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

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 (WSOC p ) is mainly from secondary organic aerosol (SOA) production. Comparisons between WSOC p , organic carbon (OC) and elemental carbon (EC) indicate that WSOC p was a nearly comprehensive measure of SOA in the Atlanta summertime. WSOC p and gas-phase WSOC (WSOC g ) concentrations both exhibited afternoon maxima, indicating that photochemistry was a major route for SOA formation. An additional nighttime maximum in the WSOC g concentration indicated a dark source for oxidized organic gases, but this was not accompanied by detectable increases in WSOC p . To study SOA formation mechanisms, WSOC gas/particle partitioning was investigated as a function of temperature, RH, NO x , O 3 , 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 WSOC p and WSOC g with temperature. Relative humidity data indicated a linear dependence between partitioning and predicted fine particle liquid water. Lower NO x concentrations were associated with greater partitioning to particles, but WSOC partitioning had no visible relation to O 3 or fine particle OC mass concentration. There was, however, a relationship between WSOC partitioning and the WSOC p concentration, Correspondence to: R. Weber (rweber@eas.gatech.edu) 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.

Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): a review of laboratory, field and model studies

“…A similarly elevated OM/OC ratio for total OM has been reported for the Southeast US (Simon et al, 2010). While it is possible that this result might be influenced by the assumptions made in the latter study, this finding is also consistent with a large role for aqSOA in this high-RH, photochemically-active location (Hennigan et al, 2009) (Fig. 2a-c).…”

“…Based on the measured WSOC gas/particle partitioning in Atlanta, an effective Henry's law constant K H * ∼10 9 M atm −1 for total WSOC was derived (Hennigan et al, 2009). While not pertaining to a specific compound, this value exceeds all values that are known for the physical solubility of common atmospheric organic compounds and is even higher than any derived from studies that suggest reversible oligomerization (Sect.…”

Observations of rapid aerosol optical depth enhancements in the vicinity of polluted cumulus clouds

“…Hennigan et al (2008) found that the fraction of WSOC in the particle phase increases sharply with RH. The positive correlation with liquid water rather than with organic matter (Hennigan et al, 2009) suggests that aqueous reactions were the dominant SOA formation process rather than gas-phase reactions. Moreover, the inorganic analysis, integrated over a 3-5 min period (limited to few data point per profiles), show an increase by a factor of 1.5 of the sulfate concentration (from 1.6 µg m −3 before the cloud formation up to 2.5 µg m −3 after the cloud formation), while the nitrate concentration do not show any significant evolution.…”

Glyoxal processing by aerosol multiphase chemistry: towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles

“…This trend is in agreement with laboratory evidence that suggests slightly higher uptake at RH > 80% (Figs. 3 and 7 in Volkamer et al, 2009), and field measurements of water-soluble organics in particles that have shown enhanced partitioning above RH ∼75% (Hennigan et al, 2009). The major fraction of predicted SOA is comprised of products that are formed through the photochemical pathway k photochem (Sect.…”