supersaturation spectrum in a manner which increases the availability of CCN at lower cloud supersaturations. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Aerosol particles; Aerosol growth; Cloud processing
1. Introduction
The optical properties of clouds, critical to planetary albedo, are partly determined by Ž
. Ž
. the cloud condensation nuclei CCN distribution on which they form Twomey, 1974 .
The CCN distribution is a subset of the aerosol distribution which ranges from 0.001 to over 100 mm. Aerosols, which are effective as CCN, must be small enough to mix
Ž .
vertically to cloud condensation level typically a few hundred to a thousand metres and large enough to be activated at supersaturations typically encountered in the atmospheric
Ž .
boundary layer - 1 . Furthermore, the aerosol generally contains some soluble
aerosol species. Convective clouds such as cumulus clouds generally possess peak supersaturations of around 0.5 or greater and, thus, can activate aerosol 0.02 mm
radius, while peak supersaturations reached in stratiform clouds are of the order of 0.1–0.2, typically activating aerosol of around 0.05 mm and larger.
The sub-micron atmospheric aerosol size distribution is multi-modal in shape, reflecting the different formation and loss mechanisms, and typically possesses an
Ž .
Ž .
accumulation mode 0.05–0.2 mm radius , an Aitken mode 0.02–0.4 mm with a minimum at around 0.04–0.05 mm, and a nucleation mode with radii - 5–10 nm
Ž .
O’Dowd et al., 1993, 1998; Quinn et al, 1993; Makela et al., 1997 . The nucleation
¨ ¨
mode is infrequently observed and is indicative of recent homogeneous nucleation events forming new particles directly from the gas phase. The Aitken mode is always
observed and is thought to result from condensation of precursor gases and the Ž
. coagulation of recently formed ultra-fine particles Raes and Van Dingenen, 1992 ,
while the accumulation mode is generally observed in the presence of cloud fields or after cloud evaporation.
Although aerosols are known to strongly influence cloud micro-physics, clouds are also thought to play an important role in shaping the aerosol size distribution due to
chemical and physical interactions. When CCN are activated into cloud droplets, physical properties are altered through increases in surface area resulting in enhanced
scavenging of trace gases, and their chemical properties are altered through rapid changes in pH and ionic composition resulting from oxidation of dissolved species
Ž
. Seinfeld and Pandis, 1998; O’Dowd et al., 2000 . Both of these processes result in an
increase in the soluble mass of the activated nuclei. The composition of the activated nuclei can also be changed through coalescence of nuclei activated upon different
aerosol species and through the diffusive coagulation of interstitial aerosol of different composition with the activated droplets. Diffusive coagulation of interstitial aerosol
transfers some mass from interstitial aerosol into the size range of activated nuclei, however, while it does increase the CCN mass, it does not increase CCN concentration.
By comparison, coalescence results in the activated nuclei reducing in concentration but
increasing in mean mass. Chemical processing, as opposed to the aforementioned physical processing of the aerosol, results in the addition of aerosol mass through
aqueous phase oxidation processes, which tend to increase the size of activated nuclei while maintaining the concentration constant. The primary composition of the activated
nuclei is thought to be predominantly nss-sulphate aerosol in both the marine and continental environment; however, nitrate, organic and sea-salt nuclei are also found to
Ž contribute to CCN composition Hegg and Hobbs, 1982; Leaitch, 1996; Leaitch et al.,
. 1986; Noone et al., 1996 . The much reduced solute concentrations in cloud droplets
provides a suitable environment for rapid oxidation of dissolved soluble precursors, such as SO
scavenged from the gas phase, by H O or O
depending on droplet pH
2 2
2 3
Ž .
Seinfeld and Pandis, 1998 . Ž
. Hoppel et al. 1986 suggested that the minimum in the size distribution separating
the Aitken and accumulation modes resulted from chemical processing of activated nuclei in non-precipitating clouds. This minimum results from in-cloud scavenging and
activation of aerosol, around 0.03–0.05 mm and larger, into cloud droplets where they accumulate extra mass through cloud-chemistry oxidation and consequently, grow into
accumulation mode aerosol, thus leaving a deficit of aerosol in the 0.03–0.05 mm size
Ž .
range. Hegg 1992 also postulated that, at least in the marine environment, CCN activated in stratiform cloud cannot be formed without first being activated in cumulus
clouds where they grow to sizes readily activated by the lower supersaturation found in stratiform clouds.
Ž .
Ž .
Ž .
Measurements by Radke and Hobbs 1969 , Dinger et al. 1970 , Hobbs 1971 indicated that CCN concentrations active at a given supersaturation are often higher in
air that has been cycled through clouds than in the ambient air and that additional Ž
. sulphate mass was produced during this type of cloud cycling. Hegg and Hobbs 1982
found that although the amount of sulphate produced by cloud cycling was quite variable, it could be as high as 10.9 mg m
y3
. Average values were found to range from 0.9 mg m
y3
in the Pacific Northwest coast to 2.84 mg m
y3
in the mid-Atlantic coast of the US — the difference presumed to be due to the background levels of pollution.
Ž .
Hegg 1985 concluded that the conversion of SO to aerosol sulphate in the troposphere
2
as a whole was dominated by the in-cloud heterogeneous conversion mechanism with as much as 10–15 times more SO oxidised in this manner compared with homogeneous
2
Ž gas-phase oxidation
which provides the material for new particle formation and .
condensation growth . In a similar manner, a large fraction of nitrate aerosol is also produced by cycling of aerosol through clouds with values ranging from 0.06 mg m
y3 y3
Ž in the clean Pacific Northwest to 2.9 mg m
in the polluted East coast US Hegg and .
Ž .
Hobbs, 1988 . Leaitch et al. 1986 found strong evidence for aerosol nitrate production in cloud, however, knowledge of the mechanisms for nitrate production is rather more
limited than that of sulphate production. We present further evidence of the growth of accumulation mode aerosol after
passage through cloud during two intensive campaigns conducted over the rural north- west of England. The primary objectives of these campaigns were to examine the
vertical structure of aerosol and it’s relationship to boundary layer thermodynamics, the Ž
. results of which are reported in detail in O’Dowd and Smith 1996 . In this study, the
differences between cloudy and cloud-free aerosol characteristics are discussed along
with aerosol properties in the vicinity of clouds. An examination of possible aerosol- growth mechanisms is also presented.
2. Experimental
