Comparison of measurements of peroxyacyl nitrates and primary carbonaceous aerosol concentrations in Mexico City determined in 1997 and 2003

摘要

The concentrations of peroxyacetyl nitrate (PAN) in ambient air can be agood indicator of air quality and the effectiveness of control strategiesfor reducing ozone levels in urban areas. As PAN is formed by the oxidationof reactive hydrocarbons in the presence of nitrogen dioxide (NO₂), itis a direct measure of the peroxyacyl radical levels produced from reactiveorganic emissions in the urban air shed. Carbon soot, known as black carbon(BC) or elemental carbon (EC), is a primary atmospheric aerosol species andis a good indicator of the levels of combustion emissions, particularly fromdiesel engines, in major cities. Mexico City is the second largest megacityin the world and has long suffered from poor air quality. Reported here areatmospheric measurements of PAN and BC obtained in Mexico City during theMexico Megacity 2003 field study. These results are compared withmeasurements obtained earlier during the Investigación sobreMateria Particulada y Deterioro Atmosférico – Aerosol and VisibilityResearch (IMADA-AVER) campaign in 1997 to obtain an estimate of the changesin emissions in Mexico City and the effectiveness of control strategiesadopted during that time. Concentrations of PAN in 1997 reached a maximum of34 ppb with an average daily maximum of 15 ppb. The PAN levels recorded in2003 were quite different, with an average daily maximum of 3 ppb. Thisdramatic reduction in PAN levels observed in 2003 indicate that reactivehydrocarbon emissions have been reduced in the city due to controls onolefins in liquefied petroleum gas (LPG) and also due to the significantnumber of newer vehicles with catalytic converters that have replaced olderhigher emission vehicles. In contrast, black/elemental carbon levels weresimilar in 1997 and 2003 indicating little improvement likely due to thelack of controls on diesel vehicles in the city. Thus, while air quality andozone production have improved, Mexico City and other megacities continue tobe a major source of black carbon aerosols, which can be an importantspecies in determining regional radiative balance and climate.