Aircraft-based measurements of High Arctic springtime aerosol show evidence for vertically varying sources, transport and composition
Abstract. 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◦ 300 N and below potential temperatures of 283.5–287.5 K. 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 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, altitude10 days) in the Arctic, while air masses in the upper polar dome had entered
the Arctic more recently. Variations in aerosol composition were closely related to transport history. In the lower polar dome, the measured sub-micron aerosol mass was dominated by sulfate (mean 74 %), with lower contributions from rBC (1 %), ammonium (4 %) and OA (20 %). At higher altitudes and higher potential temperatures, OA, ammonium 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 highlight the differences in Arctic aerosol chemistry observed at surface-based sites and the aerosol transported throughout the depth of the Arctic troposphere in spring.
Arctic regions are warming faster than the global average, with significant impacts on local ecosystems and local people (e.g, Bindoff et al., 2013; Hinzman et al., 2013). While Arctic warming is driven largely by increasing concentrations of anthropogenic greenhouse gases and local feedback mechanisms, short-lived climate forcing agents also impact Arctic climate. In particular, short-lived species such as aerosol, tropospheric ozone and methane are important climate forcers (e.g., Law and Stohl, 2007; Quinn et al., 2008). The impact of pollution aerosol, transported northward over long distances, on Arctic climate has been significant. For example, a large fraction of greenhouse-gas-induced warming (∼ 60 %) has been offset by anthropogenic aerosol over the past century, such that reductions in sulfur emissions in Europe since 1980 can explain a large amount of Arctic warming since that time (∼ 0.5 K) (Fyfe et al., 2013; Najafi et al., 2015; Navarro et al., 2016). These estimates are compelling, and at the same time global models that form the basis of our predictive capability often struggle to reproduce key characteristics of Arctic aerosol, such as the seasonal cycle and vertical distribution (Shindell et al., 2008; Emmons et al., 2015; Monks et al., 2015; Eckhardt et al., 2015; Arnold et al., 2016). Our incomplete understanding of Arctic aerosol processes results in diverse and frequently poor model skill in simulating Arctic aerosol both at the surface and through the troposphere, and therefore also in accurately simulating aerosol–climate interactions (Arnold et al., 2016). This challenge arises in part due to a lack of vertically resolved observations in Arctic regions.