Abstract
Abstract. The effect of dry and wet deposition of semi-volatile organic compounds (SVOCs) in the gas phase on the concentrations of secondary organic aerosol (SOA) is reassessed using recently derived water solubility information. The water solubility of SVOCs was implemented as a function of their volatility distribution within the WRF-Chem regional chemistry transport model, and simulations were carried out over the continental United States for the year 2010. Results show that including dry and wet removal of gas-phase SVOCs reduces annual average surface concentrations of anthropogenic and biogenic SOA by 48 and 63% respectively over the continental US. Dry deposition of gas-phase SVOCs is found to be more effective than wet deposition in reducing SOA concentrations (−40 vs. −8% for anthropogenics, and −52 vs. −11% for biogenics). Reductions for biogenic SOA are found to be higher due to the higher water solubility of biogenic SVOCs. The majority of the total mass of SVOC + SOA is actually deposited via the gas phase (61% for anthropogenics and 76% for biogenics). Results are sensitive to assumptions made in the dry deposition scheme, but gas-phase deposition of SVOCs remains crucial even under conservative estimates. Considering reactivity of gas-phase SVOCs in the dry deposition scheme was found to be negligible. Further sensitivity studies where we reduce the volatility of organic matter show that consideration of gas-phase SVOC removal still reduces average SOA concentrations by 31% on average. We consider this a lower bound for the effect of gas-phase SVOC removal on SOA concentrations. A saturation effect is observed for Henry's law constants above 108 M atm−1, suggesting an upper bound of reductions in surface level SOA concentrations by 60% through removal of gas-phase SVOCs. Other models that do not consider dry and wet removal of gas-phase SVOCs would hence overestimate SOA concentrations by roughly 50%. Assumptions about the water solubility of SVOCs made in some current modeling systems (H* = H* (CH3COOH); H* = 105 M atm−1; H* = H* (HNO3)) still lead to an overestimation of 35%/25%/10% compared to our best estimate.
Highlights
Organic compounds represent a major, often dominant mass fraction of ambient aerosol (e.g., Murphy et al, 2006; Jimenez et al, 2009)
We investigated the effect of considering removal of semivolatile organic compounds on secondary organic aerosols concentrations according to recent findings that suggest SVOCs are highly water soluble (Hodzic et al, 2014a)
In a number of sensitivity studies spanning the months of June and July of 2010 we investigate the robustness of these findings by varying the volatility distribution of the organic matter, the Henry’s law constants used, and key parameters of the dry deposition scheme
Summary
Organic compounds represent a major, often dominant mass fraction of ambient aerosol (e.g., Murphy et al, 2006; Jimenez et al, 2009). Recent findings from explicit oxidation chemistry modeling (Hodzic et al, 2013, 2014a) with the Generator of Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A Aumont et al, 2005) together with structure-activity estimation of solubility (Raventos-Duran et al, 2010) suggests that many SVOCs are highly water soluble, with Henry’s law constants H ∗ between 105 and 1010 M atm−1. This makes them very susceptible to removal processes in the atmosphere (wet deposition and dry deposition to wet surfaces/vegetation). Given that gas and particle phases are in equilibrium, this implies that removal of gas-phase SVOCs could be an important indirect sink of secondary organic aerosol (SOA) mass
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