The summertime Arctic lower troposphere is a relatively pristine background aerosol environment dominated by nucleation and Aitken mode particles. Understanding the mechanisms that control the formation and growth of aerosol is crucial for our ability to predict cloud properties and therefore radiative balance and climate. We present an analysis of an aerosol growth event observed in the Canadian Arctic Archipelago during summer as part of the NETCARE project. Under stable and clean atmospheric conditions, with low inversion heights, carbon monoxide less than 80aEuro-ppb(v), and black carbon less than 5aEuro-ngaEuro-m(-3), we observe growth of small particles, < aEuro-20aEuro-nm in diameter, into sizes above 50aEuro-nm. Aerosol growth was correlated with the presence of organic species, trimethylamine, and methanesulfonic acid (MSA) in particles similar to aEuro-80aEuro-nm and larger, where the organics are similar to those previously observed in marine settings. MSA-to-sulfate ratios as high as 0.15 were observed during aerosol growth, suggesting an important marine influence. The organic-rich aerosol contributes significantly to particles active as cloud condensation nuclei (CCN, supersaturationaEuro- = aEuro-0.6aEuro-%), which are elevated in concentration during aerosol growth above background levels of similar to 100 to similar to 220aEuro-cm(-3). Results from this case study highlight the potential importance of secondary organic aerosol formation and its role in growing nucleation mode aerosol into CCN-active sizes in this remote marine environment

Abstract. Observations addressing effects of aerosol particles on summertime Arctic clouds are limited. An airborne study, carried out during July 2014 from Resolute Bay, Nunavut, Canada, as part of the Canadian NETCARE project, provides a comprehensive in situ look into some effects of aerosol particles on liquid clouds in the clean environment of the Arctic summer. Median cloud droplet number concentrations (CDNC) from 62 cloud samples are 10 cm−3 for low-altitude cloud (clouds topped below 200 m) and 101 cm−3 for higher-altitude cloud (clouds based above 200 m). The lower activation size of aerosol particles is  ≤  50 nm diameter in about 40 % of the cases. Particles as small as 20 nm activated in the higher-altitude clouds consistent with higher supersaturations (S) for those clouds inferred from comparison of the CDNC with cloud condensation nucleus (CCN) measurements. Over 60 % of the low-altitude cloud samples fall into the CCN-limited regime of Mauritsen et al. (2011), within which increases in CDNC may increase liquid water and warm the surface. These first observations of that CCN-limited regime indicate a positive association of the liquid water content (LWC) and CDNC, but no association of either the CDNC or LWC with aerosol variations. Above the Mauritsen limit, where aerosol indirect cooling may result, changes in particles with diameters from 20 to 100 nm exert a relatively strong influence on the CDNC. Within this exceedingly clean environment, as defined by low carbon monoxide and low concentrations of larger particles, the background CDNC are estimated to range between 16 and 160 cm−3, where higher values are due to activation of particles  ≤  50 nm that likely derive from natural sources. These observations offer the first wide-ranging reference for the aerosol cloud albedo effect in the summertime Arctic.

The Arctic region is a harbinger of global change and is warming at a rate higher than the global average. While Arctic warming is driven by increases in anthropogenic greenhouse gases' in combination with local feedback mechanisms, short‐lived climate forcing agents, such as tropospheric aerosol, are also important drivers of Arctic climate. Arctic aerosol‐climate impacts vary seasonally as a result of the interplay between aerosol and different cloud types, available solar radiation, sea ice, surface albedo, Arctic and lower latitude removal processes, and atmospheric transport patterns. Photochemistry and efficient wet aerosol removal have low impact in winter but become important in spring to summer, dramatically altering aerosol chemical composition, and driving the size distribution from a pronounced accumulation mode toward a dominance of smaller particles. Retreating sea ice, increasing solar insolation and warmer temperatures in summer result in enhanced emissions from Arctic marine and terrestrial ecosystems, and anthropogenic sources, with impacts on the composition of gas and particle phases. Fractional cloud cover reaches a maximum in Arctic summer, in parallel with decreasing sea ice extent and surface albedo. This seasonal variation corresponds to significant changes in the net cloud radiative effect; changes that are affected by aerosol. This review summarizes our current knowledge of processes that control Arctic aerosol properties. We highlight both natural and anthropogenic processes that will be impacted by current and future sea ice loss. Efforts are needed to better constrain aerosol removal rates, characterize aerosol precursors, and constrain the seasonality and magnitude of aerosol‐cloud‐climate impacts.

Summertime Arctic shipboard observations of oxygenated volatile organic compounds (OVOCs) such as organic acids, key precursors of climatically active secondary organic aerosol (SOA), are consistent with a novel source of OVOCs to the marine boundary layer via chemistry at the sea surface microlayer. Although this source has been studied in a laboratory setting, organic acid emissions from the sea surface microlayer have not previously been observed in ambient marine environments. Correlations between measurements of OVOCs, including high levels of formic acid, in the atmosphere (measured by an online highresolution time-of-flight mass spectrometer) and dissolved organic matter in the ocean point to a marine source for the measured OVOCs. That this source is photomediated is indicated by correlations between the diurnal cycles of the OVOC measurements and solar radiation. In contrast, the OVOCs do not correlate with levels of isoprene, monoterpenes, or dimethyl sulfide. Results from box model calculations are consistent with heterogeneous chemistry as the source of the measured OVOCs. As sea ice retreats and dissolved organic carbon inputs to the Arctic increase, the impact of this source on the summer Arctic atmosphere is likely to increase. Globally, this source should be assessed in other marine environments to quantify its impact on OVOC and SOA burdens in the atmosphere, and ultimately on climate.Arctic | chemical ionization mass spectrometry | oxygenated volatile organic compounds | sea surface microlayer | marine boundary layer

Electrospray and Electrosonic Spray Ionization Mass Spectrometry (ESI-MS and ESSI-MS) have been widely used to report evidence that many chemical reactions in micro- and nano-droplets are dramatically accelerated by factors...