Impacts of decadal variations in natural emissions due to land-cover changes on ozone production in southern China

The decadal variations in emissions of high-reactivity biogenic volatile organics (BVOCs), as a result of land-cover changes, could significantly impact ozone (O-3) production. In this study, the Weather Research and Forecasting/Chemistry (WRF/Chem) modelling system, coupled with dynamic vegetation data sets derived from Moderate Resolution Imaging Spectroradiometer (MODIS, 2001-2012) and Advanced Very High Resolution Radiometer (AVHRR, early 1990s) measurements, were used to investigate the impacts of land-cover changes on natural emissions, and consequently O-3 production, in the Pearl River Delta (PRD) region of southern China over the past two decades. Model results indicate that BVOC emissions were highly dependent on forest area. The total BVOC emissions in the modelling domain increased by a factor of two due to afforestation since the early 1990s, declined slowly (-5.8% yr(-1)) until 2006 and then increased continuously (+ 9.1% yr(-1)) to 2012. The decadal variations in BVOC emissions have complex implications for summer O-3 production in PRD, depending on the chemical regimes and prevailing winds. The impacts on O-3 production were most sensitive in downwind areas, and it was found that the large increase in BVOC emissions during 2006-2012 tended to reduce surface O-3 concentrations by 1.6-2.5 ppb in rural regions, but caused an increment of O-3 peaks by up to 2.0-6.0 ppb in VOC-limited urban areas (e.g., Guangzhou, Foshan and Zhongshan). The opposite was true in the period 2001-2006, when the reduced BVOC emissions resulted in 1.3-4.0 ppb increases in daytime O-3 concentrations over northern rural regions. Impact of the two-fold increase in BVOC emissions since the early 1990s to 2006 was a 0.9-4.6 ppb increment in surface O-3 concentrations over the downwind areas. This study suggests that the potential impacts on ozone chemistry should be considered in long-term land-use planning and air-quality management.