Insights into atmospheric oxidation capacity and its impact on PM2.5 in megacity Beijing via volatile organic compounds measurements

Abstract

This study conducted a VOCs monitoring campaign in Jul and Oct of 2018, Jan and Apr of 2019 in Beijing urban area, to explore the atmospheric oxidation capacity and its impact on PM2.5 there through investigating the chemical behaviors of VOCs at a 1-h time resolution. We totally detected 53 VOCs in the atmosphere over Beijing, sum of them (TVOCs) reaching 29.0 ± 12.9, 47.6 ± 29.4, 45.7 ± 56.4 and 35.8 ± 19.3 ppb (mol/mol) in Jul, Oct, Jan and Apr respectively. The gap between VOCs and CO diurnal amplitudes presented the trend of Jul > Oct and Apr > Jan, which implies the more chemical loss of VOCs in non-winter seasons compared to winter. Then, according to the notable difference in OH- and O3- reactivity of various VOCs, hourly OH exposure ([OH]Δt), O3 exposure ([O3]Δt) and OH concentration were estimated. In result, in Jul, Oct, Jan and Apr, the daily [OH]Δt was 10.39 ± 4.95, 11.10 ± 6.01, 6.39 ± 3.74 and 8.96 ± 5.05 × 1010 molecules·s·cm−3, the nighttime [O3]Δt was 9.43 ± 2.83, 5.22 ± 3.62, 5.79 ± 3.65 and 8.60 ± 4.74 × 1015 molecules·s·cm−3, and the daily OH was 9.76 ± 5.17, 5.11 ± 2.98, 2.95 ± 2.18 and 6.26 ± 3.18 × 106 molecules·cm−3, in respective. The OH peak magnitudes agreed well with the OH measured in-site, which indicates the reliability of our estimates on the oxidation capacity of atmosphere over Beijing. During our studied periods, [OH]Δt, OH and atmospheric oxidability combining OH and O3 (ACO) were all positively related to PM2.5 in Jul and Apr; only [OH]Δt was positively related to PM2.5 in Oct; while [OH]Δt, OH and ACO were all negatively related to PM2.5 in Jan. The proportion of SO2 and VOCs oxidized through gaseous OH reactions was found to increase with PM2.5 increasing in non-winter seasons, but decrease with PM2.5 increasing in winter. It means that high PM2.5 concentrations are attributed more to gas-phase oxidations in non-winter seasons and potentially boosted by photochemistry in summer and spring. But, in winter high PM2.5 concentrations effectively inhibit photolysis reactions and weaken atmospheric oxidation capacity, which makes high PM2.5 less dependent on the gas-phase oxidations.