with aerosol properties in the vicinity of clouds. An examination of possible aerosol- growth mechanisms is also presented.
2. Experimental
The UMIST Cessna 184, modified for atmospheric research studies, was fitted with a Ž
. Particle Measuring Systems PMS ASASP-X and FSSP-100 optical particle counters to
examine atmospheric aerosol concentrations and size distributions. The ASASP-X measures the accumulation mode aerosol size distribution from 0.05 mm radius up to 1.5
mm. The FSSP was configured for in situ sampling of coarse mode aerosol ranging from 0.25 to 8 mm. To examine aerosol volatility and, thus, infer chemical composition, a
thermal analytical volatility system was also installed which comprised four heater tubes set to specific temperatures to volatilise common aerosol species. The four heater tubes
were configured to temperatures of 408C, 808C, 1508C and 3408 C. Aerosol measured at 408C is considered a dry aerosol and is therefore independent of relative humidity
Ž .
changes O’Dowd et al., 1993 . The loss of aerosol between 408C and 808C is thought to correspond to loss of low volatility aerosol such as nitric acid andror light organic
aerosol, while the loss between 808C and 1508C corresponds to the loss of sulphuric acid. Ammonium sulphate volatilises at 2008C and therefore is determined by the loss of
mass and concentration between 1508C and 3508C. The residual aerosol remaining at
Ž 3508C is normally soot carbon in polluted and non-marine air masses Jennings et al.,
. 1994; Smith and O’Dowd, 1996 . For a more detailed explanation of the volatility
Ž .
technique, the readers are referred to O’Dowd and Smith 1993 . The aerosol measured by the ASASP-X was sampled from an intake duct channelling air into the fuselage
where it could be selected for direct sampling or volatility analysis. Particle losses were calculated to be negligible for direct sampling and comparison between the direct and
volatility sampling indicated approximately 10 loss in the volatility system for sizes
Ž .
larger than 0.5 mm O’Dowd and Smith, 1996 . The observations reported here were taken in the vicinity of Carlisle in the northwest
Ž .
of England 54.38N 2.78W and the Eden Valley about 200 km south of Carlisle. Meteorological conditions during the two projects, Winter and Summer, were charac-
terised by persistent high-pressure systems. During the Winter project, the centre of the high pressure was a few tens of kilometres to the west of our sampling area at the
beginning of the week, however, later in the week the centre strengthened and moved towards central Europe. Two high-pressure systems straddled the UK during the
duration of the Summer project.
3. Results
