Significance of Nitric Acid Photolysis in low NOx troposphere: Model Simulations

Both field observations and laboratory results have shown that photolysis of the NOy “permanent sink” (HNO3) on surfaces is fast, at a rate of ~3x10-5 s-1, releasing photochemically reactive nitrogen (HONO and NOx) back into the overlying atmosphere. However, how significant this process is as a re-NOx-ification pathway in the low NOx atmospheric chemistry is still not clear. To bridge the knowledge gap, a 20-layer one-dimensional (1D) photochemical model has been developed and evaluated. The 1D model includes a complete set of gas phase reactions from the Regional Atmospheric Chemistry Mechanism (RACM), vertical eddy transport between the ground surface and the top of planetary boundary layer (PBL), and hydrolysis of photoexcited NO2 in the gas phase. The 1D model also includes heterogeneous HONO sources, including photolysis of surface-adsorbed HNO3 from ground surfaces, HONO formation from “dark” NOx reactions, in situ photolytic HONO source from aerosols, and dew formation/evaporation mechanism as temporary HONO reservoir. Sensitivity of model output to O3, NOx, HCHO, isoprene, HNO3, surface-adsorbed HNO3, UV intensity and surface roughness (Zo) was performed. Model simulated HONO concentrations are compared with field observations at a low NOx, rural site at Pinnacle State Park, NY. Linear regression analysis between simulated HONO and composite diurnal observations yields a slope near unity (intercept was set to zero) and correlation coefficient (R2) of 0.84. The Mean bias (MB), Normalized Mean Bias (NMB%), the Root Mean Square Error (RMSE), the Normalized Mean Error (NME%) and the fraction of predictions within a factor of two of observations (FAC2) of the model are 1.3 pptv, 2.2%, 6.6 pptv, 6.8%, and 1.0 respectively. Including the photoexcited NO2 hydrolysis mechanism in the model increases the daily and midday HONO concentrations by 2.1% and 2.4% respectively, and increases OH by 1.4%. Adding these heterogeneous and photoexcited NO2 hydrolysis mechanism to the base model (RACM) brings simulated daily mean HONO concentrations from 30.4% to 102.2% of observation and midday mean from 3.6% to 103.9%, and 19.7% increase in OH level. Model results show that photolysis of surface-adsorbed HNO3 is by far the most important daytime HONO source, which accounts for 60% of midday and 32% of daily HONO observation. We conclude that this surface photolysis mechanism is an important re-NOx-ification pathway in the rural troposphere.