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Retama A.,Direccion de Monitoreo Atmosferico | Baumgardner D.,Droplet Measurement Technologies Inc. | Raga G.B.,National Autonomous University of Mexico | McMeeking G.R.,Droplet Measurement Technologies Inc. | Walker J.W.,Droplet Measurement Technologies Inc.
Atmospheric Chemistry and Physics

The Mexico City metropolitan area (MCMA) is a region that continues to grow in population and vehicular traffic as well as being the largest source of short-lived climate pollutants (SLCP) in Latin America. The local city government has made significant progress in controlling some of these pollutants, i.e., ozone (O3) and carbon monoxide (CO), but particulate matter (PM2.5 and PM10) and black carbon (BC) have shown a less positive response to mitigation strategies that have been in place for almost 3 decades. For the first time, extended measurements of equivalent black carbon (eBC), derived from light absorption measurements, have been made using a Photoacoustic Extinctiometer (PAX) over a 13 month period from March 2013 through March 2014. The daily trends in workdays (Monday through Saturday) and Sunday eBC, PM2.5 and the co-pollutants CO, O3 and NOx are evaluated with respect to the three primary seasons in the MCMA: rainy, cold and dry and warm and dry. The maximum values in all of the particle and gas concentrations were significantly larger (Student's t test, P < 0.05) during the dry periods than in the rainy season. The changes from rainy to dry seasons for eBC, PM2.5, CO, O3 and NOx were 8.8 to 13.1 μg m-3 (40 %), 49 to 73 μg m-3 (40 %), 2.5 to 3.8 ppm (40 %), 73 to 100 ppb (30 %) and 144 to 252 ppb (53 %), respectively. The primary factors that lead to these large changes between the wet and dry seasons are the accelerated vertical mixing of boundary layer and free tropospheric air by the formation of clouds that dilutes the concentration of the SLCPs, the decreased actinic flux that reduces the production of ozone by photochemical reactions and the heavy, almost daily rain that removes particulate matter. A significant "weekend effect" was also identified, particularly the decrease in BC due to fewer large transport vehicles that are fueled by diesel, which produces a large fraction of the BC. The other co-pollutant concentrations are also significantly less on weekends except for O3 that shows no change in maximum values from workdays to Sundays. This lack of change is a result of the balancing effects of lower precursor gases, i.e., VOCs, offset by lower concentrations of NOx, that is an O3 inhibitor. A comparison of the average maximum value of eBC measured during the 1 year period of the current study, with maximum values measured in shorter field campaigns in 2000 and 2006, shows no significant change in the eBC emissions over a 14 year period. This suggests that new methods may need to be developed that can decrease potentially toxic levels of this particulate pollutant. © Author(s) 2015. Source

Moya M.,Instituto Nacional Of Enfermedades Respiratorias Ismael Cosio Villegas | Moya M.,National Autonomous University of Mexico | Madronich S.,U.S. National Center for Atmospheric Research | Retama A.,Direccion de Monitoreo Atmosferico | And 5 more authors.
Atmospheric Environment

As part of the MIRAGE (MILAGRO) study conducted 7-30 March 2006 in Mexico City and its Metropolitan Area (MCMA), fine particulate matter (PM2.5) was collected using two Tapered Element Oscillating Microbalance (TEOM) systems, and a Partisol instrument at the T1 super-site (Tecamac, State of Mexico). Inorganic analysis was performed on filter-based (PM1, PM2.5-URG) measurements also collected at this site. Measurements from the gravimetric (TEOMs, Partisol) and URG systems were inter-compared with chemical speciation measurements using a Particle Into Liquid Sampler (PILS) and Thermal Optical methods.Mass and chemical balances applied over the first part of the study (11-22 March) showed that a TEOM using a device (SES) which reduces particle-bound water and retains a fraction of semi-volatile compounds (SVM) gives readings ∼30% larger than a conventional TEOM. In the second part of the study (26-30 March), the loss of SVM during TEOM-heated filter collection (both systems) represented a significant fraction of PM2.5 mass due to changes in particle composition. Overall, when nonvolatile nitrate dominated (i.e., when associated with crustal species and not NH4 +) and/or sulfate dominates (SO4 2-/NO3 - molar ratio is >1), PM2.5 mass readings are in agreement with those reported for the T1 site if TEOM is using a SES device. However, when volatile nitrate dominates (i.e., NH4NO3) or SO4 2-/NO3 - molar ratio is <1, a larger fraction is lost from both TEOMs (with or without the SES device). Under the latter regime, uncertainties are large and gravimetric losses may reach 30%-50%. The gravimetric PARTISOL instrument recorded lower readings under all of the aforementioned conditions with differences versus TEOMs decreasing with increasing RH. These findings call for a careful characterization of such volatilization biases to improve current PM (PM10, PM2.5) measurements/networks, especially in alkaline-rich environments that can favor such biases.With regards to PM1 and PM2.5 filter-based measurements, findings are: 1) crustal-related elements are important features in the PM2.5-1 size fraction; 2) a factor of ∼2 overestimation of SO4 2- concentrations is recorded on substrates during PM collection and 3) main elements of a typical urban aerosol size distribution are concentrated in the 1μm (versus 2.5μm) size fraction. © 2010 Elsevier Ltd. Source

Jaimes P.M.,National Autonomous University of Mexico | Bravo A.H.,National Autonomous University of Mexico | Sosa E.R.,National Autonomous University of Mexico | Sanchez P.A.,National Autonomous University of Mexico | Retama A.H.,Direccion de Monitoreo Atmosferico
Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA

Continuous monitoring of VOC and NO x in the Mexico City Metropolitan Area (MCMA) is critical to evaluate the ozone generation. These measurements will provide important information for the evaluation and management of the air quality, and improve proposed control strategies, to minimize serious ozone problem in the MCMA. Up to date, there is no continuous monitoring of VOC that allows to characterize the formation of ozone and the factors influencing their production in the MCMA. The site-selection for the location of the monitors is one of the most important tasks of the air-monitoring network's design, since it selects the most representative area to monitored for the air quality conditions; for the implementation of the monitoring of precursors of ozone in the metropolitan area; the experience that other cities have, will be very useful. The selection of monitoring photochemical precursor sites at Mexico City was discussed. This is an abstract of a paper presented at the 104th AWMA Annual Conference and Exhibition 2011 (Orlando, FL 6/21-24/2011). Source

Salcedo D.,Autonomous University of the State of Morelos | Castro T.,National Autonomous University of Mexico | Ruiz-Suarez L.G.,National Autonomous University of Mexico | Garcia-Reynoso A.,National Autonomous University of Mexico | And 14 more authors.
Science of the Total Environment

Results from the first study of the regional air quality in Morelos state (located south of Mexico City) are presented. Criteria pollutants concentrations were measured at several sites within Morelos in February and March of 2007 and 2009; meteorological data was also collected along the state for the same time periods; additionally, a coupled meteorology-chemistry model (Mesoscale Climate Chemistry Model, MCCM) was used to gain understanding on the atmospheric processes occurring in the region. In general, concentrations of almost all the monitored pollutants (O3, NOx, CO, SO2, PM) remained below the Mexican air quality standards during the campaign; however, relatively high concentrations of ozone (8-hour average concentrations above the 60ppb level several times during the campaigns, i.e. exceeding the World Health Organization and the European Union maximum levels) were observed even at sites with very low reported local emissions. In fact, there is evidence that a large percentage of Morelos vegetation was probably exposed to unhealthy ozone levels (estimated AOT40 levels above the 3ppmh critical limit). The MCCM qualitatively reproduced ozone daily variations in the sites with an urban component; though it consistently overestimated the ozone concentration in all the sites in Morelos. This is probably because the lack of an updated and detailed emission inventory for the state. The main wind patterns in the region corresponded to the mountain-valley system (downslope flows at night and during the first hours of the day, and upslope flows in the afternoon). At times, Morelos was affected by emissions from surrounding states (Distrito Federal or Puebla). The results are indicative of an efficient transport of ozone and its precursors at a regional level. They also suggest that the state is divided in two atmospheric basins by the Sierras de Tepoztlán, Texcal and Monte Negro. © 2011 Elsevier B.V. Source

Jaimes P.M.,National Autonomous University of Mexico | Bravo A.H.,National Autonomous University of Mexico | Sosa E.R.,National Autonomous University of Mexico | Cureno G.I.,National Autonomous University of Mexico | And 3 more authors.
Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA

The trends of surface O3 concentration in Mexico City metropolitan area were evaluated. Four statistical measures using five monitoring stations were considered for the data trend analysis, i.e., the hourly concentration average, the daily maximum of the 8-hr average, the number of hours, and days above the Mexican Air Quality Standard for O3 (110 ppb). A generalized additive modeling approach was used to assess trends. All statistical measures indicated a negative tendency during the first 15 yr of the study period with a more moderate rate of decrease during the last 5 yr. However, the daily maximum of the 8-hr average indicated a positive tendency during 2011 in those stations located to the north and center of Mexico City. Despite the downward trend and management programs that were implemented, residents of the MCMA still breathe unhealthy air and their health risk increase because the air quality standard for O3 has not yet been complied. This means that it is important to develop and implement new and better control strategies that take into account the complexity of O3 formation at its sources. This is an abstract of a paper presented at the 105th AWMA Annual Conference and Exhibition (San Antonio, TX 6/19-22/2012). Source

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