Ozone precursors and boundary layer meteorology before and during a severe ozone episode in Mexico city.

Volatile organic compounds (VOCs) data in conjunction with other inorganic pollutants, surface meteorological data and continuous measurement of the Planetary Boundary Layer height (PBLH) at an urban site in Mexico City were performed from 6 to 18 March 2016. Positive Matrix Factorization (PMF) identified four emission source factors of VOCs along with equivalent black carbon (eBC), gaseous pollutants (CO, NO, NO2, SO2, NH3) and ions (Na+, Mg2+, Ca2+, NO3-, NH4+): (1) secondary aerosol precursors, (2) evaporation and non-LPG fuel combustion, (3) geogenic source and (4) vehicle exhaust. Propylene Equivalent and Maximum Incremental Reactivity (MIR) methods identified isoprene and ethylene as the highest oxidant and O3 forming species. Pollutant data normalized to the variation of the PBLH revealed continued production of O3 precursors in the afternoon beyond the typical morning rush hour. In particular this could be observed during the second part of the measurement period (12-15 March) when a strong O3 episode occurred under weak wind and lower PBLH conditions compared to the preceding period (6-11 March) when well mixed conditions due to elevated daytime PBLH and strong advection led to overall reduced pollutant mixing ratios in the afternoon hours.

Ultraviolet Radiation Environment of a Tropical Megacity in Transition: Mexico City 2000-2019.

Tropical regions experience naturally high levels of UV radiation, but urban pollution can reduce these levels substantially. We analyzed 20 years of measurements of the UV index (UVI) at several ground-level locations in the Mexico City Metropolitan Area and compared these data with the UVI values derived from the satellite observations of ozone and clouds (but not local pollution). The ground-based measurements were systematically lower than the satellite-based estimates by ca. 40% in 2000 and 25% in 2019. Calculations with a radiative transfer model using observed concentrations of air pollutants explained well the difference between satellite and ground-based UVI and showed specific contributions from aerosols, O3, NO2, and SO2 in decreasing order of importance. Such large changes in UV radiation between 2000 and 2019 have important implications ranging from human health (skin cancer and cataract induction) to air pollution control (photochemical smog formation).

Historical trends of metals concentration in PM10 collected in the Mexico City metropolitan area between 2004 and 2014.

In this work, we report metals concentrations in 80 PM10 samples collected at four sites in the Mexico City Metropolitan Area (MCMA): Tlalnepantla (NE), Xalostoc (NE), Merced (C), and Pedregal (S), during the dry/cold season (October to January) for the 2004-2014 period. Mean PM10 mass concentration (66.1 µg m-3) significantly exceeds the annual mean air quality guidelines recommended by the World Health Organization. The statistical analysis of concentration data and meteorological parameters allows us to recognize the importance of wind intensity speed (Wsp) and wind direction conditions in the enrichment of PM concentrations. The proximity and magnitude of the emitting source is also relevant for PM concentration. Such conditions favored that higher metal concentration was recognized at the north of the studied area. By means principal component analysis (PCA) was difficult to identify the groups of metals associated with specific sources (anthropogenic and geogenic) given the high complexity of the study area and the long period of time evaluated. Metal concentration trend shows an important positive trend for Pt, V and Cr, while PM10, Ni, Cu, Ag and Sb show a trend of moderate increase. In contrast, Pb and Co registered a strong percentage reduction, while Hg, Mn, As and Cd show a slight reduction, probably resulting from the implementation of regulatory measures and influenced by urban changes associated at the north of the studied area. The results of this research provide information that should be considered for evaluating the impact of anthropogenic sources and applying regulatory measures to control emissions.

Spatial and temporal distribution of metals in PM2.5 during 2013: assessment of wind patterns to the impacts of geogenic and anthropogenic sources.

The Mexico City Metropolitan Area (MCMA) was the object of a chemical elemental characterization (Ti, V, Cr, Mn, Co, Ni, Cu, Mo, Ag, Cd, Sb, Pb, La, Sm, Ce, and Eu) of PM2.5 collected during 2013 and analyzed by inductively coupled plasma mass spectrometry (ICP-MS). Sampling campaigns were carried out at five locations simultaneously-northwest, northeast, center, southwest, and southeast-during dry-warm season (April), rainy season (August), and dry-cold season (November). By means of enrichment factor (EF) and principal component analysis (PCA), it was possible to attribute the analyzed elements to geogenic and anthropogenic sources, as well as to identify a group of elements with mixed provenance sources. The highest concentrations for most metals were found in northwest and northeast, and during dry-warm (DW), confirming the trend observed in PM2.5 samples collected in 2011. Despite similarities between 2011 and 2013, an increase of 17% in PM2.5 mass concentration was observed, mainly attributable to geogenic sources, whereby the importance of wind intensity to the impact of emission sources is highlighted. The effect of wind intensity was revealed, by means of polar plots, as the controlling mechanism for this increase. This allowed us to conclude that high-speed episodes (5 m s-1) were responsible for raising geogenic metal concentrations rather than wind direction.

Estimating particle exposure in the Mexico City metropolitan area.

STUDY QUESTION: We examined whether methods for measuring exposure to airborne particles less than 10 microm in aerodynamic diameter (PM10) in the Mexico City metropolitan area give different estimates of PM10 levels, and the nature of these differences, and developed a model for estimating missing PM10 data for one measurement method. METHODS: Government PM10 measurements using two different technologies at five sites (the Sierra-Anderson PM10 High-Volume Air Sampler System, Hi-Vol) (every sixth day) and the Rupprecht and Patashnik Tapered Element Oscillating Microbalance (TEOM) monitor (daily), as well as Harvard Impactor (HI) data collected for research purposes from one monitoring station were matched by day and monitoring site, then compared visually and with basic descriptive statistics. We fit linear regression models with airport visual range measurements, meteorological data, and information on other air pollutants to predict the Hi-Vol measured PM10 levels for those days when direct measurements were not available. RESULTS: We found relatively low correlations (r ranging from 0.46 to 0.63) between PM10 measured with the TEOM and Hi-Vol methods, and highly variable differences (0-70 microg/m(3)) between the means of these measurements, depending on monitoring site. The HI measurements had a relatively high correlation with the Hi-Vol measurements (r=0.90). The models developed for the missing Hi-Vol measurements provided a series of estimated values similar to the actual Hi-Vol measurements, although the estimated series did not have high values in the range observed in the measured data. CONCLUSIONS: The differences we observed in the PM10 measurements across methods in Mexico City may be important when studying health effects associated with particle exposure, evaluating method performance under conditions and operating protocols similar to those in Mexico City, and determining compliance with air quality standards. The estimated series of PM10 measures may be a useful index of exposure for use in studies of the effects of air pollution on health.