This paper describes the radiative transfer model (RTM) MOCRA (MOnte Carlo Radiance Analysis), developed in the frame of DOAS (Differential Optical Absorption Spectroscopy) to correctly interpret remote sensing measurements of trace gas amounts in the atmosphere through the calculation of the Air Mass Factor. Besides the DOAS-related quantities, the MOCRA code yields: 1- the atmospheric transmittance in the vertical and sun directions, 2- the direct and global irradiance, 3- the single- and multiple- scattered radiance for a detector with assigned position, line of sight and field of view. Sample calculations of the main radiometric quantities calculated with MOCRA are presented and compared with the output of another RTM (MODTRAN4). A further comparison is presented between the NO2 slant column densities (SCDs) measured with DOAS at Evora (Portugal) and the ones simulated with MOCRA. Both comparisons (MOCRA-MODTRAN4 and MOCRA-observations) gave more than satisfactory results, and overall make MOCRA a versatile tool for atmospheric radiative transfer simulations and interpretation of remote sensing measurements.

In 2019 the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission was selected to be the 9th Earth Explorer mission of the European Space Agency (ESA). In the preparatory phase of the mission there was the need for accurate and versatile codes to compute the spectrally resolved Earth radiation escaping to space ( outgoing long-wave radiation, OLR), targets for the FORUM measurements.Moreover, for the study of planetary atmospheres, several instruments measuring the planetary radiation escaping to space have been deployed (i.e., the planetary Fourier spectrometer on Mars express or composite infrared spectrometer on Cassini). For both the analysis of the measurements of these instruments and the design of new instruments, reliable radiative transfer codes need to be available. In this paper, we describe two full physics codes, Geofit broadband-Nadir (GBB-Nadir) and Kyoto protocol-informed management of adaptation (KLIMA), both able to compute the OLR spectrum, while GBB-Nadir is only a forward model, and therefore computes the spectra only, KLIMA implements the computation of spectral radiance derivatives with respect to atmospheric parameters and therefore it is suitable to be used in retrieval codes. The GBB-Nadir code can be interfaced with radiative transfer solvers that include representations of multiple scatterings, making it suitable to compute the radiances in all-sky conditions. KLIMA has been extensively validated comparing its radiances to ones generated by the widely used line-by-line radiative transfer model (LBLRTM) code. In this paper, we describe the latest version of both codes and their comparison. We compared the optical depth computed by GBB-Nadir and KLIMA for given values of pressure, temperature and gas columns for most gases active in the far-infrared and thermal-infrared spectral regions. We show that the optical depths computed by the two codes are in very good agreement. We compared the simulated spectra in clear sky conditions for three different atmospheres (equatorial, mid-latitude and polar) at resolutions of the FORUM instrument. The differences found are well below the expected noise of the FORUM instrument. The KLIMA code has already been used to simulate the observations of the Mars atmosphere, while the limb version of the GBB code has been used to simulate the radiances measured in the limb geometry of planetary atmospheres (Titan and Jupiter). Therefore, we may safely affirm that both codes can be used to simulate the nadir measurements of planetary atmospheres.

This paper presents an evaluation of the nitrogen dioxide (NO₂) trend inferred from UV–visible zenith-sky measurements performed at the ‘Ottavio Vittori’ ground-based research station (http://www.isac.cnr.it/cimone) at Monte Cimone (44.18° N, 10.7° E, 2165 m above sea level (a.s.l.)). Experimental data series are obtained from the measured spectra adopting a retrieval procedure according to differential optical absorption spectroscopy (DOAS) methodology. Data series cover the period from 1993 to 2009, with limited interruptions due to technical maintenance of the instrument or poor meteorological conditions. The NO₂ trends are calculated with a linear regression model considering different factors assumed to contribute to NO₂ atmospheric content. Before running the model, a procedure for data quality control was applied to reduce or exclude the influence of measurements affected by poor meteorological conditions or the vertical transport of polluted air masses from the lower troposphere. The negative trends obtained for sunrise (a.m.) and sunset (p.m.) NO₂ measurements are in good agreement with previous studies accomplished for ground-based stations in the northern hemisphere.

The emissions of the cruise ships, in terms of nitrogen dioxide (NO₂) and sulphur dioxide (SO₂), are evaluated with the DOAS scanning spectrometer TropoGAS (Tropospheric Gas Analyser Spectrometer) developed at ISAC CNR in close collaboration with the CGE-UE. The slant columns amounts of the above mentioned compounds are obtained with the application of the Differential Optical Absorption Spectroscopy (DOAS) technique to the spectral measurements carried out with the TropoGAS instrument. This last is linked with an optical fibre to a simple scanning optical system allowing for measurements in multiple axis configurations. The measurements are carried out across the Giudecca Channel in Venice, during two field campaigns performed in July and in October 2007. The instrumental setup, the DOAS method and the technique for the evaluation of the ships emissions, are described. The results of flow rate emissions for NO₂ and SO2 are presented and discussed. Their mean values are about 12g/s and 4 g/s for NO₂ and SO₂ respectively.