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

To investigate photochemical ozone (O) 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, O 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 O and its precursors (i.e. NO and VOCs) showed a reasonably good agreement with the observations at both TC and WSI, indicating that the PTM was capable of simulating O 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 decreased in the morning while O 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 at TC consumed more radicals, whereas the concentration of radicals increased from TC to WSI because of NO dilution and destruction. Subsequently, the photochemical cycling of radicals accelerated, leading to high O 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 O formation were evaluated. The results suggested that gasoline vehicles exhaust and solvent usage largely contributed the O 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 O formation was quantified.