WRF/CHEM modeling of impacts of weather conditions modified by urban expansion on secondary organic aerosol formation over Pearl River Delta

In this paper, 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) 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: (1) urbanization can increase monthly averaged temperatures by about 0.63 °C, 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 2–4 ppbv and 4–12 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, 55–65% 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.