Urban influences on solar radiation in CAPLTER

Rapid growth of cities feeds back into the climate system in many ways. As Phoenix, AZ has undergone rapid growth and expansion into the surrounding desert in the 1990s, local aerosol production and particulate emissions are occurring at ever increasing rates. These population-related activities have apparently altered the solar energy climatology in a manner consistent with other findings on solar reduction in cities. Recent research found over short distances sizeable differences in short wave radiation receipt between fixed urban and rural stations within metropolitan Phoenix and the surrounding rural desert. At the same time urban/rural radiation differences have been increasing at the rate of 0.5% per year in the decade of the 1990s. This paper reveals results from direct observations of solar and UV radiation through the method of detailed mobile environmental transects across the urban LTER region of CAPLTER. These data include 1 minute time intervals and measurements of air temperature, dew point, pressure, LICOR solar radiation, and UV radiation for selected days in autumn, winter, and spring of 1999-2000. Some of these days were during high pollution alert conditions, which were more frequent than normal this past season due to the drier winter and more stable atmospheric conditions. The mechanics of conducting transects for measurements of solar radiation require time corrections of the data utilizing standard solar equations. Pollution data for specific days were archived; detailed notes on the brown cloud conditions were obtained; and observations from fixed stations were collected to determine the city's overall effects on solar radiation. For example, during a high pollution alert day at noon (during clear, calm conditions on January 10, 2000), transect data indicate a 11% reduction in direct+diffuse solar radiation and 13% reduction in UV radiation through the bulk of the brown cloud that was evident in the western sector of the city on that date. The transect data allow for more spatial resolution of these reductions than standard weather station networks and is an adequate alternative to the small number of fixed points measuring solar radiation in and around the city.