Abstract
Abstract We present a detailed analysis of long-term aerosol measurements from four sun photometer sites (from west to east: Plymouth, Chilbolton, Dunkirk, Oostende) and four Department for Environment, Food & Rural Affairs surface sites (from west to east: Plymouth, Southampton, Portsmouth, Eastbourne) near the English Channel. From the early 2000s to about 2016, annual mean Aerosol Optical Depth (AOD) from all sun photometer sites decreased by an overall average of 23% decade-1 (range of 15–28% decade-1). From 2010 to 2017, annual mean concentration of particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5) from all the surface sites decreased by an overall average of 44% decade-1 (range of 7–64% decade-1). Seasonally, the highest aerosol loading is generally found around the springtime, and this maximum has been decreasing much faster over recent years than during the other seasons. This is driven by the interaction between the seasonal weather patterns (e.g. reduced westerly flow and drier weather in the spring) and the main emission sources being predominantly from the European Continent. We find clear spatial gradients in the aerosol loading as well as aerosol composition. From west to east along the English Channel, PM2.5 concentration increases with a mean gradient of about 0.007 μg m-3 km-1. At the westernmost site Plymouth, sea spray is estimated on average to account for 16% of the AOD and 13% of the particulate matter with aerodynamic diameter less than 10 μm (PM10). The importance of sea spray is reduced by at least a factor of two at the more eastern sites. The long-term decrease in aerosol loading along the English Channel appears to be more strongly driven by the reduced anthropogenic emissions, rather than by changes in the large-scale circulation such as the North Atlantic Oscillation. Clean ups in road vehicles and ship emissions, however, do not appear to be strong drivers for the long-term trends in aerosol loading at these coastal sites.
Highlights
Aerosols affect atmospheric chemistry, visibility, and the Earth’s radiative balance
Aerosol loading along the English Channel is subject to the strong contrast between the marine airmasses coming off the North Atlantic from the southwest quadrant, and the European continental airmasses from the east/southeast
Ground based observations clearly show that the loadings of small aerosols in coastal cities along the English Channel have decreased over the last decade and a half
Summary
At the confluence of several interfaces, coastal environments (especially coastal cities) are exposed to multiple sources of aerosols or their precursors depending on wind direction and airmass history These include terrestrial pollution, ship emissions, and sea spray (e.g. Zielin ski, 2004). Several studies have reported reductions in the aerosol loading over Europe since the 2000s (de Meij et al, 2012; Mao et al, 2014; Yoon et al, 2016; Zhao et al, 2017; Ningombam et al, 2019), coinciding with the aforementioned emission reductions Despite this welcoming trend, exceedances in aerosol concentrations over air quality regulations over Europe still occur (https://www.eea.europa.eu/da ta-and-maps/indicators/main-an thropogenic-air-pollutant-emissions/assessment-4). There is considerable uncertainty about which sectors are responsible for the trend in the aerosol loading and in the present day aerosol composition (e.g. Curci et al, 2015; Kristiansen et al, 2016)
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