Impact of afforestation on surface ozone in the North China Plain during the three-decade period

Abstract

Surface ozone (O3) in the North China Plain (NCP) was simulated using a coupled meteorology and atmospheric chemistry model WRF-Chem for the warm seasons (May to September) in four selected years at 10-year intervals (1980, 1990, 2000, and 2010) to examine the effects of increasing forest cover on the ozone budget. Model simulations were performed under two biogenic emission scenarios of precursor gases, with the first one using year-specific annual emissions, which increased from 1980 to 2010 in association with the growing forest cover, and the second one using the fixed annual emissions of 1980, which assumed no change in forest cover across the NCP for a sensitivity test purpose. In both scenarios, year-specific annual anthropogenic emissions were used, which had increased during the three decades due to economic development. Increasing anthropogenic and biogenic emissions together caused markedly increasing surface O3 concentrations in most areas of the NCP. Anthropogenic emissions of O3 precursors generally dominated biogenic emissions on ozone formation. The contributions of biogenic emissions were significant in certain areas of NCP with extensive afforestation activities from 1980 to 2010. Afforestation alone contributed 1%–2% to ozone formation in the Beijing metropolitan area and 4%–5% to low-populated cities with high forest cover. The average ozone removal attributed to dry deposition was about 5% in the NCP with the highest removal occurring in the forest-covered and mountainous regions. The biogenic VOC emission had a greater contribution to the O3 budget as compared to the atmospheric removal by dry deposition, resulting in a net increase in O3 level over the NCP.