American Association for Aerosol Research - Abstract Submission

AAAR 36th Annual Conference
October 16 - October 20, 2017
Raleigh Convention Center
Raleigh, North Carolina, USA

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Aerosol Size Distribution, Chemical Composition and CCN Activities in Eastern North Atlantic

GUANGJIE ZHENG, Tamara Pinterich, Janek Uin, Thomas Watson, Stephen Springston, Robert Bullard, Chongai Kuang, Allison Aiken, Rob Wood, Jian Wang, Brookhaven National Laboratory

     Abstract Number: 355
     Working Group: Aerosols, Clouds, and Climate

Abstract
The responses of marine low cloud systems to changes in aerosols are among major sources of uncertainty that limit our ability to predict future climate. Major contributions to the uncertainty derive from poor understanding of aerosol under natural conditions and the perturbation by anthropogenic emissions. The Eastern North Atlantic Ocean (ENA) is a region of persistent but diverse marine boundary layer (MBL) clouds, and whose albedo and precipitation are highly susceptible to perturbation in aerosol properties. In this study, we examine aerosol properties, trace gases, and meteorological parameters measured on Graciosa Island in the ENA since 2014. The concentrations of CO and O3 are well correlated, suggesting entrainment of free tropospheric air is a common source of both species in the boundary layer. Monthly averages of total aerosol number concentration (CN) in 2016 varied from 220 cm-3 in December to 620 cm-3 in April. The seasonal variation in CN is likely due to a combination of more efficient particle removal during the winter months and more frequent transport of continental pollution during the spring time. Observed particle volume size distribution in the accumulation size range typically consists of a condensation sub-mode (centered around 200 nm) and a droplet sub-mode (centered around 800 nm). This bimodal pattern could arise from cloud processing and/or production of sea spray aerosol. Non-refractory submicron aerosol composition measured by an Aerodyne Aerosol Chemical Speciation Monitor shows that sulfate and organics are the dominating species, while the contribution of nitrate is much lower. In addition, cloud condensation nuclei (CCN) concentrations are measured at five super-saturations (ss) of 0.1%, 0.2%, 0.5%, 0.8%, and 1%. The observed CCN spectra can be well fitted using a power-law function, i.e. CCN(ss)=C ssk. Monthly averages of the fitted C and k ranged from 222 to 440 cm-3 and from 0.4 to 0.7, respectively, in agreement with previous observations in this region. The hygroscopicity parameter, κ, is derived from the concurrent CCN spectrum and size distribution measurements, with the monthly averages ranging 0.3 to 0.6. The value of κ decreases with decreasing particle diameter, suggesting particles composition is increasingly dominated by organics at small particle sizes.