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The air quality of megacities can be influenced by external emissionsources on both global and regional scales. At the same time their outflowemissions can exert an impact to the surrounding environment. The presentstudy evaluates an SO2 peak observed on 24 March 2006 at the suburbansupersite T1 and at ambient air quality monitoring stations located in thenorthern region of the Mexico City Metropolitan Area (MCMA) during theMegacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. We found that this peak could be related to an importantepisodic emission event coming from Tizayuca region, northeast of the MCMA.Back-trajectory analyses suggest that the emission event started in theearly morning at 04:00 LST and lasted for about 9 h. The estimated emissionrate is about 2 kg s−1. To the best of our knowledge, sulfur dioxideemissions from the Tizayuca region have not been considered in previous studies.This finding suggests the possibility of "overlooked" emission sources inthis region that could influence the air quality of the MCMA. This furthermotivated us to study the cement plants, including those in the state ofHidalgo and in the State of Mexico. It was found that they can contribute to the SO2 levels inthe northeast (NE) region of the basin (about 42%), at the suburbansupersite T1 (41%) and that at some monitoring stations theircontribution can be even higher than the contribution from the TulaIndustrial Complex (TIC). The contribution of the Tula Industrial Complex to regional ozone levels isestimated. The model suggests low contribution to the MCMA (1 to 4 ppb) andslightly higher contribution at the suburban T1 (6 ppb) and rural T2(5 ppb) supersites. However, the contribution could be as high as 10 ppb inthe upper northwest region of the basin and in the southwest andsouth-southeast regions of the state of Hidalgo. In addition, the resultsindicated that the ozone plume could also be transported to northwestTlaxcala, eastern Hidalgo, and farther northeast of the State of Mexico, butwith rather low values. A first estimate of the potential contribution fromflaring activities to regional ozone levels is presented. Results suggestthat up to 30% of the total regional ozone from TIC could be related toflaring activities. Finally, the influence on SO2 levels from technological changes in theexisting refinery is briefly discussed. These changes are due to theupcoming construction of a new refinery in Tula. The combination of emissionreductions in the power plant, the refinery and in local sources in the MCMAcould result in higher reductions on the average SO2 concentration.Reductions in external sources tend to affect more the northern part of thebasin (−16 to −46%), while reductions of urban sources in themegacity tend to diminish SO2 levels substantially in the central,southwest, and southeast regions (−31 to −50%)

atmospheric chemistry and physics

Impact of external industrial sources on the regional and local SO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; levels of the Mexico megacity