Measurements of HNOsub3/sub and Nsub2/subOsub5/sub using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign

摘要

An ion drift-chemical ionization mass spectrometer (ID-CIMS) was deployed inMexico City between 7 and 31 March to measure gas-phase nitric acid(HNO₃) and dinitrogen pentoxide (N₂O₅ during the Mexico CityMetropolitan Area (MCMA)-2006 field campaign. The observation site waslocated at the Instituto Mexicano del Petróleo in the northern part ofMexico City urban area with major emissions of pollutants from residential,vehicular and industrial sources. Diurnally, HNO₃ was less than 200parts per trillion (ppt) during the night and early morning. Theconcentration of HNO₃ increased steadily from around 09:00 a.m. centralstandard 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 thepeak value near 05:00 p.m. CST. An inter-comparison between the ID-CIMS andan ion chromatograph/mass spectrometer (ICMS) showed a good agreement betweenthe two HNO₃ measurements (R2=0.75). The HNO₃ mixing ratiowas found to anti-correlate with submicron-sized aerosol nitrate, suggestingthat the gas-particle partitioning process was a major factor in determiningthe gaseous HNO₃ concentration. Losses by irreversible reactions withmineral dust and via dry deposition also could be important at this site.Most of the times during the MCMA 2006 field campaign, N₂O₅ wasfound to be below the detection limit (about 30 ppt for a 10 s integrationtime) 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, whenabout 40 ppt N₂O₅ was observed during the late afternoon andearly evening hours under cloudy conditions before the build-up of NO at thesurface site. The results revealed that during the MCMA-2006 field campaignHNO₃ was primarily produced from the reaction of OH with NO₂ andregulated by gas/particle transfer and dry deposition. The production ofHNO₃ from N₂O₅ hydrolysis during the nighttime was smallbecause of high NO and low O₃ concentrations near the surface.