Impacts Of Weather Conditions Modified By Urban Expansion On Secondary Organic Aerosol And Ozone Formation with WRF/CHEM

The online Weather Research and Forecasting and Chemistry (WRF/CHEM) model, coupled with urban canopy (UCM) and biogenic-emission models, is used to explore impacts of urban expansion on secondary organic aerosols (SOA) and ozone formation. Two scenarios of urban maps are used in WRF/CHEM to represent early 1990s (pre-urbanization) and current urban distribution in the Pearl River Delta (PRD). Month-long simulation results using the above land-use scenarios for March 2001 show prominent differences in meteorological conditions and surface ozone concentrations. Urbanization increases both the day- and night-time 2-m temperatures by about 0.6°C and 1.4°C, respectively, while it decreases both the day- and night-time 10-m wind speed. Daytime reduction in the wind speed by about 3.0 m s-1 is larger than that for the nighttime (0.5 to 2 m s-1). Urbanization also increases both the day- and night-time boundary layer depths. The daytime increase in the PBL height (> 200 m) is also larger than the nighttime (50-100m). The meteorological conditions modified by urbanization lead to detectable ozone and SOA concentration changes in the PRD: (1) Urbanization can increase monthly averaged temperatures by about 0.63°æ, decrease monthly averaged 10-m wind speeds by 38%, increase monthly averaged boundary-layer depths by 80 m, and decrease monthly averaged water mixing ratio by 0.2 g/kg. (2) Changes in meteorological conditions can result in detectable concentration changes of NOx, VOC, O3 and NO3 radicals. Urbanization decreases surface NOx and VOC concentrations by a maximum of 4 ppbv and 1.5 ppbv, respectively. Surface O3 and NO3 radical concentrations over major cities increase by about 2¨C4 ppbv and 4¨C12 pptv, respectively; areas with increasing O3 and NO3 radical concentrations generally coincide with the areas of temperature increase and wind speed reduction where NOx and VOC decrease. (3) Urbanization can induce 9% increase of SOA in Foshan, Zhongshan and west Guangzhou and 3% decrease in Shenzhen and Dongguan. Over PRD major cities, SOA from Aitken mode reduces by 30% but with more than 70% SOA from accumulate mode. Urbanization has stronger influence on SOA formation from Aitken mode. (4) Over the PRD, 55¨C65% SOA comes from aromatics precursors. Urbanization has strongest influence on aromatics precursors to produce SOA (14% increase), while there is less influence on alkane precursors. Alkene precursors have negative contribution to SOA formation under urbanization situation.