of the most important routes for producing fine atmospheric particles
WSOCp measurements were conducted via a particleinto-liquid sampler (PILS) coupled to a Total Organic Carbon analyzer, a method which has been detailed by Sullivan et al (2004)
WSOCp was highly correlated with organic carbon (OC) (R2=0.73) and the slope (0.70μg C/ μg C) indicates that a high fraction of the OC was soluble and likely to be secondary. (Note, the results are based on a Deming linear regression that minimizes the distance between the observed data and fitted line in both the x and y directions and is more appropriate than regular linear regression for this type of data (Cornbleet and Gochman, 1979))

Summary

Introduction

The gas/particle partitioning of oxidized semi-volatile organic compounds (SVOCs) is an integral process in the formation of secondary organic aerosol (SOA), making it one of the most important routes for producing fine atmospheric particles. Matsunaga et al (2005) simultaneously measured ten carbonyls in the gas and particle phases at a site near Tokyo, while Fisseha et al (2006) made simultaneous measurements of four carboxylic acids in the gas and particle phases in Zurich. One limitation to these studies, and others like them, is that they characterize a very minor fraction of the total SOA (and organic carbon aerosol (OC)) mass, so their behavior may not be representative of the SOA. Heterogeneous reactions may alter the condensed phase component to forms difficult to detect, which would not be considered in single component analysis partitioning studies

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