Long-term monitoring of cloud chemical composition in the UK and implications for estimating wet deposition

Cape, J.N.; Smith, R.I.; Fowler, D.; Beswick, K.; Choularton, T.. 2010 Long-term monitoring of cloud chemical composition in the UK and implications for estimating wet deposition. Edinburgh, NERC Centre for Ecology & Hydrology, 31pp. (CEH Project No: C03789) (Submitted)

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• Continuous monitoring of rain and cloud water amount and composition has been conducted at Holme Moss (southern Pennines) since 1994. • Year-to-year variation in rain and cloud amount has been large, but with no temporal trend. • Wind speed at Holme Moss has increased over the period, especially from the prevailing SW wind direction. The effect on transporting pollutants to the site is not known. • Concentrations of non-marine sulphate in rain and cloud have decreased significantly over the period, consistent with the large decrease in UK emissions of sulphur dioxide. Concentrations of nitrate in rain and cloud have decreased slightly, not matching changes in NOx emissions in the UK, as noted for other sites in the UK rainfall network. Ammonium concentrations show no significant temporal trend. There is no temporal trend in sea-salt concentrations. • Deposition of ions follows similar temporal trends to concentrations. • There were no temporal trends in the ratio of cloud to rain water concentrations of ions at Holme Moss, nor in the ‘scavenging factor’ used to calculate seeder-feeder enhancement of deposition. • The average scavenging factor for sea-salts of 3.6 was higher than the default value of 2 used across the whole UK for modelling seeder-feeder enhancement of all ions. Average scavenging factors for other ions were all less than 2. • Scavenging factors of 2 or less are related to the presence of gas-phase components of sulphate, nitrate and ammonium which are not included in the seeder-feeder model, and/or to local sources that affect only one of a pair of monitoring sites. • Uncertainties in the use of the seeder-feeder model are shown to be within the year-to-year variability for modelled total deposition across the UK as a whole. Uncertainties in Critical Load exceedance are also small for a Critical Load of deposited nitrogen of 10 kg ha-1 y-1 which is typical of many upland areas, but could be large for areas with higher Critical Loads if they occurred in areas where seeder-feeder enhancement is an important component of total deposition. • The results point to deficiencies in our knowledge of the spatial variation of the scavenging factor for different ions across the UK, and in the variation of trace gas concentrations with elevation. • Options are provided for the extension, continuation or cessation of this monitoring work, with the associated pros, cons and approximate cost implications. • Recommendations include: stopping cloud water monitoring, monitoring of additional paired high- and low-elevation sites for rainfall composition, and installation of (conditional) trace gas and particle integrating samplers at paired high- and low-elevation sites for a restricted period.

Item Type: Publication - Report
Programmes: CEH Topics & Objectives 2009 - 2012 > Biogeochemistry > BGC Topic 1 - Monitoring and Interpretation of Biogeochemical and Climate Changes
UKCEH and CEH Sections/Science Areas: Billett (to November 2013)
Funders/Sponsors: Department of Environment, Food and Rural Affairs (Defra)
Additional Keywords: long-term monitoring, deposition, cloud
NORA Subject Terms: Chemistry
Date made live: 26 Jun 2014 13:54 +0 (UTC)

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