Characterization of Nonmethane Hydrocarbons at Three Urban Sites in Western Saudi Arabia, in Lahore (Pakistan), and in Singapore

Abstract:

Favorable conditions to tropospheric ozone (O₃) formation are present over the Persian Gulf Region. Ozone is a well known pollutant affecting human health and natural ecosystems. Among the several factors contributing to the formation of the O₃ hot spot over the Middle East, the presence of local emissions of its precursors needs to be considered. We report initial measurements of a suite of nonmethane hydrocarbons (NMHCs), an important component of volatile organic compounds (VOCs), which, coupled with nitrogen oxides in the presence of sunlight, are key chemical precursors of tropospheric O₃.

We measured 63 speciated C₂-C₁₀ NMHCs, in addition to methane (CH₄) and carbon monoxide (CO) in three cities of Saudi Arabia (Jeddah, Mecca, and Madina; October-November 2012 and April 2013) and in the city of Lahore (Pakistan; December 2012). To put these data into perspective, we compare our results to data collected in Singapore (August-November 2012). We observed enhanced levels in all three Saudi Arabian cities compared to the local background and to those measured in Singapore. However, the Saudi levels are much lower than those measured in Lahore, where the sum of quantified NMHCs is about six times higher. For Madina, enhanced levels of the alkenes, ethyne and CO indicated that vehicle exhaust was the dominant source. In Jeddah and Mecca, the most abundant NMHC were the alkanes (47-61% of total measured NMHCs), which are more closely associated with emissions from natural gas, liquid petroleum gas (LPG), and gasoline evaporation. In Lahore, the hydroxyl radical (OH) reactivity, used to evaluate the importance of the different measured species toward ozone production, is three to six times higher than for the Saudi cities, and more than 20 times higher than for Singapore. For all urban areas reported here, among the measured compounds, the alkenes (especially ethene and propene) dominate in terms of OH reactivity because of a combination of their great abundance and relatively fast reaction rates with the hydroxyl radical. In light of the ozone hot spot detected over the Persian/Arabia Gulf our study shows the importance of different classes of NMHCs, or individual species, to the OH reactivity. This information can help policy makers regulate emissions in an attempt to reduce O₃ formation by targeting key atmospheric pollutants.