Measurements of HNo₃ and N₂O₠using ion drift-chemical ionization mass spectrometry during the MILAGRO/MCMA-2006 campaign

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, N₂O₠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 N2O₠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 N2O₠hydrolysis during the nighttime was small because of high NO and low O₃ concentrations near the surface.