Influence of atmospheric aerosols on solar spectral irradiance in an urban area

Abstract

Solar radiation reaching the earth's surface at different wavelengths has been extensively discussed during the last decades. Great emphasis has been placed on the potential increase in surface UV radiation due to the depletion of stratospheric ozone. The present study reports the variation of solar spectral irradiance and its relation with aerosols over a typical urban environment in India. Synchronous measurements of aerosol optical depth, UV irradiance, aerosol-particle size, black carbon (BC) concentration and solar irradiance have been carried out at the urban station of Hyderabad located in central India. Considerable reduction in the UV intensity has been observed during periods of high aerosol loading. A comparison of the erythemal UV (UV_ery) intensities on normal day with those of high aerosol loading suggested a ∼24% decrease in the UV_ery reaching the ground. Satellite observations showed forest fire occurrence over the region. PAR and diffuse-to-direct-beam ratio of solar irradiance showed marked differences under varying aerosol-loading conditions.

Introduction

Atmospheric aerosols are fine particles that can scatter and absorb the incident solar radiation contributing to cooling of the earth's surface and a simultaneous warming of the lower atmosphere (Keil and Haywood, 2003; Pace et al., 2006). Besides this direct radiative effect, aerosols act as condensation nuclei in the formation of clouds modifying their microphysical properties. The aerosol-number density, chemical composition and size distribution can influence the albedo and lifetime of clouds as well as the rate and the amounts of precipitation (Abel et al., 2005; Lohmann and Feichter, 2005). Aerosols degrade the air quality in urban areas and reduce visibility. Continental aerosols are mainly wind-blown mineral dust as well as carbonaceous and sulfate particles produced by forest fires, land use and industrial activities, while marine aerosols are mainly sea-salt particles produced by wave-breaking and sulfate particles produced by the oxidation of dimethyl sulfide released by phytoplankton. As the oceans cover more than 70% of the earth's surface, they consist one of the largest sources of natural aerosols. Being hygroscopic in nature, marine aerosols are crucial in cloud formation in the marine boundary layer and are also important in the radiative coupling between ocean and atmosphere. Continental aerosols can be both scattering and absorbing, while marine aerosols are mostly of scattering type (Dubovik et al., 2002); thus, there can be an argument about the planetary albedo. Aerosols in urban environments are physically and chemically different from aerosol in remote areas with the most obvious differences being the high concentration of sulfur and heavy metals in urban aerosols (Latha and Badarinath, 2004). The variety of sources, natural and anthropogenic, the short lifetimes of aerosols and their influence by the meteorological parameters, especially by relative humidity (Day et al., 2000; Hänel, 1976; Horvath, 1996), result in a spatially and temporally heterogeneous aerosol field, making aerosol characterization and modeling a real challenge (Smirnov et al., 2002).

On the other hand, the amount of solar ultraviolet (UV) radiation penetrating the earth's surface is critically important for the health of biological systems (Feister and Grasnick, 1992; Németh et al., 1996; Sutherland et al., 1991); practically no solar radiation reaches the ground at wavelengths shorter than 290 nm due to its strong absorption by stratospheric ozone. The biologically harmful UV-B radiation lies in the spectral range 280–320 nm, while erythemal response of the human skin is maximum at about 297 nm. Erythema, which is defined as a reddening of human skin in response to solar radiation, extends through both UV_ery and UV-A (315–400 nm) (Herman et al., 1996). Autocorrelation between total column ozone and surface UV radiation is a complex function of many variables, including solar zenith angle, altitude, cloud cover, aerosol loading, surface albedo and vertical profile of ozone. The effect of aerosols on the UV radiation constitutes a great scientific issue. Therefore, many studies have been carried out (Liu et al., 1991; Kylling et al., 1998); these researchers have reported that high loading of the absorbing particles could cause reduction of UV flux at the surface by more than 50%. In the last decades, a continuous increase in the biological active solar UV-B radiation due to a decrease in the ozone amount emerges at a global scale (Zerefos et al., 1995). Nevertheless, at regional scales even a decrease of the ozone amount of 50 DU in combination with an increase in the aerosol loading can lead to a decrease in the UV radiation (Balis et al., 2002; Papayannis et al., 1998). Therefore, continuous ground-based observations play an important role in improving the understanding of some of these effects (Madronich and Flocke, 1997).

The precise determination role of all the above parameters in quantifying and modifying the UV_ery levels is very difficult to be distinguishable due to the combined involvement of all these parameters in the radiation processes in the atmosphere. Therefore, the use of solar radiation models (e.g. SMARTS) is necessary for the improvement of the knowledge at the role of each parameter. Indeed, systematic investigations on the temporal variation of UV_ery radiation and its influencing parameters are still sparse (Gueymard, 1995). This paper provides a case study of changes in ground-level solar irradiance, diffuse-to-direct-beam ratio and UV_ery as well as their relationship with the aerosols under different turbidity conditions over the tropical urban area of Hyderabad, India, using simultaneous measurements. Such measurements are very limited all over India.