The most important of these are sulfur dioxide (SO2), nitrogen oxides (NOx) and hydrocarbons or volatile organic compounds (VOCs). Sulfur dioxide and nitrogen oxides contribute to the acidification of the atmosphere and soil, hydrocarbons and nitrogen oxides contribute to smog formation.
The SO2 and NOx emissions at ExxonMobil's plants are far below legal standards, because they only use gas for their heating needs. VOC emissions are low, thanks to the use of special seals or internal floating storage tank covers, vapor recovery when loading barges and trucks, and special measuring and maintenance programs to detect and repair leaks.
In order to improve the air quality in its member states, the European Union has drawn up a series of new environmental regulations. The desulfurization requirements are thus becoming increasingly stringent. As of 2009, diesel may not contain more than 10 ppm (parts per million) of sulfur. That's why ExxonMobil built a new [[LINK (/Benelux-Dutch/Newsroom/Publications/20070115_ReflexHTM_NL/heatingoilupgradingproject.htm):desulfurization plant]] in Rotterdam.