Fabrizio Ravegnani

First comparison between ground‐based and satellite‐borne measurements of tropospheric nitrogen dioxide in the Po basin

in the Mount Cimone area is good (R 2 = 0.9) with the mixing properties of the atmosphere being the most important parameter for a valid comparison of the measurements. However, even when the atmospheric mixing properties are optimal for comparison, the ratio between GOME and ground-based tropospheric column data may not be unity. It is demonstrated that the values obtained (less than 1) are related to the fraction of the satellite ground pixel occupied by the NO2 hot spot. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; KEYWORDS: tropospheric NO2, satellite validation, Po basin

Differential optical absorption spectrometer for measurement of tropospheric pollutants.

Our institute has recently developed a differential optical absorption spectrometry system called the gas analyzer spectrometer correlating optical absorption differences (GASCOAD), which features as a detector a linear image sensor that uses an artificial light source for long-path tropospheric-pollution monitoring. The GASCOAD, its method of eliminating interference from background sky light, and subsequent spectral analysis are reported and discussed. The spectrometer was used from 7 to 22 February 1993 in Milan, a heavily polluted metropolitan area, to measure the concentrations of SO(2), NO(2), O(3), and HNO(2) averaged over a 1.7-km horizontal light path. The findings are reported and briefly discussed.

Tropospheric and stratospheric NO2 amount deduced by slant column measurements at Mt. Cimone station

Abstract An UVVis spectrometer was installed at Mt. Cimone Station in 1993. Since then it carried out zenith scattered solar radiation measurements at sunrise and sunset in the 407–464 nm spectral region. Data has been processed through DOAS methodology in order to obtain NO 2 slant column. An inversion algorithm is used to calculate the gas vertical distribution from ground based column amount measurements so that the gas content in stratosphere and troposphere is evidenced. Two years data (1995–1996) are shown and discussed.

MOCRA: a Monte Carlo code for the simulation of radiative transfer in the atmosphere

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.

Stratospheric nitrogen dioxide in the Antarctic

Several UV–visible spectrometers have been developed at the ISAC‐CNR Institute. Differential Optical Absorption Spectroscopy (DOAS) methodology is applied to their measurements to monitor the amounts of stratospheric trace gases: mainly ozone (O3) and nitrogen dioxide (NO2) which is involved in the ozone cycle. Observations of the scattered zenith‐sky light were performed with one of these instruments installed at the Terra Nova Bay station (TNB), Antarctica. GASCOD (Gas Analyzer Spectrometer Correlating Optical Differences) is described briefly and a method for data analysis and validation of the results introduced. Some aspects of the DOAS technique are presented: the algorithm allowing the best spectral alignment between spectra obtained with GASCOD and a high‐resolution wavelength calibrated spectrum, is explained. Simple considerations allow for calculation of the NO2 concentration in the background spectrum used in DOAS analysis. For the period of activity of the GASCOD at TNB (1996–2003), the results of NO2 vertical column density (VCD) at twilight show a maximum in the summer and a minimum in the winter. Three years of measurements (2001–2003) are analysed in terms of stratospheric temperature and potential vorticity to obtain information about stratospheric warming that occurred in 2002 over Antarctica. The correlation between NO2 atmospheric content and stratospheric temperature is highlighted. The diurnal variations of NO2, which are controlled by photochemistry, show an unusual behaviour at high latitudes. Analysis of the a.m./p.m. ratios—the sunrise NO2 VC (a.m.) over the sunset VC (p.m.)—during different seasons and at various Solar Zenith Angles (SZA) is presented and discussed.

Spectrometric measurements of NO2 slant column amount at Stara Zagora Station (42°N, 25°E)

Abstract Nitrogen dioxide (NO 2 ) is a representative of a family of chemically very active radicals NO x =NO+NO 2 . These radicals catalytically destroy the ozone and play a significant role in other ozone-destroying catalytic cycles. In the polar night NO 2 can deactivate ClO by a termolecular reaction to form the reservoir gas Cl0NO 2 . In this way NO 2 restricts the ozone destruction in the polar vortex. NO 2 also takes part in heterogeneous reactions, which occur on the surface of polar stratospheric clouds. For the trace gases study and particularly for the nitrogen dioxide a spectrometric instrument GASCOD-BG was installed at the Solar Terrestrial Influences Laboratory - Bulgarian Academy of Sciences , Stara Zagora Department in August 1999. The instrument was developed at the Institute of Atmospheric and Oceanic Sciences at the Council of National Research, Italy, Bologna. The results of the NO 2 observation for more than two years are presented in this paper.

Cruise ships flow rate emission evaluated by means of a passive DOAS instrument

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.

A Monte Carlo simulation of radiative transfer in the atmosphere applied to ToTaL-DOAS

In the frame of DOAS, a Monte Carlo code has been developed, to calculate, for a given detector with assigned diameter and field of view, the single and multiple scattering radiance. Very general 3-D geometry is foreseen. Spatial distribution along the detector axis for the single and total scattering radiance are computed. Ground reflected contributions to the solar radiance are estimated. Differential effects due to small perturbations in physical parameters, such as ozone density, can simultaneously be taken into account in the same calculation. The code has been applied to ToTaL-DOAS (Topographic Target Light scattering-Differential Optical Absorption Spectroscopy) measurements.

Retrieval of Gas Pollutants Vertical Profile in the Boundary Layer by Means of Multiple-Axis DOAS

This paper presents a methodology for the retrieval of the vertical profile of atmospheric gas pollutants in the boundary layer from ground-based remote sensing measurements. Nitrogen dioxide (NO2) and ozone (O3) slant column amounts were obtained with the differential optical absorption spectroscopy (DOAS) technique used in the multiple-axis configuration (referred to as multiaxis DOAS). The measurements were carried out in the Presidential Estate at Castel Porziano (near Rome) from September to November 2006, within a program started in 1994 for studying and monitoring the Estates environment. The retrieval of information on trace gas vertical profiles from the slant column amounts requires as follows: (1) the simulation of the radiative transfer in the atmosphere for air mass factor calculation and (2) the application of inversion schemes. This paper illustrates and discusses the vertical profiles of NO2 and O3 obtained from multiaxis DOAS measurements and their daily evolution on October 29, 2006.

Stratospheric minor gas distribution over the Antarctic Peninsula during the APE–GAIA campaign

This work presents measurements obtained with the multi‐input spectroradiometer GASCOD/A4π, the Fourier transform far‐infrared interferometer SAFIRE‐A and the two‐channel chromatograph HAGAR, on board the M‐55 Geophysica stratospheric aircraft during the APE–GAIA campaign, held in September–October 1999 over the Antarctic Peninsula. The paper aims to investigate the gas mixing ratio distributions at the flight altitude, during a recovery phase of the reservoir species in the lower stratosphere in the Antarctic spring. Attention is focused on the daytime flight of 23 September, when the aircraft flew inside the vortex at two different altitudes (about 16.5 km and 18.5 km). Concentration distributions were examined for NO2, N2O, O3, BrO, ClO, HCl, H2O and HNO3, during flight inside and outside the vortex. The NO2 maxima were well defined in the edge region both on entry into and exit from the vortex at different flight altitudes. Simultaneous concentration measurements of HNO3 and H2O showed a denitrification but not dehydration inside the vortex core. The reformation of chlorine reservoirs is investigated. It was evident that the re‐conversion of HCl was favoured in the Cl+CH4 branches, and its high mixing ratio up to 1 ppbv at 425K surface confirms that low ozone air masses inside the vortex and strong sunlight greatly favour a more rapid formation of passive chlorine reservoirs.