James B. Kerr

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.

New methodology for deriving total ozone and other atmospheric variables from Brewer spectrophotometer direct sun spectra

Received 17 August 2001; revised 28 December 2001; accepted 8 January 2002; published 14 December 2002. [1] A new method has been developed for taking high-quality spectral measurements of ultraviolet radiation with the Brewer spectrophotometer. Spectral measurements of direct solar radiation made routinely with the new method at Toronto between 1996 and 2001 were used to determine total ozone, aerosol optical depth, sulfur dioxide and ozone temperature. Corrections to laboratory-based ozone absorption coefficients have been derived from the data set of these new measurements as have the temperature dependencies of standard Brewer and Dobson total ozone measurements. It was found that the temperature of atmospheric ozone has little effect on the total ozone derived from the standard algorithm used for the Brewer instrument. The new measurement method and the calibration information required to derive the atmospheric variables from the spectra are described. Results of calibrations carried out at Mauna Loa Observatory in 1997 and 2000 are presented. The records of atmospheric variables measured at Toronto between 1996 and 2001 are given. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: total ozone, ozone temperature, aerosol optical depth, spectral measurements, solar radiation

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 .

UV index climatology over the United States and Canada from ground‐based and satellite estimates

[1] Long-term monthly mean UV index values for Canada and the United States were calculated using information from two sources: from noon erythemal UV estimated from Total Ozone Mapping Spectrometer (TOMS) total ozone and reflectivity data and from UV index values derived from observations of global solar radiation, total ozone, dew point, and snow cover. The results are presented as monthly maps of mean noon UV index values. Mean UV index values in summer range from 1.5 in the Arctic to 11.5 over southern Texas. Both climatologies were validated against spectral UV irradiance measurements made by Brewer spectrophotometers. With snow on the ground the TOMSbased data underestimate UV by up to 60% with respect to Brewer measurements and UV derived from global solar radiation and other parameters. In summer, TOMS UV index climatology values are from 10 to 30% higher than those derived from global solar radiation and other parameters. The difference is probably related to aerosol absorption and pollution effects in the lower troposphere that are not currently detected from space. For 21 of 28 midlatitude Brewer sites, long-term mean summer UV measured values and UV derived from global solar radiation and other parameters agree to within +5 to 7%. The remaining seven sites are located in ‘‘clean’’ environments where TOMS estimates agree with Brewer measurements while UV derived from global solar radiation and other parameters is 10–13% lower. Brewer data also demonstrate that clean and ‘‘typical’’ sites can be as little as 70–120 km apart. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); KEYWORDS: UV index, Brewer, TOMS, pyranometer, climatology, ozone

The 1991 WMO International ozonesonde intercomparison at Vanscoy, Canada

Abstract An intercomparison of ozonesondes was held at Vanscoy, Saskatchewan, from 13 to 24 May 1991. The intercomparison, which was sponsored by the WMO and hosted by the Atmospheric Environment Service (AES) of Canada, was attended by scientists from six countries: Canada, Finland Germany, India, Japan and the United States. Four different makes of ozonesondes were used: the ECC sonde, the Brewer‐Mast sonde, the Indian ozonesonde and the Japanese RSH‐KC79 ozonesonde. These represent most of the sonde types that are in routine operation in the Global Ozone Observing System. A balloon payload and telemetry system was developed to accommodate up to eight ozonesondes that could operate independently and transmit data simultaneously to a ground receiver. Ten flights were launched, each carrying 7 or 8 sondes, and a total of 65 successful profile measurements were made. The payloads were carried to altitudes between 35 and 40 km. The measured profiles are used to determine statistically meaningful evaluations...

Surface ultraviolet radiation

Abstract One of the main concerns regarding a decrease in stratospheric ozone is the consequential increasein the amount of ultraviolet (UV) radiation that reaches the lower atmosphere and the Earths surface. Radiationat UV wavelengths where ozone absorbs strongly is detrimental to most biological species, including humanbeings, so a decrease in stratospheric ozone could have a significant impact on the biosphere. This concern hasled to a significant increase in surface UV radiation research over the last two decades since the ratification ofthe Montreal Protocol. Studies include investigations into understanding the complicated absorption and scatteringprocesses involved in the radiative transfer of UV through the atmosphere as well as research on theimpacts of changes in UV radiation. Factors affecting surface UV radiation will be discussed, resources used tostudy surface UV radiation will be described and progress made in our understanding of surface UV radiationover the past two decades will be reviewed.

Estimating UV Index Climatology over Canada

Abstract Hourly UV index values at 45 sites in Canada were estimated using a statistical relationship between UV irradiance and global solar radiation, total ozone, and dewpoint temperature. The estimation method also takes into account the enhancement of UV irradiance by snow using an empirical correction derived from Brewer UV measurements. Different characteristics of UV index distribution over Canada were estimated from the derived UV irradiance for the period 1979–87 and then presented in the form of monthly maps. Direct comparisons of Brewer measurements at seven Canadian sites with derived UV irradiance show agreement within 2%–3% except for periods of melting snow when variations in snow albedo yield higher errors in the derived UV irradiance.

Understanding the factors that affect surface ultraviolet radiation

Spectral measurements of solar ultraviolet (UV) radiation have been made at several ground-based locations and for more than 10 yr at some sites. These measurements are important for two main rea- sons. First, the measurements combined with results of radiative transfer models contribute toward our understanding of the many complicated radiative transfer processes in the atmosphere and at the Earths sur- face. These processes include absorption of radiation by atmospheric gases such as ozone and sulfur dioxide, scattering by atmospheric aero- sols and clouds, and scattering from the earths surface. Knowledge of these processes is required for operational applications such as the es- timation of surface UV radiation from satellite data and the forecasting of the UV index. Also, our ability to estimate UV climatology in the past, as well as in the future, requires thorough knowledge of the UV radiative transfer processes. The second reason for making systematic ground- based measurements of UV radiation is to determine whether long-term changes are occurring as a result of ozone depletion or climate change and to identify specific causes. Examples of how long-term ground- based data records have contributed to our understanding of surface UV radiation are presented.