Gas-phase kinetics, POCPs and an investigation of the contributions of VOCs to urban ozone production in the U.K.

Ambient concentrations of 22 volatile organic compounds (VOCs) measured at London Marylebone Road (LMR); an urban traffic site and London Eltham (LE); an urban background site, were analysed over a period of 23 years (1997-2019) to assess the impact of pollution control strategies. A significant decrease in ambient concentration is seen for the majority of VOCs analysed with total VOC burden decreasing by 76% and 59% for LMR and LE, respectively, across the period studied. This is likely as a result of legislative controls. This analysis was extended to consider the dominant contribution of VOCs to ozone formation at the sites utilising Photochemical Ozone Creation Potential (POCP) values. Similarly, the overall reactivity of the VOC burden at the sites has resulted in a significant decrease of 11% and 7% per year in ozone formation potential for LMR and LE, respectively. At LMR, the declines in ozone formation potential for VOCs associated with road traffic emissions are all in good agreement at 11-13% decrease per year. Reasonable agreement is also seen at LE with a decrease of 6-11% per year in ozone formation potential of the VOCs related to traffic sources. VOCs related to non-traffic sources, namely ethane and propane from natural gas leakage, did not see a significant decline over the study period at either site. The variation and composition of the overall VOC burden was compared across three decadal time periods (1997-2000, 2001-2010, 2011-2019) and saw an increase in significance of these pollutants at both sites. At LMR, ethane and propane moved from the fifth and eleventh largest contributors to total VOC burden in 1997-2000 to the first and second largest contributors in 2011-2019. At LE, a similar trend is seen with ethane and propane becoming the first and second largest contributors in the most recent time period, up from second and eighth at the beginning of the dataset. The similarity between these sites suggests such pollutants are not sufficiently controlled under current legislation. The increase in significance of ethane and propane was mirrored in their contribution to ozone generation potential at both sites but ethene continues to dominate in contribution to ozone formation potential. At LMR, ethene dominates by a factor of 4 and 5 compared with ethane and propane, respectively, however, at LE, ethene dominates to a lesser extent over ethane and propane at factors of 1 and 3, respectively. Alkanes are typically considered to be less important in the context of ozone formation potential due to their low reactivity in comparison to other VOCs. Analysis presented herein demonstrates the negative impact of ignoring such emissions as their influence begins to grow such that alkanes now represent 3 of the 5 highest contributors to tropospheric ozone formation at both sites (in order of contribution at LMR: ethene > propene > n-butane > ethane > propane; and LE: ethene > ethane > n-butane > propane > propene). The importance of high-quality gas-phase kinetic studies to determine the impact of VOCs in ozone production is clear and the usefulness of metrics such as POCPs is demonstrated.