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Centre for Atmospheric Science

Holme Moss PROCLOUD Project - Measurements

A full listing of all measurements made and the institute responsible for these measurements can be found in the table here. These measurements are sumarised in the following paragraphs.


Gas phase

The following species were measured continuously at HM1 and HM2: NO, NO2, SO2, O3. H2O2 was measured at HM1 during intensive run periods only, and NO2 was measured at HM3. All above species were measured with a time resolution of one minute. Rotating annular batch denuders were deployed at HM1 and HM3, using a stripping solution designed to collect NH3 and HNO3. These denuders collected hourly batch samples which were analysed later with ion chromatography.


Aerosol size distribution

DMPS systems were deployed at all four measurement sites, and with the exception of Manchester were set up to measure particles in the size range of 3 – 850 nm (for the Manchester system the size range was 20 – 450 nm). At sites HM1 and HM3 ASASP-X optical particle spectrometers measuring from 100-3000 nm were also used, thus giving a total aerosol size spectra from 3 – 3000 nm. In addition condensation particle counters were deployed at HM1 and HM3 to measure total particle number in the size ranges 3 – 1000 nm, and 20 – 1000 nm, and hence also 3 – 20 nm from the difference between the two. These measurements were carried out with a time resolution of one minute. At HM3 a second DMPS system was deployed, but rather than scanning through different particles sizes, this system was set to sample two sizes continuously. These sizes were 6 and 40 nm, and were sampled with a time resolution of 1Hz. This was done to look for new particle formation in air exiting from the cloud.


Aerosol solubility and chemical composition

Aerosol hygroscopic growth factors were measured using a TDMA system. During Holme Moss PROCLOUD these measurements were only available for the upwind site, and covered the size range 25 – 440 nm in 9 discrete sizes. A full scan through all sizes took about 40 minutes. Six stage Berner impactors were used to collect size segregated aerosol samples for chemical analysis. These samples were analysed for Na+, NH3+, K+, Mg+, Ca+, NO2-, NO3-, Cl-, and SO42- ions using ion chromatography, and were also analysed for organics. These impactors were deployed at HM1 and HM3, and were operated at a controlled relative humidity of 60%. The reason for this was to minimise bounce from the impactor foils. These impactors covered particles in the size range of 0.1 – 10 µm aerodynamic diameter (at 60% RH), with an integration time of three hours. Black carbon content was measured at HM1 and HM3 using optical absorption spectroscopy.


Cloud microphysical properties

The Lund Droplet Aerosol Analyser (DAA) and a DMT Forward Scattering Spectrometer Probe (FSSP-100) were used to measure cloud droplet number and diameter, and hence liquid water content (LWC). However, due to radio frequency interference from the transmitter mast at Holme Moss, the Particulate Volume Monitor (PVM) (which directly measures LWC) would not work. In addition to these instruments, as part of the long-term measurements at Holme Moss, a VPF-730 present weather sensor was deployed, which measures visibility, precipitation, and cloud, by extinction and back-scatter of IR radiation. Coupled with droplet number data from the FSSP or DAA, the VPF IR extinction data was used to calculate LWC. A short intercomparison between the VPF, a PVM and an FSSP carried out at Great Dun Fell in April 1997, showed LWC calculated from the VPF and FSSP to be within about 20% of each other most of the time, over a wide range of cloud conditions. It was noted, however, that there were more significant differences at low LWC with high droplet number.


Cloud water chemical composition

Cloud water was collected using active size segregated and bulk collectors, and bulk passive collectors. Water from these collectors was analysed on site for pH, and H2O2, and later for Na+, NH3+, K+, Mg+, Ca+, NO2-, NO3-, Cl-, and SO42- ions using ion chromatography.