Erythemally weighted UV trends over northern latitudes derived from Nimbus7 TOMS measurements

This study examines the distribution of long-term trends in ground level er ythemally weighted ultraviolet (UV) exposures in the northern latitudes for the period 1979-1991 using measurements from the Nimbus 7 Total Ozone Mapp ing Spectrometer (TOMS) instrument. A new erythemal UV data set (now availa ble to the public via World Wide Web) was produced recently by NASA and has been tested by NASA at the Goddard Space Flight Center against a previous NASA erythemal UV product, which was used in a former study that included s imilar adjustments for aerosols and clouds but not aerosol absorption. Zona l mean erythemal UV data from both products show similar, similar to 3-7% p er decade, increases in the midlatitudes to high latitudes. The detection o f regional patterns in trends in erythemal UV favors summer months when sur face UV is strongest and noise factors such as clouds and aerosols are not as influential. Analysis of the zonal patterns in trends around summer mont hs indicates that most of the regional increases (exceeding 6% per decade) in the Northern Hemisphere in the latitude range 30 degrees N-40 degrees N originate from the Pacific and Atlantic oceanic regions. Increases (also ex ceeding 6% per decade) in latitudes 40 degrees-60 degrees N appear to origi nate from the North American and Asian continents and also central Europe. Trends over the east Asian continent in high latitudes indicate increases e xceeding 10% per decade for May-August. The important conclusion is that po sitive trends in the northern subtropical latitudes originate mostly over o ceanic regions, whereas positive trends at higher latitudes originate mostl y over landmasses. Some of the increases in erythemal UV over central Europ e and the east Asian continent in summer months can be attributed to decada l decreases in cloudiness for the 1979-1991 time period.