Vitali E. Fioletov

Assessment of temperature, trace species, and ozone in chemistry-climate model simulations of the recent past

Simulations of the stratosphere from thirteen coupled chemistry-climate models (CCMs) are evaluated to provide guidance for the interpretation of ozone predictions made by the same CCMs. The focus of the evaluation is on how well the fields and processes that are important for determining the ozone distribution are represented in the simulations of the recent past. The core period of the evaluation is from 1980 to 1999 but long-term trends are compared for an extended period (1960–2004). Comparisons of polar high-latitude temperatures show that most CCMs have only small biases in the Northern Hemisphere in winter and spring, but still have cold biases in the Southern Hemisphere spring below 10 hPa. Most CCMs display the correct stratospheric response of polar temperatures to wave forcing in the Northern, but not in the Southern Hemisphere. Global long-term stratospheric temperature trends are in reasonable agreement with satellite and radiosonde observations. Comparisons of simulations of methane, mean age of air, and propagation of the annual cycle in water vapor show a wide spread in the results, indicating differences in transport. However, for around half the models there is reasonable agreement with observations. In these models the mean age of air and the water vapor tape recorder signal are generally better than reported in previous model intercomparisons. Comparisons of the water vapor and inorganic chlorine (Cly) fields also show a large intermodel spread. Differences in tropical water vapor mixing ratios in the lower stratosphere are primarily related to biases in the simulated tropical tropopause temperatures and not transport. The spread in Cly, which is largest in the polar lower stratosphere, appears to be primarily related to transport differences. In general the amplitude and phase of the annual cycle in total ozone is well simulated apart from the southern high latitudes. Most CCMs show reasonable agreement with observed total ozone trends and variability on a global scale, but a greater spread in the ozone trends in polar regions in spring, especially in the Arctic. In conclusion, despite the wide range of skills in representing different processes assessed here, there is sufficient agreement between the majority of the CCMs and the observations that some confidence can be placed in their predictions.

Satellite estimation of spectral surface UV irradiance in the presence of tropospheric aerosols: 1. Cloud‐free case

The algorithm for determining spectral UVA (320-400 nm) and UVB (290-320 nm) flux in cloud-free conditions is discussed, including estimates of the various error sources (uncertainties in ground reflectivity, ozone amount, ozone profile shape, surface height, and aerosol attenuation). It is shown that the Brewer-measured spectral dependence of UV flux can be accurately reproduced using just total column ozone amount and the solar flux spectrum. The presence of aerosols tends to reduce the logarithm of the absolute UV flux linearly with aerosol optical depth. Using Brewer measurements of UV flux and aerosol optical depth on clear days at Toronto, the estimated slope falls in the range 0.2 to 0.3 (aerosol single-scattering albedo about 0.95). The Brewer measurements of UV flux can be reproduced using the aerosol model derived within uncertainties of the instrument calibration. We have applied the algorithm to the data collected by the total ozone mapping spectrometer (TOMS) instruments that have been flown by NASA since November 1978. It was demonstrated that in the absence of clouds and UV-absorbing aerosols, TOMS measurements of total column ozone and 380 nm (or 360 nm) radiances can be used in conjunction with a radiative transfer model to provide estimates of surface spectral flux to accuracies comparable to that of typical ground-based instruments. A newly developed technique using TOMS aerosol index data also allows estimation of UV flux transmission by strongly absorbing aerosols. The results indicate that over certain parts of the Earth, aerosols can reduce the UV flux at the surface by more than 50%. Therefore the most important need for reducing errors in TOMS-derived surface UVB spectra is to improve the understanding of UV aerosol attenuation.

Long‐term variations of UV‐B irradiance over Canada estimated from Brewer observations and derived from ozone and pyranometer measurements

Routine uniform spectral UV-B measurements with Brewer spectrophotometers in the Canadian network began in 1989. This relatively short duration of UV measurements militates against reliable detection of long-term changes in UV. A statistical model has been developed to extend the record of UV back to the early 1960s. It estimates UV values (at individual wavelengths and spectrally integrated) from global solar radiation, total ozone, dew point temperature, and snow cover. The model results are demonstrated to be in good agreement with the measurements. For example, the standard deviation of the difference between monthly values of measured and derived erythemally weighted UV irradiation is 3.3% for summer months. The major source of error in the model estimates is probably linked to rare occurrences of absorbing aerosols in the atmosphere. Long records of reliable measurements of total ozone, global solar radiation, and other parameters made it possible to derive UV-B values at three Canadian stations from the mid-1960s. Trends in derived erythemally weighted UV at two stations (Toronto and Edmonton) are similar to those expected from total ozone trends although the estimated error of the UV trends is more than 2 times larger. However, the increase in annual UV at Churchill (59°N) in 1979–1997 was found to be more than twice that expected from the ozone decline. This is a result of longterm changes in snow cover and clouds.

Estimating the global ozone characteristics during the last 30 years

All available total ozone data from over 150 past and present Global Ozone Observing System (GO3OS) stations, after careful quality control and reevaluation, have been analyzed in order to deduce the basic global ozone characteristics both for pre-ozone-hole and during “ozone hole” time periods. Utilizing Total Ozone Mapping Spectrometer (TOMS) data, the longitudinal inhomogeneity of the total ozone distribution was estimated. That permitted the use of ground-based data for establishing long-term zonal as well as hemispheric and global ozone variations for the 1964–1994 period. The difference between the estimations of monthly zonal variations from ground-based and TOMS data for the overlapping period of 1979–1993 is less than 1% in latitudes 40°S–60°N. The ozone changes are several times larger than possible errors of the estimated values; therefore the results are highly reliable. They show that the northern hemisphere average ozone was ∼312 and the southern average was ∼300 matm cm in the pre-ozone-hole decades (1964–1980) and that the global average for the 1984–1993 period was lower by ∼3% (from 306.4±1.0 down to 297.7±2.2 matm cm). The southern hemisphere contributed ∼64% of the overall ozone decline. The levels of annual ozone maximum have been reduced by 5.8% in the southern hemisphere and 3.2% in the northern hemisphere, and the levels of ozone minimum have been reduced by 2.1% and 1.2%, respectively. The ozone trends for midlatitudinal bands (35–65°) show a pronounced seasonal dependence varying from ∼3% to 8 % (and even more for the southern hemisphere) for the cumulative decline since 1970. The ozone decline calculated in percent per decade from 1980 is almost twice as large as the decline calculated from 1970. The cumulative year-round global ozone decline is 4.8±0.6%; however, the cumulative year-round decline over middle and polar latitudes is more than 7%. The advantages of establishing ozone “norms” for estimations of long-term ozone variations from ground-based data are emphasized.

Validation of daily erythemal doses from Ozone Monitoring Instrument with ground‐based UV measurement data

[1] The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600 km wide viewing swath and it is capable of daily, global contiguous mapping. The Finnish Meteorological Institute and NASA Goddard Space Flight Center have developed a surface ultraviolet irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths, noontime erythemal dose rate (UV index), and the erythemal daily dose. The overpass erythemal daily doses derived from OMI data were compared with the daily doses calculated from the ground-based spectral UV measurements from 18 reference instruments. Two alternative methods for the OMI UV algorithm cloud correction were compared: the plane-parallel cloud model method and the method based on Lambertian equivalent reflectivity. The validation results for the two methods showed some differences, but the results do not imply that one method is categorically superior to the other. For flat, snow-free regions with modest loadings of absorbing aerosols or trace gases, the OMI-derived daily erythemal doses have a median overestimation of 0–10%, and some 60 to 80% of the doses are within ±20% from the ground reference. For sites significantly affected by absorbing aerosols or trace gases one expects, and observes, bigger positive bias up to 50%. For high-latitude sites the satellite-derived doses are occasionally up to 50% too small because of unrealistically small climatological surface albedo.

Satellite estimation of spectral surface UV irradiance. 2. Effects of homogeneous clouds and snow

This paper extends the theoretical analysis of the estimation of the surface UV irradiance from satellite ozone and reflectivity data from a clear-sky case to a cloudy atmosphere and snow-covered surface. Two methods are compared for the estimation of cloud-transmission factor CT, the ratio of cloudy to clear-sky surface irradiance: (1) the Lambert equivalent reflectivity (LER) method and (2) a method based on radiative transfer calculations for a homogeneous (plane parallel) cloud embedded into a molecular atmosphere with ozone absorption. The satellite-derived CT from the NASA Total Ozone Mapping Spectrometer (TOMS) is compared with ground-based CT estimations from the Canadian network of Brewer spectrometers for the period 1989 -1998. For snow-free conditions the TOMS derived CT at 324 nm approximately agrees with Brewer data with a correlation coefficient of ;0.9 and a standard deviation of ;0.1. The key source of uncertainty is the different size of the TOMS FOV (;100 km field of view) and the much smaller ground instrument FOV. As expected, the standard deviations of weekly and monthly C T averages were smaller than for daily values. The plane-parallel cloud method produces a systematic CT bias relative to the Brewer data (17% at low solar zenith angles to 210% at large solar zenith angles). The TOMS algorithm can properly account for conservatively scattering clouds and snow/ice if the regional snow albedo RS is known from outside data. Since RS varies on a daily basis, using a climatology will result in additional error in the satellite-estimated CT. The CT error has the same sign as the R S error and increases over highly reflecting surfaces. Finally, clouds polluted with absorbing aerosols transmit less radiation to the ground than conservative clouds for the same satellite reflectance and flatten spectral dependence of CT. Both effects reduce C T compared to that estimated assuming conservative cloud scattering. The error increases if polluted clouds are over snow.

Air quality over the Canadian oil sands : a first assessment using satellite observations

Results from the first assessment of air quality over the Canadian oil sands–one of the largest industrial undertakings in human history–using satellite remote sensing observations of two pollutants, nitrogen dioxide (NO2) and sulfur dioxide (SO2), are presented. High-resolution maps were created that revealed distinct enhancements in both species over an area (roughly 30 km × 50 km) of intensive surface mining at scales of a few kilometers. The magnitude of these enhancements, quantified in terms of total mass, are comparable to the largest seen in Canada from individual sources. The rate of increase in NO2between 2005 and 2010 was assessed at 10.4 ± 3.5%/year and resulted from increases both in local values as well as the spatial extent of the enhancement. This is broadly consistent with both surface-measurement trends and increases in annual bitumen production. An increase in SO2 was also found, but given larger uncertainties, it is not statistically significant.

Total ozone trends from quality-controlled ground-based data (1964-1994)

Seasonal and year-round trend analyses of ground-based total ozone data that have been carefully quality controlled are presented for 46 Dobson stations, for four regions of the former USSR and for the two polar regions. The trend model incorporated the 10.7-cm solar flux and equatorial stratospheric wind as indicators for removal of the solar and quasi-biennial components in ozone variations. The trends were calculated for two main time intervals: from January 1964 through March 1994, with slope since 1970, and from January 1979 through March 1994. The analyses show continuous year-round ozone decline in middle latitudes (35°–60°) where the estimated values are −4.3 and −4.1% per decade for the period since January 1979 in the northern and southern hemispheres respectively. The northern hemisphere data permit estimation of the trends over vast regions such as North America −4.1±1.5%, Europe −5.1±2.0%, Siberia and Far East −5.6±1.8% (values and two sigma intervals are given in percent per decade for year-round trend). The ozone decline over the Arctic is about −5.6±2.0% per decade for year-round trend and −7.5±3.8% for the winter-spring season. The most dramatic ozone decline occurs in spring over Antarctica, −22±7.2% per decade. The trends estimated for the 1979–1994 period are about 1.5% per decade stronger in northern and southern middle and high latitudes than the trends with slope since 1970, which shows a substantial acceleration of the rates of ozone decline. The sensitivity of the ozone trends to the extremely low ozone during the 1992–1993 period is discussed. Most of the ozone data and calibrations in this paper were the base for the Ozone Trends section of the World Meteorological Organization/United Nations Environment Program Ozone Assessment 1994. The results presented here in general concur with the assessment and provide further estimates of the ozone changes in the northern middle latitudes in view of another extremely low winter-spring season of 1994–1995.

An assessment of the world ground-based total ozone network performance from the comparison with satellite data

Total ozone data available from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) have been compared with Version 7 of the Nimbus 7 (for the 1978-1993 period) and Meteor 3 total ozone mapping spectrometer (TOMS) data sets (for the 1991-1994 period) and with Version 6 of the Nimbus 7 solar backscattered ultraviolet (SBUV) data set (for the 1978-1990 period). Comparison between the ground-based and satellite observations resulted in parameters such as bias, scattering, relative trend, and seasonal component. About 80% of all Dobson, Brewer, and filter ozonometer stations have standard deviations of monthly mean differences with TOMS that are less than 2.5%. Typically, results of the comparisons between ground-based stations and SBUV are similar to those for ground-based and TOMS comparisons. For the ground-based direct sun total ozone measurements the standard deviations between TOMS and Dobson daily mean values are about 2.4%. The standard deviations for Brewer and filter ozonometer stations are 2.2 and 3.5%, respectively. For the less accurate zenith sky measurements, standard deviations are 3.8% for Dobsons, 4% for Brewers, and 4.7% for filter ozonometer data. Comparisons of individual ground-based measurements with satellite overpasses yield standard deviations which for middle latitudes increase approximately linearly from about 2% at coincidence to 6% for a 24 hour difference in time or a 600 km spatial difference. Typical problems with ground-based observations and results of the comparisons for individual stations are also discussed.

Comparison of Brewer ultraviolet irradiance measurements with total ozone mapping spectrometer satellite retrievals

Comparison of measured UV irradiance with estimates from satellite observation is potentially effective for the validation of data from the two sources. Summer data from ten Canadian Brewer sites were compared in this study with noon UV irradiance estimated from total ozone mapping spectrometer (TOMS) measurements. In general, TOMS estimates can successfully reproduce long-term and major short-term UV variations. However, there are some systematic differences between the measurements at the ground and satellite-retrieved UV irradiance. From 3 to 11% of the Brewer-TOMS difference can be attributed to the Brewer angular response error. This error depends on the solar zenith angle and cloud conditions, and is different from instrument to instru- ment. When the angular response of the Brewer instrument is consid- ered and applied, the Brewer data are still lower than TOMS-estimated UV irradiance by 9 to 10% on average at all sites except one. The dif- ference is close to zero at one station (Saturna Island), possibly due to its much cleaner air. The bias can be seen in clear sky conditions and at the 324-nm wavelength, i.e., it is not related to local cloud conditions or absorption by ozone or SO2 .