Abstract
Abstract. We present an overview of the background, scientific goals, and execution of the Aerosol, Radiation, and Cloud Processes affecting Arctic Climate (ARCPAC) project of April 2008. We then summarize airborne measurements, made in the troposphere of the Alaskan Arctic, of aerosol particle size distributions, composition, and optical properties and discuss the sources and transport of the aerosols. The aerosol data were grouped into four categories based on gas-phase composition. First, the background troposphere contained a relatively diffuse, sulfate-rich aerosol extending from the top of the sea-ice inversion layer to 7.4 km altitude. Second, a region of depleted (relative to the background) aerosol was present within the surface inversion layer over sea-ice. Third, layers of dense, organic-rich smoke from open biomass fires in southern Russia and southeastern Siberia were frequently encountered at all altitudes from the top of the inversion layer to 7.1 km. Finally, some aerosol layers were dominated by components originating from fossil fuel combustion. Of these four categories measured during ARCPAC, the diffuse background aerosol was most similar to the average springtime aerosol properties observed at a long-term monitoring site at Barrow, Alaska. The biomass burning (BB) and fossil fuel layers were present above the sea-ice inversion layer and did not reach the sea-ice surface during the course of the ARCPAC measurements. The BB aerosol layers were highly scattering and were moderately hygroscopic. On average, the layers produced a noontime net heating of ~0.1 K day−1 between 3 and 7 km and a slight cooling at the surface. The ratios of particle mass to carbon monoxide (CO) in the BB plumes, which had been transported over distances >5000 km, were comparable to the high end of literature values derived from previous measurements in wildfire smoke. These ratios suggest minimal precipitation scavenging and removal of the BB particles between the time they were emitted and the time they were observed in dense layers above the sea-ice inversion layer.
Highlights
Global temperature records show a statistically significant warming in the last century, with most of the change attributed to anthropogenically emitted greenhouse gases and associated feedbacks (IPCC, 2007)
Movies showing RAQMS-simulated sulfate, OC + BC, and dust transport are provided in the Supplement. The remainder of this manuscript contains a brief discussion of aerosol measurement consistency, and focuses on observations made in four different regimes: 1. background haze – regions above the top of the surface boundary layer, over both land and sea-ice, to 7.2 km that did not contain layers of locally enhanced aerosol or gas-phase species directly transported within the last 20 days from identifiable sources
Airborne measurements from the National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft in April 2008 in the Alaskan Arctic could be placed into one of four categories based on trace gas mixing ratios: (1) air measured within the Arctic boundary layer over sea-ice (ABL) that had been in chemical and physical contact with the sea-ice surface; (2) air that was strongly influenced by biomass burning (BB) originating primarily from sources in southern Russia and in southeastern Siberia; (3) air that was affected primarily by anthropogenic fossil fuel combustion sources; and (4) air that was not influenced by any recent source, but that represented the aged, polluted background of the springtime Arctic
Summary
Global temperature records show a statistically significant warming in the last century, with most of the change attributed to anthropogenically emitted greenhouse gases and associated feedbacks (IPCC, 2007). The PALMS instrument reported that 64% by number of the fine mode particles were classified as containing a biomass burning component based on potassium and organic carbon marker ions, following the scheme of Hudson et al (2004) This fraction is noticeably higher than typical levels of 20 to 40% that are consistently observed throughout the midlatitude and tropical troposphere (Hudson et al, 2004; Froyd et al, 2009). In northern Europe during the cold season, wood burned for heating creates smoke that is co-emitted with anthropogenic pollution (Gelencser et al, 2007; Yttri et al, 2009) Both this cold-season biofuel source and mid-latitude springtime fires likely contribute to the seasonal background of BB aerosol material found in free tropospheric Arctic air
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