**Table 1.**

*Characteristics of some greenhouse gases.*


#### **Table 2.**

*Potential characteristics of aerosol pollutants.*

#### **4.2 Aerosols**

Air quality may be eroded by particulates. Particles of public health attention are defined by an aerodynamic diameter in the range of ≤10 μm (PM10). Fragments with diameters less than 2.5 μm (PM2.5) are identified as fine particles. Aerosol is the technical parlance used to denote airborne liquid and solid droplets and particles. Aerosols are ever-present (inside/outside homes, buildings, atmosphere); could be of human or natural origin; primary (when emitted directly from pollution sources) or secondary (when made from precursor pollutants); and could exhibit greenhouse gas properties or function as reflectors. Aerosols include sulfates (SO4 2−), nitrates (NO3 − ), black carbon (BC), and a plethora of other compounds, and chemical species [41, 42]. Examples of natural aerosols are desert dust, sea salt particles, meteoritic fragments, as well as ash and sulfates from volcanic eruptions. Aerosols affect atmospheric and ocean conditions including cloud droplets, circulation, snow covers, precipitation, temperature, and heat transfer; thereby influencing planetary energy and water budgets [18, 26]; and the references in these citations. Those aerosols that absorb radiation from the sun (e.g., BC, mineral dust) could fulfill the effects of GHGs and warm the earth, while those that reflect and scatter radiant energy (e.g., SO4 2−, NO3 − ) may impart cooling effect. Collectively and on average, aerosols appear to have a net cooling impact [18]. In 2007, the solar direct radiative effect (DRE) for aerosols estimated from satellite remote sensing studies was reported to be −5.4 ± 0.9 W/m<sup>2</sup> [40]. The possible properties of some aerosols are presented in **Table 2**, and their pollution impacts exhibited in **Figure 3**.
