Abstract
Abstract. Atmospheric sulfur dioxide (SO2) was measured continuously from the Penlee Point Atmospheric Observatory (PPAO) near Plymouth, United Kingdom, between May 2014 and November 2015. This coastal site is exposed to marine air across a wide wind sector. The predominant southwesterly winds carry relatively clean background Atlantic air. In contrast, air from the southeast is heavily influenced by exhaust plumes from ships in the English Channel as well as near Plymouth Sound. A new International Maritime Organization (IMO) regulation came into force in January 2015 to reduce the maximum allowed sulfur content in ships' fuel 10-fold in sulfur emission control areas such as the English Channel. Our observations suggest a 3-fold reduction in ship-emitted SO2 from 2014 to 2015. Apparent fuel sulfur content calculated from coincidental SO2 and carbon dioxide (CO2) peaks from local ship plumes show a high level of compliance to the IMO regulation (> 95 %) in both years (∼ 70 % of ships in 2014 were already emitting at levels below the 2015 cap). Dimethyl sulfide (DMS) is an important source of atmospheric SO2 even in this semi-polluted region. The relative contribution of DMS oxidation to the SO2 burden over the English Channel increased from about one-third in 2014 to about one-half in 2015 due to the reduction in ship sulfur emissions. Our diel analysis suggests that SO2 is removed from the marine atmospheric boundary layer in about half a day, with dry deposition to the ocean accounting for a quarter of the total loss.
Highlights
The trace gas sulfur dioxide (SO2) is important for atmospheric chemistry (Charlson and Rodhe, 1982) as a principal air pollutant
Important natural sources of SO2 include the atmospheric oxidation of dimethyl sulfide (DMS, which is formed by marine biota) and volcanic eruptions
We present 1.5 years of continuous atmospheric SO2 measurements from the Penlee Point Atmospheric Observatory (PPAO) in the English Channel, one of the busiest shipping lanes in the world
Summary
The trace gas sulfur dioxide (SO2) is important for atmospheric chemistry (Charlson and Rodhe, 1982) as a principal air pollutant (e.g., contributor to acid rain). Atmospheric oxidation of SO2 leads to sulfate aerosols, which influence the Earth’s radiative balance directly by scattering incoming radiation and indirectly by affecting cloud formation (Charlson et al, 1987). SO2 is removed from the lower atmosphere by dry deposition and oxidation in both the gas phase and the aqueous phase. The relatively slow gas phase oxidation of SO2 leads to sulfuric acid vapor, which usually condenses upon pre-existing aerosols but can nucleate to form new particles under specific conditions (e.g., Clarke et al, 1998). The much faster aqueous phase oxidation of SO2 takes place primarily in cloud water (e.g., Hegg, 1985; Yang et al, 2011b) and leads to particulate sulfate, which is removed from the atmosphere mainly by wet deposition
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