17. Aerosols and Climate Change: Present Understanding, Challenges, and Future Outlook

Atmospheric aerosols are a mixture of solid or liquid particles suspended in the medium of air. Their physical (size, shape, and texture) and chemical properties vary over sub-micron and super-micron radius range, consequently their removal processes, and their residence times differ greatly. Aerosols play a vital role in cloud formation and air pollution among others. The space-time variations in aerosol characteristics, and aerosol types arise due to different aerosol sources, and atmospheric dynamics which include boundary layer, temperature, relative humidity variations, and long-range transport. The optical, physical, and chemical characteristics of aerosols over continent and ocean exhibit distinct variations. Atmospheric aerosols through direct and indirect radiative effects influence the Earth-atmosphere radiation budget and climate. The challenges involved in estimating the direct and indirect radiative effects of aerosols, and further detection and attribution of the temperature response due to aerosols which include delineating more quantitatively the natural and man-made contribution, and scattering versus absorbing aerosols are detailed in this chapter. The presence of different aerosol species over a location produced by sources of local origin, and long-range transport can give rise to different mixing states of aerosols owing to aging and interaction among them which in turn can change their characteristics and life cycle. Results reveal that single scattering albedo, which is non-linearly related to aerosol radiative forcing, is significantly different for external, core-shell, and internal mixtures. Aerosol measurements from a remote sensing perspective and the advances that have been made in this regard are enlisted. The future requirements in terms of measurements, remote sensing techniques, and modeling studies for better detection and attribution of climate change due to aerosols is envisioned.