Hydration of nitric acid and its collection in the atmosphere by diffusion denuders

A High-Precision Mid-Infrared Spectrometer for Ambient HNO3 Measurements

Precise and accurate measurements of ambient HNO3 are crucial for understanding various atmospheric processes, but its ultra-low trace amounts and the high polarity of HNO3 have strongly hindered routine, widespread, direct measurements of HNO3 and restricted field studies to mostly short-term, localized measurement campaigns. Here, we present a custom field-deployable direct absorption laser spectrometer and demonstrate its analytical capabilities for in situ atmospheric HNO3 measurements. Detailed laboratory characterizations with a particular focus on the instrument response under representative conditions for tropospheric measurements, i.e., the humidity, spectral interference, changing HNO3 amount fractions, and air-sampling-related artifacts, revealed the key aspects of our method: (i) a good linear response (R2 > 0.98) between 0 and 25 nmol·mol−1 in both dry and humid conditions with a limit of detection of 95 pmol·mol−1; (ii) a discrepancy of 20% between the spectroscopically derived amount fractions and indirect measurements using liquid trapping and ion chromatography; (iii) a systematic spectral bias due to water vapor. The spectrometer was deployed in a three-week field measurement campaign to continuously monitor the HNO3 amount fraction in ambient air. The measured values varied between 0.1 ppb and 0.8 ppb and correlated well with the daily total nitrates measured using a filter trapping method.

Characterization of condensed phase nitric acid particles formed in the gas phase

The formation of nitric acid hydrates has been observed in a chamber during the dark reaction of NO2 with 03 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) < or = 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 HNO3 x 3H2O and NO3- x H2O (x > or = 4) at low RH, whereas at high RH HNO3 x H2O, HNO3 x 2H2O, HNO3 x 3H2O and NO3- x xH2O (x > or = 4) all exist in the condensed phase. At high RH HNO3 x xH2O (x < or = 3) collected on the glass fiber membrane is greatly increased, while NO3- x xH2O (x > or = 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.