Microbial Soil Emissions of Nitrogen Oxides and their Impact on Atmospheric Composition in a Deciduous Forest Clearing

Reactive nitrogen oxides (NOy = nitric oxide, NO, nitrogen dioxide, NO2, and nitrous acid, HONO) are important trace gases that control the oxidative capacity of the troposphere. Improvements in pollution control, rising surface temperatures, and increases in fertilizer application mean that biogenic sources of NOy due to soil emissions are increasing in their importance relative to anthropogenic sources. The objective of this study is to understand how biogenic NOy emissions impact local atmospheric chemistry in a clearing within a mixed deciduous forest in Indiana (U.S.A.). Over a two-week period, three dynamic flux chambers were deployed on three experimental plots that differed in their extent of vegetative ground cover. The site experienced relatively high positive fluxes of NO, with a peak in mid-day; nitrogen dioxide fluxes were always negative, with peak deposition occurring in morning. Temperature, soil moisture, and vegetation played an important role in determining the amount of NOy released from soil; the latter suggests an important role of soil microbe-plant interactions. A 0-D model was used to evaluate the relative importance of biogenic emissions on ambient NOy concentrations. We found that biogenic emissions have the greatest impact on local NOy levels following precipitation events when they are the sole source of NOy to the area. Furthermore, a model including biogenic HONO emissions reproduces the ambient HONO concentration profile measured at the site during a previous field campaign. The implication of soil NOy emissions on the local oxidant (e.g., hydroxyl and hydroperoxyl radicals) budget will be discussed.