G. Sosa

Particulate Air Pollution in Mexico City: A Collaborative Research Project

PM10, PM25, precursor gas, and upper-air meteorological measurements were taken in Mexico City, Mexico, from February 23 to March 22, 1997, to understand concentrations and chemical compositions of the citys particulate matter (PM). Average 24-hr PM10 concentrations over the period of study at the core sites in the city were 75 H g/m3. The 24-hr standard of 150 μ g/m3 was exceeded for seven samples taken during the study period; the maximum 24-hr concentration measured was 542 μ g/m3. Nearly half of the PM10 was composed of fugitive dust from roadways, construction, and bare land. About 50% of the PM10 consisted of PM2.5, with higher percentages during the morning hours. Organic and black carbon constituted up to half of the PM2.5. PM concentrations were highest during the early morning and after sunset, when the mixed layers were shallow. Meteorological measurements taken during the field campaign show that on most days air was transported out of the Mexico City basin during the afternoon with little day-to-day carryover.

Development and testing of meteorology and air dispersion models for Mexico City

Abstract Los Alamos National Laboratory and Instituto Mexicano del Petroleo are completing a joint study of options for improving air quality in Mexico City. We have modified a three-dimensional, prognostic, higher-order turbulence model for atmospheric circulation (HOTMAC) and a Monte Carlo dispersion and transport model (RAPTAD) to treat domains that include an urbanized area. We used the meteorological model to drive models which describe the photochemistry and air transport and dispersion. The photochemistry modeling is described in a separate paper. We tested the model against routine measurements and those of a major field program. During the field program, measurements included: (1) lidar measurements of aerosol transport and dispersion, (2) aircraft measurements of winds, turbulence, and chemical species aloft, (3) aircraft measurements of skin temperatures, and (4) Tethersonde measurements of winds and ozone. We modified the meteorological model to include provisions for time-varying synoptic-scale winds, adjustments for local wind effects, and detailed surface-coverage descriptions. We developed a new method to define mixing-layer heights based on model outputs. The meteorology and dispersion models were able to provide reasonable representations of the measurements and to define the sources of some of the major uncertainties in the model-measurement comparisons.

Project aguila: In situ measurements of Mexico City air pollution by a research aircraft

Abstract Measurements of aerosol concentrations, chemical species and meteorological quantities in the air above Mexico City were obtained from an instrumented research aircraft. Concentrations of particles in the size range between 0.12 and 3.12 μm were nearly invariant with height, and typical values were of the order of 5000 cm−3. However, particles smaller than 0.12 μm were confined to the lowest few hundred meters of the atmosphere until the morning temperature inversion dissipated, after which time those particles, together with newly formed particles created by secondary processes, mixed to a greater height above the city. Total particle concentrations near the surface attained values in excess of 60,000 cm−3. An examination of the corresponding profiles of SO2 suggests that combustion processes are likely sources for the additional small particles.

Fine and coarse particulate matter chemical characterization in a heavily industrialized city in central Mexico during Winter 2003.

Abstract This paper presents the results of the first reported study on fine particulate matter (PM) chemical composition at Salamanca, a highly industrialized urban area of Central Mexico. Samples were collected at six sites within the urban area during February and March 2003. Several trace elements, organic carbon (OC), elemental carbon (EC), and six ions were analyzed to characterize aerosols. Average concentrations of PM with aerodynamic diameter of less than 10 μm (PM10) and fine PM with aerodynamic diameter of less than 2.5 μm (PM2.5) ranged from 32.2 to 76.6 μg m-3 and 11.1 to 23.7 μg m-3, respectively. OC (34%), SO4 = (25.1%), EC (12.9%), and geological material (12.5%) were the major components of PM2.5. For PM10, geological material (57.9%), OC (17.3%), and SO4 = (9.7%) were the major components. Coarse fraction (PM10 –PM2.5), geological material (81.7%), and OC (8.6%) were the dominant species, which amounted to 90.4%. Correlation analysis showed that sulfate in PM2.5 was present as ammonium sulfate. Sulfate showed a significant spatial variation with higher concentrations to the north resulting from predominantly southwesterly winds above the surface layer and by major SO2 sources that include a power plant and refinery. At the urban site of Cruz Roja it was observed that PM2.5 mass concentrations were similar to the submicron fraction concentrations. Furthermore, the correlation between EC in PM2.5 and EC measured from an aethalometer was r2 = 0.710. Temporal variations of SO2 and nitrogen oxide were observed during a day when the maximum concentration of PM2.5 was measured, which was associated with emissions from the nearby refinery and power plant. From cascade impactor measurements, the three measured modes of airborne particles corresponded with diameters of 0.32, 1.8, and 5.6 μm.

Air Pollution Science in the MCMA: Understanding Source-Receptor Relationships through Emissions Inventories, Measurements, and Modeling

Throughout the world, air pollution science has been important in defining and characterizing the air pollution problem and the health risks it imposes. Air pollution by sulfur dioxide, nitrogen oxides, carbon monoxide, hydrocarbons, ozone, and particulates has been linked unequivocally to anthropogenic emissions. Air pollution science has also played a central role in evaluating the extent to which various control actions might reduce ambient exposures, despite the fact that uncertainties in relating emissions at sources and concentrations at receptors are often significant. In the 1980s, for example, modeling studies of ozone formation in the US suggested that controls on VOC (volatile organic compound) emissions would be sufficient to reduce concentrations of ozone. Because VOC controls were thought to be inexpensive, they were emphasized at that time(NRC, 1991).

Soot and SO 2 contribution to the supersites in the MILAGRO campaign from elevated flares in the Tula Refinery

This work presents a simulation of the plume tra- jectory emitted by flaring activities of the Miguel Hidalgo Refinery in Mexico. The flame of a representative sour gas flare is modeled with a CFD combustion code in order to estimate emission rates of combustion by-products of inter- est for air quality: acetylene, ethylene, nitrogen oxides, car- bon monoxide, soot and sulfur dioxide. The emission rates of NO2 and SO2 were compared with measurements ob- tained at Tula as part of MILAGRO field campaign. The rates of soot, VOCs and CO emissions were compared with es- timates obtained by Instituto Mexicano del Petr ´ oleo (IMP). The emission rates of these species were further included in WRF-Chem model to simulate the chemical transport of the plume from 22 to 27 March of 2006. The model presents re- liable performance of the resolved meteorology, with respect to the Mean Absolute Error (MAE), Root Mean Square Er- ror (RMSE), mean bias (BIAS), vector RMSE and Index of Agreement (IOA). WRF-Chem outputs of SO2and soot were compared with surface measurements obtained at the three supersites of MI- LAGRO campaign. The results suggest a contribution of Tula flaring activities to the total SO 2 levels of 18 % to 27 % at the urban supersite (T0), and of 10 % to 18 % at the suburban su- persite (T1). For soot, the model predicts low contribution at the three supersites, with less than 0.1 % at three supersites. According to the model, the greatest contribution of both pol- lutants to the three supersites occurred on 23 March, which coincides with the third cold surge event reported during the campaign.

Modeling inorganic aerosols and their response to changes in precursor concentration in Mexico City.

Abstract Based on data from the 1997 Investigación sobre Materia Particulada y Deterioro Atmosférico-Aerosol and Visibility Evaluation Research (IMADA-EVER) campaign and the inorganic aerosol model ISORROPIA, the response of inorganic aerosols to changes in precursor concentrations was calculated. The aerosol behavior is dominated by the abundance of ammonia and thus, changes in ammonia concentration are expected to have a small effect on particle concentrations. Changes in sulfate and nitrate are expected to lead to proportional reductions in inorganic fine particulate matter (PM2.5). Comparing the predictions of ISORROPIA with the observations, the lowest bias and error are achieved when the aerosols are assumed to be in the efflorescence branch. Including crustal species reduces the bias and error for nitrate but does not improve overall model performance. The estimated response of inorganic PM2.5 to changes in precursor concentrations is affected by the inclusion of crustal species in some cases, although average responses are comparable with and without crustal species. Observed concentrations of particle chloride suggest that gas phase concentrations of hydrogen chloride may not be negligible, and future measurement campaigns should include observations to test this hypothesis. Our ability to model aerosol behavior in Mexico City and, thus, design control strategies, is constrained primarily by a lack of observations of gas phase precursors. Future campaigns should focus in particular on better understanding the temporal and spatial distribution of ammonia concentrations. In addition, gas phase observations of nitric acid are needed, and a measure of particle water content will allow stable versus metastable aerosol behavior to be distinguished.

Mexico City and the biogeochemistry of global urbanization

Mexico City is far advanced in its urban evolution, and cities in currently developing nations may soon follow a similar course. This paper investigates the strengths and weaknesses of infrastructures for the emerging megacities. The major driving force for infrastructure change in Mexico City is concern over air quality. Air chemistry data from recent field campaigns have been used to calculate fluxes in the atmosphere of the Valley of Mexico, for compounds that are important to biogeochemistry including methane (CH4), carbon monoxide (CO), nonmethane hydrocarbons (NMHCs), ammonia (NH3), sulfur dioxide (SO2), nitrogen oxides (NOx and NOy), soot, and dust. Leakage of liquified petroleum gas approached 10% during sampling periods, and automotive pollutant sources in Mexico City were found to match those in developed cities, despite a lower vehicle-to-person ratio of 0.1. Ammonia is released primarily from residential areas, at levels sufficient to titrate pollutant acids into particles across the entire basin. Enhancements of reduced nitrogen and hydrocarbons in the vapor phase skew the distribution of NOy species towards lower average deposition velocities. Partly as a result, downwind nutrient deposition occurs on a similar scale as nitrogen fixation across Central America, and augments marine nitrate upwelling. Dust suspension from unpaved roads and from the bed of Lake Texcoco was found to be comparable to that occurring on the periphery of the Sahara, Arabian, and Gobi deserts. In addition, sodium chloride (NaCl) in the dust may support heterogeneous chlorine oxide (ClOx) chemistry. The insights from our Mexico City analysis have been tentatively applied to the upcoming urbanization of Asia.

Air Pollutant Characterization in Tula Industrial Corridor, Central Mexico, during the MILAGRO Study

Pollutant emissions and their contribution to local and regional air quality at the industrial area of Tula were studied during a four-week period as part of the MILAGRO initiative. A recurrent shallow stable layer was observed in the morning favoring air pollutants accumulation in the lower 100 m atmospheric layer. In the afternoon the mixing layer height reached 3000 m, along with a featuring low level jet which was responsible of transporting air pollutants at regional scales. Average PM10 at Jasso (JAS) and Tepeji (TEP) was 75.1 and 36.8 μg/m3, respectively while average PM2.5 was 31.0 and 25.7 μg/m3. JAS was highly impacted by local limestone dust, while TEP was a receptor of major sources of combustion emissions with 70% of the PM10 constituted by PM2.5. Average hourly aerosol light absorption was 22 Mm−1, while aerosol scattering (76 Mm−1) was higher compared to a rural site but much lower than at Mexico City. δ 13C values in the epiphyte Tillandsia recurvata show that the emission plume directly affects the SW sector of Mezquital Valley and is then constrained by a mountain range preventing its dispersion. Air pollutants may exacerbate acute and chronic adverse health effects in this region.