FTIR time series of stratospheric NO₂ over Hefei, China, and comparisons with OMI and GEOS-Chem model data

We present the trend and seasonal variability of stratospheric NO₂ column for the first time over the polluted atmosphere at Hefei, China, retrieved using Fourier transform infrared spectroscopy (FTIR) between 2015 and 2018. The FTIR observed stratospheric NO₂ columns over Hefei show a peak in June and reach a minimum in January. The mean stratospheric NO₂ column concentration in June is (3.49 ± 0.25) × 10¹⁵ molecules*cm^-2, and is 39.20% ± 8.95% higher than that in January with a mean value of (2.51 ± 0.21) × 10¹⁵ molecules*cm^-2. We find a negative trend of (-0.34 ± 0.05) %/yr in the FTIR observations of stratospheric NO₂ column. The FTIR data are compared to the satellite OMI observations to assess the new data set quality and also applied to evaluate the GEOS-Chem model simulations. We find in general the OMI observations and GEOS-Chem model results are in good agreement with the coincident FTIR data, and they all show similar seasonal cycles with strong correlation coefficients of 0.84-0.86. The annual average OMI minus FTIR difference is (1.48 ± 5.33) × 10¹⁴ molecules*cm^-2 (4.82% ± 17.37%), and average GEOS-Chem minus FTIR difference is (2.36 ± 2.33) × 10¹⁴ molecules*cm ^-2 (7.66% ± 7.49%). Their maximum differences occur in April and May with mean differences of 12-16%. We also found negative trends in the stratospheric NO₂ column over Hefei for 2015-2018 with both OMI observations (-0.91 ± 0.09%/yr) and GEOS-Chem model results (-0.31 ± 0.05%/yr), demonstrating some consistency among them.