1Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Bldg. 54, 77 Massachusetts Ave, Cambridge, MA 02139, USA
2Aerodyne Research Inc., 45 Manning Road, Billerica MA 01821-3876, USA
3Laboratory for Atmospheric Research, Department of Civil and Environmental Engineering, Washington State University, 101 Sloan Hall, Spokane Street, Pullman, WA 99164-2910, USA
4Centro de Ciencias de la Atmósfera, UNAM, Ciudad Universitaria, 04 510 Mexico City, Mexico
5Gobierno del Distrito Federal, Agricultura 21, Piso 1, Col. Escandon, Del. M. Hidalgo, CP 11 800, Mexico, D.F., Mexico
6Centro Nacional de Investigacion y Capacitacion Ambiental-INE, Av. Periférico 5000, Col. Insurgentes Cuicuilco, CP 04530, Mexico, D.F., Mexico
*now at: University of Colorado at Boulder, Cooperative Institute for Research in Environmental Sciences, UCB 216, Boulder, CO 80309, USA
Abstract. The performance of the EPA Federal Equivalent Method (FEM) technique for monitoring ambient concentrations of O3 via ultraviolet absorption (UV) has been evaluated using data from the Mexico City Metropolitan Area (MCMA-2003) field campaign. Comparisons of UV O3 monitors with open path Differential Optical Absorption Spectroscopy (DOAS) and open path Fourier Transform Infrared (FTIR) spectroscopy instruments in two locations revealed average discrepancies in the measured concentrations of +13% to −10%. Excellent agreement of two separate open path DOAS measurements at one location indicated that spatial and temporal inhomogeneities were not substantially influencing comparisons of the point sampling and open path instruments. The poor agreement between the UV O3 monitors and the open path instruments was attributed to incorrect calibration factors for the UV monitors, although interferences could not be completely ruled out. Applying a linear correction to these calibration factors results in excellent agreement of the UV O3 monitors with the co-located open path measurements; regression slopes were 0.94 to 1.04 and associated R2 values were >0.89. A third UV O3 monitor suffered from large spurious interferences, which were attributed to extinction of UV radiation within the monitor by fine particles (<0.2 μm) due to a particulate filter with too large a pore size. The overall performance of this particular monitor was poor owing to a combination of interferences from a contaminated particle filter and/or ozone scrubber. Suggestions for improved operation practices of these UV O3 monitors and recommendations for future testing are made.