Long-term trends in rain and cloud chemistry in a region of complex topography

Abstract

Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. Highlights (Cape et al. Long-term trends in rain and cloud chemistry in a region of complex topography)Cloud and rain water analysis from low and high elevation sites in Pennines, UK Orographic (seeder-feeder) enhancement of cloud ion concentrations estimated Long-term (1994Long-term ( -2008 downward trends in non-sea sulfate and nitrateNo trends in cloud:rain scavenging ratios, implying consistent mechanism Uncertainties in extrapolation to upland UK are quantifiedLong-term trends in rain and cloud chemistry in a region of complex topography. Long-term trends in ion concentrations in cloud and rain showed significant downward trends 19 in non-sea sulphate, and a weaker downward trend in nitrate, but no trend in other ions. There 20 was also no trend in the orographic scavenging ratios, implying that the methods used to 21 estimate orographic enhancement across the U.K. are robust over time. in the northern Pennines, which showed that not only rainfall amount but also the 31 concentrations of ions in rain increased with altitude, leading to much greater wet deposition 32 of pollutants on western hills than had been expected (Choularton et al., 1988; Fowler et al., 33 1988). The intensive field campaign measurements of individual rain events also 34 demonstrated the mechanism for these increases, in terms of the 'seeder-feeder' effect 35(Supplement Figure S1). As rain from high-level clouds falls through the atmosphere to low 36 elevation sites, it does not efficiently scavenge the fine (sub-micron) aerosols in the 37 atmosphere, so that rain composition measured at low levels reflects the composition of the Dollard et al., 1983;Fowler et al., 1988). All items were rinsed with deionised water after 107 each sample was taken. For each weekly sample, the total volume was measured, and a sub- with bird droppings. Samples with very small volumes (< 100 mL) were also discounted. 139Similar procedures were applied to the rain and cloud composition data measured at the Results 152 Rain and cloud water amounts 153The amount of rain and cloud collected has consequences for the concentrations of ions in the 154 samples averaged over long periods. In general, the greater the rainfall amount, the more 155 dilute the samples are likely to be to be; raindrops will on average have passed through a 156 greater thickness of cloud, thereby enhancing their volume, but scavenging larger and more 157 dilute cloud droplets. Rainfall amounts used for deposition calculations are derived from the 158 tipping-bucket collector at ground level. 159The data in Figure 2 show that the difference in recorded precipitation amount between Figure S2), with a major peak in the S to W sector, 187and a secondary peak in the NNE to E sector. These peaks became noticeably better defined 188 over the mea...

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Review of methods for assessing deposition of reactive nitrogen pollutants across complex terrain with focus on the UK

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