Abstract. 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.