Aircraft measurements of High Arctic springtime aerosol show evidence for vertically varying sources, transport and composition.

The sources, chemical transformations and removal mechanisms of aerosol transported to the Arctic are key factors that control Arctic aerosol-climate interactions. Our understanding of sources and processes is limited by a lack of vertically resolved observations in remote Arctic regions. We present vertically resolved observations of trace gases and aerosol composition in High Arctic springtime, made largely north of 80° N, during the NETCARE campaign. Trace gas gradients observed on these flights defined the polar dome as north of 66-68.5° N and below potential temperatures of 283.5-287.5 K (Bozem et al., 2018). In the polar dome, we observe evidence for vertically varying source regions and chemical processing. These vertical changes in sources and chemistry lead to systematic variation in aerosol composition as a function of potential temperature. We show evidence for sources of aerosol with higher organic aerosol (OA), ammonium (NH4) and refractory black carbon (rBC) content in the upper polar dome. Based on FLEXPART-ECMWF calculations, air masses sampled at all levels inside the polar dome (i.e., potential temperature < 280.5 K, altitude <∼ 3.5 km) subsided during transport over transport times of at least 10 days. Air masses at the lowest potential temperatures, in the lower polar dome, had spent long times (> 10 days) in the Arctic, while air masses in the upper polar dome had entered the Arctic more recently. These differences in transport history were closely related to aerosol composition. In the lower polar dome, the measured sub-micron aerosol mass was dominated by sulphate (mean 74%), with lesser contributions from rBC (1%), NH4 (4%) and OA (20%). At higher altitudes and warmer potential temperatures, OA, NH4 and rBC contributed 42%, 8% and 2% of aerosol mass, respectively. A qualitative indication for the presence of sea salt showed that sodium chloride contributed to sub-micron aerosol in the lower polar dome, but was not detectable in the upper polar dome. Our observations suggest that long-term, surface-based measurements underestimate the contribution of OA, rBC and NH4 to aerosol transported to the Arctic troposphere in spring. [ABSTRACT FROM AUTHOR]