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 (2s), 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.
A particulate nitrate analyzer based on a scrubber difference/NO-O3 chemiluminescence method (SD-CL method) has been developed for measuring nitrate concentrations in remote areas. Particulate nitrate concentrations (NO3(-)(p)) were analyzed by the difference between the concentrations of NOy - gaseous nitric acid (HNO3) and NOy - HNO3 - NO3(-)(p). Annular denuders coated with NaCl and PTFE filter were used as the scrubbers for HNO3 and NO3(-)(p), respectively. The transmission efficiency of coarse particles in the denuder was found to be 93.4 ± 5.8%, so the loss of NO3(-)(p) to the denuder was within the uncertainty of the particulate nitrate analyzer (±20%). The measurements of NOy, HNO3, and NO3(-)(p) were conducted from March 15 to April 31, 2008, at Cape Hedo, Okinawa, Japan. Over 99.5% of the observed concentrations of NO3(-)(p) for 10 min integration times were higher than the detection limit of the SD-CL method (0.18 μg m(-3)). The least-squares fit of the R&P nitrate monitor against the SD-CL method yielded a slope of 0.67 ± 0.02 and a correlation coefficient of R = 0.73. This result indicates that this method could also measure NO3(-)(p) when the diameter of aerosols was larger than 10 μm. The SD-CL method was found to be useful as a measurement system for NO3(-)(p) in remote areas where coarse NO3(-)(p) dominates.
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