Preprints
https://doi.org/10.5194/acp-2022-340
https://doi.org/10.5194/acp-2022-340
 
22 Jun 2022
22 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Observation-Based Constraints on Modeled Aerosol Surface Area: Implications for Heterogeneous Chemistry

Rachel A. Bergin1, Monica Harkey2,3, Alicia Hoffman2,3, Richard H. Moore4, Bruce Anderson4, Andreas Beyersdorf4,a, Luke Ziemba4, Lee Thornhill4,5, Edward Winstead4,5, Tracey Holloway2,3, and Timothy H. Bertram1 Rachel A. Bergin et al.
  • 1Department of Chemistry, University of Wisconsin - Madison, Madison, WI 53703, USA
  • 2Nelson Institute Center for Sustainability and the Global Environment, University of Wisconsin–Madison, Madison, WI 53703, USA
  • 3Department of Atmospheric and Oceanic Sciences, University of Wisconsin–Madison, Madison, WI 53703, USA
  • 4NASA Langley Research Center, Hampton, VA 23666, USA
  • 5Science Systems and Applications, Inc., Hampton, VA 23666, USA
  • anow at: California State University, San Bernardino, San Bernardino, CA 92407

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, Sa,mod and Sa,meas respectively, are modestly correlated (r2 = 0.52) and on average agree to within a factor of two (Sa,mod/Sa,meas = 0.44) over the course of the 13 research flights. We show that Sa,mod/Sa,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 (Nmod/Nmeas = 0.87, r2 = 0.63). The persistent underestimate of Sa 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 Sa and the reactive uptake coefficient (γ). To assess the importance of uncertainty in modeled Sa for the representation of heterogeneous reactions in models, we compare both the mean and the variance in Sa,mod/Sa,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 Sa,mod and Sa,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.

Rachel A. Bergin et al.

Status: open (until 03 Aug 2022)

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Rachel A. Bergin et al.

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Observation-Based Constraints on Modeled Aerosol Surface Area: Implications for Heterogeneous Chemistry Rachel A. Bergin, Monica Harkey, Alicia Hoffmann, Richard H. Moore, Bruce Anderson, Andreas Beyersdorf, Luke Ziemba, Lee Thornhill, Edward Winstead, Tracey Holloway, and Timothy H. Bertram https://minds.wisconsin.edu/handle/1793/83241

Rachel A. Bergin et al.

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Short summary
Correctly predicting aerosol surface area concentrations is important for determining the rate of heterogeneous reactions in chemical transport models. Here, we compare aircraft measurements of aerosol surface area with a regional model. In polluted air masses, we show that the model under predicts aerosol surface area by a factor of two. Despite this disagreement, the representation of heterogeneous chemistry still dominates the overall uncertainty in the loss rate of molecules such as N2O5.
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