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

Abstract

Studies of O₃ at higher elevations, especially in mountain (Mt.) areas, can provide information of regional background pollutants, the photochemistry of biogenic VOCs, and the influence of meteorological factors on O₃ chemistry. In Hong Kong, field measurements were simultaneously conducted at a mountain site (Tai Mao Shan, Mt. TMS) and an urban site (Tsuen Wan, TW) at the foot of the Mt. TMS. An interesting event, with consecutive high-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 (TW). The observed data were detailed analyzed and a comprehensive air quality model (WRF-CMAQ) was used to understand this event. It was found high pressure systems with inversion layers controlled the meteorology of Hong Kong during the whole event. Northerly wind dominated with noticeably high wind speed observed at the height of ~1000m in Hong Kong. Higher temperature and lower RH were found at TW (22.3 ± 0.4°C and 57% ±2.4 %, respectively) compared to those at Mt. TMS (15.9 ± 0.3°C and 71.5% ± 2.0%, respectively).The model performance was evaluated by comparing the observed data with the simulated results. 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. The vertical difference in O₃ values between the higher locations and the ground level indicated that a vertical O₃ gradient existed 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, indicating that O₃ values at lower locations could be affected by O₃ at higher locations via vertical advection and diffusion over Hong Kong. For the O₃ formation mechanism at Mt. TMS and TW, it was concluded that the high O₃ values observed at Mt. TMS were attributed to the higher O₃ mixing ratio in the middle of planetary boundary layer (PBL) than that at ground level, the reduced inversion layer which was unfavorable to vertical dispersion of air pollutants, the positive contribution of vertical transport (advection and diffusion), and less NO titration at this mountainous location. At TW site, both vertical transport and horizontal transport contributed to the O₃ formation while gas-phase chemistry was photochemically consumed.