Observational Study Advances of Haze and Photochemical Pollution in China

In recent decades, haze pollution and photochemical pollution in China have become severe environment-meteorological disasters, causing broad and deep concern of government and public. Many research projects focusing on China's haze and photochemical pollution have been carried out by Chinese scientists in collaboration with scientists from abroad. A large number of peer-reviewed papers have been published to report results and outcomes from these projects. A review of reported results and outcomes from the study of haze and photochemical pollution in China is given. To limit the length, the paper can only include a concise review of the literature published mainly in the last decade and summarize the outcomes from observations, observation-based analyses, with impact and damage studies. Studies of haze pollution are reviewed, with foci on the haze phenomenon and haze-fog differentiation, the major compositions of haze particles and their sources, meteorological factors influencing haze formation, long-term trends of haze and potential causes. Advances in the study of photochemical pollution are summarized, in terms of characteristics and extent of photochemical pollution, the regional extension of photochemical pollution, the vertical distribution of ozone, factors influencing surface ozone, long-term trends of surface and tropospheric ozone. In addition, it introduces impacts of haze and ozone pollution on human health, impacts of haze on the safety of traffic and electricity transfer, and impacts of ozone pollution on crops, vegetables and natural vegetion. Methods to differentiate haze from fog or mist have been questioned for a long time. The major issue has been the upper limit of relative humidity (RH) for haze, which have influenced the data quality of historical haze and fog records. Recent studies suggest that the RH upper limits for haze (80%) and haze-fog mixture (95%), which are given in the current meteorological standard, should be lowered. Including PM_2.5 data may facilitate the haze-fog differentiation. However, more studies are necessary in different regions. Although impacted by mineral dust, the major compositions of PM_2.5 are from anthropogenic sources, including organic compounds, sulfate, nitrate, ammonium, etc. Secondary aerosols dominate the formation severe haze. There have been long-term increases in haze days over many regions of China. These trends can more or less be attributed to factors related with climate change, such as lower windspeed, lower RH, lower boundary layer height, etc., but should mainly be caused by increased emissions of air pollutants. After the discovery of photochemical smog in Lanzhou, western China, high levels of surface ozone have been often observed in other cities, particularly those in the North China Plain (NCP), Yangtze River Delta (YRD) and Pearl River Delta (PRD) regions. In recent decades, even higher ozone levels have been observed at rural and regional background sites in the above regions, indicating that photochemical pollution in China has become a regional or super-regional phenomenon. Only a few sites in China have been collecting long-term data of surface ozone and ozone profiles are observed only at one site for a longer time. Limited datasets show that levels of ozone in the NCP and PRD regions have been signifcantly increasing, and there has also been an increase trend in surface ozone at Mt Waliguan, a baseline station in western China. Ozone formation at most urban and rural sites in the NCP, YRD and PRD has been found to be more sensitive to VOCs, species that have been increasingly emitted. It implies that photochemical pollution would become even more severe in the above regions and take a long time to control. In future studies, more attention should be paid to photochemical pollution, to interactions between haze and photochemical pollution, and to coupling among pollutants, meteorology, chemistry and ecosystem.