Radiative effects of aerosols over land and ocean

Abstract

The last two centuries have witnessed tremendous growth on all fronts (especially industrial) compared to what had occurred in the past thousands of years. This growth brought along, in addition to the awareness of safety and comfort in our lives, environmental pollution. It has become essential for humans to understand the importance of the influence of these pollutants, especially those in the air, on their environment. Of late, there has been a lot of focus on global warming and its influence on the climate.

Long ago, in 1896, Svante Arrhenius asked, "Is the mean temperature of the ground in any way influenced by the presence of heat-absorbing gases in the atmosphere?" Thus opened the question on global warming. It is now well recognized that not only do greenhouse gases (GHGs), but also aerosols (particles suspended in the atmosphere), contribute significantly. The recent IPCC (2001) report on climate change clearly highlights the importance and influence of these particles. Moreover, recent experiments such as the Tropospheric Aerosol Radiative Forcing Experiment (TARFOX) and Indian Ocean Experiment (INDOEX) have also focused on the anthropogenic (man-made) aspect of this.

Aerosols, the solid or liquid particles suspended in the atmosphere, have been found to have considerable influence on the Earth's radiation budget and, hence, the climate. The uncertainty in the effect of aerosols and their high variability in space and time make aerosol studies critical. In the present study, aerosol observations were made at Bangalore, an urban continental station, and over the Arabian Sea, a maritime location. Aerosols, in addition to their direct effect on radiation, also act as condensation nuclei (CN) for the formation of clouds, thus affecting cloud microphysics and indirectly the radiation. This indirect radiative effect has been studied, with a particular focus on the anthropogenic aerosols. In the present study, preliminary estimates of the indirect effect of sea-salt aerosols have been made over the Arabian Sea.

Aerosol optical depth (AOD) measurements were carried out both over Bangalore, an urban continental site, and over the Arabian Sea, a maritime location. Aerosol optical depth measured over as long as about two years, since April 2001, at Bangalore was utilized in this study to assess the impact of aerosols on the radiation budget. This study was carried out as part of the Indian Space Research Organisation - Geosphere Biosphere Programme (ISRO-GBP). It was observed that AOD variations over Bangalore show a systematic seasonal pattern, with a maximum during April/May (0.5 at 500nm) and a minimum during November/December/January (0.2 at 500nm). The Angstrom wavelength exponent was as high as 1.45 during the turbid period, which, in conjunction with the high optical depth, indicates significant anthropogenic influence. The value of the wavelength exponent was larger than 0.8 throughout the period of observation.

By examining various meteorological parameters, it was observed that one of the factors controlling the seasonal variation of AOD over Bangalore could be rainfall. Though rainfall over Bangalore did not show a one-to-one correspondence with AOD variation, the rainfall pattern over a larger area surrounding Bangalore showed that it influences AOD. The onset of monsoon rain causes a drastic decrease in AOD due to wet removal, whereas the buildup during the dry season is relatively slow. This influence of rainfall on the aerosol characteristics is very clearly seen by the contrast in aerosol parameters during 2001 and 2002 (a rain-deficit year).

Aerosol forcing at the top of the atmosphere (TOA), surface, and atmosphere was as high as -1.61 to -3.12 W/m², -27.95 to -28.91 W/m², and 24.82 to 27.30 W/m² respectively during the turbid period and -0.88 to -1.96 W/m², -17.08 to -17.61 W/m², and 15.12 to 16.73 W/m² respectively during clean periods. The atmospheric forcing ranged from 15 to 33 W/m² from April 2001 to January 2003. The consistently high atmospheric forcing throughout the year suggests the presence of absorbing aerosols.

AOD observations have been made over the northern and central Arabian Sea during the summer monsoon season for the first time, and its radiative forcing has been estimated. During the summer monsoon season over the Arabian Sea, aerosol visible optical depths are as high as 0.4 to 0.7, with an Angstrom wavelength exponent of 0.35 and aerosol single scattering albedo of 0.97, indicating the presence of large non-absorbing aerosols. Aerosol optical depth increases nearly exponentially with sea-surface wind speed. Our estimates show that, on average, sea-salt contributes more than 60% of the composite aerosol optical depth. The presence of aerosol over the Arabian Sea decreases the shortwave radiation arriving at the surface by 21 W/m² and increases the top of the atmosphere reflected radiation by 18 W/m². The radiative forcing thus estimated over the Arabian Sea during the summer monsoon season has been compared with that obtained during other periods. The study also shows the effect of high wind speeds on aerosol properties and, consequently, high sea-salt aerosol production and its effect on radiation.

Though there have been numerous studies devoted to the indirect effect of anthropogenic aerosols, studies on the indirect effect of natural aerosols are sparse. In this thesis, a simple approach was used to determine the indirect effect of aerosols over the Arabian Sea and demonstrated that the indirect effect of sea-salt (natural) aerosols is considerably large when compared to the direct radiative effect, and hence cannot be ignored in modeling studies. The indirect effect of natural (sea-salt) aerosol has been estimated for different seasons over the Arabian Sea. This study demonstrates the importance of wind speed on aerosol characteristics and its impact on direct and indirect radiative forcing. The magnitude of the indirect radiative effect (and uncertainty) is several-fold more than the direct radiative effect of sea-salt aerosols.