Ability of the MAX-DOAS method to derive profile information for NO₂: can the boundary layer and free troposphere be separated?

Abstract. Multiple Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments can measure from the ground the absorption by nitrogen dioxide (NO₂) of scattered sunlight seen in multiple viewing directions. This paper studies the potential of this technique to derive the vertical distribution of NO₂ in the troposphere. Such profile information is essential for detailed comparisons of MAX-DOAS retrievals with other measurement techniques for NO₂, e.g. with a lidar or from space.

The retrieval algorithm used is based on a pre-calculated look-up table and assumes homogeneous mixing of aerosols and NO₂ in layers extending from the surface to a variable height. Two retrieval models are compared: one including and one excluding an elevated NO₂ layer at a fixed altitude in the free troposphere. An ensemble technique is applied to derive retrieval uncertainties.

Sensitivity studies demonstrate that NO₂ in the free troposphere can only be retrieved accurately if: (i) the retrieved boundary layer profiles for aerosols and NO₂ correspond to the real ones, (ii) if the right a-priori choice is made for the (average) height of free tropospheric NO₂, and (iii) if all other error sources are very low. It is shown that retrieval models that are capable of accurate NO₂ retrievals in the free troposphere, i.e. models not constrained too much by a-priori assumptions, have as a major disadvantage that they will frequently find free tropospheric NO₂, also when it is not present in reality. This is a consequence of the fact that NO₂ in the free troposphere is poorly constrained by the MAX-DOAS observations, especially for high aerosol optical thickness values in the boundary layer. Retrieval of free tropospheric NO₂ is therefore sensitive to a large number of error sources. For this reason it is advised to firmly constrain free tropospheric NO₂ in MAX-DOAS retrieval models used for applications such as satellite validation. This effectively makes free tropospheric NO₂ a source of error for MAX-DOAS retrieval of NO₂ profiles in the boundary layer.

A comparison was performed with independent data, based on MAX-DOAS observations done at the CINDI campaign, held in the Netherlands in 2009. Comparison with lidar partial tropospheric NO₂ columns showed a correlation of 0.78, and an average difference of 0.1× 10¹⁵ molec cm⁻². The diurnal evolution of the NO₂ volume mixing ratio measured by in-situ monitors at the surface and averaged over five days with cloud-free mornings, compares well to the MAX-DOAS retrieval: a correlation was found of 0.94, and an average difference of 0.04 ppb.