Simulation of ozone formation at different elevations in mountainous area of Hong Kong using WRF-CMAQ model

Field measurements were simultaneously conducted at a mountain (Mt.) site (Tai Mao Shan, TMS) and an urban site (Tsuen Wan, TW) at the foot of the Mt. TMS in Hong Kong. An interesting event with consecutive high-ozone (O3) days from 08:00 on 28 Oct. to 23:00 on 03 Nov., 2010 was observed at Mt. TMS, while no such polluted event was found at the foot of the mountain. The Weather Research and Forecasting (WRF)-Community Multiscale Air Quality (CMAQ) models were used to understand this event. Model performance evaluation showed that the simulated meteorological parameters and air pollutants were well in agreement with the observations. The index of agreement (IOA) of temperature, relative humidity, wind direction and wind speed were 0.93, 0.83, 0.46 and 0.60, respectively. The multi-day high O3 episode at Mt. TMS was also reasonably reproduced (IOA = 0.68). Horizontally, the photochemical processes determined the O3 levels in southwestern Pearl River Delta (PRD) and the Pearl River Estuary (PRE), while in eastern and northern PRD, the O3 destruction was over the production during the event. Vertically, higher O3 values at higher levels were found at both Mt. TMS and TW, indicating a vertical O3 gradient over Hong Kong. With the aid of the process analysis module, we found positive contribution of vertical transport including advection and diffusion to O3 mixing ratios at the two sites, suggesting that O3 values at lower locations could be affected by O3 at higher locations via vertical advection and diffusion over Hong Kong.