Santiago authorities need to control LPG in order to improve air quality

By controlling the leakage and alkene content of liquid petroleum gas (LPG), Santiago authorities would be able to significantly reduce ozone levels in the polluted air above the Chilean city, says new research.

According to a study by researchers at the University of California, in the Chilean capital of Santiago approximately 5% of unburnt LPG, which is used extensively in the city for domestic heating and cooking, leaks into the atmosphere, contributing as much as 15% of the city’s ozone pollution. According to the study, the impact of this leakage needs to be recognised by the city authorities in order for reductions of nonmethane hydrocarbons (NMHC’s) – an important precursor of ozone – to succeed.

Santiago is home to five million people, accounting for 40% of the country’s population, as well as 800,000 vehicles, and 70% of the nation’s industries. Combined with geographical and meteorological factors, the high concentration of people and industry has made Santiago one of Latin America’s most polluted cities, with ozone concentrations exceeding the one hour quality standard of 80 parts per billion throughout the year, and particularly in the summer. However, programmes are already in place in the city to reduce industrial and vehicle pollution, as well as a conversion from coal and oil to natural gas.

Improvements in both local and regional air quality can be made by minimising LPG leakage, such as through practical tasks such as the periodical checking and replacing of o-rings and seals, say the researchers.

According to the study, a further complement to the city authorities’ attempts to cut down on pollution would be to change the composition of LPG to remove the more reactive components. The C3-C4 alkanes from LPG (propane and butane) contribute approximately 10% to the ozone formation in Santiago, with alkenes furnishing another 5%. However, whereas propane and butane make up 74% and 22% respectively of the average LPG formulation, the alkenes comprise less than 3%, making them very effective in their contribution to ozone production.

The researchers report that a similar situation also exists in Mexico City, with regards to pollution resulting from LPG, although, in this city, the LPG formulation is higher in the more reactive butanes and alkenes, and lower in the less reactive propane. Here, the situation has lead to legislation limiting the alkene content of LPG. In the city of Los Angeles, on the other hand, the cumulative ozone productivity is roughly half that of Mexico City due to a low alkene and high propane content of LPG.

The paper is published in the June issue of the American Geophysical Union’s journal, Geophysical Research Letters.

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