Giorgio Giovanelli

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.

Mask correlation spectrophotometry advanced methodology for atmospheric measurements

Abstract An advanced methodology in mask correlation spectrophotometry is presented. By operating the instrument with its oscillating correlation mask placed in two chosen positions, a system ot two equations can be solved in terms of the unknown optical depth of the gas. The methodology does not require the comparison of field instrumentation responses with laboratory calibration curves nor the in situ establishment of an absolute baseline. The relative error associated with each measurement can also be computed. Onlv a “normal” U.V. source of liaht need be used.

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.

Monitoring of atmospheric ozone and nitrogen dioxide over the south of Portugal by ground-based and satellite observations.

The SPATRAM (Spectrometer for Atmospheric TRAcers Monitoring) instrument has been developed as a result of the collaboration between CGE-UE, ISAC-CNR and Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multi-purpose UV-Vis-scanning spectrometer (250 - 950 nm) and it is installed at the Observatory of the CGE, in Evora, since April 2004. A brief description of the instrument is given, highlighting the technological innovations with respect to the previous version of similar equipment. The need for such measurements automatically taken on a routine basis in south-western European regions, specifically in Portugal, has encouraged the development and installation of the equipment and constitutes a major driving force for the present work. The main features and some improvements introduced in the DOAS (Differential Optical Absorption Spectroscopy) algorithms are discussed. The results obtained applying DOAS methodology to the SPATRAM spectrometer measurements of diffused spectral sky radiation are presented in terms of diurnal and seasonal variations of nitrogen dioxide (NO(2)) and ozone (O(3)). NO(2) confirms the typical seasonal cycle reaching the maximum of (6.5 +/- 0.3) x 10(+15) molecules cm(-2) for the sunset values (PM), during the summer season, and the minimum of (1.55 +/- 0.07) x 10(+15) molecules cm(-2) for the sunrise values (AM) in winter. O(3) presents the maximum total column of (433 +/- 5) Dobson Unit (DU) in the spring season and the minimum of (284 +/- 3) DU during the fall period. The huge daily variations of the O(3) total column during the spring season are analyzed and discussed. The ground-based results obtained for NO(2) and O(3) column contents are compared with data from satellite-borne equipment (GOME - Global Ozone Monitoring Experiment; SCIAMACHY - Scanning Imaging Absorption Spectrometer for Atmospheric CHartographY; TOMS - Total Ozone Monitoring Spectrometer) and it is shown that the two data sets are in good agreement. The correlation coefficient for the comparison of the ground-based/satellite data for O(3) is of 0.97.

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.

Measurements of stratospheric ozone and nitrogen dioxide at Èvora, Portugal

The Spectrometer for Atmospheric TRAcers Monitoring (SPATRAM) has been developed as a result of collaboration between the Geophysics Centre of Évora University (CGE-UE), the Institute for Atmospheric Sciences and Climate of the National Research Council (ISAC-CNR) in Italy and the Italian National Agency for New Technologies, Energy and the Environment (ENEA). SPATRAM is a multipurpose ultraviolet (UV)–visible scanning spectrometer (250–950 nm). It has been installed at the Observatory of the CGE, in Évora, since April 2004 and is currently used to carry out measurements of the zenith scattered radiation, the so-called ‘Passive mode’, to retrieve the vertical content and distribution of some atmospheric tracers such as ozone (O3) and nitrogen dioxide (NO2) using Differential Optical Absorption Spectroscopy (DOAS) methodology. The lack of such measurements taken automatically on a routine basis in southwestern European regions, specifically in Portugal, motivated the effort for its installation and constitutes a major driving force for the present work. For continuous NO2 monitoring the 425–455 nm spectral range is investigated. For O3 retrieval the spectral interval 320–340 nm is chosen. The measurements are in good agreement with the photochemical theory of NO2 (O3), showing maximum values during the summer (spring) and minimum values during the winter (autumn) seasons. Moreover, the application of sophisticated inversion schemes to the output of the DOAS program, using the Air Mass Factor (AMF) matrix as the kernel of the inversion algorithm, allowed for the determination of the vertical distribution of NO2 and O3 atmospheric compounds. In addition, the influence of desert dust aerosol absorption on ozone retrieval is assessed, revealing values of about 3.5% for an aerosol optical depth (AOD) of 1.0, in the case simulated. A correction factor is derived and applied whenever desert dust is detected. The ground-based results obtained for the ozone column content are compared with data from the satellite-borne Ozone Monitoring Instrument (OMI), and the two data sets are found to be in good agreement, with a correlation coefficient of 0.96.

Application features of mask correlation spectrophotometry to long horizontal paths

Abstract Fluctuations in the atmospheric refractive index due to turbulence cause difficulties when measuring optical properties of the atmosphere with a collimated light beam. Mask correlation spectrophotometry (M.C.S.) proved to be strongly affected by this phenomenon even though it is based on a methodology which gives results practically independent of the optical depth of the atmosphere. Measurements performed in the field can also be affected by rapid variations in the target gas optical depth and by the atmospheric opacity. Our methodology of M.C.S., both in its physical and technical aspects, has been reviewed, and a series of improvements are herein presented. The result is that the measurements can be carried out in short time intervals and so undesirable light intensity fluctuations are cut-off. The troubles caused by atmospheric opacity and target gas variations are also avoided. The improvements made meet field operation requirements.

A new multipurpose UV-Vis spectrometer for air quality monitoring and climatic studies

The multipurpose ultraviolet-visible (UV-Vis) remote sensing equipment SPATRAM (SPectrometer for Atmospheric TRAcers Monitoring) is a scanning spectrometer for measurement of electromagnetic radiation in the 250–950 nm spectral range. In this paper SPATRAM will be presented and new solutions will be discussed. The monochromator is based on the one installed in GASCOD (Gas Analyzer Spectrometer Correlating Optical Differences) developed during the 1990s. The most important improvements of SPATRAM relative to GASCOD are: (i) the wider spectral range scanned, allowing for the detection of more atmospheric compounds than with GASCOD; (ii) an increased number of inputs, resulting in the possibility of quasi-simultaneous measurements from different optical devices; (iii) the focusing optic system, which permits a simple optical alignment procedure and low cost; (iv) electronic self-thermoregulation, allowing for reliable spectral measurements unaffected by mechanical deformation caused by variation of temperature; (v) adoption of a CCD sensor, resulting in an increase of equipment sensitivity and therefore an enhancement in time resolution of the measurements; (vi) the use of an advanced CPU and a standard OS, guaranteeing full stability of the equipment; and (vii) the development of a new software tool for complete control of the whole instrument and for pre-processing of the measured data.