Georg Hansen

Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and ...

A trajectory-based estimate of the tropospheric ozone column using the residual method

We estimate the tropospheric column ozone using a forward trajectory model to increase the horizontal resolution of the Aura Microwave Limb Sounder (MLS) derived stratospheric column ozone. Subtracting the MLS stratospheric column from Ozone Monitoring Instrument total column measurements gives the trajectory enhanced tropospheric ozone residual (TTOR). Because of different tropopause definitions, we validate the basic residual technique by computing the 200-hPa-to-surface column and comparing it to the same product from ozonesondes and Tropospheric Emission Spectrometer measurements. Comparisons show good agreement in the tropics and reasonable agreement at middle latitudes, but there is a persistent low bias in the TTOR that may be due to a slight high bias in MLS stratospheric column. With the improved stratospheric column resolution, we note a strong correlation of extratropical tropospheric ozone column anomalies with probable troposphere-stratosphere exchange events or folds. The folds can be identified by their colocation with strong horizontal tropopause gradients. TTOR anomalies due to folds may be mistaken for pollution events since folds often occur in the Atlantic and Pacific pollution corridors. We also compare the 200-hPa-to-surface column with Global Modeling Initiative chemical model estimates of the same quantity. While the tropical comparisons are good, we note that chemical model variations in 200-hPa-to-surface column at middle latitudes are much smaller than seen in the TTOR.

Ozone depletion in and below the Arctic vortex for 1997

The winter 1996/97 was quite unusual with late vortex formation and polar stratospheric cloud (PSC) development and subsequent record low temperatures in March. Ozone depletion in the Arctic vortex is determined using ozonesondes. The diabatic cooling is calculated with PV-theta mapped ozone mixing ratios and the large ozone depletions, especially at the center of the vortex where most PSC existence was predicted, enhances the diabatic cooling by up to 80%. The average vortex chemical ozone depletion from January 6 to April 6 is 33, 46, 46, 43, 35, 33, 32 and 21 % in air masses ending at 375, 400, 425, 450, 475, 500, 525, and 550 K (about 14–22 km). This depletion is corrected for transport of ozone across the vortex edge calculated with reverse domain-filling trajectories. 375 K is in fact below the vortex, but the calculation method is applicable at this level with small changes. The column integrated chemical ozone depletion amounts to about 92 DU (21%), which is comparable to the depletions observed during the previous four winters.

Match observations in the Arctic winter 1996/97: High stratospheric ozone loss rates correlate with low temperatures deep inside the polar vortex

With the Match technique, which is based on the coordinated release of ozonesondes, chemical ozone loss rates in the Arctic stratospheric vortex in early 1997 have been quantified in a vertical region between 400 K and 550 K. Ozone destruction was observed from mid February to mid March in most of these levels, with maximum loss rates between 25 and 45 ppbv/day. The vortex averaged loss rates and the accumulated vertically integrated ozone loss have been smaller than in the previous two winters, indicating that the record low ozone columns observed in spring 1997 were partly caused by dynamical effects. The observed ozone loss is inhomogeneous through the vortex with the highest loss rates located in the vortex centre, coinciding with the lowest temperatures. Here the loss rates per sunlit hour reached 6 ppbv/h, while the corresponding vortex averaged rates did not exceed 3.9 ppbv/h.

Evidence of substantial ozone depletion in winter 1995/96 over northern Norway

In winter 1995/96, the stratospheric ozone layer over most of the European Arctic was exposed to extremely low temperatures and strong PSCs. At the same time, total ozone monitoring instruments at Tromso and Andoya, Northern Norway, observed values up to 50% below the long-term average values of the Dobson instrument at Tromso. The deviation increased systematically with time of the year, reaching a maximum in early March. Measurements with the ozone DIAL system at the ALOMAR facility at Andoya showed significant reduction of ozone mixing ratios in the 430 to 580 K potential temperature region. A maximum reduction of almost 50% occurred at about 500 K between early January and early March. Comparison with 3D model calculations revealed large reductions with respect to passive tracer ozone profiles in the 400 to 600 K region, reaching a maximum of about 60% by the end of March at the 475 K level. The 3D chemical model showed ozone depletion in the same altitude region as the measurements but seemed to underestimate the depletion systematically. The comparison also indicated ozone depletion of up to 30% in the 475 to 675 K region in extra-vortex air which remained close to the vortex edge during a period in mid-March.

Determining the index of refraction of polar stratospheric clouds above Andoya (69°N) by combining size‐resolved concentration and optical scattering measurements

Observations within two polar stratospheric clouds (PSCs) of aerosol scattering and size-resolved aerosol concentration were compared to infer the index of refraction of the PSC particles. The observations were completed in situ with balloon-borne aerosol counters and a laser scatterometer (692, 830 nm) and remotely with an ozone (308, 353 nm) and Rayleigh (532, 1064 nm) lidar. A Monte Carlo analysis, accounting for the errors of the individual measurements, indicates the comparison method has a precision of ±0.03 for an index of refraction range of 1.30–1.60. Measurements from all instruments were obtained in one PSC with relatively little vertical or horizontal structure. The comparison suggested that the index of refraction of the PSC particles was near 1.47±0.01 in the nondepolarizing region of the cloud and 1.52–1.56±0.04 in the depolarizing region. These values were consistent for the observations at 308, 353, 692, and 830 nm. The comparisons with the Rayleigh lidar were not as consistent. Aerosol volumes inferred from the particle measurements agree closely with volumes expected for liquid ternary aerosol (LTA) at the base of the cloud, with nitric acid trihydrate (NAT) above 23 km, in the depolarizing region, and with both LTA and NAT in the bulk of the nondepolarizing portion of the cloud. A much more limited set of measurements was obtained in a second PSC with strong vertical structure, evident in the temperature and aerosol profiles. Comparisons in this cloud were difficult because of the inherent problems in comparing in situ and remote measurements in clouds with strong vertical and horizontal structure. In this PSC the comparisons of in situ aerosol size distribution and remote aerosol scattering did not converge to a clear index of refraction.

Ozone depletion at the edge of the Arctic polar vortex 1996/1997

In winter 1996/1997 the Arctic polar stratospheric vortex was extremely long-lived. During most of its lifetime the vortex was centered at the pole, and its edge was almost permanently located over Northern Scandinavia. The ozone lidar at the Arctic Lidar Observatory for Middle Atmosphere Research (ALOMAR) was operated from mid-December 1996 until mid-May 1997 when the final breakup of the vortex occurred. Comparison of the measurements with three-dimensional model calculations reveals ozone depletion of up to 40% at the 475 and 550 K level. Maximum depletion occurred by around May 5 at levels up to 550 K and by around April 20 at 675 K. Analysis of the chemical model shows that while much of the early spring ozone depletion was due to halogen chemistry, associated with chlorine activation on polar stratospheric clouds, the ongoing depletion in late April and early May was due to “summertime” NOx (= NO + NO2) chemistry. The unusual persistence of the vortex, with the isolation of high-latitude air masses until early May, permitted the occurrence of this depletion. The observations of air masses with low O3 show a very sudden end around May 10, indicating that even in the final phase of its existence, with continuously decreasing potential vorticity, some vortex air masses were well-confined and mixing with the surroundings was small. A possible explanation of the strong confinement of vortex air is the scarcity of disturbances due to wave activity from below the vortex. It is supported by the frequent observation of intravortex layering in spring 1997.

Evaluation of Global Ozone Monitoring Experiment (GOME) ozone profiles from nine different algorithms

An evaluation is made of ozone profiles retrieved from measurements of the nadir-viewing Global Ozone Monitoring Experiment (GOME) instrument. Currently four different approaches are used to retrieve ozone profile information from GOME measurements, which differ in the use of external information and a priori constraints. In total nine different algorithms will be evaluated exploiting the Optimal Estimation (Royal Netherlands Meteorological Institute, Rutherford Appleton Laboratory, University of Bremen, National Oceanic and Atmospheric Administration, Smithsonian Astrophysical Observatory), Phillips-Tikhonov Regularization (Space Research Organization Netherlands), Neural Network (Center for Solar Energy and Hydrogen Research, Tor Vergata University), and Data Assimilation (German Aerospace Center) approaches. Analysis tools are used to interpret data sets that provide averaging kernels. In the interpretation of these data, the focus is on the vertical resolution, the indicative altitude of the retrieved value, and the fraction of a priori information. The evaluation is completed with a comparison of the results to lidar data from the NDSC (Network for Detection of Stratospheric Change) stations in Andoya (Norway), Observatoire Haute Provence (France), Mauna Loa (USA), Lauder (New Zealand) and Dumont d’Urville (Antarctic) for the years 1997–1999. In total the comparison involves nearly 1000 ozone profiles, and allows the analysis of GOME data measured in different global regions and hence observational circumstances. The main conclusion of this paper is that unambiguous information on the ozone profile can at best be retrieved in the altitude range 15–48 km with a vertical resolution of 10 to 15 km, precision of 5–10%, and a bias up to 5% or 20% depending on the success of recalibration of the input spectra. The sensitivity of retrievals to ozone at lower altitudes varies from scheme to scheme and includes significant influence from a priori assumptions.

Narrowband filter radiometer for ground-based measurements of global ultraviolet solar irradiance and total ozone

The ultraviolet narrowband filter radiometer (UV-RAD) designed by the authors to take ground-based measurements of UV solar irradiance, total ozone, and biological dose rate is described, together with the main characteristics of the seven blocked filters mounted on it, all of which have full widths at half maxima that range 0.67 to 0.98 nm. We have analyzed the causes of cosine response and calibration errors carefully to define the corresponding correction terms, paying particular attention to those that are due to the spectral displacements of the filter transmittance peaks from the integer wavelength values. The influence of the ozone profile on the retrieved ozone at large solar zenith angles has also been examined by means of field measurements. The opportunity of carrying out nearly monochromatic irradiance measurements offered by the UV-RAD allowed us to improve the procedure usually followed to reconstruct the solar spectrum at the surface by fitting the computed results, using radiative transfer models with field measurements of irradiance. Two long-term comparison campaigns took place, showing that a mean discrepancy of +0.3% exists between the UV-RAD total ozone values and those given by the Brewer #63 spectroradiometer and that mean differences of +0.3% and -0.9% exist between the erythemal dose rates determined with the UV-RAD and those obtained with the Brewer #63 and the Brewer #104 spectroradiometers, respectively.

Comparison of observed and calculated incoherent scatter spectra from the D region

During August 1989, extended twilight and nighttime measurements with the European incoherent scatter (EISCAT) UHF radar were performed under PCA conditions. This provided an excellent data quality in the altitude region of 70 to 90 km throughout the three nights of August 12 to 15, 1989. A sophisticated control program allowed the measurement of the spectral width in the altitude region mentioned. In general, the measured spectral width deviates significantly from model values based on temperatures measured simultaneously by Na lidar combined with CIRA 88 temperature and density values. The observed spectra are up to 2 or 3 times narrower. In our observations the deviation tends to increase with increasing altitude. We also find that earlier spectral width measurements published by other workers are often narrower than current D region theory predicts. The possible reasons for this phenomenon are discussed.