Abstract
Effects of tropospheric aerosols on the radiation budget in the lower atmosphere have been studied through aircraft measurements of aerosols and the solar and infrared radiative fluxes combined with a concurrent ground-based spectral attenuation measurement of the direct solar beam. Six cases of observations during cloudless winter midday hours with the solar zenith angles around 60°, were carefully analyzed. Of the six cases, four were made over land (Tsukuba) and two cases were conducted off the coast (Kashimanada).
The vertical profiles of aerosol concentrations were associated with those of water vapor, and the aerosol size distributions were typically bi-modal. Tropospheric aerosols effectively scatter and absorb the solar radiation, greatly influencing the vertical profiles of the upward and downward solar fluxes. Over the lower troposphere, the average aerosol absorption effects were found to be at least the same order of magnitude as those due to water vapor. In dense haze layers, however, the instantaneous solar heating rate was as large as 5°C/day, and the contribution from aerosols was about three times larger than that of water vapor. Since the IR flux profiles were mainly determined by the distribution of gaseous constituents and temperature, the effects of tropospheric aerosols were not appreciably large. The lower troposphere experienced IR cooling of the order of 1°C/day, but the cooling was lessened in the surface layers when the surface temperature was much higher than the surface air temperature.
Presence of the tropospheric aerosols had only a small effect on the radiation budget at the top boundary of the lower troposphere-surface system. However, the tropospheric aerosols greatly affected the distribution of solar energy in the system. Compared to an aerosol-free case, the lower troposphere experienced a substantial amount of additional solar heating due to aerosol absorption, at the expense of a comparable amount (of the order of 10W/m2 as the 24-hour mean) of the solar energy absorbed by the surface. As a consequence of the solar heating combined with the IR cooling, the lower troposphere has a substantial net heating in its lower part and a net cooling in its upper part. This, as well as the net solar heating of the surface may be responsible for the formation of the well-developed mixed layer over Tsukuba.
