In the urban atmosphere, heterogeneous conversion of NOx on the surface of soot particles has been suggested to occur efficiently to form nitrous acid (HONO). This process leads to accumulation of elevated levels of HONO at night. Subsequent photolysis of HONO during the morning results in a sudden rise in the hydroxyl radical (OH) concentration and, hence, promotes efficient oxidation of volatile organic compounds (VOCs) and rapid ozone production. Model calculations have demonstrated that inclusion of the heterogeneous conversion of NO2 to HONO on the surfaces of soot aerosols accelerates the O3 production by about 1 hour in the morning and leads to a noticeable increase of about 7 ppb on average in the daytime O3 level over the Houston area. In this talk, we present measurements of HONO using ion drift – chemical ionization mass spectrometry (ID-CIMS) during the 2009 SHARP/SOOT Campaign. Preliminary results will be discussed on inter-comparison of our ID-CIMS instrument for HONO measurements with several other HONO instruments conducted during this field study, including a mist chamber/ion chromatography, differential optical absorption spectroscopy (DOAS), the long path absorption photometer (LOPAP), and the Aerodyne quantum cascade laser absorption spectrometer (QCLAS).