Slower CCN growth kinetics of anthropogenic aerosol compared to biogenic aerosol observed at a rural site

Abstract

Abstract. Growth rates of water droplets were measured with a static diffusion cloud condensation chamber in MayJune 2007 at a rural field site in Southern Ontario, Canada, 70 km north of Toronto. The observations include periods when the winds were from the south and the site was impacted by anthropogenic air from the U.S. and Southern Ontario as well as during a 5-day period of northerly wind flow when the aerosol was dominated by biogenic sources. The growth of droplets on anthropogenic size-selected particles centred at 0.1 µm diameter and composed of approximately 40% organic and 60% ammonium sulphate (AS) by mass, was delayed by on the order of 1 s compared to a pure AS aerosol. Simulations of the growth rate on monodisperse particles indicate that a lowering of the water mass accommodation coefficient from α c =1 to an average of α c =0.04 is needed (assuming an insoluble organic with hygroscopicity parameter, κ org , of zero). Simulations of the initial growth rate on polydisperse anthropogenic particles agree best with observations for α c =0.07. In contrast, the growth rate of droplets on size-selected aerosol of biogenic character, consisting of >80% organic, was similar to that of pure AS. Simulations of the predominantly biogenic polydisperse aerosol show agreement between the observations and simulations when κ org =0.2 (with upper and lower limits of 0.5 and 0.07, respectively) and α c =1. Inhibition of water uptake by the anthropogenic organic applied to an adiabatic cloud parcel model in the form of a constant low α c increases the number of droplets in a cloud compared to pure AS. If the α c is assumed to increase with increasing liquid water on the droplets, then the number of droplets decreases which could diminish the Correspondence to: N. C. Shantz (nicole.shantz@ec.gc.ca) indirect climate forcing effect. The slightly lower κ org in the biogenic case decreases the number of droplets in a cloud compared to pure AS.

Subarctic atmospheric aerosol composition: 3. Measured and modeled properties of cloud condensation nuclei

“…[45] Some studies indicated that organic films could delay particle growth and thus influence detection of CCN [e.g., Ruehl et al, 2008;Shantz et al, 2010]. Comparison of droplet size spectra detected in the CCNC's optical particle counter for ambient aerosol and ammonium sulfate calibration aerosol gave no indication of strongly delayed growth in this study.…”

A coupled observation – modeling approach for studying activation kinetics from measurements of CCN activity

“…CCN concentrations are not significantly affected, but even moderate particle concentrations can have an observable effect on droplet size (Lathem and Nenes, 2011). The second limitation of TDGA is that it cannot provide numerical values for vapor uptake coefficients; a droplet growth model such as those used by Shantz et al (2010), Ruehl et al (2008), Asa-Awuku et al (2009 and Kumar et al (2011a) is instead required.…”

“…Chuang, 2003Chuang, , 2006Sjogren et al, 2007;Drisdell et al, 2009;Tong et al, 2011) and supersaturated (e.g. Winkler et al, 2004Winkler et al, , 2006Ruehl et al, 2008Ruehl et al, , 2009Lance et al, 2009;Asa-Awuku et al, 2009Shantz et al, 2010) conditions have been used. Most of the latter are done with CCN instruments such as the Droplet Measurement Technologies (DMT) CCN counter (Lance et al, 2006), which is based on the Continuous Flow Streamwise Thermal Gradient Chamber (CFSTGC) design (Roberts and Nenes, 2005).…”

The hygroscopicity parameter (κ) of ambient organic aerosol at a field site subject to biogenic and anthropogenic influences: relationship to degree of aerosol oxidation

“…The optimal residence time can also depend on the aerosol chemical composition. This effect was not explored systematically during this study, although Shantz et al (2010) discuss the kinetics of activation during the Egbert 2007 study.…”