Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195;
Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195.
Proc Natl Acad Sci U S A. 2018 Aug 7;115(32):8110-8115. doi: 10.1073/pnas.1803295115. Epub 2018 Jul 23.
Sulfate ([Formula: see text]) and nitrate ([Formula: see text]) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas-particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter [Formula: see text] and [Formula: see text] GEOS-Chem reproduces observed [Formula: see text]-[Formula: see text]-[Formula: see text] particulate concentrations (2.45 μg [Formula: see text]) and composition ([Formula: see text]: 47%; [Formula: see text]: 32%; [Formula: see text]: 21%) during WINTER. Only 18% of [Formula: see text] emissions were regionally oxidized to [Formula: see text] during WINTER, limited by low [HO] and [OH]. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter [Formula: see text] emissions from 2007 to 2015 and find that the HO limitation on [Formula: see text] oxidation weakened, which increased the fraction of [Formula: see text] emissions oxidizing to [Formula: see text] Simultaneously, NOx emissions decreased by 35%, but the modeled [Formula: see text] particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [[Formula: see text]] and no change in [[Formula: see text]], as observed. Wintertime [[Formula: see text]] and [[Formula: see text]] are expected to change slowly between 2015 and 2023, unless [Formula: see text] and NOx emissions decrease faster in the future than in the recent past.
硫酸盐([Formula: see text])和硝酸盐([Formula: see text])占美国东部细颗粒物质量的一半。尽管前体排放量大幅减少，但在过去十年中，它们的冬季浓度几乎没有变化。其原因仍不清楚，因为缺乏对冬季气粒化学系统进行约束的详细观测。我们利用 2015 年冬季传输、排放和反应性调查（WINTER）期间在美国东部进行的广泛的机载观测；地面观测；以及 GEOS-Chem 化学输送模型，来确定冬季[Formula: see text]和[Formula: see text]的控制因素。GEOS-Chem 模拟出观测到的[Formula: see text]-[Formula: see text]-[Formula: see text]颗粒物浓度（2.45μg[Formula: see text]）和组成（[Formula: see text]：47%；[Formula: see text]：32%；[Formula: see text]：21%）在 WINTER 期间。在 WINTER 期间，只有 18%的[Formula: see text]排放被区域性氧化为[Formula: see text]，这受到低[HO]和[OH]的限制。相对酸性的细颗粒物（pH∼1.3）允许 45%的硝酸盐分配到颗粒物相。我们使用 GEOS-Chem 来研究 2007 年至 2015 年冬季[Formula: see text]排放量减少 58%的影响，发现[Formula: see text]氧化的 HO 限制减弱，这增加了[Formula: see text]排放氧化为[Formula: see text]的比例。同时，NOx 排放量减少了 35%，但由于细颗粒物酸度降低，模型中[Formula: see text]颗粒分数增加。这些反馈导致模型中[Formula: see text]和[Formula: see text]分别减少了 40%，与观测结果一致。除非未来[Formula: see text]和 NOx 排放量的减少速度快于过去，否则预计 2015 年至 2023 年期间冬季[Formula: see text]和[Formula: see text]的变化将缓慢。
