The Dark Side of Atmospheric Chemistry

Nitrogen oxides (NOx = NO+NO2) emitted from combustion and natural sources are reactive gases that regulate the composition of Earth's atmosphere. Nocturnal chemical processes initiated through formation of the nitrate radical, NO3, are a major but poorly understood component of these cycles. This so-called nighttime chemistry affects the lifetime of NOx itself and its availability for daytime O3 photochemistry. Nighttime heterogeneous (i.e., gas to particle) reactions influence particulate matter production, represent an important halogen activation mechanism, and influence the abundance of O3 throughout the global atmosphere. Dark oxidation chemistry influences the lifetime and fates of biogenic volatile organic compounds, the production of organic nitrogen and organic aerosol, and the marine sulfur cycle. Wildfires, now a major midlatitude emission of reactive particles and gases, exhibit NO3-driven oxidation and heterogeneous chemistry that is thought to be a source of absorbing aerosol. This talk will survey the development of instruments and observational approaches to characterize nighttime chemistry, and briefly highlight a few key results from nearly two decades of measurements across environments from megacities to forests, marine areas, fires and the upper atmosphere.