Measurements of peroxyacyl nitrates (PANS) in Mexico City: implications for megacity air quality impacts on regional scales

Peroxyacyl nitrates (PANs) were measured using gas chromatography with elec tron capture detection (GC/ECD) in north central Mexico City during Februar y-March of 1997. Peroxyacetyl nitrate (PAN) was observed to exceed 30 ppb d uring five days of the study, with peroxypropionyl nitrate (PPN) and peroxy butryl nitrate (PBN) reaching 6 and 1 ppb maximum, respectively. Levels of total PANs typically exceeded 10 ppb during the period of measurement and s howed a very strong diurnal variation with PANs maximum during the early af ternoon and falling to less than 0.1 ppb during the evening hours. These le vels of PANs are the highest reported values in North America (and the worl d) for an urban center, since levels of approximately 30 ppb were reported during the late 1970s in the Los Angeles area (South Coast Air Basin, Tuazo n et al., 1978), Hydrocarbon measurements indicate that the levels of olefi ns, specifically butenes are significant in Mexico City. A time series take n of source indicator hydrocarbons taken before and during a Mexican Nation al Holiday with reduced automobile traffic clearly show that mobile sources of butenes are as important as liquefied petroleum gas. Observations of 10 -40 ppb C methyl-t-butyl ether (MTBE) are consistent with MTBE/gasoline fue l usage as a source of isobutene and formaldehyde. Both these reactive spec ies can lead to increased oxidant and PAN formation. The strong diurnal pro files of PANs are consistent with regional clearing of the Mexico City air basin on a daily basis. Estimates are given using a simple box model calcul ation for a number of key primary and secondary pollutant emissions From th is megacity on an annual basis. These calculations indicate that megacities can be important sources of both primary and secondary pollutants: and tha t PANs produced in megacity environments are likely to contribute strongly to regional scale ozone and aerosol productions during long range transport . (C) 1999 Elsevier Science Ltd. All rights reserved.