An ozone episode in the Pearl River Delta: Field observation and model simulation

In the fall of 2007 concurrent air sampling field measurements were conducted for the first time in Guangzhou (at Wan Qing Sha (WQS)) and Hong Kong (at Tung Chung (TC)), two cities in the rapidly developing Pearl River Delta region of China that are only 62 km apart. This region is known to suffer from poor air quality, especially during the autumn and winter months, when the prevailing meteorological conditions bring an outflow of continental air to the region. An interesting multiday O-3 pollution event (daily maximum O-3 > 122 ppbv) was captured during 9-17 November at WQS, while only one O-3 episode day (10 November) was observed at TC during this time. The mean O-3 mixing ratios at TC and WQS during the episode were 38 +/- 3 (mean +/- 95% confidence interval) and 51 +/- 7 ppbv, respectively, with a mean difference of 13 ppbv and a maximum hourly difference of 150 ppbv. We further divided this event into two periods: 9-11 November as Period 1 and 12-17 November as Period 2. The mixing ratios of O-3 and its precursors (NOx and CO) showed significant differences between the two periods at TC. By contrast, no obvious difference was found at WQS, indicating that different air masses arrived at TC for the two periods, as opposed to similar air masses at WQS for both periods. The analysis of VOC ratios and their relationship with O-3 revealed strong O-3 production at WQS during Period 2, in contrast to relatively weak photochemical O-3 formation at TC. The weather conditions implied regional transport of O-3 pollution during Period 1 at both sites. Furthermore, a comprehensive air quality model system (Weather Research and Forecasting-Community Multiscale Air Quality model (WRF-CMAQ)) was used to simulate this O-3 pollution event. The model system generally reproduced the variations of weather conditions, simulated well the continuous high O-3 episode event at WQS, and captured fairly well the elevated O-3 mixing ratios in Period 1 and low O-3 levels in Period 2 at TC. The modeled surface O-3 distributions and flow structures clearly illustrated the occurrence of O-3 formation and the impact of regional transport on O-3 levels in Period 1 in the Pearl River Delta. Further analysis of O-3 formation indicated that horizontal transport was the main contributor to the O-3 increase at TC during Period 1, while at WQS O-3 levels were dominated by photochemical production during both periods. The low O-3 levels at TC during Period 2 were attributable to lower temperatures and the arrival of fresh maritime air masses brought in by strong easterly winds. This study highlights how contrasting precursor concentrations and photochemical conditions can occur over a very small distance, and it provides a rare opportunity to better understand ozone production and precursor source origins on a finer scale in this region.