Arctic spring and summertime aerosol optical depth baseline from long-term observations and model reanalyses – Part 2: Statistics of extreme AOD events, and implications for the impact of regional biomass burning processes

Abstract

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.

Spatiotemporal Variations in Summertime Arctic Aerosol Optical Depth Caused by Synoptic‐Scale Atmospheric Circulation in Three Reanalyses

“…The lower OC contribution in other seasons was compensated for by a higher SS contribution (blue bars in Figure 1), with total AOD thus exhibiting weak seasonal variability. A similar predominance of oceanic sources over the NAtlantic region was reported by Xian, Zhang, O'Neill, Reid, et al (2022). CAMSRA area-averaged AODs in summer are similar to those in spring for all areas except NEurasia (Figure S1 in Supporting Information S1).…”

“…investigated the monthly variability and long-term trends of aerosol optical depth (AOD) over the Arctic, showing that monthly climatological variabilities are associated with negative and positive trends in AOD during spring and summer, respectively, with aerosol reanalyses ensemble and available satellite observations over the Arctic. The peak month of AOD over the Arctic has changed from spring to summer in recent decades, likely due to a shift in extreme smoke events to later in the summer season and to changes in cyclonic activity over the North Atlantic (Xian, Zhang, O'Neill, Reid, et al, 2022). Synoptic-scale disturbances over the Arctic are most frequently observed in summer (Crawford & Serreze, 2016;Serreze & Barrett, 2008;Tilinina et al, 2014;Vessey et al, 2020;Zhang et al, 2004).…”

Arctic spring and summertime aerosol optical depth baseline from long-term observations and model reanalyses – Part 1: Climatology and trend

“…Some of the BB smoke events caused short-term record-high AOD, and some lasted weeks to months, resulting in high monthly mean AOD. The statistics of extreme AOD events and implications for the impact of regional biomass burning processes are provided in Part 2 (Xian et al, 2022).…”

“…The goal of the study is to provide a baseline of AOD distribution, magnitude, speciation, and interannual variability over the Arctic during the sea ice melting season. Statistics of Arctic extreme AOD events are provided in a companion paper (Xian et al, 2022). The baseline can be used for evaluating aerosol models, calculating aerosol radiative forcing, and providing background information for field campaign data analysis and future field campaign planning in a larger climate context.…”

Assessing the Impact of Self‐Lofting on Increasing the Altitude of Black Carbon in a Global Climate Model

“…Absorbing aerosol haze layers are commonly found over the Arctic, especially during the summer months (e.g., Quinn et al., 2007; Shaw, 1995) and originate from a range of sources, including industrialized zones in the northern Hemisphere, biomass burning events in the Boreal forest (e.g., Xian et al., 2022) and to a lesser extent localized sources in and around the Arctic perimeter (Law & Stohl, 2007; Stohl et al., 2013; Zheng et al., 2021). The haze typically arrives in elevated layers but can mix down into the boundary layer and interact with mixed‐phase clouds.…”