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 (59øN) 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.

[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 À7%. 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.

[1] Measurements of aerosol optical depth have become more numerous since the mid1990s with the onset of commercially available, high-quality, low-maintenance automatic instrumentation. The development of several networks for aerosol measurements, and the next day availability of preliminary data for some, have further enhanced interest in the products this type of measurement can provide. With several networks operating globally and others operating either regionally or continentally within North America the comparability of the data emanating from the various archive centers is an important issue. The Bratt's Lake Observatory operates four separate types of Sun photometers in conjunction with three different networks: Aerosols in Canada, Global Atmosphere Watch, and the U.S. Department of Agriculture UV-B Monitoring Program. Data collected during the summer of 2001, following the protocols established by the networks and the Meteorological Service of Canada, were analyzed to determine the comparability among these networks. As the instruments and conversion algorithms are similar to other networks from around the globe, it is believed that the results of this comparison can be transferred, at least in part, to other operational networks. The results of the 3-month study indicate that the data obtained from the networks that operate direct-pointing instruments are very comparable, being within ±0.01 of an optical depth for instantaneous measurements during cloud-free line-of-sight conditions. Over the length of the comparison the root mean square difference of aerosol optical depth at 500 nm between the direct sun-pointing instruments was 0.0069. The rotating shadowband instruments did not perform as well. These results indicate that the data from well-maintained networks of direct sun-pointing photometers can provide data of the quality necessary to compare stations from across the globe.

No abstract

[1] Sunphotometry, supported by TOMS imagery and the NRL-NAAPS global aerosol model, was used to monitor the 2001 springtime Asian dust plume across Canada and the U.S. Comparative analysis between these data sets indicated that the dust plume was remarkable in its extent and persistence and that its evolution could be inferred at western, mid-western and eastern sunphotometer stations for a period of about a week. The dust influence on sunphotmeter monthly statistics is shown to be weak, but systematic.