Abstract
Abstract. A new approximation is proposed to estimate O3 and NO2 mixing ratios in the northern subtropical free troposphere (FT). The proposed method uses O4 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 Izaña observatory (28° 18' N, 16° 29' W) are used in this study. The results show that under low aerosol loading, O3 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 O3 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 O3 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.
Highlights
The knowledge of reactive trace gas distributions in populated areas around the world has greatly improved recently due to the extensive deployment of measurement networks on local to regional scales
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
Two independent methods were used for the optical path calculation: the first method, O4-modified geometrical approach (MGA), uses MAXDOAS slant column densities (SCDs) of O4; the second method, radiative transfer models (RTMs)-MGA, obtains optical paths from air mass factor (AMF) obtains optical paths from a RTM
Summary
The knowledge of reactive trace gas distributions in populated areas around the world has greatly improved recently due to the extensive deployment of measurement networks on local to regional scales. Field instrumentation generally provides information on the surface concentrations, rather than the vertical distributions, of the measured species. Knowledge of free troposphere (FT) tracer distributions is lacking. Remote sensing instrumentation based on spectroscopic analysis Multi-axis differential optical absorption spectroscopy (MAX-DOAS) has demonstrated success in obtaining profiles in polluted areas and has been used extensively for pollution studies A BL thermal inversion limits the upward ventilation of pollutants and other surface-produced tracers; as a result, the FT concentrations of gases such as NO2 are much lower (possibly by orders of magnitude). Due to favourable weather conditions or to mechanical forcing on given orographies, polluted air masses may be injected into the FT (Thakur et al, 1999)
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