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 Sload Hall, Spokane Street, Pullman, WA 99164-2910, USA
4University of Arkansas at Little Rock, 2801 South University Avenue, Little Rock, AR 72204-1099, USA
5Centro de Ciencias de la Atmysfera, Universidad Nacional Autónoma de México, Mexico, D.F., Mexico
6Centro Nacional de Investigacion y Capacitacion Ambiental-INE, Av. Periférico 5000, Col. Insurgentes Cuicuilco, CP 04530, Mexico, D.F., Mexico
7Gobierno del Distrito Federal, Agricultura 21, Piso 1, Col. Escandon, Del. M. Hidalgo, CP 11800, Mexico, D. F., Mexico
Abstract. Data from a recent field campaign in Mexico City are used to evaluate the performance of the EPA Federal Reference Method for monitoring ambient concentrations of NO2. Measurements of NO2 from standard chemiluminescence monitors equipped with molybdenum oxide converters are compared with those from Tunable Infrared Laser Differential Absorption Spectroscopy (TILDAS) and Differential Optical Absorption Spectroscopy (DOAS) instruments. A significant interference in the chemiluminescence measurement is shown to account for up to 50% of ambient NO2 concentration during afternoon hours. As expected, this interference correlates well with non-NOx reactive nitrogen species (NOz as well as with ambient O3 concentrations, indicating a photochemical source for the interfering species. A combination of ambient gas phase nitric acid and alkyl and multifunctional alkyl nitrates is deduced to be the primary cause of the interference. Observations at four locations at varying proximities to emission sources indicate that the percentage contribution of HNO3 to the interference decreases with time as the air parcel ages. Alkyl and multifunctional alkyl nitrate concentrations are calculated to be reach concentrations as high as several ppb inside the city, on par with the highest values previously observed in other urban locations. Averaged over the MCMA-2003 field campaign, the CL NOx monitor interference resulted in an average measured NO2 concentration up to 22% greater than that from co-located spectroscopic measurements. Thus, this interference has the potential to initiate regulatory action in areas that are close to non-attainment and may mislead atmospheric photochemical models used to assess control strategies for photochemical oxidants.