The formation of nitric acid hydrates has been observed in a chamber during the dark reaction of NO 2 with O 3 in the presence of air.The size of condensed phase nitric acid was measured to be 40-100 nm and 20-65 nm at relative humidity (RH) 5% and RH=67% under our experimental conditions,respectively.The nitric acid particles were collected on the glass fiber membrane and their chemical compositions were analyzed by infrared spectrum.The main components of nitric acid hydrates in particles are HNO 3 ·3H 2 O and NO 3 ·xH 2 O (x 4) at low RH,whereas at high RH HNO 3 ·H 2 O,HNO 3 ·2H 2 O,HNO 3 ·3H 2 O and NO 3 ·xH 2 O (x 4) all exist in the condensed phase.At high RH HNO 3 ·xH 2 O (x 3) collected on the glass fiber membrane is greatly increased,while NO 3 ·xH 2 O (x 4) decreased,compared with low RH.To the best of our knowledge,this is the first time to report that condensed phase nitric acid can be generated in the gas phase at room temperature. The formation of nitric acid hydrates has been observed in a chamber during the dark reaction of NO 2 with O 3 in the presence of air. The size of condensed phase nitric acid was measured to be 40-100 nm and 20-65 nm at relative humidity (RH) 5% and RH = 67% under our experimental conditions, respectively. The nitric acid particles were collected on the glass fiber membrane and their chemical compositions were analyzed by infrared spectrum. The main components of nitric acid hydrates in particles are HNO 3 · 3H 2 O and NO 3 · xH 2 O (x 4) at low RH, while at high RH HNO 3 · H 2 O, HNO 3 · 2H 2 O, HNO 3 · 3H 2 O and NO 3 · xH 2 (X 4) all exist in the condensed phase. At high RH HNO 3 · xH 2 O (x 3) collected on the glass fiber membrane is greatly increased, while NO 3 · xH 2 O (x 4) decreased, compared with low RH.To the best of our knowledge, this is the first time to report that condensed phase nitric acid can be generated in the gas phase at room temperature.