Franco Evangelisti

A 6‐year analysis of stratospheric intrusions and their influence on ozone at Mt. Cimone (2165 m above sea level)

were analyzed. Moreover, three-dimensional backward trajectories calculated by the FLEXTRA model and potential vorticity values along these trajectories were used. In order to identify SI and evaluate their contribution to the tropospheric ozone at Mt. Cimone, a statistical methodology was developed. This methodology consists of different selection criteria based on observed and modeled stratospheric tracers as well as on tropopause height values recorded by radio soundings. On average, SI effects affected Mt. Cimone for about 36 days/year. The obtained 6-year SI climatology showed a clear seasonal cycle with a winter maximum and a spring-summer minimum. The seasonal cycle was also characterized by an interannual variation. In particular, during winter (autumn), SI frequency could be related to the intensity of the positive (negative) NAO phase. In order to separate direct SI from indirect SI, a restrictive selection criterion was set. This criterion, named Direct Intrusion Criterion (DIC), requested that all the analyzed tracers were characterized by stratospheric values. Direct SI affected Mt. Cimone for about 6 days/year, with frequency peaks in winter and early summer. At Mt. Cimone, SI contribution to background ozone concentrations was largest in winter. On average, an ozone increase of 8% (3%) with respect to the monthly running mean was found during direct (indirect) SI. Finally, the typical variations of stratospheric tracers during SI events were analyzed. The analysis of in situ atmospheric pressure values suggested that direct SI were connected with intense fronts affecting the region, while indirect SI were possibly connected with subsiding structures related with anticyclonic areas.

Intercomparison of UV/visible spectrometers for measurements of stratospheric NO2 for the Network for the Detection of Stratospheric Change

During the period May 12–23, 1992, seven groups from seven countries met in Lauder, New Zealand, to intercompare their remote sensing instruments for the measurement of atmospheric column NO2 from the surface. The purpose of the intercomparison was to determine the degree of intercomparability and to qualify instruments for use in the Network for the Detection of Stratospheric Change (NDSC). Three of the instruments which took part in the intercomparison are slated for deployment at primary NDSC sites. All instruments were successful in obtaining slant column NO2 amounts at sunrise and sunset on most of the 12 days of the intercomparison. The group as a whole was able to make measurements of the 90° solar zenith angle slant path NO2 column amount that agreed to about ±10% most of the time; however, the sensitivity of the individual measurements varied considerably. Part of the sensitivity problem for these measurements is the result of instrumentation, and part is related to the data analysis algorithms used. All groups learned a great deal from the intercomparison and improved their results considerably as a result of this exercise.

Performance of a diode-array spectrometer in DOAS applications

A photodiode-array spectrometer for the detection of atmospheric trace gases has been developed to take diffuse solar zenith and, using a n artificial light source, horizontal long path measurements. Of the numerous factors involved in determining the minimum amount of a detectable gas, including its spectral characteristics, atmospheric phenomena and the algorithm used, the present study examines only the features of the spectrometer and sensor. The spectrometers spectral dispersion is accurately calculated and its internal stray light kept to a minimum. The linear image sensors dynamic range and aperture-response function are analyzed along with those phenomena that can alter the actual signal such as veining glare, blooming, etaloning and dark current; the latter two are treated in depth. Etaloning is linked to optical interference in the sensors passivating layer. A simple model of it, as well as a check of the relative stability over time of the transmission peaks, are included. The analysis of the dark current indicates the existence of a photo-induced component that is inversely proportional to the incident radiation flux. It yields a simple analytical equation describing the phenomenon for the sensors various elements. This makes it possible to derive the correct dark current value in relation to both the incident radiant energy and the spectral range investigated.

Measurements of atmospheric pollutants by a DOAS spectrometer in urban areas

A monitoring campaign of atmospheric pollutants was conducted in February 1993 by several of Italys CNR institute in heavily polluted greater Milan. This metropolitan area, the largest one in northern Italy, is situated in the northernmost part of the Po Valley and, because of its topography and orography is frequently marked by low ventilation and inversion phenomena, a fast that promotes the accumulation and vertical layering over the city of pollutants. The need for more detailed information on air circulation and changes occurring in the lower atmospheric layers, as well as to understand why air-mass exchange does not take place, thereby impeding the dispersion of pollutants, was the projects goal- orientation. Measurement of NO2, SO2, O3, HNO2 were carried out over a 1.7 Km path in the city center by means of a DOAS system called GASCOD developed by remote sensing group of FISBAT-CNR at Bologna. The light source has been equipped with a remote-controlled occulting devices in order to separate the sky light scattered into the field of bye of the receiving system, which can interfere with the lamp spectra during daytime. The light from the source is collected by a Cassegrain telescope and focused on the spectrographs entrance slit receiving system; the detector is a linear image sensor featuring an array of 512 MOS photodiodes. Data recorded in the same and boundary areas by a conventional analyzer from citys air-pollution monitoring network are reported for comparison. The statistical correlation of concentration values to the main weather and atmospheric stability parameters are stressed.

Ground-based NO2 and O3 analysis at Monte Cimone station during 1995 and 1996: a case study for spring 1995 NO2 concentration profiles

A UV/Vis DOAS spectrometer (GASCOD, Gas Analyzer Spectrometer Correlating Optical Differences) was installed at Monte Cimone station in 1993 and since then it has been measuring zenith scattered solar radiation at sunset and sunrise. During 1995 it was possible to investigate two spectral regions, about 50 nm width, centered at 365 nm and 436 nm while later we only have measurements at 436 nm available. The spectra obtained during the 1995 - 96 period have been processed with DOAS technique to obtain column amounts of NO2 and O3. The seasonal and diurnal variation of the NO2 column amounts is shown with a summer maximum (about 1.2 X 1017 mol(DOT)cm-2 for p.m. value and 6 X 1016 mol(DOT)cm-2 for a.m.) and winter minimum (about 2 X 1016 mol(DOT)cm-2 for a.m. and 5 X 1016 mol(DOT)cm-2 for p.m.). An anomalous spring increase in p.m. NO2 value during 1995 is investigated through a vertical distribution analysis. The gas profile is retrieved through a Chahine inversion algorithm applied to the slant columns measured at different solar zenith angle. In fact the air mass factor variation with solar zenith angle can be used to extract information about the gas concentration at each atmospheric layers. A consistent and frequent tropospheric increase in NO2 a.m. concentration is evident. The method and the results obtained are discussed.

Study of ozone transport at Mt. Cimone

Mt. Cimone, the highest peak in Italys north-central Apennines, divides two distinct climatic regions: the continentally temperate Po Valley to the north and the Mediterranean Basin to the south. It is considered a nonpolluted site, and the Italian Meteorological Service Observatory atop it, which is part of the WMO BAPMoN for the measurement of CO2, turbidity, and suspended particulate matter, has been continuously monitoring surface ozone since March 1991 together with the FISBAT Insititute. The present, preliminary study of ozone transport episodes indicates that the air masses reaching Mt. Cimone exhibit low surface-ozone concentrations under certain weather conditions, i.e. the arrival of dust-laden air from the Sahara area or in concomitance with particular low-pressure systems.