Articles | Volume 8, issue 22
Atmos. Chem. Phys., 8, 6823–6838, 2008

Special issue: MILAGRO/INTEX-B 2006

Atmos. Chem. Phys., 8, 6823–6838, 2008

  28 Nov 2008

28 Nov 2008

Measurements of HNO3 and N2O5 using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign

J. Zheng1, R. Zhang1, E. C. Fortner1,*, R. M. Volkamer3,2, L. Molina2, A. C. Aiken4,3, J. L. Jimenez4,3, K. Gaeggeler5, J. Dommen5, S. Dusanter6, P. S. Stevens6, and X. Tie7 J. Zheng et al.
  • 1Dept. of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843, USA
  • 2Molina Center for Energy and the Environment, La Jolla, California and Massachusetts Institute of Technology, Cambridge, Massachusetts, USA
  • 3Dept. of Chemistry and Biochemistry, University of Colorado at Boulder, Boulder CO, USA
  • 4Cooperative Institute for Research in the Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA
  • 5Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Switzerland
  • 6Dept. of Chemistry, Indiana University, Bloomington, Indiana, USA
  • 7Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, Colorado, USA
  • *now at: Dept. of Chemistry and Biochemistry, Montana State University, Bozeman, Montana, USA

Abstract. An ion drift-chemical ionization mass spectrometer (ID-CIMS) was deployed in Mexico City between 7 and 31 March to measure gas-phase nitric acid (HNO3) and dinitrogen pentoxide (N2O5 during the Mexico City Metropolitan Area (MCMA)-2006 field campaign. The observation site was located at the Instituto Mexicano del Petróleo in the northern part of Mexico City urban area with major emissions of pollutants from residential, vehicular and industrial sources. Diurnally, HNO3 was less than 200 parts per trillion (ppt) during the night and early morning. The concentration of HNO3 increased steadily from around 09:00 a.m. central standard time (CST), reached a peak value of 0.5 to 3 parts per billion (ppb) in the early afternoon, and then declined sharply to less than half of the peak value near 05:00 p.m. CST. An inter-comparison between the ID-CIMS and an ion chromatograph/mass spectrometer (ICMS) showed a good agreement between the two HNO3 measurements (R2=0.75). The HNO3 mixing ratio was found to anti-correlate with submicron-sized aerosol nitrate, suggesting that the gas-particle partitioning process was a major factor in determining the gaseous HNO3 concentration. Losses by irreversible reactions with mineral dust and via dry deposition also could be important at this site. Most of the times during the MCMA 2006 field campaign, N2O5 was found to be below the detection limit (about 30 ppt for a 10 s integration time) of the ID-CIMS, because of high NO mixing ratio at the surface (>100 ppb) during the night. An exception occurred on 26 March 2006, when about 40 ppt N2O5 was observed during the late afternoon and early evening hours under cloudy conditions before the build-up of NO at the surface site. The results revealed that during the MCMA-2006 field campaign HNO3 was primarily produced from the reaction of OH with NO2 and regulated by gas/particle transfer and dry deposition. The production of HNO3 from N2O5 hydrolysis during the nighttime was small because of high NO and low O3 concentrations near the surface.

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Final-revised paper