Measurements of Criteria Pollutants in Suburban Locations in the Mexico City Metropolitan Area

In the town of Santa Ana in the southern part of the Mexico City Metropolitan Area (MCMA) at an approximate height of 370 meters above the Valley of Mexico, a series of criteria pollutants (nitrogen dioxide, ozone, sulfur dioxide, and carbon monoxide) and meteorological parameters (wind speed and direction, temperature and relative humidity) were measured during the MCMA-2003 field campaign during April 2003. Santa Ana is considered a border site, agriculture being the predominant activity. The generated data in Santa Ana was compared with those from two representative atmospheric monitoring stations from the southeast zone (CENICA Supersite) and southwest (Revolucion Station) of Mexico City. The carbon monoxide and ozone concentrations measured in Santa Ana are atypical for a rural area. The sulfur dioxide and nitrogen dioxide concentrations are characteristic of the local activity. The average obtained during the measurement time for carbon monoxide was 0.86 ppm, the presence of the contaminant was within a 0.3 to 1.5 ppm range, 5 to 95 percentile, and does not follow the observed behavior of the monitoring stations that are located inside the urban area, although during the night (10 pm to 6 am) the carbon monoxide in Santa Ana was preset in levels from 0.4 to 1.5 ppm, 5 to 95 percentiles, average 0.92 ppm, above the reported concentrations at Revolucion Station, levels from 0.1 to 1.7 ppm, 5 to 95 percentiles; average 0.72 ppm. The presence of this contaminant can be attributed to transportation and accumulation phenomena. The ozone daytime behavior is similar to the one observed in the CENICA and Revolucion stations, but with a lower magnitude, the daily maximum generally occuring 2 or 3 hours after the urban stations. The one-hour average maximum values were 133 ppb in Santa Ana area and 188 ppb in the city. During the night the average concentrations were 37 ppb in Santa Ana and 17 ppb in the urban area. Nevertheless, the ozone average value in Santa Ana during the study was 49 ppb, slightly higher than the one obtained in the urban area, which was 44 ppb. The presence of this contaminant can be possibly attributed to the transportation and accumulation factors as well as to the low reactivity due to the absence of NOx, as explained in Dommen Josef and Prévôt, S.H. André, 2002, Characterization of the photooxidant formation in the metropolitan area of Milan from aircraft measurements, Journal of Geophysical Research, 107 (D22): 8197, doi: 10.1029/2000JD000283.