Processes controlling the annual cycle of Arctic aerosol number and size distributions

Abstract

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.

Measurements of aerosol and CCN properties in the Mackenzie River delta (Canadian Arctic) during spring–summer transition in May 2014

“…Zeppelin and also similar to those discussed in Tunved et al (2013). Croft et al (2016a) suggest that the observed similarities at these two stations, which are 1000 km apart, and between the different years examined at Mt. Zeppelin indicate the existence of an annual cycle that spans the high Arctic.…”

“…From this connection follows an increased new particle production due to the current decrease in the sea ice pack extent (Dall'Osto et al, 2017a). Croft et al (2016a) reported data collected in the years 2011 to 2013 from Mt. Zeppelin, i.e.…”

“…Indeed, these precipitation-related scavenging processes, which are effective from late spring throughout the summer, were shown to be among the drivers of the yearly cycle in Arctic PNSDs (Browse et al, 2012;Croft et al, 2016a). Resulting low number concentrations of particles in the accumulation-mode size range enable new particle formation (NPF).…”

Molecular-resolution simulations of new particle formation: Evaluation of common assumptions made in describing nucleation in aerosol dynamics models

“…Assessing these assumptions for different atmospheric environments is important especially now that much effort is put to obtaining more accurate estimates for NPF rates in different conditions (Almeida et al 2013;Riccobono et al 2014;Glasoe et al 2015), and climate and air-quality models aim at increasingly better description of NPF by implementing the state-of-the-art formation rate schemes (Riccobono et al 2014;Baranizadeh et al 2016;Croft et al 2016).…”

Evidence for marine biogenic influence on summertime Arctic aerosol

“…In the Arctic, interactions between aerosol, clouds, and climate are complicated by a combination of high surface albedo; the dependence of cloud emissivity on droplet size and aerosol properties [e.g., Curry, 1995;Lubin and Vogelmann, 2006;Kay and Gettelman, 2009]; and the seasonal cycle in aerosol concentration, size, and composition [e.g., Shaw, 1995;Quinn et al, 2007;Sharma et al, 2012;Breider et al, 2014;Croft et al, 2016a]. Aerosol can exert a strong influence on Arctic climate, and changes in anthropogenic aerosol concentrations likely contributed to the magnitude of Arctic warming and impacted sea ice concentrations in recent decades [Najafi et al, 2015;Acosta Navarro et al, 2016;Gagné et al, 2017].…”