David I. Wardle

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.

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 .

Record low ozone values over Canada in early 1993

Measurements of ozone made over Canada during the first four months of 1993 indicate that total ozone has been about 11 to 17% below normal. These low values were recorded, without exception, at all nine of the Canadian mid-latitude ground-based monitoring sites. Record low averages for the period from January to April were measured at three of the four mid-latitude stations where measurements have been made since the 1960’s. Comparison of January to April, 1993 ozonesonde profile measurements with those from earlier years indicates that the ozone deficit is in the lower stratosphere between 40 and 200 mb, the same altitudes where aerosols from the Mount Pinatubo volcanic eruption have been observed. The peak loss is 30% at 100 mb (16 Km).

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 ACE-MAESTRO instrument on SCISAT: description, performance, and preliminary results

The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite is a simple, compact spectrophotometer for the measurement of atmospheric extinction, ozone, nitrogen dioxide, and other trace gases in the stratosphere and upper troposphere as part of the Atmospheric Chemistry Experiment (ACE) mission. We provide an overview of the instrument from requirements to realization, including optical design, prelaunch and on-orbit performance, and a preliminary examination of retrievals of ozone and NO(2).

The relationship between total ozone and spectral UV irradiance from Brewer observations and its use for derivation of total ozone from UV measurements

Spectral UV irradiance measurements made by Brewer spectrophotometers at 7 stations over periods from 4 to 7 years together with Dobson and Brewer direct sun total ozone measurements have been used to establish a statistical relationship between total ozone, solar zenith angle, and UV irradiance. Using the relationship it is possible to estimate daily mean total ozone from routine UV measurements with a standard error of about 3%. Also the spectral irradiance for wavelengths between 300 and 325 nm can be estimated with error of about 25% at 300 nm and lower at longer wavelengths. The relationship is helpful for quality control of UV data. It can also be used to estimate total ozone at sites where spectral UV irradiance is measured and no direct ozone observations are made.

Long‐term ozone decline over the Canadian Arctic to early 1997 from ground‐based and balloon observations

Column ozone measurements in the Canadian High Arctic (north of 70°N) started in 1957 and there have been regular ozone sonde flights there since 1966. Column ozone values over the High Arctic were as much as 45% below normal for some days in March 1997. During March 1996 arctic ozone values were also very low. In both years, temperatures in the arctic stratosphere were extremely cold and, especially in 1997, the vortex wind pattern was unusual for the arctic and quite similar to the antarctic spring vortex. Similar cold stratospheric temperatures were present in 1967, but ozone deviations were much smaller. Despite the very low values in the High Arctic during March 1997, the average column ozone from three Canadian sites in the 50–60°N latitude range was only 3.5% below normal.

Tropospheric ozone trends over Canada: 1980–1993

An analysis of ozone in the troposphere at five sites in Canada is presented, using ECC sonde data from 1980-1993. The entire data record has been carefully reexamined for quality, and where necessary reevaluated from the raw telemetry. Trend analysis is performed on both the raw and the corrected (nor- malized to the total ozone measurement) profiles. The correction factors are also examined for trends. Where required, the data are adjusted for diurnal variation introduced by changes in time of launch. We find statistically significant (2a) decreases in con- centration at all stations except Alert, and in most cases, at all levels, including the surface. The tropospheric trends we derive here are compared with corresponding lower stratospheric trends. Although the long-term trends are similar, stratospheric varia- tions on shorter time scales are not well-correlated with those in the troposphere.

The 1996 North American Interagency Intercomparison of Ultraviolet Monitoring Spectroradiometers

Concern over stratospheric ozone depletion has prompted several government agencies in North America to establish networks of spectroradiometers for monitoring solar ultraviolet irradiance at the surface of the Earth. To assess the ability of spectroradiometers to accurately measure solar ultraviolet irradiance, and to compare the results between instruments of different monitoring networks, the first North American Intercomparison of Ultraviolet Monitoring Spectroradiometers was held September 19–29, 1994 at Table Mountain outside Boulder, Colorado, USA. This Intercomparison was coordinated by the National Institute of Standards and Technology and the National Oceanic and Atmospheric Administration (NOAA). Participating agencies were the Environmental Protection Agency, National Science Foundation, Smithsonian Environmental Research Center, and Atmospheric Environment Service, Canada. Instruments were characterized for wavelength accuracy, bandwidth, stray-light rejection, and spectral irradiance responsivity, the latter with a NIST standard lamp calibrated to operate in the horizontal position. The spectral irradiance responsivity was determined once indoors and twice outdoors, and demonstrated that, while the responsivities changed upon moving the instruments, they were relatively stable when the instruments remained outdoors. Synchronized spectral scans of the solar irradiance were performed over several days. Using the spectral irradiance responsivities determined with the NIST standard lamp, and a simple convolution technique to account for the different bandwidths of the instruments, the measured solar irradiances agreed within 5 %.

Influence of volcanic sulfur dioxide on spectral UV irradiance as measured by Brewer Spectrophotometers

Spectra of UV irradiance measured by Brewer spectrophotometers at 13 stations in Japan and Canada have been examined to determine the effect of absorption by sulfur dioxide (SO2). A simple algorithm to estimate the total column amount of SO2 from global UV irradiance measurements has been developed. The algorithm which overestimates SO2 amounts at low altitudes is useful for identifying large SO2 absorption and for establishing upper limits of the SO2 values. Except at one station, the overall occurrence of days with SO2 amounts greater than 10 Dobson Units (DU) was about 0.22%. The exception was the Kagoshima observatory located 10 km from the volcano, Sakurajima. Volcanic activity resulted in frequent observation of column SO2 amounts of more than 20 DU over Kagoshima and in extreme cases up to 100 DU. The reduction in the long-term erythemally weighted UV irradiation due to absorption by SO2 is between one and two percent at Kagoshima and negligible at the twelve other stations.