资源环境科技发展态势分析平台

Abstract. Heterogeneous reactions occurring at the surface of atmospheric aerosol particles regulate the production and lifetime of a wide array of atmospheric gases. Aerosol surface area plays a critical role in setting the rate of heterogeneous reactions in the atmosphere. Despite the central role for aerosol surface area, there are few assessments of the accuracy of aerosol surface area concentrations in regional and global models. In this study, we compare aerosol surface area concentrations in the EPA’s Community Multiscale Air Quality (CMAQ) model with commensurate observations from the 2011 NASA flight-based DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality) campaign. The study region includes the Baltimore and Washington, DC metropolitan area. Dry aerosol surface area was measured aboard the NASA P-3B aircraft using an Ultra-High Sensitivity Aerosol Spectrometer (UHSAS). We show that modeled and measured dry aerosol surface area, S_a,mod and S_a,meas respectively, are modestly correlated (r² = 0.52) and on average agree to within a factor of two (S_a,mod/S_a,meas = 0.44) over the course of the 13 research flights. We show that S_a,mod/S_a,meas does not depend strongly on photochemical age or the concentration of secondary biogenic aerosol, suggesting that the condensation of low-volatility gas-phase compounds does not strongly affect model-measurement agreement. In comparison, there is strong agreement between measured and modeled aerosol number concentration (N_mod/N_meas = 0.87, r² = 0.63). The persistent underestimate of S_a in the model, combined with strong agreement in modeled and measured aerosol number concentrations, suggests that model representation of the size distribution of primary emissions or secondary aerosol formed at the early stages of oxidation may contribute to the observed differences.

For reactions occurring on small particles, the rate of heterogeneous reactions is a linear function of both S_a and the reactive uptake coefficient (γ). To assess the importance of uncertainty in modeled S_a for the representation of heterogeneous reactions in models, we compare both the mean and the variance in S_a,mod/S_a,meas to that in γ(N₂O₅)_mod/γ(N₂O₅)_meas. We find that the uncertainty in model representation of heterogeneous reactions is primarily driven by uncertainty in the parametrization of reactive uptake coefficients, although the discrepancy between S_a,mod and S_a,meas is not insignificant. Our analysis suggests that model improvements to aerosol surface area concentrations, in addition to more accurate parameterizations of heterogeneous kinetics, will advance the representation of heterogeneous chemistry in regional models.