Cooperative Institute for Research in Environmental Sciences

Nighttime Tropospheric Chemistry

The primary drivers for atmospheric chemical transformation are radical species and oxidants that are photochemical in origin, such as ozone and OH. Nighttime chemical transformations, which are also important in certain environments, often rely on photochemical energy stored in the form of compounds such as ozone and can provide significant feedbacks that affect daytime photochemical cycles. Perhaps one of the most important classes of dark reactions are the cycles involving the higher nitrogen oxides, NO₃ and N₂O₅, which form from the oxidation of NO₂ by O₃ at night. These compounds are unstable in sunlight, but can build to appreciable concentrations in the dark. Production and loss of these nocturnal nitrogen oxides (NNO_x = NO₃ + N₂O₅) influences tropospheric budgets of reactive nitrogen, ozone, VOC, halogens, sulfur compounds and aerosol. Understanding this dark chemistry is therefore important to regional air quality and climate.

Recent advances in optical spectroscopy have led to the development of new instruments for in-situ atmospheric observations of both NO₃ and N₂O₅. These observations have yielded new scientific insights based on measurements of these and related compounds from ground sites, tall towers, ships and aircraft, as well as in laboratory studies. Participation of these instruments is planned for future field studies, and will focus particularly on the following topics.

• Organic aerosol formation from nighttime oxidation of biogenic VOCs.
• Halogen activation through heterogeneous uptake of N₂O₅ to chloride aerosol.
• Rates of N₂O₅ hydrolysis and the implications for transport and / or loss of reactive N.
• The interaction of micrometeorology and the shallow nocturnal boundary layer with nighttime chemical transformation

NOAA ESRL Contact
Steve Brown
Steven.S.Brown@noaa.gov

CIRES Contact
Ben Balsley
balsley@cires.colorado.edu
Veronica Vaida
vaida@colorado.edu