Urban UV measurements: rationale for the establishment of long-term monitoring in the Baltimore Ecosystem Study

Monitoring of the solar UV irradiance at the earth's surface is routinely carried out at about 70 locations in the United States. The majority of these sites are located in rural locations to assess UV impacts on agriculture and rural ecosystems and to determine long-term irradiance trends. However, most of the population lives in metropolitan areas where scattering by primary and secondary atmospheric pollutants make it difficult to extrapolate above-canopy and below canopy UV irradiance from rural UV irradiance measurements. Greater scattering by pollutants in the urban boundary layer may enhance the penetration of UV radiation to ground the level under tree canopies. An 'urban' UV monitoring site consisting of a UVB broadband and global solar irradiance sensor has been established in the Baltimore Ecosystem Study (BES), a Long-Term Ecological Research site in Baltimore, Maryland. A nearby 'rural' USDA UV Monitoring Network site to the east of Baltimore provides the ability to make urban/rural comparisons under certain weather patterns and provides measures of the column ozone and aerosol optical thickness. Supplementary surface measurements in Baltimore include the BES reference meteorological station; surface PM10, NO2, SO2 and O3. UV actinic and radiant fluxes are being modeled at scales ranging from local neighborhoods to the Baltimore-Washington metropolitan area. Studies will include evaluation of the interactions between the UV radiation and atmospheric pollutants. Educational and public-interaction activities will translate the measured UVB irradiance in terms of erythemal exposure for area citizens. Urban/rural differences in UV irradiance associated with 'urban' and 'rural plumes' are summarized.