This is the authors' final draft of the paper subsequently published in a shortened form and with a modified title, as Atmospheric Environment, 2006, 40(39), pp.7598-7613. The definitive version ("Ozone photochemistry and elevated isoprene during the UK heatwave of August 2003") is available from http://www.sciencedirect.com/science/journal/13522310 ).A wide range of chemical and physical parameters have been observed over the course of a severe Europe-wide air pollution episode in August 2003. Detailed surface observations made at the rural perimeter edge of London, U.K. indicated significantly elevated levels of primary VOCs, ozone (>110ppb), other photochemical by-products such as PAN, HCHO, and higher oxygenates but not NOx. Reactive organic tracers in combination with surface Doppler wind radar and back trajectory analysis have been used to establish that initial rapid morning rises in O3 during the episode were caused by entrainment of air from aloft, polluted on regional scales from mainland Europe. Total VOC reactivity to OH approximately doubled during this episode, with similar distribution between functional groups, but showing a temperature dependant exponentially increasing contribution from biogenic isoprene (max 1.2 ppbV). In addition to entrainment of regional air pollution, ozone formation rates within the U.K. boundary layer on the day of observation have been determined using measured peroxy radicals in combination with other chemical data. Under episodic conditions total peroxy radicals in excess 120pptV were observed in late afternoon with strong correlation to a later and higher peak in ozone when compared to non-episodic conditions. During the daytime under episodic conditions alkyl peroxy radical formation was dominated by PAN thermolysis, whose afternoon lifetime averaged only 18.3 min, but which was sustained at a concentration greater than 750 pptV. Low episodic wind speeds resulted in a relatively small possible spatial footprint for emissions of reactive precursors to PAN, and a strong correlation between isoprene and PAN production rate suggest this species may have contributed to the later afternoon increases seen in surface O3
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