Observations and Modeling of Downward Radiative Fluxes (Solar and Infrared) in Urban/Rural Areas

Abstract

Pollutants (gaseous and aerosol) contained in urban atmospheres alter radiative fluxes at the surface.Numerous radiative models have been developed, and while few experimental data are available, results areoften contradictory. We have taken measurements, over several weeks, of downward radiation (solar andinfrared) over the city of Toulouse and a rural reference site. The downward IR flux was larger at the urbansite by day and night (increase between 15 and 25 W m2). Attenuation of the incident solar radiation atthe urban site was observed (30 W m2 in the middle of the day). A radiative model enabled us to show thatthe IR flux increase is mainly due to higher air temperature associated with the urban "heat island." Theenhissivity increase due to the addition of absorbing constituents in the urban area was very weak. Theattenuation of solar radiation was due to absorption by urban aerosol. Total incoming radiation (solar+ infrared) was similar at the two sites by day and was slightly higher at the urban site during the night.