An investigation of the contributions of VOCs to urban ozone production in the U.K.

Ambient concentrations of 23 volatile organic compounds (VOCs) measured at London Marylebone Road (LMR), an urban traffic site in the UK were analysed over a period of 22 years (1998-2019) 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% across the period studied, likely as a result of legislative controls. This analysis was extended to consider the contribution of VOCs to ozone formation at this site utilising POCP values. Similarly, the overall reactivity of the VOC burden at LMR has resulted in a significant decrease of just under 12% per year in ozone formation potential over 1998-2019 at this site. The declines in ozone formation potential for VOCs associated with road traffic emissions are all in good agreement at 11-13% decrease per year. VOCs related to non-traffic sources, namely ethane and propane from natural gas leakage, did not see a significant decline over the study period. The variation and composition of the overall VOC burden was compared across three decadal time periods (1998-2000, 2001-2010, 2011-2019) and saw an increase in significance of these pollutants (with ethane and propane moving from the fifth and eleventh largest contributors in 1998-2000 to the first and second largest contributors in 2011-2019, respectively) suggesting they are not sufficiently controlled under current legislation. The increase in significance of ethane and propane was mirrored in their contribution to ozone generation potential but ethene continues to substantially dominate in contribution to ozone formation potential by a factor of 4 and 5 compared with ethane and propane, 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 this site (in order of contribution: ethene > propene > n-butane > ethane > propane). 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.