OH-reactivity of volatile organic compounds at urban and rural sites across Canada: Evaluation of air quality model predictions using speciated VOC measurements

Abstract

The total OH-reactivity resulting from volatile organic compound (VOC) oxidation (defined here as ∑ k OH[VOC]) is a measure of the potential of an air mass to produce organic peroxy radicals. Organic peroxy radicals are key intermediate species in the production of ozone and secondary organic aerosol. In this study, routine VOC measurements from 18 selected urban and rural sites across Canada (data from the National Air Pollution Surveillance (NAPS) network) were used to calculate OH-reactivity. Observationally-derived OH-reactivity for VOCs was compared to predictions from Environment Canada's unified regional air quality modelling system (AURAMS). The model was run for two summer periods: between July 8 and August 19, 2004 for evaluation over an eastern Canada domain and between August 23 and September 9, 2005 for evaluation over a western Canada domain. Total OH-reactivity was predicted within a factor of two for most sites; however, OH-reactivity model-measurement differences for individual model VOC species were often larger and offsetting contributions were common. In general, total OH-reactivity was under-predicted in urban centres with the exception of the site in Calgary, Alberta. The higher alkene species, ALKE, was largely responsible for the model under-prediction at urban sites. Calgary, Alberta showed an over-prediction in the higher alkane species, ALKA. The observationally-derived OH-reactivities for aromatic compounds were in good agreement with the two model aromatic VOC species (AROM, TOLU). In rural areas in eastern Canada, total OH-reactivity was over-predicted largely due to over-predictions in the contribution from isoprene (ISOP). A sensitivity modelling run with a new Canadian vegetation database improved ISOP predictions in the rural east; however, systematic ISOP over-predictions were still apparent.