Abstract
Abstract. This study investigated the air pollution characteristics of synoptic-scale circulation in the Beijing megacity, and provided quantitative evaluation of the impacts of circulation patterns on air quality during the 2008 Beijing Summer Olympics. Nine weather circulation types (CTs) were objectively identified over the North China region during 2000–2009, using obliquely rotated T-mode principal component analysis (PCA). The resulting CTs were examined in relation to the local meteorology, regional transport pathways, and air quality parameters, respectively. The FLEXPART-WRF model was used to calculate 48-h backward plume trajectories for each CT. Each CT was characterized with distinct local meteorology and air mass origin. CT 1 (high pressure to the west with a strong pressure gradient) was characterized by a northwestern air mass origin, with the smallest local and southeasterly air mass sources, and CT 6 (high pressure to the northwest) had air mass sources mostly from the north and east. On the contrary, CTs 5, 8, and 9 (weak pressure field, high pressure to the east, and low pressure to the northwest, respectively) were characterized by southern and southeastern trajectories, which indicated a greater influence of high pollutant emission sources. In turn, poor air quality in Beijing (high loadings of PM10, BC, SO2, NO2, NOx, O3, AOD, and low visibility) was associated with these CTs. Good air quality in Beijing was associated with CTs 1 and 6. The average visibilities (with ±1σ) in Beijing for CTs 1 and 6 during 2000–2009 were 18.5 ± 8.3 km and 14.3 ± 8.5 km, respectively. In contrast, low visibility values of 6.0 ± 3.5 km, 6.6 ± 3.7 km, and 6.7 ± 3.6 km were found in CTs 5, 8, and 9, respectively. The mean concentrations of PM10 for CTs 1, 6, 5, 8, and 9 during 2005–2009 were 90.3 ± 76.3 μg m−3, 111.7 ± 89.6 μg m−3, 173.4 ± 105.8 μg m−3, 158.4 ± 90.0 μg m−3, and 151.2 ± 93.1 μg m−3, respectively. Analysis of the relationship between circulation pattern and air quality during the emission control period suggests that CTs are the primary drivers of day-to-day variations in pollutant concentrations over Beijing and its vicinity. During the Olympics period, the frequency of CT 6 was twice that of the mean in August from 2000 to 2009. This CT had northerly transport pathways and favorable meteorological conditions (e.g. frequent precipitation) for clean air during the Olympics. Assuming that relationships between CTs and air quality parameters in the same season are fixed in different years, the relative contributions of synoptic circulation to decreases in PM10, BC, SO2, NO2, NOx, CO, and horizontal light extinction during the Olympics were estimated as 19 ± 14%, 18 ± 13%, 41 ± 36%, 12 ± 7%, 10 ± 5%, 19 ± 11%, and 54 ± 25%, respectively.
Highlights
Air pollution has become a major environmental problem in China as a consequence of industrialization and urbanization during years of rapid economic growth (Chan and Yao, 2008; Fang et al, 2009)
Because of the significant differences in meteorological conditions and footprints of 48-h backward trajectories among the circulation types described in Sect. 3, we evaluated the relationship between circulation type and air quality in Beijing
Episode 1 (23–28 July) was persisence was that circulation types (CTs) 6 occurrence was doubled and CT 5 disappeared during the Olympic Games. with southwestern origins and shorter transport distances; This situation was indicated by the average location of episode 2 (4–8 August) had more air masses with southeastthe subtropical high during the Olympics in 2008 and in Au- ern origins and faster transport speeds
Summary
Air pollution has become a major environmental problem in China as a consequence of industrialization and urbanization during years of rapid economic growth (Chan and Yao, 2008; Fang et al, 2009). The dependence of the control effect on synoptic weather patterns has not yet been studied using a circulation-to-environment approach (Yarnal, 1993). The effects of emission control measures must be evaluated under different meteorological conditions so that the relative contribution of reduced anthropogenic emissions to improved air quality can be calculated more accurately. More recent studies (Chen et al, 2008; Wei et al, 2011) showed that the evolution and different sections of an anticyclonic system play an important role in controlling air quality These studies of the dependence of air quality on the synoptic weather situation have been performed on a caseby-case basis over short periods, using subjective methods.
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