From slant column densities to trace gas profiles: Post processing data from the new MAX-DOAS network in Mexico City

Abstract:

The new MAX-DOAS network in Mexico City provides results of O₄, HCHO and NO₂ slant column densities (SCD). Here, we present a new numerical code developed to retrieve gas profiles of NO₂ and HCHO using radiative transfer simulations. We present first results of such profiles from the MAX-DOAS station located at UNAM campus.
The code works in two steps: First, the O₄ slant column density information is used to retrieve an aerosol profile. As an a-priori aerosol profile, we use averaged ceilometer data measured at UNAM and scaled to the total optical depth provided by the Aeronet data base. In the second step, the retrieved aerosol profile information is used together with the trace gas (HCHO or NO₂) SCDs to retrieve the trace gas profiles.
The inversion is based on a gauss-newton iteration scheme and uses constrained least square fitting with either optimal estimation or Tihkonov regularization. For the latter, the regulation matrix is currently constructed from the discrete first derivative operator.
The forward model uses the radiative transfer code VLIDORT. The inputs to VLIDORT are calculated using temperature and pressure information from daily radiosounde measurements and aerosol single scattering optical depths and asymmetry factors from the Aeronet data base for Mexico City. For the gas absorption cross sections we use the same values as were used for the SCD calculation from the recorded spectra using QDOAS. Besides demonstrating the functionality of the algorithm showing profile retrievals of simulated SCDs with added random noise, we present HCHO and NO₂ profiles retrieved from SCDs calculated from the MAX-DOAS measurements at UNAM campus at selected days.