Numerical study on seasonal variations of gaseous pollutants and particulate matters in Hong Kong and Pearl River Delta Region

This study presents Community Multiscale Air Quality (CMAQ) model results for Hong Kong (HK) and the Pearl River Delta region in January, April, July, and October 2004, representing winter, spring, summer, and autumn seasons. Two sets of observations/measurement data are referred to in order to evaluate model performance. The first set consists of hourly observations of criteria pollutants at HK Environmental Protection Department (HKEPD) ambient stations throughout HK, while the second set contains speciated PM measurements, sampled at 6-day intervals, at six of the HKEPD ambient stations. Model-observation agreement is good in PM species sulfate, organic carbon (OC), and elemental carbon (EC). The good performance for OC and EC, when compared to similar evaluation for North America and Europe, demonstrates the importance of correct speciation in an emission sector. Underprediction of NOx and NH3 suggests a desperate need of emission estimation improvement in these species. PM levels are higher in autumn and winter, lower in spring, and lowest in summer, coinciding with northeasterly winds due to continental outflow in autumn and winter, and with southwesterly monsoon in summer. The model is also able to reproduce the seasonal and spatial patterns for PM and O-3. In terms of PM composition, the model agrees with the measured fractions of sulfate, OC, and EC. Additionally, CMAQ was rerun with emission sources partially removed, with results suggesting that pollution sources beyond the Pearl River Delta also contribute to the PM and sulfate levels in HK, particularly during the winter season.