Evolution of Meteorological Conditions during a Heavy Air Pollution Event under the Influence of Shallow Foehn in Urumqi, China

The air pollution in Urumqi which is located on the northern slope of the Tianshan Mountains in northwestern China,is very serious in winter.Of particular importance is the influence of terrain-induced shallow foehn,known locally as elevated southeasterly gale(ESEG).It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution.During 2013-17,Urumqi had an average of 50 d yr-1 of heavy pollution(daily average PM2.5 concentration>150μg m-3),of which 41 days were in winter.The majority(71.4%)of heavy pollution processes were associated with the shallow foehn.Based on microwave radiometer,wind profiler,and surface observations,the surface meteorological fields and boundary layer evolution during the worst pollution episode in Urumqi during 16-23 February 2013 are investigated.The results illustrate the significant role of shallow foehn in the building,strengthening,and collapsing of temperature inversions.There were four wind field patterns corresponding to four different phases during the whole pollution event.The most serious pollution phase featured shallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m.Intense convergence caused by foehn and mountain-valley winds was sustained during most of the phase,resulting in pollutants sinking downward to the lower boundary layer and accumulating around urban area.The key indicators of such events identified in this study are highly correlated to particulate matter concentrations and could be used to predict heavy pollution episodes in the feature.

Exploring the heavy air pollution in Beijing in the fourth quarter of 2015： assessment of environmental benefits for red alerts

In recent years, Beijing has experienced severeair pollution which has caused widespread public concern.Compared to the same period in 2014, the first threequarters of 2015 exhibited significantly improved airquality. However, the air quality sharply declined in thefourth quarter of 2015, especially in November andDecember. During that time, Beijing issued the first redalert for severe air pollution in history. In total, 2 red alerts,3 orange alerts, 3 yellow alerts, and 3 blue alerts wereissued based on the adoption of relatively temporaryemergency control measures to mitigate air pollution. Thisstudy explored the reasons for these variations in airquality and assessed the effectiveness of emergency alertsin addressing severe air pollution. A synthetic analysis ofemission variations and meteorological conditions wasperformed to better understand these extreme air pollutionepisodes in the fourth quarter of 2015. The results showedthat compared to those in the same period in 2014, thedaily average emissions of air pollutants decreased in thefourth quarter of 2015. However, the emission levels ofprimary pollutants were still relatively high, which was themain intrinsic cause of haze episodes, and unfavorablemeteorological conditions represented important externalfactors. Emergency control measures for heavy airpollution were implemented during this red alert period,decreasing the emissions of primary air pollutants byapproximately 36% and the PMa.5 concentration by 11%-21%.

Impacts of meteorology and emission variations on the heavy air pollution episode in North China around the 2020 Spring Festival

Based on the Weather Research and Forecasting model and the Models-3 community multi-scale air quality model(WRF-CMAQ),this study analyzes the impacts of meteorological conditions and changes in air pollutant emissions on the heavy air pollution episode occurred over North China around the 2020 Spring Festival(January to Februray 2020).Regional reductions in air pollutant emissions required to eliminate the PM2.5 heavy pollution episode are also quantified.Our results found that meteorological conditions for the Beijing-Tianjin-Hebei and surrounding"2+26"cities are the worst during the heavy pollution episode around the 2020 Spring Festival as compared with two other typical heavy pollution episodes that occurred after 2015.However,because of the substantial reductions in air pollutant emissions in the"2+26"cities in recent years,and the32%extra reduction in emissions during January to February 2020 compared with the baseline emission levels of the autumn and winter of 2019 to 2020,the maximum PM2.5 level during this heavy pollution episode around the 2020 Spring Festival was much lower than that in the other two typical episodes.Yet,these emission reductions are still not enough to eliminate regional heavy pollution episodes.Compared with the actual emission levels during January to February 2020,a 20%extra reduction in air pollutant emissions in the"2+26"cities(or a 45%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide severe pollution episodes,and avoid heavy pollution episodes that last three or more consecutive days in Beijing;a 40%extra reduction in emissions(or a 60%extra reduction compared with baseline emission levels of the autumn and winter of 2019 to 2020)could help to generally eliminate regionwide and continuous heavy pollution episodes.Our analysis finds that during the clean period after the heavy pollution episode around the 2020 Spring Festival,the regionwide heavy pollution episode would only occur with at least a 10-fold increase in air pollutant emissions.