growth is observed in all size ranges. This growth compares well with the growth of Ž
. aerosol in moderately clean cumulus observed by Leaitch 1996 where a shift to larger
Ž .
sizes in the accumulation mode was observed. Further, Leaitch 1996 also observed an increase in number concentration of evaporated nuclei at sizes - 0.08 mm, similar to
that observed here after passage through a cumulus cloud. This increase tended to preserve the general characteristics of the distribution by compensating for the growth of
pre-existing accumulation mode nuclei, even though the mass of the distribution had increased. The explanation for this observation is that under the higher supersaturations
encountered in cumulus clouds, Aitken mode nuclei readily are activated into the accumulation mode, thereby increasing the concentration in this mode.
Ž .
Leaitch 1996 also reported aerosol characteristics after passage through stratocumu- lus under much more polluted conditions than encountered here and illustrated the
presence of a bi-modal accumulation mode. Comparison of interstitial aerosol with that observed outside of cloud suggests that the minimum size of aerosol activated under
these conditions was significantly higher than that observed under their cumulus case study and the study presented here. No bi-modal accumulation mode was observed in
these measurements, however, the radius of the single mode was observed to increase in size. The complete growth of the mode presented here suggests that almost all of the
accumulation mode nuclei were activated into cloud droplets compared to the Leaitch Ž
. 1996 case where only a moderate fraction were activated. The differences between
these stratocumulus cases are likely to be due to the much more polluted environment in the Leaitch case, and thus, higher aerosol loadings, which will tend to reduce the peak
supersaturation reached in cloud as more nuclei compete for the same liquid water. This reduction will increase the size of the smallest activated size along with lowering the
Ž .
fraction of accumulation mode nuclei being activated. Hallberg et al. 1994 also saw a reduced fraction of aerosol activated in the more polluted cases during the Kleiner
Feldberg cloud experiment, along with an increase in the 50 partitioning fraction from f 0.05 to f 0.12 mm, when compared to cleaner aerosol conditions.
4. Discussion
The large changes observed in the vicinity of clouds have been attributed to micro-physical processes, specifically, chemical processing of aerosol in cloud; how-
ever, another possibility for the observations could be sampling artefacts such as shattering of cloud droplets in the inlet duct, which warrants further investigation.
Shattering of cloud droplets seems unlikely as it was ensured that all data presented here
Ž .
were sampled out of cloud using the FSSP concentration as an indicator . Furthermore, penetration through clouds, both cumulus and stratocumulus, during other flights on the
campaign resulted in a reduction of accumulation mode aerosol as these particles were removed from the accumulation mode upon activation into cloud droplets, thus refuting
this possibility. Ruling out sampling artefacts, a closer examination of both macro- and micro-scale possibilities must be conducted.
4.1. Macro-scale possibilities Changes in air mass type, and consequently aerosol characteristics, associated with
large scale meteorological conditions can also be ruled out since no frontal passage was observed throughout both campaigns. Furthermore, over the time scales of the observa-
tions, typically 2–3 h, no significant change in wind direction, and thus, aerosol sources were observed.
4.2. Micro-scale possibilities 4.2.1. Aerosol growth mechanisms
If micro-scale processes are the cause of the observed aerosol enhancement in the vicinity of clouds, these process must be capable of growing new particles, or sub-accu-
Ž .
mulation mode particles r - 0.05 mm , into the accumulation mode size range over a time scale significantly shorter than the lifetime of a particle; and, a time scale shorter
than that for large scale meteorological change. Furthermore, if these observations result, as suggested, from the processing of aerosol in cloud, then the process must be able to
proceed on a time scale comparable to that of which a parcel spends being cycled
Ž .
Ž through a cloud of the type observed here , i.e. typically less than 0.5 h Pruppacher
. and Klett, 1978 .
Possible growth processes are: condensation growth, coagulation growth, cloud-free heterogeneous aqueous phase oxidation, andror heterogeneous aqueous phase oxidation
in cloud. Time scale calculations associated with these growth processes are contained in the Appendix. Growth factor limitations are defined from the observations as illustrated
in Fig. 5. We examine the changes in the size distribution below cumulus cloud base and
Ž .
Fig. 5. Spectral differences between aerosol observed below cumulus base i.e. surface layer and below Ž
. Ž
. stratocumulus base i.e. decoupled cloud layer . A marks the size region r -0.05 mm from which particles
Ž .
are grown into the measurable size range r 0.05 mm , B marks the region from which particles are depleted from as they grow into region C.
Ž .
under stratocumulus cloud base shown in Fig. 2 by splitting the observed distributions into three regions. We then can examine the growth time scales capable of producing the
Ž .
observed spectral changes: A — Aitken mode aerosol r - 0.05 mm ; B — where the Ž
. reduction is observed 0.05–0.08 mm ; and C — the mid-point of where the increase in
Ž .
concentration is observed 0.08–0.2 mm . The changes between the surface layer
aerosol and the sub-cloud aerosol correspond to an increase in concentration of 55 cm
y3
around region C and a decrease in concentration from 96 to 44 cm
y3
in region B. It appears that 55 cm
y3
have grown from region B to C, while a further 10 cm
y3
have grown from A to B, resulting in a narrowing of the accumulation mode and a deepening
of the gap between the Aitken mode and the accumulation mode. If we consider the Ž
. growth of aerosol from the geometric centre of region B 0.063 mm to the accumulation
Ž .
mode peak in region C 0.1 mm , we can examine the possible growth processes and time scales involved in growing particles from region B to region C.
4.2.2. Condensation growth The condensation of gas phase aerosol precursors such as H SO
vapour onto
2 4
existing aerosol can increase the mass of aerosol particles. To observe an increase in accumulation mode aerosol number concentration through condensation growth, aerosol
from the sub-accumulation mode, or Aitken mode, must grow into accumulation mode sizes. Assuming a steady state condition where the oxidation of SO by the OH radical
2
is equal to the condensation rate onto existing aerosol, an SO concentration of 0.5 ppb
2
Ž .
results in a H SO vapour concentration of 3 ppt
Eq. 2 . Inserting this vapour
2 4
Ž . concentration into the particle condensation growth, Eq. 3 results in a characteristic
growth time of 7.5 days to grow a particle of radius 0.063 to 0.1 mm by condensation.
4.2.3. Coagulation growth Self-coagulation of Aitken mode particles, with a mode-radius around 0.03 mm, can
increase the number of accumulation mode particles as particles in the self-coagulating Ž .
Ž . mode increase in size. This process is characterised by Eqs. 4 and 5 in the Appendix.
Using a relatively high rural Aitken mode concentration of 10,000 cm
y3
, the self-coagu- lation growth time scale is approximately 80 days.
Below cloud diffusive coagulation of Aitken mode particles with particle size Ž .
r s 0.063 mm is characterised by Eq. 7 . Assuming that the wet ambient particle radius Ž
. is twice that of its dry value corresponding to a relative humidity of 80 , and taking
the mean radius of the capturing particle to be the arithmetic mean of its initial Ž
. Ž
. r s 0.063 mm and final size r s 0.1 mm , the diffusive coagulation coefficient K
1 2
12
is 2 = 10
y9
cm
y3
s
y1
. Initialising the Aitken mode concentration at n s 10,000 cm
y3 Ait
results in a growth time scale of 15.6 days. In-cloud diffusive coagulation would be expected to proceed at a faster rate due to the
increased surface area associated with the growing particles activated into cloud Ž .
droplets. Modifying Eq. 7 for diffusive coagulation between Aitken mode particles and cloud droplets of radius 8 mm, using a derived coagulation coefficient K s 3.6 = 10
y8 12
cm
y3
s
y1
, results in a required time scale of 21.8 h to grow 0.062 mm particles into 0.1 mm sized particles.
4.2.4. Cloud-free heterogeneous oxidation Wet aerosol particles provide an effective site for the aqueous phase oxidation of
Ž .
dissolved aerosol precursors Chameides and Stelson, 1992; Sievering et al., 1992 . In the absence of catalysts, the conversion of dissolved SO into aerosol sulphate mass is
2
Ž .
achieved through oxidation by ozone and hydrogen peroxide. In acidic aerosol H SO ,
2 4
where the pH is low, oxidation by hydrogen peroxide is the dominant reaction while for Ž
. high pH aerosol formed on NH
SO , the dominant oxidant is ozone. The conversion
4 2 4
rate of SO to sulphate by hydrogen peroxide oxidation is independent of the aerosol
2
pH, however, the conversion rate by ozone oxidation proceeds significantly faster than Ž
. oxidation by hydrogen peroxide at high pH
5–7 . Based on the aqueous phase Ž
. Ž
. production rate of sulphate mass Eq. 8 from Lin et al. 1992 , we can derive a
Ž .
heterogeneous oxidation growth law for a particular size of particle Eq. 11 . Taking a Ž
. wet radius as twice that of its dry size corresponding to a relative humidity of 80 and
assuming that the pH in the wet aerosol particle remains constant at pH s 4, the time required to grow particles of r s 0.063 mm to r s 0.1 mm for O , SO , and H O
1 2
3 2
2 2
concentrations of 30, 0.5, and 1 ppb, respectively, is 792 days. 4.2.5. In-cloud heterogeneous oxidation
In-cloud heterogeneous oxidation is expected to proceed at a faster rate since the droplet solute concentration will be very much reduced and the pH of the water droplet
Ž .
Ž .
increased O’Dowd et al., 2000 . Eq. 11 can be scaled up to estimate the growth rate if Ž
. the growing particle is activated into an 8-mm radius cloud droplet Eq. 12 , and
accounting for the uptake of NH
q
by maintaining the pH constant, leads to a growth
4
time scale of 443 s for a conservative pH value of 5. 4.2.6. In-cloud coalescence
Coalescence of two cloud droplets leads to a shift in the aerosol mass to larger sizes, however, this process results in a reduction in number concentration. To produce the
observed mass enhancement, at least 500 cm
y3
particles from below point A would have to be activated and coalesced in order to produce the accumulation mode increase.
Ž .
Ž .
Measurements from Raga and Jonas 1993 and Martin et al. 1994 suggest that for the given concentration of sub-cloud accumulation mode aerosol, it is very unlikely that
cloud droplet concentrations of this magnitude would occur. Since no penetrations of cloud were conducted during this flight, this conclusion cannot be corroborated by the
measurements.
Of the above possible mechanisms, only in-cloud aqueous phase gas-to-particle conversion appears to be a viable mechanism over the required time scales. The most
likely explanation for these observations is that the supersaturations reached in the clouds observed during this study are sufficient to activate aerosol around 0.05 mm
radius into cloud droplets, which, once activated, provide an ideal site for the aqueous phase oxidation of dissolved species scavenged from the gas phase. The in-cloud
aqueous phase oxidation of trace gases results in enhancement of the initial CCN mass and an accompanying shift in radius to larger sizes — the greatest increase in size
occurring in the smallest activated nuclei.
5. Effect of cloud processing on the CCN supersaturation spectrum
