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Abstract. Measurements at high-Arctic sites (Alert, Nunavut, and Mt. Zeppelin, Svalbard) during the years 2011 to 2013 show a strong and similar annual cycle in aerosol number and size distributions. Each year at both sites, the number of aerosols with diameters larger than 20 nm exhibits a minimum in October and two maxima, one in spring associated with a dominant accumulation mode (particles 100 to 500 nm in diameter) and a second in summer associated with a dominant Aitken mode (particles 20 to 100 nm in diameter). Seasonal-mean aerosol effective diameter from measurements ranges from about 180 in summer to 260 nm in winter. This study interprets these annual cycles with the GEOS-Chem-TOMAS global aerosol microphysics model. Important roles are documented for several processes (new-particle formation, coagulation scavenging in clouds, scavenging by precipitation, and transport) in controlling the annual cycle in Arctic aerosol number and size.Our simulations suggest that coagulation scavenging of interstitial aerosols in clouds by aerosols that have activated to form cloud droplets strongly limits the total number of particles with diameters less than 200 nm throughout the year. We find that the minimum in total particle number in October can be explained by diminishing new-particle formation within the Arctic, limited transport of pollution from lower latitudes, and efficient wet removal. Our simulations indicate that the summertime-dominant Aitken mode is associated with efficient wet removal of accumulation-mode aerosols, which limits the condensation sink for condensable vapours. This in turn promotes new-particle formation and growth. The dominant accumulation mode during spring is associated with build up of transported pollution from outside the Arctic coupled with less-efficient wet-removal processes at colder temperatures. We recommend further attention to the key processes of new-particle formation, interstitial coagulation, and wet removal and their delicate interactions and balance in size-resolved aerosol simulations of the Arctic to reduce uncertainties in estimates of aerosol radiative effects on the Arctic climate.

Abstract. Atmospheric new particle formation (NPF) is an important phenomenon in terms of global particle number concentrations. Here we investigated the frequency of NPF, formation rates of 10 nm particles, and growth rates in the size range of 10–25 nm using at least 1 year of aerosol number size-distribution observations at 36 different locations around the world. The majority of these measurement sites are in the Northern Hemisphere. We found that the NPF frequency has a strong seasonal variability. At the measurement sites analyzed in this study, NPF occurs most frequently in March–May (on about 30 % of the days) and least frequently in December–February (about 10 % of the days). The median formation rate of 10 nm particles varies by about 3 orders of magnitude (0.01–10 cm−3 s−1) and the growth rate by about an order of magnitude (1–10 nm h−1). The smallest values of both formation and growth rates were observed at polar sites and the largest ones in urban environments or anthropogenically influenced rural sites. The correlation between the NPF event frequency and the particle formation and growth rate was at best moderate among the different measurement sites, as well as among the sites belonging to a certain environmental regime. For a better understanding of atmospheric NPF and its regional importance, we would need more observational data from different urban areas in practically all parts of the world, from additional remote and rural locations in North America, Asia, and most of the Southern Hemisphere (especially Australia), from polar areas, and from at least a few locations over the oceans.

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Abstract. The first multi-year contributions from organic functional groups to the Arctic submicron aerosol are documented using 126 weekly-integrated samples collected from April 2012 to October 2014 at the Alert Observatory (82.45 • N, 62.51 • W). Results from the particle transport model FLEXPART, linear regressions among the organic and inorganic components and positive matrix factorization (PMF) enable associations of organic aerosol components with source types and regions. Lower organic mass (OM) concentrations but higher ratios of OM to non-sea-salt sulfate mass concentrations (nss-SO = 4 ) accompany smaller particles during the summer (JJA). Conversely, higher OM but lower OM / nss-SO = 4 accompany larger particles during winter-spring. OM ranges from 7 to 460 ng m −3 , and the study average is 129 ng m −3 . The monthly maximum in OM occurs during May, 1 month after the peak in nss-SO = 4 and 2 months after that of elemental carbon (EC). Winter (DJF), spring (MAM), summer and fall (SON) values of OM / nss-SO = 4 are 26, 28, 107 and 39 %, respectively, and overall about 40 % of the weekly variability in the OM is associated with nss-SO = 4 . Respective study-averaged concentrations of alkane, alcohol, acid, amine and carbonyl groups are 57, 24, 23, 15 and 11 ng m −3 , representing 42, 22, 18, 14 and 5 % of the OM, respectively. Carbonyl groups, detected mostly during spring, may have a connection with snow chemistry. The seasonally highest O / C occurs during winter (0.85) and the lowest O / C is during spring (0.51); increases in O / C are largely due to increases in alcohol groups. During winter, more than 50 % of the alcohol groups are associated with primary marine emissions, consistent with Shaw et al. (2010) and Frossard et al. (2011). A secondary marine connection, rather than a primary source, is suggested for the highest and most persistent O / C observed during the coolest and cleanest summer (2013), when alcohol and acid groups made up 63 % of the OM. A secondary marine source may be a general feature of the summer OM, but higher contributions from alkane groups to OM during the warmer summers of 2012 (53 %) and 2014 (50 %) were likely due to increased contributions from combustion sources. Evidence for significant contributions from biomass burning (BB) was present in 4 % of the weeks. During the dark months (NDJF), 29, 28 and 14 % of the nss-SO = 4 , EC and OM were associated with transport times over the gas flaring region of northern Russia and other parts of Eurasia. During spring, those percentages dropped to 11 % for each of nss-SO = 4 and EC values, respectively, and there is no association of OM.