Urban Aerosol Observations and Comparisons of daytime and Nighttime characteristics, In Aerosols, Chemistry, Environmental Impact, ed D Peretz

Understanding of atmospheric aerosol optical properties and their effect on urban and global climate is biased by lack of observations at night. Sunlight scattering and extinction provide satellite and automated sun-photometer observations on a global and local scale. Nighttime observations are very rare and usually include only point measurements of dry aerosol physical and optical properties. We have analyzed aerosol stellar extinction data over many years at an observatory in Oakland, California. This observatory was located for many years at a relatively low altitude below the nocturnal inversion layer depth (104 m). It was recently moved to a higher location (470 m) approximately at or slightly below the San Francisco Bay Area boundary layer depth. The results of our analysis show that both the aerosol light scattering magnitude and light scattering properties as a function of wavelength are different at night than during the day. This is caused by the nighttime increase in relative humidity and associated growth of hydroscopic aerosols. This growth in aerosol and nighttime haze reinforces the nocturnal inversion by heating the air both through the latent heat of condensation as well as by the trapping of infrared surface radiation. This in turn, causes more pollution trapping on the morning of the following day. This effect has not been included in urban pollution models. It is also expected that similar effects of smaller magnitude can be expected over the marine boundary layer where day / night differences in relative humidity are significant and thereby have an effect on global climate. We also show that for coastal regions (like the San Francisco Bay Area) the optical properties of the aerosols change depending on the season. As the inner regions of California warm in summer clouds and marine aerosols are drawn in from the ocean throughout the night. In the late