PNNL

Abstract

Many atmospheric chemical processes, including the formation of secondary organic aerosol (SOA), are strongly modulated by the reactions of NO and NO2 (NOx). Though NOx is controlled by anthropogenic emissions near urban areas, in rural areas soil microbial nitrification and denitrification can be a significant contribution to NO emission, thought to make up 20-25% of the global total. The relative rates of emissions of different nitrogen-containing species (e.g., NO, N2O, HONO, N2) are strong functions of soil properties such as temperature, moisture content, pH, and soil carbon and nitrogen pools. However, typical large-scale geochemical models either express these emissions simplistically, or not at all. Here we investigate the potential impact of soil NO emissions on atmospheric NOx chemistry and SOA formation over regional and monthly time scales, specifically the 2016 spring and summer Intensive Observational Periods (IOPs) of the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) field campaign. We implement a mechanistic soil NO parameterization into the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) model, supplemented by a 1-km soil moisture analysis. We then simulate both IOPs over the U.S. Great Plains, evaluating against ground stations and flight data. We show that soil NO emissions can impact some species by 10-50% regionally averaged and alleviate negative biases relative to observations. Meteorology and soil moisture and temperature changes between IOP1 and IOP2 lead to overall differences in emissions, but with large regional variability due to heterogeneous surface characteristics.