Scattering and absorbing aerosols in the climate system (original) (raw)

Nature Reviews Earth & Environment

Aerosols are small liquid or solid particles suspended in the atmosphere 1. They can be emitted directly (such as dust, sea salt, black carbon (BC) and volcanic aerosols) or formed indirectly through chemical reactions (including sulfate, nitrate, ammonium and secondary organic aerosols). Owing to their relatively short lifetime, aerosol concentrations typically peak near their sources. Desert regions (such as North Africa and the Middle East), industrial regions (such as East and South Asia) and biomass-burning regions (such as South America and South Africa) are, therefore, characterized by high mass concentrations (Fig. 1). Aerosols exhibit complicated compositions and vary substantially in shape and size, typically ranging between 0.01 and 10 μm (reF. 2). Depending on these structural and compositional characteristics, aerosols can scatter and/or absorb shortwave radiation, as quantified through the single-scattering albedo (SSA; Table 1). Purely scattering aerosols include sulfates, nitrates, ammonium and sea-salt particles, whereas absorbing aerosols are primarily BC, with dust and organic carbon partly absorbing in the ultraviolet (UV) spectrum 3. Aerosols have a direct bearing on Earth's energy balance and, therefore, on climate. For instance, aerosol scattering and absorption alters the radiation balance and atmospheric stability through perturbations to the vertical temperature profile. Aerosols can further serve as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs), which modify the reflectivity and lifetime of clouds through microphysical processes. Collectively, these influences are quantified as aerosol forcing: the change of net radiative flux at a specified level of the atmosphere, often assessed relative to estimated pre-industrial conditions 4. Globally, anthropogenic aerosols are estimated to produce a net cooling ~−1.3 ± 0.7 W m −2 at the top of the atmosphere; −0.3 ± 0.3 W m −2 is attributed to the aerosol-radiation interaction (ARI), −1.0 ± 0.7 W m −2 to aerosol-cloud interactions, ~−1.15 W m −2 to total forcing from scattering aerosols and ~+0.12 W m −2 to BC 4. This combined aerosol forcing offsets roughly one-third of the warming from anthropogenic greenhouse gases (GHGs). However, the large spread in the estimated aerosol forcing leads to large discrepancies in climate sensitivity 5,6. Thus, aerosols are considered to be the largest contributor of uncertainty in quantifying present-day climate change 4. Much of this uncertainty in aerosol forcing arises from both the lack of separate global constraints on aerosol optical and microphysical properties (optical depth, size distribution, hygroscopicity and mixing state, among others) and the inaccurate representation of them in climate models 7-10. In particular, aerosol SSA is further