Measurements of Volatile Organic Compounds Using Proton Transfer Reaction – Mass Spectrometry during the MILAGRO 2006 Campaign

Volatile organic compounds (VOCs) were mea- sured by proton transfer reaction - mass spectrometry (PTR- MS) on a rooftop in the urban mixed residential and indus- trial area North Northeast of downtown Mexico City as part of the Megacity Initiative - Local and Global Research Ob- servations (MILAGRO) 2006 field campaign. Thirty eight individual masses were monitored during the campaign and many species were quantified including methanol, acetalde- hyde, toluene, the sum of C2 benzenes, the sum of C3 ben- zenes, acetone, isoprene, benzene, and ethyl acetate. The VOC measurements were analyzed to gain a better under- standing of the type of VOCs present in the MCMA, their di- urnal patterns, and their origins. Diurnal profiles of weekday and weekend/holiday aromatic VOC concentrations showed the influence of vehicular traffic during the morning rush hours and during the afternoon hours. Plumes including ele- vated toluene as high as 216 parts per billion (ppb) and ethyl acetate as high as 183 ppb were frequently observed during the late night and early morning hours, indicating the possi- bility of significant industrial sources of the two compounds in the region. Wind fields during those peak episodes re- vealed no specific direction for the majority of the toluene plumes but the ethyl acetate plumes arrived at the site when winds were from the Southwest or West. The PTR-MS mea- surements combined with other VOC measuring techniques at the field site as well as VOC measurements conducted in other areas of the Mexico City Metropolitan Area (MCMA) will help to develop a better understanding of the spatial pat- tern of VOCs and its variability in the MCMA.