The Monterrey Metropolitan Area (MMA) in Northeast Mexico has shown high PM 2.5 concentrations since 2003. The data shows that the annual average concentration exceeds from 2 to 3 times the Mexican PM 2.5 annual air quality standard of 12 µg/m 3 . In a previous work we studied the chemical characterization of PM 2.5 in two sites of the MMA during the winter season. Among the most important components we found ammonium sulfate and nitrate, elemental and organic carbon, and crustal matter. In this work we present the results of a second chemical characterization study performed during the summer time and the application of the chemical mass balance (CMB) model to determine the source apportionment of air pollutants in the region. The chemical analysis results show that the chemical composition of PM 2.5 is similar in both sites and periods of the year. The results of the chemical analysis and the CMB model show that industrial, traffic, and combustion activities in the area are the major sources of primary PM 2.5 and precursor gases of secondary inorganic and organic aerosol (SO 2 , NOx, NH 3 , and volatile organic compounds [VOCs]). We also found that black carbon and organic carbon are important components of PM 2.5 in the MMA. These results are consistent with the MMA emission inventory that reports as major sources of particles and SO 2 a refinery and fuel combustion, as well as nitrogen oxides and ammonium from transportation and industrial activities in the MMA and ammonium form agricultural activities in the state. The results of this work are important to identify and support effective actions to reduce direct emissions of PM 2.5 and its precursor gases to improve air quality in the MMA.Implications: The Monterrey Metropolitan Area (MMA) has been classified as the most airpolluted area in Mexico by the World Health Organization (WHO). Effective actions need to be taken to control primary sources of PM 2.5 and its precursors, reducing health risks on the population exposed and their associated costs. The results of this study identify the main sources and their estimated contribution to PM 2.5 mass concentration, providing valuable information to the local environmental authorities to take decisions on PM 2.5 control strategies in the MMA.PAPER HISTORY

Environmental arsenic exposure is associated with increased risk of non-cancerous chronic diseases and a variety of cancers in humans. The aims of this study were to carry out for the first time a health risk assessment for two common arsenic exposure routes (drinking water and soil ingestion) in children living in the most important agricultural areas in the Yaqui and Mayo valleys in Sonora, Mexico. Drinking water sampling was conducted in the wells of 57 towns. A cross-sectional study was done in 306 children from 13 villages in the valleys. First morning void urine samples were analyzed for inorganic arsenic (InAs) and monomethyl and dimethyl arsenic (MMA and DMA) by HPLC/ICP-MS. The results showed a wide range of arsenic levels in drinking water between 2.7 and 98.7 μg As/L. Arsenic levels in agricultural and backyard soils were in the range of < 10-27 mg As/kg. The hazard index (HI) = ∑hazard quotient (HQ) for drinking water, agricultural soil, and backyard soil showed values > 1 in 100% of the study towns, and the carcinogenic risk (CR) was greater than 1E-04 in 85%. The average of arsenic excreted in urine was 31.7 μg As/L, and DMA had the highest proportion in urine, with averages of 77.8%, followed by InAs and MMA with 11.4 and 10.9%, respectively, percentages similar to those reported in the literature. Additionally, positive correlations between urinary arsenic levels and HI values were found (r = 0.59, P = 0.000). These results indicated that this population is at high risk of developing chronic diseases including cancer.

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