A High-Precision Mid-Infrared Spectrometer for Ambient HNO(3) Measurements

Precise and accurate measurements of ambient HNO(3) are crucial for understanding various atmospheric processes, but its ultra-low trace amounts and the high polarity of HNO(3) have strongly hindered routine, widespread, direct measurements of HNO(3) 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 HNO(3) 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 HNO(3) amount fractions, and air-sampling-related artifacts, revealed the key aspects of our method: (i) a good linear response ([Formula: see text] > 0.98) between 0 and 25 nmol [Formula: see text] mol [Formula: see text] in both dry and humid conditions with a limit of detection of 95 pmol [Formula: see text] mol [Formula: see text]; (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 HNO(3) 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.