A Review of the Direct Measurement of Total OH Reactivity: Ambient Air and Vehicular Emission

Abstract

Total OH reactivity, an index utilized to evaluate the overall effect of atmospheric reactive species on hydroxyl radicals, has been assessed over the past half century, particularly in ambient air. The direct measurement of OH reactivity for vehicular sources has also been conducted, further enhancing our understanding of chemical compounds and processes in source emissions. However, the current summary on OH reactivity dominantly focuses on ambient, and the review of OH reactivity measurements and characteristics for vehicular sources was lacking. Herein, we comprehensively reviewed and compared the measurement techniques, values of total OH reactivity, reactive chemical species, and missing OH reactivity for ambient air and vehicular sources involving exhaust and evaporation. The OH reactivity values for ambient air are comparable to those for evaporative emission (around 0–102 s−1), whereas they are all lower by 2–3 orders of magnitude than exhaust emission. In areas dominated by anthropogenic emissions, inorganic reactivity dominates the OH reactivity, while in biogenic-dominated areas, organic reactivity is the main contributor. For vehicular sources, inorganic reactivity dominates the calculated OH reactivity for exhaust emissions, while volatile organic compound reactivity (especially alkene reactivity) can almost explain all the calculated OH reactivity for evaporative emissions. The missing reactivity for ambient air and vehicular emission might derive from unmeasured, even unknown, organic species. We finally discussed possible new directions for future studies of total OH reactivity.