为什么东部美国冬季空气污染水平更为严重?

在2015年的观测结果表明，尽管减少了温室气体排放，但冬季雾霾仍然很高

In the summer, some of the emitted NOx and sulfur dioxide remains in the gas phase and gets zapped by sunlight or deposited on land, and the rest forms particulates in the form of nitrates and sulfates. As the primary ingredients drop, so do the levels of particulates.

But the new analysis shows that the chemistry of wintertime air follows a more complex path. With less sunlight and colder temperatures, more of the chemistry happens in the liquid phase, on the surfaces of existing particulates or liquid and ice clouds. In that phase, as the primary ingredients drop, the efficiency of converting sulfur dioxide to sulfate rises, because more oxidants are available. And as sulfate goes down, the particulates become less acidic, making NOx convert more easily to nitrates.

So, even though air quality regulations have reduced both types of primary emissions, the total amount of particulates that harm human health has dropped more slowly.

Why eastern U.S. air pollution levels are more stagnant in winter

https://www.sciencedaily.com/releases/2018/07/180723152550.htm

1. Viral Shah, Lyatt Jaeglé, Joel A. Thornton, Felipe D. Lopez-Hilfiker, Ben H. Lee, Jason C. Schroder, Pedro Campuzano-Jost, Jose L. Jimenez, Hongyu Guo, Amy P. Sullivan, Rodney J. Weber, Jaime R. Green, Marc N. Fiddler, Solomon Bililign, Teresa L. Campos, Meghan Stell, Andrew J. Weinheimer, Denise D. Montzka, Steven S. Brown. Chemical feedbacks weaken the wintertime response of particulate sulfate and nitrate to emissions reductions over the eastern United States. Proceedings of the National Academy of Sciences, 2018; 201803295 DOI: 10.1073/pnas.1803295115