Long-term characteristics of criteria air pollutants in megacities of Harbin-Changchun megalopolis, Northeast China: Spatiotemporal variations, source analysis, and meteorological effects.

The hourly concentration of six criteria air pollutants in the Harbin-Changchun region were used to investigate the status and spatiotemporal variation of target air pollutants and their relationships with meteorological factors. The annual concentrations of particulate matters during 2013-2017 were two times higher than the Chinese Ambient Air Quality Standards (CAAQS) Grade Ⅱ. The annual O concentration increased by two times during 2013-2018 in Harbin. The concentration of PM, SO, NO, and CO depicted a similar seasonal trend with an order of winter > autumn > spring > summer. The consistent interannual variation trends of PM/CO, NO and SO indicated that the formation of secondary inorganic aerosols in the annual scale was dominated by the concentrations of NO and SO. The interannual variations of the individual meteorological factors causing on PM and O during 2013-2018 varied significantly in seasonal scale. The interannual variations were stable in annual scale indicating that the continuous decline of PM during 2014-2018 can be attributed to the comprehensive and strict prohibition of small coal-fired boilers and straw burning in the study area. Meanwhile, the increase in O during 2013-2018 in the study area were mainly attributed to the rapid growth of the emission of its precursor (VOCs and NOx). The influence of meteorology on PM and ozone were the most stable and strongest in winter than that in the other three seasons.