Atmospheric aerosols are a combination of tiny particles of various sizes, typically in the range 0.001 to 10 μm.
Aerosols are generated from a wide range of natural and anthropogenic sources. Natural aerosols can be created from volcanic activity, phytoplankton or sea salt in the oceans, respiration by plants, minerals from arid areas, or smoke from biomass burning. In contrast, anthropogenic aerosols are the direct consequence of human activities (i.e., pollution).
Aerosols scatter and absorb sunlight, thus leading to modifications of Earth's energy balance and to climate impacts. In parallel, aerosols also act as cloud condensation nuclei and can change the microphysical and radiative properties of clouds, thus affecting their lifetime and further modulating climate.
In solar resource studies and most solar energy applications, the incident solar irradiance must be modeled because of the lack of local measurements at the specific site of a project. This incident irradiance (particularly its direct and diffuse components) are strong functions of the aerosol optical properties, most importantly the Aerosol Optical Depth (AOD). AODλ is a spectral quantity that typically decreases when wavelength increases, according to Ångström’s law:
where β is the AOD at 1 µm (usually referred to as the Ångström turbidity coefficient), λ is wavelength in µm, and α is the Ångström exponent. The latter typically varies between 0 and 2.5, depending on the average size of the aerosol particles.
In recent years, spaceborne instruments have provided regular observations of AODλ at 550 nm (AOD550) and α. One popular source of such data is NASA’s MODIS instrument, on board the Aqua and Terra satellites. Using such data to derive solar irradiance time series however entails important limitations for various reasons:
- The Level-3 MODIS database is gridded with a spatial resolution of 1x1°, which is relatively coarse.
- Depending on period and region, many grid cells can have data breaks, due to orbital characteristics or cloudiness; this is particularly critical at high latitudes.
- The AOD absolute accuracy is not always satisfactory due to limitations in the retrieval algorithms; in particular, observations above high-albedo areas (snow or sand) are known to be highly biased.
To remedy this situation, Solar Consulting Services has developed an advanced methodology to correct and expand the MODIS-derived aerosol data into the SOLSUN database. SOLSUN is available over the whole world (with no latitude restriction or data break), and is calibrated against NASA’s AERONET ground-truth data. SOLSUN has a nominal resolution of 0.5x0.5°, but can be downscaled to finer resolution upon special request. The database currently covers the period 1998–2015, and is available as either a time series of monthly values (18x12 months) or as a monthly climatology (12 months).
A comparison of the mean monthly MODIS data from its Collection 6.1 and SOLSUN appears below for January and July in the case of North and South America. Colored circles represent the AERONET ground truth. Note, in particular, the important improvements to MODIS that are made possible by SOLSUN over the Atacama region of the Andes and various other areas.
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Figure 1: January-average AOD550 over North America from MODIS.
Figure 2: January-average AOD550 over North America from SOLSUN.
Figure 3: July-average AOD550 over North America from MODIS.
Figure 4: July-average AOD550 over North America from SOLSUN.
Figure 5: January-average AOD550 over South America from MODIS.
Figure 6: January-average AOD550 over South America from SOLSUN.
Figure 7: July-average AOD550 over South America from MODIS.
Figure 8: July-average AOD550 over South America from SOLSUN.