The impact of speciated VOCs on regional ozone increment derived from measurements at the UK EMEP supersites between 1999 and 2012

Abstract

The impact of 27 volatile organic compounds (VOCs) on the regional O 3 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 O 3 in south-east England and northern UK, respectively. The monthly-diurnal regional O 3 increment was defined as the difference between the regional and hemispheric background O 3 concentrations, respectively, derived from oxidant vs. NO x correlation plots, and cluster analysis of back trajectories arriving at Mace Head, Ireland. At Harwell, which had substantially greater regional O 3 increments than Auchencorth, variation in the regional O 3 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 O 3 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 O 3 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 O 3 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 O 3 formation. In summary, this work presents a clear methodology for achieving a coherent VOC, regional-O 3 -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 O 3 concentrations.