Air pollution monitoring with two optical remote sensing techniques in Mexico City

An open-path Fourier Transform Infrared (FTIR) and a Differential Optical Absorption Spectrometer (DOAS) were installed and simultaneously operated along a 426 m optical path in downtown Mexico City. 03 and SO, were measured by both optical remote sensing techniques and the results from the comparison are presented. The instruments presented comparable sensitivities for O-3 and an excellent agreement (R-2 > 0.99) in their correlation. Although the sensitivity of the infrared technique for SO2 was limited to concentrations > 20 ppb or so, the agreement of the FTIR response with the more sensitive DOAS technique during the high levels of this pollutant was favorable (R-2 = 0.94) and accurate to within experimental error. These episodes (> 100 ppb) were found to occur several times per month. Benzene and toluene were measured by the DOAS technique and their concentrations are reported for a 3-month period during 11/2 - 12/5, 2003. The mean and highest concentration registered for benzene was 5.1 and 18.7 ppb, respectively, with an average of daily maxima at 11.5 ppb. Toluene's highest concentration during this period reached 97.3 ppb, with a mean and daily maximum average of 13.4 and 41.7 ppb,, respectively. A benzene/toluene ratio of 2.6 was determined for the entire period of study and a decrease of similar to20% in the daily ambient concentration of these aromatic hydrocarbons was observed on Sundays relative to weekdays.