Measurements of HNO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign

J Zheng,R M Volkamer,E C Fortner,P S Stevens,S Dusanter,X Tie,K Gaeggeler,J L Jimenez,L Molina,A C Aiken,R Zhang,J Dommen

doi:10.5194/acp-8-6823-2008

Abstract

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.

Highlights

The Mexico City Metropolitan Area (MCMA), one of the largest megacities in the world, has suffered from poor air quality, high concentrations of ozone (O3) and particulate matter (Molina and Molina, 2002; Molina et al, 2007)
The occurrence of the HNO3 daily peak is consistent with its photochemical production mechanism; the magnitude of the HNO3 peak is lower than that expected for the large NOx emissions in the MCMA and the strong tropical solar radiation
An ion drift-chemical ionization mass spectrometer (ID-chemical ionization mass spectrometry (CIMS)) instrument was deployed during the MCMA2006 campaign to measure HNO3 and N2O5 at the T0 urban site

Summary

Introduction

The Mexico City Metropolitan Area (MCMA), one of the largest megacities in the world, has suffered from poor air quality, high concentrations of ozone (O3) and particulate matter (Molina and Molina, 2002; Molina et al, 2007). During the MCMA-2006 field study, a component of the MILAGRO Campaign (Molina et al, 2008), daytime O3 at the urban site was frequently higher than 100 ppb and decreased to a few ppb at night due to a large amount of fresh NOx emissions, resulting in a daily average NOx concentration of over 100 ppb. Both VOCs and NOx play critical roles in the O3 formation in the troposphere (Finlayson-Pitts and Pitts, 1999; Tie et al, 2001; Zhang et al, 2003). The dominant daytime sink of NOx is through the oxidation of NO2 by the hydroxyl radical (OH) to form nitric acid (HNO3)

Objectives

Results

Conclusion