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Abstract:
Total OH reactivity measurements have been conducted in August 2013 on the Peking University campus, Beijing and from October to November 2014 in Heshan, Guangdong Province. The daily median result for OH reactivity was 19.98 ± 11.03 s−1 in Beijing and 30.62 ± 19.76 s−1 in Heshan. Beijing presented a significant diurnal variation with maxima over 27 s−1 in the early morning and minima below 16 s−1 in the afternoon. Measurements in Heshan gave a much flatter diurnal pattern. Missing reactivity was observed at both sites, with 21 % missing in Beijing and 32 % missing in Heshan. Unmeasured primary species, such as branched-alkenes could contribute to missing reactivity in Beijing, especially in morning rush hour. An observation-based model with the Regional Atmospheric Chemical Mechanism 2 was used to understand the daytime missing reactivity in Beijing by adding unmeasured oxygenated volatile organic compounds and simulated intermediates of primary VOCs degradation. However, the model failed to explain the missing reactivity in Heshan, where the ambient air was found to be more aged, and the missing reactivity was presumably to attribute to oxidized species, such as aldehydes, acids and di-carbonyls. The ozone production efficiency was 27 % higher in Beijing and 35 % higher in Heshan when constrained by the measured reactivity, compared to the calculation with measured and modeled species included, indicating the importance of quantifying the OH reactivity for better understanding ozone chemistry.
