Consumption of Hydrocarbons and Its Relationship with Ozone Formation in Two Chinese Megacities

Continuous measurements of ozone and its precursors were performed at sites in two Chinese megacities, i.e., an urban site in Beijing and a suburban site in the Pearl River Delta (PRD). At both sites, the total oxidants (O3 + NO2) varied with the ratio of ethylbenzene to m,p-xylenes, which serves as an indicator of photochemical aging. An observation-based method (OBM) was derived for calculating the photochemical consumption of individual non-methane hydrocarbons (NMHCs) based on the observed NMHC concentrations and the ratio of ethylbenzene to m,p-xylenes. The results show a strong correlation between the oxidant level and the derived consumption of precursors at the two sites (R2 = 0.81 for the PRD site and R2 = 0.83 for the Beijing site), demonstrating a strong cause–effect relationship. The relative “consumption efficiency” among NMHCs was calculated based on the integrated amount of hydroxyl radicals derived from the ratio of ethylbenzene to xylenes. Thus, the percent contributions to ozone formation from each individual NMHC can be calculated. This concept of consumption is purely observation-based and provides an easy way to bypass complicated modeling and the necessity of knowing instantaneous concentrations of hydroxyl radicals, which are highly illusive in nature.