A Gaseous Nitric Acid Analyzer for the Remote Atmosphere Based on the Scrubber Difference/NO-Ozone Chemiluminescence Method

An in situ nitric acid analyzer has been designed for sensing in the remote atmosphere. Its operation is based on measuring the concentration difference between the total odd nitrogen species (NOy) and HNO3-scrubbed NOy. NOy was measured by an NO-ozone chemiluminescent detector equipped with a molybdenum converter. A temperature-controlled annular denuder coated with NaCl was used as a scrubber of gaseous HNO3. The current detection limit of the HNO3 analyzer was estimated to be 158 and 71 pptv (parts per trillion by volume) with an integration time of 2 and 10 min (2σ), respectively. The analyzer was calibrated using standard gaseous HNO3 to verify its performance. The HNO3 concentrations measured by the scrubber difference and the NO-ozone chemiluminescence methods (SDCL method) were in agreement with those obtained by the denuder method within the experimental uncertainties (more than ±4%). We conducted limited field observations of NOy and HNO3 in the remote atmosphere using the newly designed SDCL method at Cape Hedo, Okinawa, Japan.