Petteri Taalas

The impact of greenhouse gases and halogenated species on future solar UV radiation doses

The future development of stratospheric ozone layer depends on the concentration of chlorine and bromine containing species. The stratosphere is also expected to be affected by future enhanced concentrations of greenhouse gases. These result in a cooling of the winter polar stratosphere and to more stable polar vortices which leads to enhanced chemical depletion and reduced transport of ozone into high latitudes. One of the driving forces behind the interest in stratospheric ozone is the impact of ozone on solar UV-B radiation. In this study UV scenarios have been constructed based on ozone predictions from the chemistry-climate model runs carried out by GISS, UKMO and DLR. Since cloudiness, albedo and terrain height are also important factors, climatological values of these quantities are taken into account in the UV calculations. Relative to 1979–92 conditions, for the 2010–2020 time period the GISS model results indicate a springtime enhancement of erythemal UV doses of up to 90% in the 60–90 °N region and an enhancement of 100% in the 60–90 °S region. The corresponding maximum increases in the annual Northern Hemispheric UV doses are estimated to be 14% in 2010–20, and 2% in 2040–50. In the Southern Hemisphere 40% enhancement is expected during 2010–20 and 27% during 2040–50.

Comparison of daily UV doses estimated from Nimbus 7/TOMS measurements and ground‐based spectroradiometric data

During recent years, methods have been developed for estimating UV irradiance reaching the Earths surface using satellite-measured backscattered UV radiances. The NASA-developed method is based on radiative transfer calculations and satellite measurements of parameters affecting UV radiation: extraterrestrial solar irradiance, atmospheric ozone, cloud reflectivity, aerosol amounts, and ground albedo. In this work a comparison is made between daily UV erythemal doses estimated from Nimbus-7/TOMS measurements (from 1991 to May 1993) and those calculated from ground-based spectroradiometer data. Three stations operated by the National Science Foundation were chosen for this comparison: Ushuaia, Argentina (for 573 days). Palmer, Antarctica (for 450 days), and San Diego, California, (for 149 days). These stations were selected to illustrate the differences between ground-based measurements using the same type of instrument, SUV-100 double monochromator spectroradiometers. and satellite estimates of surface UV irradiance under three different environmental conditions (mountains and snow, nearly continuous snow cover, and midlatitude urban sea level conditions). Averaging the measured and TOMS-estimated doses over periods from I week to I month improves the agreement. The daily or monthly mean bias increases during months when there is snow/ice on the surface. TOMS has a larger estimate of the UV irradiance by 25% at San Diego (no snow), in agreement with the summer-month analysis of Toronto irradiances [Herman et al., 1999]. TOMS underestimates the average daily-UV dose at Ushuaia (monthly mean bias of -13%) and at Palmer (-35%) consistent with snow/ice with cloud effects not being properly accounted for in the TOMS algorithm. When the reflectivity at all three sites is low (no snow), the TOMS irradiance estimate is larger than the SUV-100 measurements consistent with previously analyzed Brewer data at Toronto. The effects of local fog or clouds smaller than the satellite field of view and undetected UV-absorbing aerosols near the ground are discussed. In addition to uncertainties in radiometric calibrations of the spectrometers, none of the SUV-100 data are corrected for deviations of diffuser-transmittance from true cosine response.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

Effect of stratospheric ozone variations on UV radiation and on tropospheric ozone at high latitudes

A negative trend of stratospheric ozone has been observed especially at high southern and northern latitudes during the last 15 years. At the stations studied here a negative trend in total ozone was detected during 1987–1994: −10% at Marambio (64°S) and −12% at Sodankyla (67°N). The strongest negative trend was detected during spring. Mostly a negative trend in tropospheric ozone was observed at the stations during the late winter to early summer period during recent years. Because stratospheric ozone is controlling the flux of solar UV-B radiation reaching troposphere, loss of stratospheric ozone may have a strong impact on the destruction and production reactions of tropospheric ozone. We have studied the ozone sounding records of 1988–1994 at Marambio, Antarctica (64°S), and at Sodankyla, Finland (67°N), to find out observational evidence of tropospheric ozone changes due to stratospheric ozone variations. We have found that springtime stratospheric ozone loss has a pronounced impact on the upper tropospheric ozone at both hemispheres. Average ozone deviation of - 12.8% from the 1988 to 1994 means in the 6- to 8-km layer has been observed in Antarctica during the months with stratospheric ozone loss and −10.0% in the Arctic, respectively. Daily total ozone records and radiative transfer calculations were used to study the UV-B doses reaching the troposphere.

Version 2 total ozone mapping spectrometer ultraviolet algorithm: problems and enhancements

Satellite instruments provide global maps of surface UV irradiance by combining backscattered radiance measurements with radiative transfer models. The accuracy of the models is limited by uncertainties in input parameters representing the atmosphere and the Earths surface. To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow/ice, and a preliminary aerosol correction. We compare results of the version 1 TOMS UV algorithm and the proposed version. We evaluate different approaches for improved treatment for average cloud attenuation within a satellite pixel, with and without snow/ ice on the ground. In addition to treating cloud transmission based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate atmospheric UV irradiance transmission throughout the day. A new method is proposed to obtain a more realistic treatment of the effects from snow-covered terrain. The method is based on an empirical relation between UV reflectivity and measured snow depth. The new method reduces the bias between the TOMS UV estimations and ground-based UV measurements for snow periods. We also briefly discuss the complex problem of estimating surface UV radiation in presence of UV-absorbing aerosols. The improved (Version 2) algorithm can be applied to reprocess the existing TOMS UV irradiance and exposure estimates (since November 1978) and to future satellite sensors (e.g., GOME-2, OMI on EOS/Aura, and Triana/EPIC).

Ozone profiles in the high-latitude stratosphere and lower mesosphere measured by the Improved Limb Atmospheric Spectrometer (ILAS)-II: comparison with other satellite sensors and ozonesondes

A solar occultation sensor, the Improved Limb Atmospheric Spectrometer (ILAS)-II, measured 5890 vertical profiles of ozone concentrations in the stratosphere and lower mesosphere and of other species from January to October 2003. The measurement latitude coverage was 54–71°N and 64–88°S, which is similar to the coverage of ILAS (November 1996 to June 1997). One purpose of the ILAS-II measurements was to continue such high-latitude measurements of ozone and its related chemical species in order to help accurately determine their trends. The present paper assesses the quality of ozone data in the version 1.4 retrieval algorithm, through comparisons with results obtained from comprehensive ozonesonde measurements and four satellite-borne solar occultation sensors. In the Northern Hemisphere (NH), the ILAS-II ozone data agree with the other data within ±10% (in terms of the absolute difference divided by its mean value) at altitudes between 11 and 40 km, with the median coincident ILAS-II profiles being systematically up to 10% higher below 20 km and up to 10% lower between 21 and 40 km after screening possible suspicious retrievals. Above 41 km, the negative bias between the NH ILAS-II ozone data and the other data increases with increasing altitude and reaches 30% at 61–65 km. In the Southern Hemisphere, the ILAS-II ozone data agree with the other data within ±10% in the altitude range of 11–60 km, with the median coincident profiles being on average up to 10% higher below 20 km and up to 10% lower above 20 km. Considering the accuracy of the other data used for this comparative study, the version 1.4 ozone data are suitably used for quantitative analyses in the high-latitude stratosphere in both the Northern and Southern Hemisphere and in the lower mesosphere in the Southern Hemisphere.

Analysis of the ozone soundings made during the first quarter of 1989 in the Arctic

A total of 197 ozone profiles were obtained from the nine Arctic ozone sounding stations of Alert, Resolute, Ny Alesund, Heiss Island, Bear Island, Ammassalik, Scoresbysund, Sodankyla, and Lerwick during January–March 1989. The sounding stations cover the latitude range from 60°N to 83°N and the longitude range from 95°W to 58°E. It was, for the first time, possible to get a detailed picture with a 150-m resolution of the vertical distribution of ozone during winter and spring over extended areas of the north polar region. The fine structure of the Arctic ozone distribution shows considerable layering in profiles obtained outside and at the edge of the polar vortex, whereas little layering is found in profiles obtained well inside the vortex. The lowest temperatures were found at the edge of the vortex, e.g., at or below −90°C during January 30–31 at Lerwick and close to −90°C on January 24 at Bear Island at the altitudes of 20–25 km. Temperatures below −77°C, the condition for the formation of the type I polar stratospheric clouds (PSCs), were common during January and early February 1989 in the Arctic. Possibly as a consequence of the heterogeneous processes in the presence of PSCs, the ozone mixing ratios decreased, especially during late winter when the suns irradiation intensifies in high latitudes. A trend of −0.1%/d around 80°N latitude was found in the altitude range of 420–600 K potential temperature during the period January 10 to February 13, 1989, increasing to −0.4%/d when the later period January 24 to February 13, 1989, is considered. If one were to take into account the downward diabatic motion (2.6 km/month) the negative trends would increase further by about −0.4%/d in magnitude.

Two years of regular ozone soundings in the European Arctic, Sodankylä

Data have been studied from 2 years (1989 and 1990) of weekly ozone soundings performed in Sodankyla (67.4°N, 26.6°E), northern Finland. Prior to 1989, no routine ozone sounding program had been carried out in northern Europe. General behavior of ozone, temperature, and potential temperature at various levels of the stratosphere and the troposphere is reported. The seasonal means, minima, maxima, and standard deviations have been calculated within 1000-m layers from the surface to 35 km. Fairly high concentrations of ozone are found in the lower troposphere during spring and summer. The variation of stratospheric ozone in winter and spring may be explained by dynamical factors, although more chemical measurements are needed to explain the late winter-early spring episodes.

Version 2 TOMS UV algorithm: problems and enhancements

We evaluate the effects of possible enhancements of the current (version 1) TOMS surface UV irradiance algorithm. The major enhancements include more detailed treatment of tropospheric aerosols, effects of diurnal variation of cloudiness and improved treatment of snow/ice. The emphasis is on the comparison between the results of the version 1 TOMS UV algorithm and each of the changes proposed. TOMS UV algorithm does not discriminate between nonabsorbing aerosols and clouds. Absorbing aerosols are corrected by using the TOMS aerosol index data. The treatment of aerosol attenuation might have been improved by using newly derived TOMS products: optical depths and the single-scattering albedo for dust, smoke, and sulfate aerosols. We evaluate different approaches for improved treatment of pixel average cloud attenuation, with and without snow/ice on the ground. In addition to treating clouds based only on the measurements at the local time of the TOMS observations, the results from other satellites and weather assimilation models can be used to estimate attenuation of the UV irradiance throughout the day. The improved (version 2) algorithm will be applied to reprocess the existing TOMS UV data record (since 1978) and to the future satellite sensors (e.g., Quik/TOMS, GOME, OMI on EOS/Aura and Triana/EPIC).

TRANSPORT, FORMATION AND SINK PROCESSES BEHIND SURFACE OZONE VARIABILITY IN NORTH EUROPEAN CONDITIONS

Abstract Measurements of ozone, nitrogen dioxide and meteorological parameters at the two Finnish EMEP background stations of Ahtari (forested site) and Uto (an offshore island) show clear indications of the influence of the precursor source areas of Western and Eastern Europe on surface ozone behaviour at these higher latitudes. The mean ozone levels are relatively high, with maximum monthly values of 41 and 42 ppb, respectively. These values occur in April at both sites nearly irrespectively of wind direction, pointing to a global feature. Other spring, as well as summer and autumn, months have lower ozone values for wind direction sectors corresponding to clean air masses (north-westerlies and north-easterlies). Surface uptake is an important sink in the local ozone budget, especially during late spring, summer and early autumn. Chemical losses are more efficient in winter, when the ground is covered by snow.