Estimation of Daytime NO3Radical Levels in the UK Urban Atmosphere Using the Steady State Approximation Method

Abstract

The steady state approximation has been applied to the UK National Environment Technology Centre (NETCEN) data at three urban sites in the UK (Marylebone Road London, London Eltham, and Harwell) over the period of 1997 to 2012 to estimate the concentrations of daytime NO3. Despite the common assertion that NO3levels are negligible in the day as a consequence of photolysis, there are occasions where NO3reaches a few pptv. A seasonal pattern in NO3concentration was observed with higher levels in the spring with consistent peaks in April and May. A combination of temperature effects (the formation of NO3from the reaction of NO2with O3has a high activation energy barrier), a distinct pattern in O3concentration (peaking in spring), and loss via reaction with NO peaking in winter is responsible for this trend. Although reaction with OH is still the dominant loss process for VOCs during the day, there are VOCs (unsaturated) that will have an appreciable loss due to reaction with NO3in the daytime. Since the addition reaction of NO3with alkenes can lead directly to organic nitrate formation, there are implications for O3formation and secondary organic aerosol formation during daytime and these are discussed.