Effects of heterogeneous processes on NO3, HONO, and HNO3 chemistry in the troposphere


  • Brian G. Heikes,

  • Anne M. Thompson


Atmospheric measurements of trace oxides of nitrogen, i.e., NO3, HONO, and HNO3, are at variance with accepted photochemical theory. In particular, measured NO3 levels at night are lower than expected from photochemical equilibrium, observed HONO concentrations increase throughout the night, and HNO3 or NO3 is produced rapidly in cloud and plume. We investigate theoretically the potential role of wet particles in the chemistry of NO3, HONO, and HNO3 through a model that incorporates H-N-O photochemistry and a heterogeneous scavenging parameterization. The model includes effects due to temperature, pressure, photostationary state number, photolysis rate, diffusion rate, and sticking coefficient. Application of the model to suitable NO3 case studies reveals that low NO3 concentrations could result from the reaction of NO with NO3 provided NO is present at night. However, in the absence of NO, low NO3 can result from heterogeneous loss of NO3 and N2O5 provided their sticking coefficients are greater than 10−3. In the study of HONO we find that a nocturnal production by heterogeneous H2O reactions is insufficient to account for observed levels of HONO. Model sensitivity calculations demonstrate that the formation of HNO3 or NO3 in cloud can occur through two distinct and complementary mechanisms. The route, OH + NO2→ HNO3, can account for considerable HNO3 formation in cloud but only during daylight hours. A heterogeneous mechanism consisting of NO3 and N2O5 absorption and reaction on droplets is shown to be a viable source of HNO3 in cloud during all hours.