Aerosol measurements during COPE: composition, size and sources of CCN and INPs at the interface between marine and terrestrial influences

Heavy rainfall from convective clouds can lead
to devastating flash flooding, and observations of aerosols
and clouds are required to improve cloud parameterisations
used in precipitation forecasts. We present measurements of
boundary layer aerosol concentration, size, and composition
from a series of research flights performed over the southwest
peninsula of the UK during the COnvective Precipitation
Experiment (COPE) of summer 2013. We place emphasis
on periods of southwesterly winds, which locally are most
conducive to convective cloud formation, when marine air
from the Atlantic reached the peninsula. Accumulation-mode
aerosol mass loadings were typically 2–3 μgm^-3 (corrected
to standard cubic metres at 1013.25 hPa and 273.15 K), the
majority of which was sulfuric acid over the sea, or ammonium
sulfate inland, as terrestrial ammonia sources neutralised
the aerosol. The cloud condensation nuclei (CCN)
concentrations in these conditions were 150–280 cm^-3 at
0.1% and 400–500 cm^-3 at 0.9% supersaturation (SST),
which are in good agreement with previous Atlantic measurements,
and the cloud drop concentrations at cloud base
ranged from 100 to 500 cm^-3. The concentration of CCN
at 0.1% SST was well correlated with non-sea-salt sulfate,
meaning marine sulfate formation was likely the main source
of CCN. Marine organic aerosol (OA) had a similar mass
spectrum to previous measurements of sea spray OA and was
poorly correlated with CCN.

In one case study that was significantly different to the
rest, polluted anthropogenic emissions from the southern and
central UK advected to the peninsula, with significant enhancements
of OA, ammonium nitrate and sulfate, and black
carbon. The CCN concentrations here were around 6 times
higher than in the clean cases, and the cloud drop number
concentrations were 3–4 times higher.

Sources of ice-nucleating particles (INPs) were assessed
by comparing different parameterisations used to predict INP
concentrations, using measured aerosol concentrations as input.
The parameterisations based on total aerosol produced
INP concentrations that agreed within an order of magnitude
with measured first ice concentrations at cloud temperatures
as low as -12 C. Composition-specific parameterisations
for mineral dust, fluorescent particles, and sea spray OA were
3–4 orders of magnitude lower than the measured first ice
concentrations, meaning a source of INPs was present that
was not characterised by our measurements and/or one or
more of the composition-specific parameterisations greatly
underestimated INPs in this environment.