Refractory black carbon (rBC) concentrations in an ice core from Devon ice cap, Devon Island, Nunavut

Black carbon aerosol (BC) emitted from natural and anthropogenic sources (e.g., wildfires, coal burning) can contribute to magnify climate warming at high latitudes by darkening snow- and ice-covered surfaces, thus lowering their albedo. Modelling the atmospheric transport and deposition of BC to the Arctic is therefore important, and historical archives of BC accumulation in polar ice can help to validate such modelling efforts. Here we present a >250-year ice-core record of refractory BC (rBC) deposition on Devon ice cap, Canada, spanning the years 1735-1992, the first such record ever developed from the Canadian Arctic. Mass concentrations of rBC in the ice core were measured at annual to sub-annual resolution by the single-particle intracavity laser-induced incandescence photometer (sp2) method. The estimated mean deposition flux of rBC on Devon ice cap for 1963-1990 is 0.2 mg m^-2 a^-1, which is at the low end of estimates from Greenland ice cores obtained by the same analytical method (~0.1-4 mg m^-2 a^-1). The Devon ice cap rBC record also differs from Greenland records in that it shows only a modest increase in rBC deposition during the 20th century, unlike in Greenland where a pronounced rise in rBC occurred from the 1880s to the 1910s, largely attributed to mid-latitude coal burning emissions. The deposition of contaminants such as sulfate and lead increased on Devon ice cap in the 20th century but no concomitant rise in rBC is recorded in the ice. Part of the difference with Greenland could be due to local factors such as melt-freeze cycles on Devon ice cap that may limit the detection sensitivity of rBC analyses in melt-impacted core samples, and wind scouring of winter snow at the coring site. Air back-trajectory analyses also suggest that Devon ice cap receives BC from more distant North American and Eurasian sources than Greenland, and aerosol mixing and removal during long-range transport over the Arctic Ocean likely masks some of the specific BC source-receptor relationships. Findings from this study suggest that there could be a large variability in BC aerosol deposition across the Arctic region arising from different transport patterns. This variability needs to be accounted for when estimating the large-scale albedo lowering effect of BC deposition on Arctic snow/ice.

黑碳气溶胶（Black carbon aerosol, BC）由自然源与人为源（如野火、燃煤）排放产生，可通过使积雪及冰面变暗、降低其反照率，加剧高纬度地区的气候变暖。因此，对黑碳气溶胶向北极的大气传输与沉降进行模拟研究具有重要意义；而极地冰芯中黑碳累积的历史记录，可用于验证这类模拟工作。本研究报道了加拿大德文冰帽（Devon ice cap）上长达250余年的难熔黑碳（refractory BC, rBC）沉降冰芯记录，时间跨度为1735年至1992年，是加拿大北极地区首份此类冰芯记录。本研究采用单颗粒腔内激光诱导白炽光光度计（single-particle intracavity laser-induced incandescence photometer, sp2）方法，以年际至亚年际分辨率测定了该冰芯内难熔黑碳的质量浓度。1963年至1990年期间，德文冰帽难熔黑碳的平均沉降通量估算值为0.2 mg·m⁻²·a⁻¹，处于采用相同分析方法得到的格陵兰冰芯估算值区间（~0.1~4 mg·m⁻²·a⁻¹）的下限。德文冰帽的难熔黑碳记录与格陵兰的记录存在显著差异：20世纪期间，前者的难熔黑碳沉降仅出现小幅增长，而后者在1880年至1910年期间呈现显著上升趋势，这一现象主要归因于中纬度地区的燃煤排放。20世纪期间，德文冰帽上硫酸盐、铅等污染物的沉降量有所上升，但冰芯中并未记录到难熔黑碳的同步增长。与格陵兰记录的部分差异，可能源于德文冰帽的局地因素：例如冰帽的冻融循环可能会降低受融蚀影响的冰芯样品中难熔黑碳分析的检测灵敏度，以及钻探点冬季积雪的风蚀作用。气团后向轨迹分析同样表明，与格陵兰相比，德文冰帽接收的黑碳气溶胶来自更远的北美与欧亚大陆源区；而在北冰洋上空进行长距离传输的过程中，气溶胶的混合与清除作用，可能掩盖了部分黑碳的源-受体特异性关联。本研究结果表明，由于传输路径的差异，北极地区黑碳气溶胶的沉降可能存在显著空间异质性。在评估黑碳沉降对北极积雪/冰面的大范围反照率降低效应时，需要考虑这种空间变异性。