Ioannis C. Ziomas

Spectral measurements of solar UVB radiation and its relations to total ozone, SO2, and clouds

Spectral solar UV radiation measurements performed at Thessaloniki, Greece (40°N), are presented, and the influence of various atmospheric constituents such as total ozone, cloud cover, and columnar SO2 on these measurements is examined. By comparing UV radiation measurements at days with different total ozone amounts the magnification factor was calculated. Its values range from 1 to 20, depending on the wavelength and the total ozone. A relationship between the UV radiation and the cloud cover was established, being representative only for measurements at 50° solar zenith angle. In addition, the influence of columnar SO2 variations on UV irradiances was also studied. Finally, an attempt was made to compare the relative influence of these parameters on UV radiation, which proved that total ozone is the major factor controlling the solar UVB radiation received at the ground.

Record low total ozone during northern winters of 1992 and 1993

The last two winter-spring seasons (DJFM) distinguished themselves by being with the lowest ever total ozone over all three continental size regions between 45°N and 65°N of North America, Europe and Siberia. The total ozone deficiencies for the entire season over all of the above mentioned regions were about 11% and 13% below the long-term normal during the two consecutive years (1991/92 and 1992/93 respectively ). This helped to pull down the cumulative ozone decline since the winter-spring of 1969/70 to be about 14% in the latitude belt of the 45°N–65°N. Frequencies of days with ozone values deviating below the long-term mean by more than 2σ have been ten times higher than their 35-year average. There are evidences deduced from trajectories on potential temperature surfaces that transport of poor in ozone air masses forced in addition by vertical motions, could account for a number of the extreme cases. There is also evidence that cold air, known to have excess ClO content, has moved over the sun lighted latitudes on many occasions, when chemical ozone destruction could have been favored. These ozone deficiencies do not have similar rates of decline and did not reach even close to the extreme low values regularly observed during the Antarctic-spring ozone hole phenomena.

Forecasting peak pollutant levels from meteorological variables

The main objective of this paper is to present analytical models relating maximum pollutant concentrations in urban areas with meteorological and other variables. The analysis is based on measurements from Greater Athens Area and is restricted in only one pollutant of special interest, namely N02. The meteorological variables, used in analytical modeling for forecasting pollution concentrations, cover the most important atmospheric processes favoring pollution episodes. The selection of the variables was based both on extensive correlation analysis and on the existing knowledge from the scientific literature. The evaluation of the developed forecasting models showed that their degree of success is promising. The final model equations derived are simple and they can be used easily for operational forecasts from the air quality management authorities.

A note on the recent increase of Solar UV‐B radiation over northern middle latitudes

In this work we present the results from the analysis of regular solar UV-B spectral measurements obtained at Thessaloniki (40°N) during a period of three years, from November 1990 through November 1993. The results presented here refer to both clear-skies (cloudiness ≤3/8) as well as to all-skies conditions, and to solar zenith angles of 63°±1°. These results support earlier findings of increased solar UV-B irradiances, under all-skies conditions, associated with observed ozone decline at the same period. The observed acceleration of the ozone decline-of about 5% per year-during the three-year period resulted to increased solar UV irradiance levels at Thessaloniki by 12.3% per year at 305 nm and 2.7% per year at 325 nm. These figures are slightly modified under all-skies conditions, and they are in agreement with findings reported independently by other scientists.

On the relative importance of quasi-biennial oscillation and El Nino/Southern Oscillation in the revised Dobson total ozone records

The interannual variability of the ozone layer is studied with the largest available revised Dobson (RD) total ozone records through March 1991. The analysis of this new data set, as well as the revised Total Ozone Mapping Spectrometer (TOMS) zonal mean total ozone, shows that the phase of the ozone Quasi-Biennial Oscillation (QBO) relative to the 50 hPa zonal wind at Singapore varies linearly with latitude, being nearly in-phase at low latitudes and more symmetric about the equator than published in earlier studies. The phase progression of the QBO is about 2 months per 10 degrees of latitude, ozone following the 50 hPa zonal wind at Singapore. After removing the QBO and solar cycles from the RD total ozone records, a relatively large part of the remaining total ozone variance can be explained by El Nino/Southern Oscillation (ENSO) events in the tropics. It is also shown that only very large ENSO events (1982–1983 and possibly 1972) are followed within a few months time lag by low total ozone values in middle and even higher latitudes. This analysis also shows that at individual stations there are other significant total ozone anomalies which cannot be explained by the QBO or ENSO. Tentative circulational aspects of the ENSO disturbance and its relation to total ozone are also discussed.

Quasi-continuous measurements of non-methane hydrocarbons (NMHC) in the Greater Athens area during medcaphot-trace

Abstract During the comprehensive field campaign MECAPHOT-TRACE (20 August to 20 September 1994) quasi-continuous measurements of NMHC in the range of C 4 –C 12 were carried out for the first time in the Greater Athens area simultaneously at two sites, by means of on-line gas-chromatographic methods. The results show a heavy load of hydrocarbons in ambient air, especially during stagnant weather conditions that favour the development of land–sea-breeze circulations. Overall the urban hydrocarbon mix reflects the emission of traffic. This influence is confirmed through correlation analyses of hydrocarbon ratios, especially aromatic compound ratios, and their dependence on both wind direction and wind velocity at a suburban receptor site. The ethylbenzene/ m -xylene-ratio turned out to be a good indicator for the impact of anthropogenically related hydrocarbon chemistry leading to the formation of secondary pollutants. Episodes will be discussed that elucidate the importance of hydrocarbons in the course of ozone formation.

Atmospheric aerosol and gaseous species in Athens, Greece

Abstract Measurements of aerosol species including Cl - , NO - 3 , SO 2- 4 , NH + 4 , Na + , K + , Mg 2+ and Ca 2+ and gaseous pollutants like HCl, HNO 3 , NO 2 , SO 2 and O 3 were conducted at the centre of Athens. Results show moderate levels of pollution with aerosol species typical of dust emissions and secondary aerosol of anthropogenic origin. Most gaseous pollutants correlate well with aerosol species. There is a pronounced elevation in the concentration of acidic trace gases HCl and HNO 3 arriving at the city from the direction of the Saronikos gulf, indicating poor neutralisation of such species over the sea. On the contrary, air masses coming from inland are characterised by considerably higher ammonium aerosol. It was concluded that in addition to heavier aerosol load pollution episodes in Athens during the summer period are associated with higher concentrations of gaseous acids.

Role of urban and suburban aerosols on solar UV radiation over Athens, Greece

The scope of this paper is to provide an experimental and modelling contribution on the role of suspended lower tropospheric particles to the UV spectral irradiance reaching the ground in urban environments. The UV spectral irradiance was measured with a UV spectrophotometer, while the aerosol data were obtained simultaneously by a lidar system operating in the 300 nm spectral region. Both UV and lidar instruments were operated inside the Athens basin during various air pollution and meteorological conditions, in the frame of the European MEDCAPHOT-TRACE Campaign in summer 1994. Aerosol vertical profiles were also obtained by a second lidar system operated outside the Athens basin (in a sub-urban area) during the same time period and were intercompared to the urban profiles. Using the lidar data as input to a radiative transfer model (MODTRAN), we provide comparisons of measured and modelled spectral UV irradiances under distinct environmental conditions.

Boundary layer dynamics in an urban coastal environment under sea breeze conditions

Abstract A three-dimensional, higher-order turbulence closure model is applied to simulate the boundary layer dynamics in Attica peninsula, where the city of Athens is located. All model equations are transformed to a terrain influenced coordinate system, which enables studies of flow over complex terrain. Major characteristics of the sea breeze circulation including vertical extension and intensity as well as the growth rate of the thermal internal boundary layer are identified and discussed. Model predictions agree fairly well with measured profiles at one location and measured surface winds in the center of Athens. In addition to the basic simulations, a series of sensitivity tests has been performed in order to evaluate the relative importance of the various input parameters. It was found that some parameters (topography, the amplitude; of the diurnal variation of the temperature over the land) have relatively larger impact on the development of sea breeze circulation over the Athens basin than others. The sea breeze circulation developing at the east side of the Attica peninsula is particularly sensitive to the input geostrophic wind.

Tropospheric LIDAR aerosol measurements and sun photometric observations at Thessaloniki, Greece

We present measurements of the vertical structure of the aerosol backscattering coefficient in the lower troposphere, which have been performed at the city of Thessaloniki in N. Greece, during the years 1996 and 1997. A ground-based backscatter lidar system operated throughout the year, mostly around local noon hours. The lidar measurements were accompanied by measurements of the aerosol optical depth in the visible spectral region, using a CIMEL sun-tracking photometer. The seasonal variation of the aerosol loading and its vertical distribution in the lower troposphere over the city of Thessaloniki is discussed. The maximum values of the aerosol optical depth are found during the spring season. Indication about the origin of these maxima is given by inspection of the various aerosol layers observed in the lidar profiles. Most of the aerosol loading is present in the first 3 km height, and only in rare cases there are important aerosol layers detectable above 3 km, as in a case of Saharan dust transported over the city of Thessaloniki, in May 1997. Both instruments used in this study show similar seasonal variation of the aerosol load. It was found that almost 85% of the aerosol load is located in the layer below 3 km. There is a bias between the CIMEL and lidar derived optical depth at 532 nm, mainly attributed to the aerosols present between ground level and 600 m height, which represent up to 50% of the total aerosol optical depth.