Information Content of the Ground-Based FTIR Method for Atmospheric HNO3 Vertical Structure Retrieval

Nitric acid plays an important role in atmospheric chemistry; for this reason, it is currently monitored by various methods and instruments. The ground-based FTIR method based on spectral measurements of solar radiation by Bruker Optics IFS 125HR spectrometers allows one to retrieve not only the total column HNO 3 but also its content in individual atmospheric layers. Analysis of HNO 3 content series at the St. Petersburg NDACC site between 2009 and 2021 shows that the spectroscopic measurements under consideration contain about three independent parameters on the average, which makes it possible to obtain information about the HNO 3 content in several layers of the atmosphere. The mean random errors of HNO 3 measurements amount to 3.9, 14, and 1.6% for the total atmospheric, tropospheric (up to 15 km), and stratospheric (above 15 km) content, respectively. Thus, the FTIR method considered is more sensitive to changes in the stratospheric HNO 3 content. The absorption of solar radiation by nitric acid in the measured spectra overlaps with the absorption by water vapor; therefore, the information content and accuracy of HNO 3 measurements are maximal in winter and minimal in summer: in winter, measurements are carried out mainly at low sun and low humidity; in summer, vice versa.