Investigating the Effect of Surface Mixing on Boundary Layer Chemistry Using the PACT-1D Atmospheric Model

Abstract:

Photochemical smog, composed of tropospheric ozone and particulates, is a prominent issue in urban basins and is associated with health and environmental risks. Hydroxyl radicals (OH), formed by precursors such as nitrous acid (HONO), govern smog formation by oxidizing hydrocarbons, forming surface ozone through its interaction with nitrogen oxides. Although the mechanism by which smog forms is heavily researched, its sensitivity to chemical reactions on the ground and surface mixing is not well understood. This study investigates the effect of changes in surface mixing on pollutant concentrations, focusing on how the combination of surface chemistry and mixing affects OH and ozone mixing ratios. Using the PACT-1D atmospheric model, we ran sensitivity studies that varied the eddy diffusion coefficients (Kz) in the lowest 20 meters of the atmosphere and conducted budget analyses on species associated with hydrocarbon oxidation. We discovered that increases in surface mixing resulted in elevated levels of HONO above the surface, caused by increased upward transport and a decrease in HONO deposition rates. However, despite HONO being a major OH source, we did not observe similar effects in ozone mixing ratios, likely due to compensating effects, such as an increased nitric acid formation and deposition. These findings highlight the atmosphere’s non-linearity and demonstrate the importance of surface chemistry and lower atmosphere mixing on tropospheric chemistry.