Nitric acid (HNO3) represents an important oxidized nitrogen species (NOz) formed during the ozone (O3) photochemical production processes in the troposphere and contributes significantly to the problem of acid deposition in the urban and regional atmospheres. Formation of nitric acid occurs from the termolecular reaction between OH and NO2 during the daytime, and from the hydrolysis of N2O5 during the nighttime. During the TexAQS II 2006 field campaign, an ion drift – chemical ionization mass spectrometer (ID-CIMS) developed at the Texas A&M University was deployed at the Moody Tower located inside the University of Houston to measure gaseous HNO3 from August 17 to September 30, 2006. Diurnal HNO3 profile followed the solar cycle and the daily peak mixing ratios ranged from 0.2 to 2 ppbv in the early afternoon. Occasionally nighttime peaks were also observed, which likely originated from the N2O5 + H2O heterogeneous reaction or transported aged plumes. The analysis showed that both O3 and HNO3 time series had similar trends and followed each other very well, consistent with the production mechanism of HNO3 during daytime. Further inter-comparisons between HNO3 and other trace gases will also be discussed.