Long-path averaged mixing ratios of O3 and NO2 in the free troposphere from mountain MAX-DOAS

A new approximation is proposed to estimate O-3 and NO2 mixing ratios in the northern subtropical free troposphere (FT). The proposed method uses O-4 slant column densities (SCDs) at horizontal and near-zenith geometries to estimate a station-level differential path. The modified geometrical approach (MGA) is a simple method that takes advantage of a very long horizontal path to retrieve mixing ratios in the range of a few pptv. The methodology is presented, and the possible limitations are discussed. Multi-axis differential optical absorption spectroscopy (MAX-DOAS) high-mountain measurements recorded at the Izana observatory (28 degrees 18' N, 16 degrees 29' W) are used in this study. The results show that under low aerosol loading, O-3 and NO2 mixing ratios can be retrieved even at very low concentrations. The obtained mixing ratios are compared with those provided by in situ instrumentation at the observatory. The MGA reproduces the O-3 mixing ratio measured by the in situ instrumentation with a difference of 28 %. The different air masses scanned by each instrument are identified as a cause of the discrepancy between the O-3 observed by MAX-DOAS and the in situ measurements. The NO2 is in the range of 20-40 ppt, which is below the detection limit of the in situ instrumentation, but it is in agreement with measurements from previous studies for similar conditions.