Effects Of Polar Stratospheric Clouds Research Articles

Microphysical modeling of the 1999-2000 Arctic winter 2. Chlorine activation and ozone depletion

[1] The effect of polar stratospheric clouds (PSCs) on ozone depletion during the 1999–2000 Arctic winter has been assessed using a coupled microphysical/photochemical model. Scenarios spanned a large range of denitrification levels, with up to 80% vortex-averaged denitrification and localized dehydration. PSC composition was varied, exploring sensitivity to heterogeneous reaction rates. PSC formation during February was critical in causing severe ozone depletion below 500 K. Heterogeneous chemistry on these PSCs was able to continuously reactivate the newly produced ClONO2. Only 30–40% vortex-averaged ozone loss would have occurred without this chlorine reactivation; with it, 21–32% more ozone loss is possible. During February (unlike earlier in the winter), chlorine reactivation and ozone loss were sensitive to heterogeneous chemistry: varying the reactivities altered ozone loss by 11%. An analysis of the critical temperatures for heterogeneous chemistry demonstrates the importance of temperatures near the nitric acid trihydrate condensation point, where many uncertainties influence heterogeneous reaction rates. Chlorine reactivation during February also prevented denitrification from enhancing ozone loss until March: 70% vortex-averaged denitrification only enhanced ozone depletion by 3% on 10 March. The mid-March vortex breakup probably limited the extent of ozone depletion; if the vortex had remained stable until 15 April, 16% ozone loss (out of a total 68% ozone loss) could be caused by 70% denitrification. Ozone loss intensifies nonlinearly with enhanced denitrification: in individual air parcels with 90% denitrification, 40% ozone loss in mid-April can be attributed to denitrification alone.

Properties of the 1979 SAM II Antarctic 1.0‐μm extinction coefficients: Implications of dehydration and seasonal evolution of the Antarctic polar vortex

The present study investigates the 1.0‐μm extinction coefficient measurements obtained in the Antarctic region in 1979 from the Stratospheric Aerosol Measurement (SAM) II, with particular focus on the background aerosol properties. Correlative meteorological information from the National Centers for Environmental Prediction is incorporated in this investigation. The results indicate that the data frequency distribution of the background aerosol extinction coefficient in the local summer and fall can be adequately modeled by using a single‐mode normal distribution, and that a binormal distribution is needed for modeling the distribution in the local winter and spring because of the different characteristics of the aerosols inside and outside the polar vortex. In general, the vertical distribution of the aerosol mean extinction coefficient exhibits two regions of different seasonal variation. Above 16 km the extinction coefficient is the highest during the local summer, and the lowest during the local spring inside the polar vortex. Below 16 km the aerosol seasonal variation is more complex, but the winter enhancement of the aerosol extinction coefficient inside the Antarctic polar vortex is clearly evident. As the season changes from winter to spring, the results inside the Antarctic polar vortex also indicate a reduction in aerosol optical depth in the stratosphere, but no significant changes in the upper troposphere. The present study further indicates that the bottom of the winter polar vortex in Antarctica is located at an altitude as low as 8 to 9 km, which is about 4 to 5 km lower than the bottom of the Arctic polar vortex. This difference may be attributable to the different strengths of the winter polar vortex and the planetary wave activities between the two hemispheres. In summary, the properties of the Antarctic background aerosol are very consistent with the effect of polar stratospheric clouds on the aerosol vertical distribution through their formation, sedimention, and evaporation, and with the seasonal evolution of the polar vortex. Finally, the result of the present study provides valuable opportunities for fully utilizing the multiyear SAM II tropospheric and stratospheric measurements to investigate the aerosol climatology and long‐term variations in the Arctic and Antarctic regions.

Radiative feedback of polar stratospheric clouds on Antarctic temperatures

A one‐dimensional time marching radiative transfer model has been used to investigate the potential effects of polar stratospheric clouds (PSCs) on winter and spring temperatures in the Antarctic lower stratosphere. High, middle, and low PSC amounts were specified from lidar backscatter profiles and were chosen to represent the likely range of PSC amounts present in the Antarctic region. The computed effects of the PSCs on temperatures depend strongly on the surface temperature and on the extent of tropospheric cloudiness, and range from a maximum increase of 6 K for a high amount of PSCs over a warm surface and clear troposphere to a maximum decrease of 2 K for a high amount of PSCs over a cold surface and a troposphere with high clouds. The average effect is unlikely to be more than a 1 or 2 K temperature change.

Optical effects of polar stratospheric clouds on the retrieval of TOMS total ozone

Small areas of sharply reduced ozone density appear frequently in the maps produced from polar region total ozone mapping spectrometer (TOMS) data. These mini‐holes are of the order of 1000 km in extent with a lifetime of a few days. On the basis of measurements from ground‐based instruments, balloon‐borne ozonesondes, and simultaneous measurements of aerosol and ozone concentrations during aircraft flights in the Arctic and Antarctic regions, the appearance of polar stratospheric clouds (PSCs) are frequently associated with false reductions in ozone derived from the TOMS albedo data. By combining radiative transfer calculations with the observed PSC and ozone data, it is shown that PSCs located near or above the ozone density maximum (with optical thickness greater than 0.1) can explain most of the differences between TOMS ozone data and ground or in situ ozone measurements. Several examples of real and false TOMS mini‐hole phenomenon are investigated using data from the 1989 Airborne Arctic Stratospheric Expedition (AASE) and from balloon flights over Norway and Sweden.

Radiative effects of polar stratospheric clouds during the Airborne Antarctic Ozone Experiment and the Airborne Arctic Stratospheric Expedition

Daily, three dimensional fields of both Type I nitric acid trihydrate (NAT) and Type II water ice polar stratospheric clouds (PSCs) have been generated in the polar regions during the period of the Airborne Antarctic Ozone Experiment (AAOE) and the Airborne Arctic Stratospheric Expedition (AASE) aircraft missions. Mission data on particulate composition and size, together with National Meteorological Center (NMC) analyzed temperatures, have been used. A comparison of computed l‐μm extinction coefficients with those observed by the Stratospheric Aerosol Measurement (SAM II) satellite served to constrain the water ice particle size and the nitric acid mixing ratio profiles. For AAOE, both Type I and Type II clouds were formed for the time period August 23 to September 17, after which only Type I clouds formed. During AASE, while Type I clouds were formed for each day between January 3 and February 10, Type II clouds formed on only two days, January 24 and 31. Mie theory and a radiative transfer model were used to compute the radiative heating rates during the mission periods, for clear and cloudy lower sky cases. Only the Type II water ice clouds have a significant radiative effect, with the Type I NAT PSCs generating a net heating or cooling of 0.1 K/d or less. The major effect of PSCs during AAOE appears to be one of net heating, with maximum heating rates as large as 0.5 K/d for particular days. Heating rates of 0.3 K/d or greater were computed during the periods September 1–6 and September 11–14. This additional heating was generally found over the western part of Antarctica and near the coastal regions, while a small cooling was computed over the Antarctic Plateau. However, when the heating rates are averaged over the AAOE mission, a net heating on the order of only 0.1 K/d is generated by the PSCs over western Antarctica, with other regions exhibiting a very small additional cooling, less than 0.05 K/d. Tropospheric clouds turn this net heating to a small amount of net cooling, roughly 0.1 K/d. The average effect of PSCs to the radiative budget during AASE is extremely small. Therefore, on the whole, the radiative effects of PSCs appear to be too small to have an appreciable impact on vertical motions except over time scales of a few days during AAOE. This result makes it unlikely that the radiative‐dynamical mechanisms proposed for the Antarctic ozone depletion play any significant role.

Dehydration in the antarctic stratosphere: Radiative effects

A two dimensional model is used to investigate the effects of polar stratospheric clouds on the water vapor budget in the Antarctic stratosphere. It is shown that loss of stratospheric water vapor through formation and sedimentation of large ice particles decreases infrared cooling. This cooling reduction partially compensates the decreased ozone heating due to the formation of the Antarctic hole. Such a conclusion is reached by comparing a control unperturbed situation with two cases in which an ozone hole is formed with and without polar dehydration. We show that in the case of substantial loss of stratospheric water vapor since the years of the ozone hole formation, a dynamical mechanism may be necessary to explain the observed stratospheric cooling during the Antarctic spring. In this case the combined radiative perturbation (H2O + O3) is too small to force this cooling. Instead, in the case of a negligible change of water vapor content, the O3 radiative perturbation could be entirely responsible for the observed secular cooling.

Model simulation of chemical depletion of Arctic ozone during the winter of 1989

Ozone depletion in the Arctic vortex has become an important issue. As part of a Canadian research program designed to study the effect of polar stratospheric clouds on the chemistry of the Arctic ozone layer, project CANOZE (Canadian Arctic Northern OZone Experiment) 4, a series of ozonesondes were flown during January and February 1989 from Alert at 82.5°N. This time series of ozone profiles indicated a decrease of ozone during this period. We have applied a simple chemical model to estimate the possible ozone depletions that could have occurred during this period. The results suggest a chemical depletion of ∼ 4% per week is possible, in approximate agreement with the observed depletion rate at 20 km of 5% per week. The major features of the analysis confirm earlier suggestions that thermal dissociation of Cl2O2 could limit the amount of O3 destruction. For our simulation conditions of largely unperturbed O3 densities the ClO+O reaction is a major loss process for O3 above 19 km, while Cl2O2 photolysis is the major loss process below 19 km, with the BrO+ClO reaction contributing about 14% of the loss in both cases. The branch of the BrO+ClO reaction that results in the formation of BrCl also acts to limit the nighttime O3 destruction via BrO+ClO since BrO is rapidly converted to BrCl at twilight. The photolytic release of NO and NO2 from HNO3 at the latitudes encountered by the air parcels driving these late winter/early spring conditions is sufficiently slow that it has no major impact on O3 destruction by Clx and Brx on the time scale of a ∼ 1 month.

Correction to: “Radiative effects of polar stratospheric clouds” by S. Kinne and O.B. Toon

Correction to: “Radiative effects of polar stratospheric clouds” by S. Kinne and O.B. Toon

Radiative effects of polar stratospheric clouds

Radiative transfer calculations are performed for polar stratospheric clouds (PSCs) using newly acquired PSC properties and polar atmospheric data. PSC radiative effects depend strongly on upwelling thermal radiation and vary from infrared heating over warm polar surfaces, such as oceans, to cooling over cold surfaces, such as the Antarctic plateau. Heating and cooling rates of nitric acid PSCs are smaller than ±0.1K/day. Rates for optically thicker ice PSCs vary from 1.0 to −0.2K/day, those for orographically forced ice PSCs even from 3.0 to −0.5K/day. Frequently observed optically thick cirrus decks near the tropopause provide a very cold radiative surface. These clouds not oily act to prevent heating and enhance cooling in ice PSCs to −0.5K/day and orographic ice PSCs to‐day, but such cirrus cloud decks also cool the entire stratosphere by up to −0.5K/day over warm surfaces, even in the absence of PSCs.