33spheric release of NF 3 seems to be due to the manufacture of large, liquid crystal
display screens (Thomas et al. 2012 ). The present RF of climate due to NF 3 is small,
~2.4 × 10−4 W m−2.^33 Perhaps this late-comer to the GHG-club will one day be known
as the couch potato GHG.
1.2.3.6 Aerosols
Aerosols are small solid or liquid particles suspended in air. In the context of this
book, we use aerosols to refer to particles either emitted directly into the atmo-
sphere by a particular human activity (typically fossil fuel combustion or fires) or
particles that form following chemical and physical transformations in the atmo-
sphere of pollutants known as aerosol precursors. The only natural aerosols we shall
consider are those resulting from volcanic eruptions; volcanic aerosols only affect
climate if they exist in the stratosphere.
Aerosols, particularly those containing the element sulfur, reflect incoming solar
radiation, which cools the surface. Sulfate aerosols tend to be produced from pollut-
ants emitted by coal-fired power plants, ships, and diesel fueled trucks and cars,
although there is a strong movement towards use of ultra-low sulfur diesel fuel in
some parts of the world (Krotkov et al. 2016 ). Volcanic aerosols, which exert short-
term climatic cooling, are also composed of sulfate (Lacis and Mischenko 1995 ).
Sooty aerosols, termed black carbon, are likewise produced by combustion of fossil
fuels and biomass burning. Black carbon aerosols have a warming effect because
these particles absorb solar radiation (Bond et al. 2013 ).
The association of human activity with the presence of tropospheric aerosols is
well established from both ground-based (Jimenez et al. 2009 ; Yoon et al. 2016 ) and
space-based observations (Streets et al. 2013 ; Yoon et al. 2014 ; He et al. 2016 ;
McLinden et al. 2016 ). Yet, quantification of the RF of climate due to tropospheric
aerosols continues to pose a scientific challenge due to the inability to precisely
define numerical values of both the direct modulation of RF by anthropogenic aero-
sols (Myhre 2009 ; Kahn 2012 ; Bond et al. 2013 ) and the changes in RF driven by the
effect of aerosols on clouds (Morgan et al. 2006 ; Carslaw et al. 2013 ). The IPCC
( 2013 ) best estimate and uncertainty of ΔRF over the course of the Anthropocene for
these two terms, labeled Aerosol Direct Effect and Aerosol-Cloud Interaction, are
shown in Fig. 1.4.
Tropospheric aerosols lie at the nexus of public health, air quality, and climate
change. Exposure to small (Dominici et al. 2006 ) and/or toxic (Bell et al. 2007 )
aerosols has deleterious effects on human health. As a consequence, movements are
underway throughout the world to reduce both the direct emission of aerosols as
well as the emission of aerosol precursors. Reductions in the abundance of tropo-
spheric aerosols and aerosol precursors, in response to air quality legislation moti-
vated by public health concerns, have been readily observed by space-borne
(^33) Atmospheric abundance of NF 3 peaked at 0.0012 ppb in late 2011 (Arnold et al. 2012 ) and radiative
efficiency is 0.2 W m−2 ppb−1 (Table 8.A.1 of IPCC ( 2013 ); RF of NF 3 = 0.0012 ppb × 0.2 W m−2 ppb−1
= 2.4 × 10−4 W m−2.
1.2 The Anthropocene