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Abstract
Abstract. The impact of 27 volatile organic compounds (VOCs) on the regional O3 increment was investigated using measurements made at the UK EMEP supersites Harwell (1999–2001 and 2010–2012) and Auchencorth (2012). Ozone at these sites is representative of rural O3 in south-east England and northern UK, respectively. The monthly-diurnal regional O3 increment was defined as the difference between the regional and hemispheric background O3 concentrations, respectively, derived from oxidant vs. NOx correlation plots, and cluster analysis of back trajectories arriving at Mace Head, Ireland. At Harwell, which had substantially greater regional O3 increments than Auchencorth, variation in the regional O3 increment mirrored afternoon depletion of anthropogenic VOCs due to photochemistry (after accounting for diurnal changes in boundary layer mixing depth, and weighting VOC concentrations according to their photochemical ozone creation potential). A positive regional O3 increment occurred consistently during the summer, during which time afternoon photochemical depletion was calculated for the majority of measured VOCs, and to the greatest extent for ethene and m+p-xylene. This indicates that, of the measured VOCs, ethene and m+p-xylene emissions reduction would be most effective in reducing the regional O3 increment but that reductions in a larger number of VOCs would be required for further improvement. The VOC diurnal photochemical depletion was linked to anthropogenic sources of the VOC emissions through the integration of gridded anthropogenic VOC emission estimates over 96 h air-mass back trajectories. This demonstrated that one factor limiting the effectiveness of VOC gridded emissions for use in measurement and modelling studies is the highly aggregated nature of the 11 SNAP (Selected Nomenclature for Air Pollution) source sectors in which they are reported, as monthly variation in speciated VOC trajectory emissions did not reflect monthly changes in individual VOC diurnal photochemical depletion. Additionally, the major VOC emission source sectors during elevated regional O3 increment at Harwell were more narrowly defined through disaggregation of the SNAP emissions to 91 NFR (Nomenclature for Reporting) codes (i.e. sectors 3D2 (domestic solvent use), 3D3 (other product use) and 2D2 (food and drink)). However, spatial variation in the contribution of NFR sectors to parent SNAP emissions could only be accounted for at the country level. Hence, the future reporting of gridded VOC emissions in source sectors more highly disaggregated than currently (e.g. to NFR codes) would facilitate a more precise identification of those VOC sources most important for mitigation of the impact of VOCs on O3 formation. In summary, this work presents a clear methodology for achieving a coherent VOC, regional-O3-impact chemical climate using measurement data and explores the effect of limited emission and measurement species on the understanding of the regional VOC contribution to O3 concentrations.
Highlights
Production of ground-level ozone (O3) is dependent on concentrations of NOx (NO and NO2), methane, carbon monoxide, and volatile organic compounds (VOCs) (Jenkin and Clemitshaw, 2000)
Gauss et al (2014) modelled the reductions in O3 impact across Europe on human health and vegetation resulting from 15 % reductions in anthropogenic NOx and VOC emissions across the EU and showed that VOC emission reductions were more effective than NOx emission reductions in reducing the O3 impact metrics across much of north-west Europe
A chemical climate is derived through the linkage of a specific “impact” of atmospheric composition through the “state” of relevant atmospheric composition variation (VOC diurnal photochemical depletion) to its causal “drivers”
Summary
Production of ground-level ozone (O3) is dependent on concentrations of NOx (NO and NO2), methane, carbon monoxide, and volatile organic compounds (VOCs) (Jenkin and Clemitshaw, 2000). Development of policies for the mitigation of these impacts requires understanding of the influences on O3 concentrations from local, regional and hemispheric-scale processes. Gauss et al (2014) modelled the reductions in O3 impact across Europe on human health (using the SOMO35 metric) and vegetation (using the deciduous forest PODY metric) resulting from 15 % reductions in anthropogenic NOx and VOC emissions across the EU and showed that VOC emission reductions were more effective than NOx emission reductions in reducing the O3 impact metrics across much of north-west Europe. Knowledge of the contribution of individual VOCs to O3 production on the European (regional) scale will enable targeting of the most effective VOC reductions for reducing regionally derived O3 exposure relevant to O3 impacts
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