资源环境科技发展态势分析平台

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 PM_2.5 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 PM_2.5 in Jul and Apr; only [OH]Δt was positively related to PM_2.5 in Oct; while [OH]Δt, OH and ACO were all negatively related to PM_2.5 in Jan. The proportion of SO₂ and VOCs oxidized through gaseous OH reactions was found to increase with PM_2.5 increasing in non-winter seasons, but decrease with PM_2.5 increasing in winter. It means that high PM_2.5 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 PM_2.5 concentrations effectively inhibit photolysis reactions and weaken atmospheric oxidation capacity, which makes high PM_2.5 less dependent on the gas-phase oxidations.