The current seasonal summer maximum in surface fine particulate matter (PM_2.5) over the eastern United States has been well established. We find that this seasonality has historically changed substantially, based on long-term quality assured inverse visibility (1/Vis) data over 1946–1998. The median summer/winter 1/Vis ratio increased from about 0.8 over both the southeastern and northeastern United States in the late 1940s to 1.24 over the southeastern United States and to 1.04 over the northeastern United States in the mid-1970s. This ratio exhibits weaker changes in both regions afterward. The observed PM_2.5 seasonality after the year 2000 has similar spatial distribution as that in 1/Vis over the mid-1990s, with systematically higher summer/winter ratios which rapidly weaken after the mid-2000s. From 1956 to 1975, stronger increases in 1/Vis occurred in summer than in winter in both regions, associated with increases in sulfur dioxide emissions and reductions in anthropogenic carbonaceous emissions. Over the southeastern United States, the changes in aerosol seasonality, i.e., both the strengthened summer maxima over 1946–1975 and dampened summer maxima after 2007, suggest historical changes in anthropogenic influence on secondary organic aerosol (SOA) formation and suggest the prospect of reducing summer SOA through controls on anthropogenic emissions.

中文翻译：

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美国东部大气霾季节变化的年代际变化：与人为排放的关系及其对气溶胶成分的影响

美国东部地区目前的夏季夏季表面细颗粒物（PM _2.5）最大值已经确定。我们发现，根据1946年至1998年的长期质量保证的反可见性（1 / Vis）数据，该季节性历史上已经发生了重大变化。美国东南部和东北部的夏/冬1 / Vis中值比率从1940年代末期的约0.8增至美国东南部的1.24，1970年代中期的美国东北部的1.04。此比率此后在两个区域中均显示较弱的变化。观测到的PM _2.52000年代以后的季节性空间分布与1990年代中期的1 / Vis相似，而系统的更高的夏季/冬季比率在2000年代中期之后迅速减弱。从1956年到1975年，两个地区的夏季，1 / Vis的增加都大于冬季，这与二氧化硫排放量的增加和人为碳排放量的减少有关。在美国东南部，气溶胶季节的变化，即1946-1975年夏季最大值的增加和2007年之后夏季最大值的减弱，都表明人为因素对次生有机气溶胶（SOA）形成的历史变化，并提出了减少气溶胶季节的前景。通过控制人为排放来实现夏季SOA。