Articles | Volume 16, issue 8
Research article
18 Apr 2016
Research article |  | 18 Apr 2016

Attribution of atmospheric sulfur dioxide over the English Channel to dimethyl sulfide and changing ship emissions

Mingxi Yang, Thomas G. Bell, Frances E. Hopkins, and Timothy J. Smyth

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.

Short summary
Exhausts from ships are an important source of air pollution in coastal regions. We observed a ~ 3 fold reduction in the level of sulfur dioxide (a principle pollutant) from the English Channel from 2014 to 2015 after the new International Maritime Organisation regulation on ship sulfur emission became law. Our estimated ship's fuel sulfur content shows a high degree of compliance. Dimethylsulfide from the marine biota becomes a relatively more important source of sulfur in coastal marine air.
Final-revised paper