NO and NO2 interconversion downwind of two different line sources in suburban environments

Abstract

Abstract Nitrogen monoxide (NO), nitrogen dioxide (NO 2 ) and ozone (O 3 ) have been measured downwind (0–450 m) from two line sources in a suburban environment in a set of initial experiments designed to investigate the role of chemistry in determining the partitioning of NOx into NO and NO 2 and the ability of a commercial modelling package (ADMS) to reproduce downwind speciated NOx concentrations. NO concentrations decrease more rapidly than NO 2 as NO is removed both by dilution and by chemical reaction with ozone. Whilst NO is the dominant form of NOx at the roadside, NO 2 concentrations decay more slowly (due to chemical production and higher background levels) and NO 2 typically forms the majority of NOx by 50–100 m from the source. For a majority of modelling studies the input NO and NO 2 concentrations were set to match the experimental concentrations at the initial site. The measured decays of NO and NO 2 can be accurately reproduced by the ADMS dispersion model only when chemical transformation is taken into account. When NOx is estimated directly from traffic flow measurements, the modelled NO 2 tends to underestimate the measured value.