This study presents recent work examining atmospheric nitric acid (HNO3) at the NDACC St. Petersburg observational site in Peterhof for the period of 2009 to 2022. At this site, nitric acid is monitored using a Bruker 125HR Fourier spectrometer, which measures infrared solar absorption spectra that are used to retrieve profile estimates of various trace gases including nitric acid. In addition to these ground-based measurements, satellite measurements of nitric acid over the site have been made by the Aura - Microwave Limb Sounder (Aura-MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS).

The focus of this work is on both analysis of the nitric acid time series measured by the Bruker 125HR and on comparing the nitric acid stratospheric columns with coincident Aura-MLS and ACE-FTS measurements for validation purposes. For the former, analysis of nitric acid showed pronounced seasonal cycles, with annual variations of -30 to +60 % between 0 and 15 km, -25 to +25 % between 15 and 50 km, and -25 to +30 % for the nitric acid total column, as compared to a yearly average. This study also found no statistically significant trend over the 13-year data record.

In order to compare the ground-based nitric acid retrievals to measurements from Aura-MLS and ACE-FTS, a set of coincidence criteria are needed to ensure similar air masses are compared. In this work, the chosen criteria require satellite measurements to be made within 500 km of St. Petersburg, and then daily averages of the ground-based measurements are compared against individual satellite measurements. Rather than compare the retrieved profiles, the stratospheric partial columns, spanning 15-40 km for the ground-based FTIR and 15-50 km for the two satellite instruments are compared. The time series of these measurements is shown in Figure 1.

Figure 1: Time series of nitric acid stratospheric partial columns near the St. Petersburg station from the ground-based Bruker 125HR FTIR (red), Aura-MLS (blue), and ACE-FTS (green) .

Overall, it was found that the stratospheric nitric acid columns from the three instruments are in good agreement. The 489 coincident measurements between the FTIR and Aura-MLS indicated that the two datasets differed by only –5.4 ± 9.3 %, with a correlation coefficient of 0.933 ± 0.006. For comparisons with ACE-FTS, 55 coincident measurements were found, which yielded relative differences of 5.7 ± 8.5 % and a correlation coefficient of 0.88 ± 0.03.