Abstract
Abstract. The MIRAGE-Shanghai experiment was designed to characterize the factors controlling regional air pollution near a Chinese megacity (Shanghai) and was conducted during September 2009. This paper provides information on the measurements conducted for this study. In order to have some deep analysis of the measurements, a regional chemical/dynamical model (version 3 of Weather Research and Forecasting Chemical model – WRF-Chemv3) is applied for this study. The model results are intensively compared with the measurements to evaluate the model capability for calculating air pollutants in the Shanghai region, especially the chemical species related to ozone formation. The results show that the model is able to calculate the general distributions (the level and the variability) of air pollutants in the Shanghai region, and the differences between the model calculation and the measurement are mostly smaller than 30%, except the calculations of HONO (nitrous acid) at PD (Pudong) and CO (carbon monoxide) at DT (Dongtan). The main scientific focus is the study of ozone chemical formation not only in the urban area, but also on a regional scale of the surrounding area of Shanghai. The results show that during the experiment period, the ozone photochemical formation was strongly under the VOC (volatile organic compound)-limited condition in the urban area of Shanghai. Moreover, the VOC-limited condition occurred not only in the city, but also in the larger regional area. There was a continuous enhancement of ozone concentrations in the downwind of the megacity of Shanghai, resulting in a significant enhancement of ozone concentrations in a very large regional area in the surrounding region of Shanghai. The sensitivity study of the model suggests that there is a threshold value for switching from VOC-limited condition to NOx (nitric oxide and nitrogen dioxide)-limited condition. The threshold value is strongly dependent on the emission ratio of NOx / VOCs. When the ratio is about 0.4, the Shanghai region is under a strong VOC-limited condition over the regional scale. In contrast, when the ratio is reduced to about 0.1, the Shanghai region is under a strong NOx-limited condition. The estimated threshold value (on the regional scale) for switching from VOC-limited to NOx-limited condition ranges from 0.1 to 0.2. This result has important implications for ozone production in this region and will facilitate the development of effective O3 control strategies in the Shanghai region.
Highlights
Ocean Science that during the experiment period, the ozone photochemical formation was strongly under the volatile organic compounds (VOCs)-limited condition in the urban area of Shanghai
The results suggest that when the ratio of CH2O/NOy is smaller than 0.28, the ozone formation is under the VOC-limited condition, while a ratio of CH2O/NOy larger than 0.28 indicates ozone formation that is under the NOx-limited condition
In the 150 km downwind, the ozone concentrations were 100, 40, and 140 ppbv, for Run 0, Run 1, and Run 2, respectively. This result shows that ozone concentrations were increased by the emissions of VOCs in the city plume, which was under the VOC-limited condition
Summary
Ocean Science that during the experiment period, the ozone photochemical formation was strongly under the VOC (volatile organic compound)-limited condition in the urban area of Shanghai. Geng et al (2007) suggested that O3 chemical production is limited by the concentrations of volatile organic compounds (VOCs) in the city of Shanghai. Cai et al (2010) used a receptor model (PCA/APCS; Principal Component Analysis/Absolute Principal Component Scores) to identify the individual contributions of different VOC sources to VOC concentrations Their result suggested that liquefied petroleum, gasoline, and solvent usages had important contributions to the VOC concentrations in the Shanghai region. The campaign integrated observations from 7 ground stations, with 3 enhanced ground stations in different parts of the center of city as well as the rural area of the Shanghai region Another important goal of the experiment is to compare the observational results with model calculations.
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