Slower CCN growth kinetics of anthropogenic aerosol compared to biogenic aerosol observed at a rural site
Date
2010
Authors
Shantz, N. C.
Chang, R. Y. -W
Slowik, J. G.
Vlasenko, A.
Abbatt, J. P. D.
Leaitch, W. R.
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Abstract
Growth rates of water droplets were measured with a static diffusion cloud condensation
chamber in May-June 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
mu m 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 alpha(c) = 1 to an average of alpha(c) = 0.04 is needed (
assuming an insoluble organic with hygroscopicity parameter, kappa(org), of zero).
Simulations of the initial growth rate on polydisperse anthropogenic particles agree
best with observations for alpha(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 kappa(org) = 0.2
(with upper and lower limits of 0.5 and 0.07, respectively) and alpha(c) = 1. Inhibition
of water uptake by the anthropogenic organic applied to an adiabatic cloud parcel model
in the form of a constant low alpha(c) increases the number of droplets in a cloud
compared to pure AS. If the alpha(c) is assumed to increase with increasing liquid water
on the droplets, then the number of droplets decreases which could diminish the indirect
climate forcing effect. The slightly lower kappa(org) in the biogenic case decreases the
number of droplets in a cloud compared to pure AS.
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Citation
Shantz, N. C., R. Y. -W Chang, J. G. Slowik, A. Vlasenko, et al. 2010. "Slower CCN growth kinetics of anthropogenic aerosol compared to biogenic aerosol
observed at a rural site." Atmospheric Chemistry and Physics 10(1): 299-312.