Photochemical evolution of continental air masses and their influence on ozone formation over the South China Sea

Yu Wang; Hai Guo; Xiaopu Lyu; Luyao Zhang; Yangzong Zeren; Shichun Zou; Zhenhao Ling

doi:10.1016/j.scitotenv.2019.04.075

Photochemical evolution of continental air masses and their influence on ozone formation over the South China Sea

Sci Total Environ. 2019 Jul 10:673:424-434. doi: 10.1016/j.scitotenv.2019.04.075. Epub 2019 Apr 8.

Authors

Yu Wang¹, Hai Guo², Xiaopu Lyu¹, Luyao Zhang¹, Yangzong Zeren¹, Shichun Zou³, Zhenhao Ling⁴

Affiliations

¹ Air Quality Studies, Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China.
² Air Quality Studies, Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China. Electronic address: ceguohai@polyu.edu.hk.
³ School of Marine Sciences, Sun Yat-sen University, China. Electronic address: ceszsc@mail.sysu.edu.cn.
⁴ School of Atmospheric Sciences, Sun Yat-sen University, China.

PMID: 30991332
DOI: 10.1016/j.scitotenv.2019.04.075

Abstract

To investigate photochemical ozone (O₃) pollution over the South China Sea (SCS), an intensive sampling campaign was conducted from August to November simultaneously at a continental site (Tung Chung, TC) and a marine site (Wan Shan Island, WSI). It was found that when continental air masses intruded the SCS, O₃ episodes often occurred subsequently. To discover the causes, a photochemical trajectory model (PTM) coupled with the near-explicit Master Chemical Mechanism (MCM) was adopted, and the photochemical processes of air masses during the transport from TC to WSI were investigated. The simulated O₃ and its precursors (i.e. NO_x and VOCs) showed a reasonably good agreement with the observations at both TC and WSI, indicating that the PTM was capable of simulating O₃ formation for air masses traveling from TC to WSI. The modeling results revealed that during the transport of air masses from TC to WSI, both VOC and NO_x decreased in the morning while O₃ increased significantly, mainly due to rapid chemical reactions with elevated radicals over the SCS. The elevated radicals over the SCS were attributable to the fact that higher NO_x at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NO_x dilution and destruction. Subsequently, the photochemical cycling of radicals accelerated, leading to high O₃ mixing ratios over the SCS. Furthermore, based on the source profiles of the emission inventory used, the contributions of six sources, i.e. gasoline vehicle exhaust, diesel vehicle exhaust, gasoline evaporation and LPG usage, solvent usage, biomass and coal burning, and biogenic emissions, to maritime O₃ formation were evaluated. The results suggested that gasoline vehicles exhaust and solvent usage largely contributed the O₃ formation over the SCS (about 5.2 and 3.8 ppbv, respectively). This is the first time that the contribution of continental VOC sources to the maritime O₃ formation was quantified.

Keywords: Continental air masses; Ozone; Photochemical trajectory model; South China Sea; VOC sources.