The role of biogenic hydrocarbons in the production of ozone in urban plumes in southeast England

Abstract

First order approximations of the rates of emission of isoprene and α-pinene are made for the region around London. Isoprene is found to be the dominant biogenic non-methane hydrocarbon species by mass in the region. Chemical degradation schemes for isoprene and α-pinene are added to an existing urban plume model which has previously been applied to London. The influence of each hydrocarbon is investigated by comparing model runs which include biogenic chemistry to a base case scenario. Oveerall, the effect is small: ozone levels in the plume are increased by the addition of the new schemes by up to 88 ppb. A model run with biogenic hydrocarbon and NO x emissions only demonstrates the VOC-limited behaviour of the urbans plume. Conversely, removall of anthropenic VOC emissions from the rural/suburban model results in a decrease of only 5 ppb in ozone production over one day. The reason for this difference in behaviour is the different NMHC/NO x ratios within and outside the urban plume. The inclusion of biogenic NMHC chemistry in the model results in a negligible change to the calculated ‘urban ozone effect’ defined as as the difference between ozone concentrations within and outsside of the plume. Substantial enhancement to present biogenic NMHC database are required if our understanding of urban plume chemistry is to be made more accurate.