V. Rizi

EARLINET correlative measurements for CALIPSO: First intercomparison results

A strategy for European Aerosol Research Lidar Network (EARLINET) correlative measurements for Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) has been developed. These EARLINET correlative measurements started in June 2006 and are still in progress. Up to now, more than 4500 correlative files are available in the EARLINET database. Independent extinction and backscatter measurements carried out at high-performance EARLINET stations have been used for a quantitative comparison with CALIPSO level 1 data. Results demonstrate the good performance of CALIPSO and the absence of evident biases in the CALIPSO raw signals. The agreement is also good for the distribution of the differences for the attenuated backscatter at 532 nm ((CALIPSO-EARLINET)/EARLINET (%)), calculated in the 1–10 km altitude range, with a mean relative difference of 4.6%, a standard deviation of 50%, and a median value of 0.6%. A major Saharan dust outbreak lasting from 26 to 31 May 2008 has been used as a case study for showing first results in terms of comparison with CALIPSO level 2 data. A statistical analysis of dust properties, in terms of intensive optical properties (lidar ratios, Angstrom exponents, and color ratios), has been performed for this observational period. We obtained typical lidar ratios of the dust event of 49 ± 10 sr and 56 ± 7 sr at 355 and 532 nm, respectively. The extinction-related and backscatter-related Angstrom exponents were on the order of 0.15–0.17, which corresponds to respective color ratios of 0.91–0.95. This dust event has been used to show the methodology used for the investigation of spatial and temporal representativeness of measurements with polar-orbiting satellites.

Measurement of the pressure dependence of air fluorescence emission induced by electrons

The fluorescence detection of ultra high energy (>10^18 eV) cosmic rays requires a detailed knowledge of the fluorescence light emission from nitrogen molecules, which are excited by the cosmic ray shower particles along their path in the atmosphere. We have made a precise measurement of the fluorescence light spectrum excited by MeV electrons in dry air. We measured the relative intensities of 34 fluorescence bands in the wavelength range from 284 to 429 nm with a high resolution spectrograph. The pressure dependence of the fluorescence spectrum was also measured from a few hPa up to atmospheric pressure. Relative intensities and collisional quenching reference pressures for bands due to transitions from a common upper level were found in agreement with theoretical expectations. The presence of argon in air was found to have a negligible effect on the fluorescence yield. We estimated that the systematic uncertainty on the cosmic ray shower energy due to the pressure dependence of the fluorescence spectrum is reduced to a level of 1% by the AIRFLY results presented in this paper.

High-speed civil transport impact: Role of sulfate, nitric acid trihydrate, and ice aerosols studied with a two-dimensional model including aerosol physics

Recent assessment studies have shown that heterogeneous chemistry could have a significant role on the model-predicted ozone changes due to gas injection from high-speed civil transport (HSCT) aircraft. One major limitation of these numerical experiments was the highly simplified scheme adopted for the aerosol particles and, in particular, the absence of any explicit feedback between the gas phase chemistry included in the models and the total aerosol surface density available for heterogeneous reactions. In this paper we describe a two-dimensional model covering the whole stratosphere and troposphere which includes photochemical reactions for the sulfur cycle and a microphysical code for sulfuric acid aerosols. Starting from these particles, the same code predicts also the size distribution for nitric acid trihydrate (NAT) and ice aerosols, covering globally a particle radius range between 0.01 μm and about 160 μm. A rather simple scheme is described for nucleation and condensation processes leading to the formation and growth of NAT and ice particles, still using grid point temperature data taken from the zonally averaged climatology of the lower stratosphere. A discussion is made of the HSCT impact on ozone adopting different scenarios for the aerosols. Model results for the aerosol size distribution and for the available surface densities appear reasonable when compared to satellite and balloon measurements and to independent numerical calculations. As pointed out also by previous research work and assessment panels, our calculation shows that the ozone sensitivity to HSCT emissions largely decreases when heterogeneous chemistry is included with respect to a pure gas phase chemistry case. In addition, our results indicate that the ozone sensitivity to HSCT emission decreases even more when NAT and ice aerosols are present: this is a consequence of the aerosol-induced stratospheric denitrification which makes the residence time of the injected odd nitrogen shorter and the relative weight of the NOx catalytic cycle smaller. Inclusion of the sulfur dioxide feedback with the sulfate aerosol surface does not change significantly the ozone depletion in our model simulation, at least in the pure sulfate case. The additional ozone change due to aircraft injection of SO2 is larger when NAT and ice aerosols are allowed to form, due to the decreased ozone sensitivity to NOx. In this version of the model no direct aircraft emission of particulate has been included as a possible source for additional condensation nuclei.

Raman lidar observations of cloud liquid water

We report the design and the performances of a Raman lidar for long-term monitoring of tropospheric aerosol backscattering and extinction coefficients, water vapor mixing ratio, and cloud liquid water. We focus on the systems capabilities of detecting Raman backscattering from cloud liquid water. After describing the system components, along with the current limitations and options for improvement, we report examples of observations in the case of low-level cumulus clouds. The measurements of the cloud liquid water content, as well as the estimations of the cloud droplet effective radii and number densities, obtained by combining the extinction coefficient and cloud water content within the clouds, are critically discussed.

Sensitivity of stratospheric ozone to heterogeneous chemistry on sulfate aerosols

The effects of sulfate aerosols on stratospheric ozone have been studied with a 2D model. The model includes a comprehensive chemical code for homogeneous and heterogeneous reactions; both background and volcanic aerosols have been taken into account according to different scenarios. The basic temperature field is prescribed, while the perturbation introduced by the presence of volcanic aerosols is predicted. Increase of total chlorine in the stratosphere causes a well known ozone depletion by itself, but. the effects could be highly enhanced in presence of a large amount of volcanic aerosols that affect the balance of nitrogen and chlorine reservoirs through heterogeneous chemical reactions. For a volcanic eruption similar to El Chichon we show that the maximum O3 depletion (7% for a 2.5ppbv amount of stratospheric chlorine and 12% by doubling Cl) is to be expected at high latitudes in spring, because of a large chemical destruction acting in situ.

Continuous lidar measurements of stratospheric aerosols and ozone after the Pinatubo eruption Part II: Time evolution of ozone profiles and of aerosol properties

Two lidar systems, an aerosol lidar and an O3 Differential Absorption Lidar (DIAL), have been routinely operated at the same site (L‧Aquila, Italy; 42°N, 13°E) since August 1991. The multiwavelength analysis of the lidar signals allows to retrieve parameters related to equivalent aerosol size distributions and their optical properties. These are needed to correct the ozone DIAL profiles from the disturbance introduced by the stratospheric volcanic aerosols. The method and the confidence of the retrieved ozone profiles are discussed in a companion paper. Here we present the whole measurement series of ozone and backscattering ratio profiles during the period from August 1991 to December 1992. In addition, for some observations, the mode radius and the dispersion of the representative aerosol size distribution are reported. The time evolutions of aerosol surface area density and mass mixing ratio are also discussed within the uncertainties of the retrieval algorithm.

A novel method for the absolute fluorescence yield measurement by AIRFLY

One of the goals of the AIRFLY (AIR FLuorescence Yield) experiment is to measure the absolute fluorescence yield induced by electrons in air to better than 10% precision. We introduce a new technique for measurement of the absolute fluorescence yield of the 337 nm line that has the advantage of reducing the systematic uncertainty due to the detector calibration. The principle is to compare the measured fluorescence yield to a well known process—the Cherenkov emission. Preliminary measurements taken in the BFT (Beam Test Facility) in Frascati, Italy with 350 MeV electrons are presented. Beam tests in the Argonne Wakefield Accelerator at the Argonne National Laboratory, USA with 14 MeV electrons have also shown that this technique can be applied at lower energies.

Continuous lidar measurements of stratospheric aerosols and ozone after the Pinatubo eruption. Part I: Dial ozone retrieval in presence of stratospheric aerosol layers

We report stratospheric DIAL (DIfferential Absorption Lidar) ozone observations in presence of large amount of volcanic aerosols, which strongly affect the inversion of the backscattering signals. The retrieval method is based on the use of three wavelengths, two from the O3 DIAL (308 and 351nm) and a third one from an aerosol lidar (589nm) operated at the same site (L‧Aquila, Italy; 42°N,13°E). For each measurement session the relationships between the backscattering ratios at different wavelengths are fitted with an appropriate size distribution of the aerosols. The aerosol optical properties determined with this procedure allow to correct the ozone profiles. The ozone data are compared whenever possible with balloon ozonesonde measurements taken at S.Pietro Capofiume (Italy, 45°N, 11°E) or Hohenpeissenberg (Germany, 48°N, 11°E) and they generally show a good agreement. The ozone profiles can be retrieved with an indetermination that, within the aerosol layer, ranges between ±15 to ±35%. A systematic intercomparison is reported with data of other stations to assess the reliability of the method.

Simultaneous stratospheric aerosol and ozone lidar measurements after the Pinatubo volcanic eruption

Preliminary results of simultaneous ozone and aerosol lidar measurements taken in the period July-early October 1991 are presented. The main problem for retrieving the ozone profile is the correction of the on and off DIAL signals. The backscattering ratio obtained by the off signal is used for this purpose and ozone profiles are validated against ozonesonde data. A number of cases are presented including a few occasions of layers with large backscattering ratio. For these events the differences between ECC sonde and DIAL range between 5% to 20% and indicate that the method adopted for correction needs improvements especially for very thin layers. Sensitivity tests show that the retrieved ozone may change by roughly 10% depending on the choice of the aerosol optical parameters. Absolute difference between ECC and DIAL could not be completely explained in terms of the uncertainty in those parameters. The data obtained may still be used to prove the effects of heterogeneous chemistry on ozone destruction rate. Given the uncertainties introduced by the aerosol correction large amount of data need to be accumulated.

Lidar observation of spherical particles in a −65° cold cirrus observed above Sodankyla (Finland) during S.E.S.A.M.E.

Abstract The absence of LIDAR depolarization in a polar cirrus observed above Sodankyla (Finland) showed the presence of spherical particles at −65°C. The presence of pure liquid water is excluded, since homogeneous freezing should occur at or above −40°C. The cirrus layer was detected at the tropopause, during the horizontal and vertical advection above northern Scandinavia of warm and wet oceanic air. Two alternative explanations are suggested, the first one involving the presence of large, deliquescent tropospheric CN, freezing at very low temperature, and the second one involving the presence of metastable, spherical ice-particles produced in the fast adiabatic cooling of the airmasses. The second hypothesis is less reasonable because the absence of depolarized LIDAR signal implies a strict cylindrical symmetry in the particles, a symmetry that is difficult to maintain during the freezing process.