DUSTFORCE DX EPISODE 1 FREE
The oxidation of SO 2 by a stabilized Criegee intermediate (a carbonyl oxide with two free radical sites) or its derivative was recently reported 13, 16. Sulfur dioxide can be oxidized in the troposphere to form sulfate, which may occur in the gas-phase (principally via reaction with OH to yield H 2SO 4), in cloud or fog droplets, or on aerosol particles 12, 13, 14, 15. It has been found that atmospheric SO 2 concentrations are typically overestimated while sulfate tends to be underestimated 9, 10, 11. A number of models have been applied to predict the formation of sulfate aerosols on a global scale. Atmospheric sulfate originates from a variety of sources, including volcanic ash, sea spray and the oxidation of sulfur dioxide (SO 2) and other sulfur-containing species 5, 6, 7, 8. Here, we report on a new mechanism of haze formation, in which the coexistence of NOx can reduce the environmental capacity for SO 2, leading to a gas-particle conversion process and fine particle growth.Īs one of the most important components of fine particles in the atmosphere, sulfate has been the focus of research studying its formation mechanism. Field observations of heavy haze in January 2013 also found the occurrence of a dust storm from January 10 to 15 in the Beijing-Tianjin-Hebei regions. Once calm weather persists, pollutants do not disperse easily and secondary particles produced by homogeneous and heterogeneous atmospheric chemical processes of pollutants readily accumulate, resulting in the formation of haze. The external cause for the large scope and long duration of haze which frequently appeared in China in January 2013 was the stable synoptic conditions however, the internal cause was related to the PM 2.5 or PM 1 and their precursor pollutants, such as SO 2, NO 2, NH 3 and VOCs etc., which heavily exceeded the environmental capacity determined by conditions such as local weather and terrain. The haze-affected area reached 1,300,000 km 2 in January 2013 4. According to the safety standard of the World Health Organization (WHO) (10 μg/m 3), downtown Beijing air conditions exceeded the standard for nearly the whole month. Therein, downtown Beijing's PM 2.5 level exceeded the Second Grade National Standard of China (75 μg/m 3, the Ambient Air Quality Standard planned to be implemented in 2016) for 22 days and exceeded the First Grade National Standard (35 μg/m 3) for 27 days. According to the statistical data from monitoring by the Chinese Academy of Sciences (CAS), the Beijing-Tianjin-Hebei regions experienced five severe haze episodes (peak concentrations of PM 2.5 over 300 μg/m 3) in January 2013 alone. In recent years, haze in China's eastern region has increased in frequency of occurrence as well as the area of the affected region. Haze is a phenomenon caused by fine particles (PM 1 or PM 2.5), resulting in the decline of atmospheric visibility. This has led to complex air pollution and is a new situation that developed countries have not experienced ever before. Coal burning, motor vehicles and industrial emission pollution, which have been seen in developed countries at different times, all contribute to a high burden and heavy composite atmospheric pollution in China nowadays. The Chinese economy is undergoing high-speed development that is heavily dependent on coal consumption as its main energy source. However, the intrinsic cause for haze formation is still not clear. The key factors that affect the formation and evolution of haze include emission of primary pollutants, secondary aerosol formation from homogeneous and heterogeneous reactions and hygroscopic growth of aerosols 1, 2, 3. In contrast, up to now, the main cause of heavy haze in China has been widely studied but not yet fully elucidated. The primary pollutants in these events were clearly revealed in these incidents. The “Great Smog of 1952 in London” was mainly caused by SO 2 emitted from coal burning.
Reviewing the history, the “Photochemical Smog Event in Los Angeles”, which took place in the 1940s–1950s, was mainly caused by photochemical reaction of NOx ( = NO + NO 2) and volatile organic compounds (VOCs) from motor vehicle exhaust. China's air pollution has been a global concern for decades.
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