Abstract
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.
Highlights
Mass concentrations of the atmospheric aerosol in the Arctic are higher during winter than in summer due to differences in transport of anthropogenic particles and wet scavenging (e.g. Barrie, 1986; Stohl, 2006)
Summary statistics for the cloud and aerosol samples are discussed in Sect. 3.1, the microphysics of low-altitude and higher-altitude clouds are contrasted in Sect. 3.2, particle activation is summarized in Sect. 3.3 and in Sect. 3.4 the relationship between volume-weighted mean droplet diameter (VMD) and cloud droplet number concentrations (CDNC) is used to consider the transition of aerosol indirect effects from potential warming to potential cooling
The mean and median values of the microphysical properties of the cloud and pre-cloud aerosols as well as the altitudes and temperatures derived from the 62 cloud samples are given in Table 1, separated between periods 1 and 2
Summary
Mass concentrations of the atmospheric aerosol in the Arctic are higher during winter than in summer due to differences in transport of anthropogenic particles and wet scavenging (e.g. Barrie, 1986; Stohl, 2006). Leaitch et al.: Effects of 20–100 nm particles changes in CCN for ultra-low values (< 10 cm−3), where CCN concentrations are equivalent to model cloud droplet number concentration (CDNC), results in a net warming due to associated long-wave changes, whereas for concentrations greater than 10 cm−3 CCN increases are estimated to produce a net atmospheric cooling. Among the studies that have considered in situ aerosol measurements and summertime Arctic clouds, Zamora et al (2016) examined the efficiency of BB plumes on indirect forcing They estimated half of the possible maximum forcing from these plumes, mostly due to the reduction in cloud-base S by higher concentrations of larger particles that control water uptake. What is the relationship between droplet size and droplet number? In particular, what is the aerosol influence on cloud below the Mauritsen limit, and is it possible to assess a background influence of the aerosol on clouds in the Arctic summer? (Sect. 3.4)
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