Citation:

Wu, Z., L. Zhang, J. Walker, P. Makar, J. Perlinger, AND X. Wang. Extension of a gaseous dry deposition algorithm to oxidized volatile organic compounds and hydrogen cyanide for application in chemistry transport models. Journal of Advances in Modeling Earth Systems. John Wiley & Sons, Inc., Hoboken, NJ, 14(8):5093-5105, (2021). https://doi.org/10.5194/gmd-14-5093-2021

Impact/Purpose:

Existing dry deposition schemes consider a relatively small number of oxidized volatile organic compounds (oVOCs). Accurate dry deposition rates are needed to assess ecosystem N inputs and to accurately simulate the atmospheric lifetimes of oVOC species in chemical transport models. Due to the lack of field flux data of oVOCs, deposition velocities of these species are typically parameterized based on physiochemical properties, taking SO2 and O3 as references. Recent studies show that deposition velocities for oVOC derived using current dry deposition models are lower than field measurements, acknowledging that measurement data remain somewhat limited in temporal extent and geographical variability. The discrepancy between measurements and models may result from inaccurate simulation of surface uptake by non-stomatal pathways (i.e., leaf cuticle, ground, wet surfaces), fast chemical loss within and near the canopy, measurement uncertainty, or combinations thereof. To fulfill community demands of modeling dry deposition of oVOCs and to take advantage of recent flux data from the Southern Oxidant and Aerosol Study, this study extends the current Zhang et al. (2003) dry deposition scheme by including 12 additional oVOC species and hydrogen cyanide (HCN). The scheme is revised based on the effective Henry’s law constants and oxidizing capacities of the individual species.

Description:

The dry deposition process refers to flux loss of an atmospheric pollutant due to uptake of the pollutant by the Earth's surfaces, including vegetation, underlying soil, and any other surface types. In chemistry transport models (CTMs), the dry deposition flux of a chemical species is typically calculated as the product of its surface layer concentration and its dry deposition velocity (Vd); the latter is a variable that needs to be highly empirically parameterized due to too many meteorological, biological, and chemical factors affecting this process. The gaseous dry deposition scheme of Zhang et al. (2003) parameterizes Vd for 31 inorganic and organic gaseous species. The present study extends the scheme of Zhang et al. (2003) to include an additional 12 oxidized volatile organic compounds (oVOCs) and hydrogen cyanide (HCN), while keeping the original model structure and formulas, to meet the demand of CTMs with increasing complexity. Model parameters for these additional chemical species are empirically chosen based on their physicochemical properties, namely the effective Henry's law constants and oxidizing capacities. Modeled Vd values are compared against field flux measurements over a mixed forest in the southeastern US during June 2013. The model captures the basic features of the diel cycles of the observed Vd. Modeled Vd values are comparable to the measurements for most of the oVOCs at night. However, modeled Vd values are mostly around 1 cm s−1 during daytime, which is much smaller than the observed daytime maxima of 2–5 cm s−1. Analysis of the individual resistance terms and uptake pathways suggests that flux divergence due to fast atmospheric chemical reactions near the canopy was likely the main cause of the large model–measurement discrepancies during daytime. The extended dry deposition scheme likely provides conservative Vd values for many oVOCs. While higher Vd values and bidirectional fluxes can be simulated by coupling key atmospheric chemical processes into the dry deposition scheme, we suggest that more experimental evidence of high oVOC Vd values at additional sites is required to confirm the broader applicability of the high values studied here. The underlying processes leading to high measured oVOC Vd values require further investigation.

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