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Anthropogenic and natural emissions contribute to enhanced concentrations of aerosols in the Arctic winter and early spring, with most attention being paid to anthropogenic aerosols that contribute to so-called Arctic haze. Less-well-studied wintertime sea-spray aerosols (SSAs) under Arctic haze conditions are the focus of this study, since they can make an important contribution to wintertime Arctic aerosol abundances. Analysis of field campaign data shows evidence for enhanced local sources of SSAs, including marine organics at Utqiaġvik (formerly known as Barrow) in northern Alaska, United States, during winter 2014. Models tend to underestimate sub-micron SSAs and overestimate super-micron SSAs in the Arctic during winter, including the base version of the Weather Research Forecast coupled with Chemistry (WRF-Chem) model used here, which includes a widely used SSA source function based on Gong et al. (1997). Quasi-hemispheric simulations for winter 2014 including updated wind speed and sea-surface temperature (SST) SSA emission dependencies and sources of marine sea-salt organics and sea-salt sulfate lead to significantly improved model performance compared to observations at remote Arctic sites, notably for coarse-mode sodium and chloride, which are reduced. The improved model also simulates more realistic contributions of SSAs to inorganic aerosols at different sites, ranging from 20 %–93 % in the observations. Two-thirds of the improved model performance is from the inclusion of the dependence on SSTs. The simulation of nitrate aerosols is also improved due to less heterogeneous uptake of nitric acid on SSAs in the coarse mode and related increases in fine-mode nitrate. This highlights the importance of interactions between natural SSAs and inorganic anthropogenic aerosols that contribute to Arctic haze. Simulation of organic aerosols and the fraction of sea-salt sulfate are also improved compared to observations. However, the model underestimates episodes with elevated observed concentrations of SSA components and sub-micron non-sea-salt sulfate at some Arctic sites, notably at Utqiaġvik. Possible reasons are explored in higher-resolution runs over northern Alaska for periods corresponding to the Utqiaġvik field campaign in January and February 2014. The addition of a local source of sea-salt marine organics, based on the campaign data, increases modelled organic aerosols over northern Alaska. However, comparison with previous available data suggests that local natural sources from open leads, as well as local anthropogenic sources, are underestimated in the model. Missing local anthropogenic sources may also explain the low modelled (sub-micron) non-sea-salt sulfate at Utqiaġvik. The introduction of a higher wind speed dependence for sub-micron SSA emissions, also based on Arctic data, reduces biases in modelled sub-micron SSAs, while sea-ice fractions, including open leads, are shown to be an important factor controlling modelled super-micron, rather than sub-micron, SSAs over the north coast of Alaska. The regional results presented here show that modelled SSAs are more sensitive to wind speed dependence but that realistic modelling of sea-ice distributions is needed for the simulation of local SSAs, including marine organics. This study supports findings from the Utqiaġvik field campaign that open leads are the primary source of fresh and aged SSAs, including marine organic aerosols, during wintertime at Utqiaġvik; these findings do not suggest an influence from blowing snow and frost flowers. To improve model simulations of Arctic wintertime aerosols, new field data on processes that influence wintertime SSA production, in particular for fine-mode aerosols, are needed as is improved understanding about possible local anthropogenic sources. 

The ability of atmospheric aerosols to impact climate through water uptake and cloud formation is fundamentally determined by the size, composition, and phase (liquid, semisolid, or solid) of individual particles. Particle phase is dependent on atmospheric conditions (relative humidity and temperature) and chemical composition and, importantly, solid particles can inhibit the uptake of water and other trace gases, even under humid conditions. Particles composed primarily of ammonium sulfate are presumed to be liquid at the relative humidities (67 to 98%) and temperatures (−2 to 4 °C) of the summertime Arctic. Under these atmospheric conditions, we report the observation of solid organic-coated ammonium sulfate particles representing 30% of particles, by number, in a key size range (<0.2 µm) for cloud activation within marine air masses from the Arctic Ocean at Utqiaġvik, AK. The composition and size of the observed particles are consistent with recent Arctic modeling and observational results showing new particle formation and growth from dimethylsulfide oxidation to form sulfuric acid, reaction with ammonia, and condensation of marine biogenic sulfate and highly oxygenated organic molecules. Aqueous sulfate particles typically undergo efflorescence and solidify at relative humidities of less than 34%. Therefore, the observed solid phase is hypothesized to occur from contact efflorescence during collision of a newly formed Aitken mode sulfate particle with an organic-coated ammonium sulfate particle. With declining sea ice in the warming Arctic, this particle source is expected to increase with increasing open water and marine biogenic emissions. 

Abstract. Even though the Arctic is remote, aerosol properties observed there arestrongly influenced by anthropogenic emissions from outside the Arctic. Thisis particularly true for the so-called Arctic haze season (January throughApril). In summer (June through September), when atmospheric transportpatterns change, and precipitation is more frequent, local Arctic sources,i.e., natural sources of aerosols and precursors, play an important role.Over the last few decades, significant reductions in anthropogenic emissionshave taken place. At the same time a large body of literature shows evidencethat the Arctic is undergoing fundamental environmental changes due toclimate forcing, leading to enhanced emissions by natural processes that mayimpact aerosol properties. In this study, we analyze 9 aerosol chemical species and 4 particleoptical properties from 10 Arctic observatories (Alert, Kevo, Pallas,Summit, Thule, Tiksi, Barrow/Utqiaġvik, Villum, and Gruvebadet and ZeppelinObservatory – both at Ny-Ålesund Research Station) to understand changesin anthropogenic and natural aerosol contributions. Variables includeequivalent black carbon, particulate sulfate, nitrate, ammonium,methanesulfonic acid, sodium, iron, calcium and potassium, as well asscattering and absorption coefficients, single scattering albedo andscattering Ångström exponent. First, annual cycles are investigated, which despite anthropogenic emissionreductions still show the Arctic haze phenomenon. Second, long-term trendsare studied using the Mann–Kendall Theil–Sen slope method. We find in total41 significant trends over full station records, i.e., spanning more than adecade, compared to 26 significant decadal trends. The majority ofsignificantly declining trends is from anthropogenic tracers and occurredduring the haze period, driven by emission changes between 1990 and 2000.For the summer period, no uniform picture of trends has emerged. Twenty-sixpercent of trends, i.e., 19 out of 73, are significant, and of those 5 arepositive and 14 are negative. Negative trends include not only anthropogenictracers such as equivalent black carbon at Kevo, but also natural indicatorssuch as methanesulfonic acid and non-sea-salt calcium at Alert. Positivetrends are observed for sulfate at Gruvebadet. No clear evidence of a significant change in the natural aerosolcontribution can be observed yet. However, testing the sensitivity of theMann–Kendall Theil–Sen method, we find that monotonic changes of around 5 % yr−1 in an aerosol property are needed to detect a significanttrend within one decade. This highlights that long-term efforts well beyonda decade are needed to capture smaller changes. It is particularly importantto understand the ongoing natural changes in the Arctic, where interannualvariability can be high, such as with forest fire emissions and theirinfluence on the aerosol population. To investigate the climate-change-induced influence on the aerosolpopulation and the resulting climate feedback, long-term observations oftracers more specific to natural sources are needed, as well as of particlemicrophysical properties such as size distributions, which can be used toidentify changes in particle populations which are not well captured bymass-oriented methods such as bulk chemical composition. 

Abstract. Sea salt aerosols play an important role in the radiationbudget and atmospheric composition over the Arctic, where the climate israpidly changing. Previous observational studies have shown that Arctic sea ice leads are an important source of sea salt aerosols, and modeling efforts have also proposed blowing snow sublimation as a source. In this study,size-resolved atmospheric particle number concentrations and chemicalcomposition were measured at the Arctic coastal tundra site ofUtqiaġvik, Alaska, during spring (3 April–7 May 2016). Blowing snow conditions were observed during 25 % of the 5-week study period andwere overpredicted by a commonly used blowing snow parameterization based solely on wind speed and temperature. Throughout the study, open leads werepresent locally. During periods when blowing snow was observed, significantincreases in the number concentrations of 0.01–0.06 µm particles(factor of 6, on average) and 0.06–0.3 µm particles (67 %, on average) and a significant decrease (82 %, on average) in 1–4 µmparticles were observed compared to low wind speed periods. These size distribution changes were likely caused by the generation of ultrafineparticles from leads and/or blowing snow, with scavenging of supermicronparticles by blowing snow. At elevated wind speeds, both submicron andsupermicron sodium and chloride mass concentrations were enhanced,consistent with wind-dependent local sea salt aerosol production. Atmoderate wind speeds below the threshold for blowing snow as well as during observed blowing snow, individual sea spray aerosol particles were measured.These individual salt particles were enriched in calcium relative to sodiumin seawater due to the binding of this divalent cation with organic matter in the sea surface microlayer and subsequent enrichment during seawaterbubble bursting. The chemical composition of the surface snowpack alsoshowed contributions from sea spray aerosol deposition. Overall, theseresults show the contribution of sea spray aerosol production from leads onboth aerosols and the surface snowpack. Therefore, if blowing snowsublimation contributed to the observed sea salt aerosol, the snow beingsublimated would have been impacted by sea spray aerosol deposition rather than upward brine migration through the snowpack. Sea spray aerosol production from leads is expected to increase, with thinning and fracturingof sea ice in the rapidly warming Arctic.