The 11-year solar cycle in stratospheric ozone: Comparison between Umkehr and SBUVv8 and effects on surface erythemal irradiance

Ozone profiles derived from ground-based Umkehr measurements at five stations and from the merged data set of Solar Backscattered Ultra Violet (SBUVv8) satellite observations are used to estimate the seasonal influence of the 11-year solar signal in the vertical distribution of stratospheric ozone. Both data sets show a strong response (2-3% of the annual mean) in the upper stratosphere, a very small response in the middle tropical stratosphere and a secondary maximum (∼2%) in the tropical lower stratosphere, in accordance to earlier reports. Thus a sequence of high-low-high response is seen in the vertical pattern of the solar cycle effect on ozone in the tropics, with its position sensitive to season, as it appears to move across the equator in direction opposite to the location of the maximum solar insolation. Results from Umkehr and satellite observations are in good agreement at levels up to 35 km, while discrepancies (of up to 2%) found at higher levels (around 40 km) are largest in summer. Finally, the estimated erythemal irradiance response to the 11-year solar cycle, calculated for the observed ozone change during a solar cycle (minimum to maximum), showed an overall decrease in the irradiance at the surface which is maximum (-2% of annual mean) over the tropics. Copyright 2007 by the American Geophyscial Union

Sensitivity of solar UV radiation to ozone and temperature profiles at Thessaloniki (40.5°N, 23°E), Greece

Measured ozone and temperature vertical profiles from Thessaloniki, Greece, were used together with the mid-latitude standard profiles as input data in a radiative transfer model. Calculations of direct and global solar irradiance, actinic flux, UV-B and CIE weighted integrals for solar zenith angles of 30°, 70° and 85° were performed and analyzed. Variable temperature values and ozone redistribution may change UV radiation reaching the surface significantly more than the proposed measurement uncertainties for high solar zenith angles. A specific measured profile corresponding to air masses of polar origin probed over Thessaloniki was selected and the differences in vertical distribution of UV-B radiation were discussed. Obtained results revealed that the use of an inappropriate temperature and ozone profile may lead to significant changes at small UV-B wavelengths and high solar zenith angles. In this case, the use of seasonal average vertical profiles of ozone and temperature for a given area may be carefully utilized when accurate model spectral calculations are needed and comparison with measurements in the troposphere performed. © 2005 Elsevier Ltd. All rights reserved

Search for man-made cirrus contrails over Southeast Asia

In this study we examine the effect of natural perturbations on cirrus cloud cover in the tropics and we look for possible signal of cirrus contrails in tropical air traffic regions focussing on Southeast Asia, a region that has received much less attention than the well investigated North Atlantic and North American air traffic corridors. The analysis is based on the latest version of the ISCCP D2 cirrus cloud dataset and covers the period 1984-2000. Four parameters were examined for their relation with cirrus cloud trends: deep convective clouds from ISCCP, vertical velocities, relative humidity and air temperature at 200 mb from ECMWF/ERA40. The results show that there is a strong correlation between cirrus clouds and dynamical parameters (deep convective clouds, vertical velocities) over Southeast Asia, explaining a significant part of the tropical cirrus cloud variability. After removing seasonality, the ENSO signal becomes dominant on cirrus, on deep convective clouds and on vertical velocities over regions of the western and the eastern tropical Pacific Ocean. Over Southeast Asia, the average decrease in cirrus during the strong 1997/98 El Nino event was about 6 % cloud cover or ∼25 % of the regional mean. In order to search for possible signal of cirrus contrails over S. E. Asia, we calculated trends in cirrus coverage over tropical regions with high air traffic after removing the ENSO effect. The results show that there is a small positive trend in cirrus clouds over the S. E. Asia air corridors during winter (+0.1 % cover/decade), which is not statistically significant and is related to small positive trends in deep convective clouds. In summer, cirrus clouds are anti-correlated with deep convective clouds over this region and the trends are opposite. The negative trends in cirrus clouds, which are observed in the summer (4.5% cover/decade), are related to trends in dynamical and thermo-dynamical parameters. It is shown that cirrus clouds are statistically significant correlated with vertical velocities and air temperature at 200 mb (correlations of -0.7 and -0.6, respectively), explaining the highest part of the long-term variability of cirrus clouds over S. E. Asia. Over the Caribbean air corridors, on the other hand, there are significant increases in cirrus cloudiness by about 2.5% per decade in winter (99% confidence level) and 2.7% cover/ decade in the summer (95% confidence level), part of which is also related to trends in dynamical and thermo-dynamical parameters. According to our findings, it is difficult to detect possible effects of regional persistent contrails on cirrus cloud trends over the S. E. Asia air traffic corridors. This is because in winter there are not statistically significant trends in cirrus clouds and in summer trends in dynamics and thermo-dynamics mask this issue. Taking also into account that flight frequencies and fuel consumption are moderate over the tropical air traffic corridors, it makes it even more difficult to detect and quantify any possible anthropogenic effects

Optical properties of Saharan dust layers as detected by a Raman lidar at Thessaloniki, Greece

Measurements during Saharan dust transport events were performed at several stations of the European Aerosol Research Lidar Network. During the period 2001-2002, 12 cases were captured at Thessaloniki, Greece (40.5°N, 22.9°E) with a 355 nm Raman lidar. For these cases the vertical profiles of the extinction and backscatter coefficients were determined and examined, as well as profiles of extinction-to-backscatter ratio (lidar ratio) and the backscatter-related Ångström exponent between 355 and 532 nm (color index). Model calculations from the DREAM model confirmed the existence of Saharan dust in all cases examined. Within the dust plumes the lidar ratios ranged from 20 to 100 sr and the color index ranged from -1 to 3. An anticorrelation was found between the lidar ratio and the color index during the Saharan dust events. In certain cases the results suggest that the mixing of dust particles with boundary layer aerosols reduces the dust 'signature' and thus, makes difficult the separation of the dust particles from those of mostly urban origin. Copyright 2004 by the American Geophysical Union

A study on natural and manmade global interannual fluctuations of cirrus cloud cover for the period 1984-2004

The seasonal variability and the interannual variance explained by ENSO and NAO to cirrus cloud cover (CCC) are examined during the twenty-year period 19842004. CCC was found to be significantly correlated with vertical velocities and relative humidity from ECMWF/ERA40 in the tropics (correlations up to -0.7 and +0.7 at some locations, respectively) suggesting that variations in large-scale vertical winds and relative humidity fields can be the origin of up to half of the local variability in CCC over these regions. These correlations reflect mostly the seasonal cycle. Although the annual cycle is dominant in all latitudes and longitudes, peaking over the tropics and subtropics, its amplitude can be exceeded during strong El Nino/La Nina events. Over the eastern tropical Pacific Ocean the interannual variance of CCC which can be explained by ENSO is about 6.8% and it is ∼2.3 times larger than the amplitude of the annual cycle. Natural long-term trends in the tropics are generally small (about -0.3% cloud cover per decade) and possible manmade trends in those regions are also small. The contributions of NAO and QBO to the variance of CCC in the tropics are also small. In the northern mid-latitudes, on the other hand, the effect of NAO is more significant and can be very important regionally. Over northern Europe and the eastern part of the North Atlantic Flight Corridor (NAFC) there is a small positive correlation between CCC and NAO index during the wintertime of about 0.3. In this region, the interannual variance of CCC explained by NAO is 2.6% and the amplitude of the annual cycle is 3.1%. Long-term trends over this region are about +1.6% cloud cover per decade and compare well with the observed manmade trends over congested air traffic regions in Europe and the North Atlantic as have been evidenced from earlier findings

Four-year aerosol observations with a Raman lidar at Thessaloniki, Greece, in the framework of European Aerosol Research Lidar Network (EARLINET)

Regular aerosol extinction and backscatter measurements using a UV Raman lidar have been performed from January 2001 to December 2004 at Thessaloniki, Greece (40.5°N, 22.9°E), in the framework of the European Aerosol Research Lidar Network (EARLINET). Profiles of the aerosol extinction coefficient, backscatter coefficient, and extinction-to-backscatter ratio (so-called "lidar ratio") were acquired under nighttime conditions and have been used for statistical investigations. The statistical analysis was made both for the planetary boundary layer and for the free troposphere. It was found that the 4-year mean boundary layer particle optical depth at 355 nm was 0.44 ± 0.18, and the total aerosol optical depth was 0.63 ± 0.27. Free tropospheric particles account on the average for 30% of the total aerosol optical depth, ranging from 5% (clean free troposphere conditions) to 55% (mainly Saharan dust events). For the cases examined, the integral of the lidar-derived extinction coefficient was in good agreement with colocated aerosol optical depth measurements at 355 nm obtained with a Brewer spectroradiometer. The mean value of the lidar ratio at 355 nm derived, for the period of measurements at Thessaloniki, was 40 sr with a standard deviation of 21 sr. Mean height profiles of the particle lidar ratio, extinction, and backscatter coefficients are shown along with their seasonal dependence, showing a significant seasonal variability in the free troposphere. An analysis of the data using back trajectories showed also a dependence of the aerosol optical depth and the lidar ratio on the origin of the air mass, with higher values mostly corresponding to air masses originating from the northeast Balkans and eastern Europe. Copyright 2005 by the American Geophysical Union

Are EARLINET and AERONET climatologies consistent? the case of Thessaloniki, Greece

In this study we investigate the climatological behavior of the aerosol optical properties over Thessaloniki during the years 2003–2017. For this purpose, measurements of two independent instruments, a lidar and a sunphotometer, were used. These two instruments represent two individual networks, the European Lidar Aerosol Network (EARLINET) and the Aerosol Robotic Network (AERONET). They include different measurement schedules. Fourteen years of lidar and sunphotometer measurements were analyzed, independently of each other, in order to obtain the annual cycles and trends of various optical and geometrical aerosol properties in the boundary layer, in the free troposphere, and for the whole atmospheric column. The analysis resulted in consistent statistically significant and decreasing trends of aerosol optical depth (AOD) at 355 nm of ĝ'23.2 and ĝ'22.3 % per decade in the study period over Thessaloniki for the EARLINET and the AERONET datasets, respectively. Therefore, the analysis indicates that the EARLINET sampling schedule can be quite effective in producing data that can be applied to long-term climatological studies. It is also shown that the observed decreasing trend is mainly attributed to changes in the aerosol load inside the boundary layer. Seasonal profiles of the most dominant aerosol mixture types observed over Thessaloniki have been generated from the lidar data. The higher values of the vertically resolved extinction coefficient at 355 nm appear in summer, while the lower ones appear in winter. The dust component is more dominant in the free troposphere than in the boundary layer during summer. The biomass burning layers tend to arrive in the free troposphere during spring and summer. This kind of information can be quite useful for applications that require a priori aerosol profiles. For instance, they can be utilized in models that require aerosol climatological data as input, in the development of algorithms for satellite products, and also in passive remote-sensing techniques that require knowledge of the aerosol vertical distribution. © 2018 by the authors

On the Retrieval of Volcanic Sulfur Dioxide Emissions from GOME Backscatter Measurements.

Abstract not availableJRC.I-Institute for Health and Consumer Protection (Ispra

Raman lidar and sunphotometric measurements of aerosol optical properties over Thessaloniki, Greece during a biomass burning episode

The influence of biomass burning smoke on the aerosol loading in the free troposphere over Thessaloniki, Greece (40.5°N, 22.9°E) is discussed in this paper. A selected case during summer 2001 is presented, when very high aerosol optical depth values were observed, benefiting from the synergy of various remote sensing instruments. The data that were collected allow the characterization of the optical properties of the aerosols in this region, where only little information has so far been available. Four-day back trajectories indicated that air masses were advected from Bulgaria and the northern coast of the Black Sea, where strong forest fires occurred in early August 2001. In order to investigate the optical properties of biomass burning aerosols, we used a two-wavelength lidar system that combines Raman and elastic-backscatter observations, in addition to a Brewer spectrophotometer, a nephelometer and a multi-filter rotating shadowband radiometer. The lidar measurements on 9 August 2001 recorded an integrated aerosol optical depth at 355nm of the order of 1.35 during cloud-free conditions. The estimated mean extinction-to-backscatter ratios from the Raman lidar were 60sr for 355nm and 50sr for 532nm. Estimated values of the single scattering albedo, using spectral UV measurements and modeling were of the order of 0.90, consistent with previous findings, indicating a weak contribution of absorption to the total extinction. The Angstrom exponent, calculated from the multi-filter rotating shadowband radiometer exhibited also high values around 1.78, indicating the presence of rather small particles. © 2003 Elsevier Ltd. All rights reserved