Abstract
Abstract. Reduced visibility is an indicator of poor air quality. Moreover, degradation in visibility can be hazardous to human safety; for example, low visibility can lead to road, rail, sea and air accidents. In this paper, we explore the combined influence of atmospheric aerosol particle and gas characteristics, and meteorology, on long-term visibility. We use visibility data from eight meteorological stations, situated in the UK, which have been running since the 1950s. The site locations include urban, rural and marine environments. Most stations show a long-term trend of increasing visibility, which is indicative of reductions in air pollution, especially in urban areas. Additionally, the visibility at all sites shows a very clear dependence on relative humidity, indicating the importance of aerosol hygroscopicity on the ability of aerosol particles to scatter radiation. The dependence of visibility on other meteorological parameters, such as wind speed and wind direction, is also investigated. Most stations show long-term increases in temperature which can be ascribed to climate change, land-use changes (e.g. urban heat island effects) or a combination of both; the observed effect is greatest in urban areas. The impact of this temperature change upon local relative humidity is discussed. To explain the long-term visibility trends and their dependence on meteorological conditions, the measured data were fitted to a newly developed light-extinction model to generate predictions of historic aerosol and gas scattering and absorbing properties. In general, an excellent fit was achieved between measured and modelled visibility for all eight sites. The model incorporates parameterizations of aerosol hygroscopicity, particle concentration, particle scattering, and particle and gas absorption. This new model should be applicable and is easily transferrable to other data sets worldwide. Hence, historical visibility data can be used to assess trends in aerosol particle properties. This approach may help constrain global model simulations which attempt to generate aerosol fields for time periods when observational data are scarce or non-existent. Both the measured visibility and the modelled aerosol properties reported in this paper highlight the success of the UK's Clean Air Act, which was passed in 1956, in cleaning the atmosphere of visibility-reducing pollutants.
Highlights
The meteorological definition of visibility is the “distance at which the contrast of a given object with respect to its background is just equal to the contrast threshold of an observer” (WMO, 1992, 2015)
The effect of human-to-automated-measurement changeovers at Heathrow, Leuchars, Nottingham, Ringway and Waddington sites appears to be minimal, with the pre-changeover long-term trends being continued after the changeover
Long-term trends in visibility for eight meteorological stations situated in the UK have been investigated
Summary
The meteorological definition of visibility is the “distance at which the contrast of a given object with respect to its background is just equal to the contrast threshold of an observer” (WMO, 1992, 2015). Good visibility is a desirable feature of any geographical location, and its importance should not be neglected (Doyle and Dorling, 2002). Poor visibility ( < 2.0 km; Founda et al, 2016) can affect the transportation of goods and people, whether it is by rail, road, sea or air. Low visibility can lead to accidents and is a concern for public safety. Tourism is often dependent on good visibility for appreciation of points of interest (Singh and Dey, 2012). For example a study at Grand Canyon Park in the USA has shown that visitor frequency in the park has reduced as visibility decreased (Trijonis et al, 1990)
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