Airborne DOAS measurements in Arctic: vertical distributions of aerosol extinction coefficient and NO₂ concentration

Abstract. We report on airborne Differential Optical Absorption Spectroscopy (DOAS) measurements of aerosol extinction and NO₂ tropospheric profiles performed off the North coast of Norway in April 2008. The DOAS instrument was installed on the Safire ATR-42 aircraft during the POLARCAT-France spring campaign and recorded scattered light spectra in near-limb geometry using a scanning telescope. We use O₄ slant column measurements to derive the aerosol extinction at 360 nm. Regularization is based on the maximum a posteriori solution, for which we compare a linear and a logarithmic approach. The latter inherently constrains the solution to positive values and yields aerosol extinction profiles more consistent with independently measured size distributions. We present results from two soundings performed on 8 April 2008 above 71° N, 22° E and on 9 April 2008 above 70° N, 17.8° E. The first profile shows aerosol extinction and NO₂ in the marine boundary layer with respective values of 0.04 ± 0.005 km⁻¹ and 1.9 ± 0.3 × 10⁹ molec cm⁻³. A second extinction layer of 0.01 ± 0.003 km⁻¹ is found at 4 km altitude where the NO₂ concentration is 0.32 ± 0.2 × 10⁹ molec cm⁻³. During the second sounding, clouds prevent retrieval of profile parts under 3 km altitude but a layer with enhanced extinction (0.025 ± 0.005 km⁻¹) and NO₂ (1.95 ± 0.2 × 10⁹ molec cm⁻³) is clearly detected at 4 km altitude.

From CO and ozone in-situ measurements complemented by back-trajectories, we interpret the measurements in the free troposphere as, for the first sounding, a mix between stratospheric and polluted air from Northern Europe and for the second sounding, polluted air from Central Europe containing NO₂. Considering the boundary layer measurements of the first flight, modeled source regions indicate closer sources, especially the Kola Peninsula smelters, which can explain the NO₂ enhancement not correlated with a CO increase at the same altitude.