In a companion paper (Xian et al., 2022, part 1 of the study), we present an Arctic aerosol optical depth (AOD) climatology and trend analysis for 2003–2019 spring and summertime periods derived from a combination of aerosol reanalyses, remote-sensing retrievals, and ground observations. Continued from the previous discussion and as the second part of the study, we report the statistics and trends of Arctic AOD extreme events using the U.S. Navy Aerosol Analysis and Prediction System ReAnalysis version 1 (NAAPS-RA v1), the sun photometer data from the AErosol RObotic NETwork (AERONET) sites, and the oceanic Maritime Aerosol Network (MAN) measurements. Here, extreme AOD events are defined as events with AOD exceeding the 95th percentile (denoted “AOD₉₅”) of AOD distributions for given locations using 6-hourly or daily AOD data. While AERONET and MAN data estimate the Arctic median 550 nm AOD value to be 0.07, the 95th percentile value is 0.24. Such extreme events are dominated by fine-mode aerosol particles, largely attributable to biomass burning (BB) smoke events for the North American Arctic, the Asian Arctic, and most areas of the Arctic Ocean. However, extreme AOD events for the lower European Arctic are more attributable to anthropogenic and biogenic fine particles. The extreme-event occurrence dominance of sea salt is largely limited to the North Atlantic and Norwegian Sea. The extreme AOD amplitudes of anthropogenic and biogenic fine-mode and sea salt AOD are, however, significantly lower than those regions where extreme smoke AOD is dominant. Even for sites distant from BB source regions, BB smoke is the principal driver of AOD variation above the AOD₉₅ threshold.Maximum AOD values in the high Arctic in 2010–2019 have increased compared to 2003–2009, indicating stronger extreme BB smoke influence in more recent years. The occurrence of extreme smoke events tended to be more equally distributed over all months (April–August) during the 2003–2009 period while being more concentrated in the late season (July–August) during the 2010–2019 period. The temporal shift of the occurrence of AOD extreme events is likely due to improved control of early-season agriculture burning, climate-change-related increases in summertime lightning frequencies, and a reduction in anthropogenic pollution over the 2010–2019 period.

中文翻译：

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来自长期观测和模型再分析的北极春季和夏季气溶胶光学深度基线 - 第 2 部分：极端 AOD 事件的统计，以及对区域生物质燃烧过程影响的影响

在一篇配套论文（Xian 等人，2022 年，研究的第 1 部分）中，我们提出了 2003-2019 年春季和夏季期间的北极气溶胶光学深度 (AOD) 气候学和趋势分析，该分析来自气溶胶再分析、远程- 传感检索和地面观测。继续前面的讨论，作为研究的第二部分，我们使用美国海军气溶胶分析和预测系统再分析版本 1 (NAAPS-RA v1) 报告北极 AOD 极端事件的统计数据和趋势，太阳光度计数据来自气溶胶机器人网络 (AERONET) 站点和海洋海洋气溶胶网络 (MAN) 测量。此处，极端 AOD 事件定义为 AOD 超过 95% 的事件（表示为“AOD ₉₅”) 使用 6 小时或每日 AOD 数据得出给定位置的 AOD 分布。虽然 AERONET 和 MAN 数据估计北极中值 550 海里 AOD 值为 0.07，但第 95 个百分位值为 0.24。此类极端事件以精细模式气溶胶颗粒为主，主要归因于北美北极、亚洲北极和北冰洋大部分地区的生物质燃烧 (BB) 烟雾事件。然而，欧洲北极下游的极端 AOD 事件更多地归因于人为和生物产生的细颗粒。海盐的极端事件发生优势主要局限于北大西洋和挪威海。然而，人为和生物精细模式和海盐 AOD 的极端 AOD 幅度明显低于极端烟雾 AOD 占主导地位的区域。₉₅阈值。2010-2019 年北极高地的最大 AOD 值与 2003-2009 年相比有所增加，表明近年来极端 BB 烟雾影响更强。极端烟雾事件的发生在 2003-2009 年期间倾向于在所有月份（4-8 月）更均匀地分布，而在 2010-2019 年期间更集中在季末（7-8 月）。AOD 极端事件发生的时间变化可能是由于对早季农业燃烧的控制得到改善、与气候变化相关的夏季闪电频率增加以及 2010-2019 年期间人为污染的减少。