Abstract
Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate. These extremes are characterized by exceedingly high concentrations of fine particulate matter (smaller than 2.5 µm, or PM2.5) and occur with extensive temporal (on a daily, weekly, to monthly timescale) and spatial (over a million square kilometers) coverage. Although significant advances have been made in field measurements, model simulations, and laboratory experiments for fine PM over recent years, the causes for severe haze formation have not yet to be systematically/comprehensively evaluated. This review provides a synthetic synopsis of recent advances in understanding the fundamental mechanisms of severe haze formation in northern China, focusing on emission sources, chemical formation and transformation, and meteorological and climatic conditions. In particular, we highlight the synergetic effects from the interactions between anthropogenic emissions and atmospheric processes. Current challenges and future research directions to improve the understanding of severe haze pollution as well as plausible regulatory implications on a scientific basis are also discussed.
Highlights
Regional severe haze represents an enormous environmental problem in China, influencing air quality, human health, ecosystem, weather, and climate
High levels of PM gas precursors lead to significant production of secondary PM, as documented by large fractions and high abundances of secondary organic aerosol (SOA) and secondary inorganic aerosol (SIA) from field measurements [8, 9, 12, 16]
Wang et al [71] showed distinct size growth and sulfate formation between ammonium sulfate and oxalic acid seed particles exposed to vapors of SO2, NO2, and NH3 at high relative humidity (RH), indicating that a particle mixture of inorganic salts adopted by the previous studies does not represent a suitable model system and that the acidity and sulfate formation cannot be reliably inferred without accounting for the effects of multiaerosol compositions during severe haze events in China
Summary
Severe haze in northern China: A synergy of anthropogenic emissions and atmospheric processes. The haze episodes in northern China have become more frequent and severe in recent decades [1, 18, 19], and a variety of regulatory measures have been implemented to improve air quality by Chinese central and local governments These regulatory measures have resulted in significant reduction in primary PM emissions from industry and other sources [54, 55], but emissions of gas precursors for secondary aerosols, including SO2, NOx, NH3, and volatile organic compounds (VOCs), remain at high levels and contribute to high abundances of secondary PM, most noticeably SOA and SIA [8, 9, 17].
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