An ion drift ? chemical ionization mass spectrometry (ID-CIMS) was deployed in Mexico City between 5 and 31 March to measure HNO₃ and N₂O₅ during the 2006 Mexico City Metropolitan Area (MCMA) field campaign. The observation site, T0, was located at the Instituto Mexicano del Petróleo at the center of the Mexico City Basin with major emissions of pollutants from both domestic and industrial sources. Diurnally, HNO₃ was less than 200 parts per trillion (ppt) during the night and in the early morning, 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 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 correlation in the HNO₃ measurements (R²=0.75). The HNO₃ mixing ratio was found to anti-correlate with aerosol nitrate, suggesting that the gaseous HNO₃ concentration was controlled by the gas-particle partitioning process. During most times of the MCMA 2006 field campaign, N₂O₅ was found to be under the detection limit (about 20 ppt for a 10 s integration time) of the ID-CIMS, because of high NO mixing ratio (>100 ppb) during the night. With one exception on 26 March 2006, about 40 ppt N₂O₅ was observed during the late afternoon and early evening hours under a cloudy condition, before NO built up at the surface site. The results revealed that during the 2006 MCMA field campaign HNO₃ was primarily produced by the reaction of OH with NO₂ and regulated by gas/particle partitioning, and HNO₃ production from N₂O₅ hydrolysis during the nighttime was small because of high NO and low O₃ concentrations near the surface.