This is a very complicated question, and is dealt with in more detail in the Intergovernmental Panel on Climate Change (IPCC) reports and elsewhere.
In brief, most aerosols scatter the Sun's light, which results in a local cooling effect. However, some aerosols also absorb light, which can result in a local warming effect, depending on the amount of aerosols and the brightness of the underlying surface at the place in question. The net direct effect of this scattering and absorption is, on global average, cooling. This means that aerosols have offset part of the positive forcing (warming) due to greenhouse gases such as CO2. However because the lifetimes of aerosols in the atmosphere are very short compared to greenhouse gases, and because aerosols have a lot of spatial and temporal variability, it is not correct to say that aerosols cancel out the effects of greenhouse gases. Additionally, aerosols can have complex influences on cloud formation and evolution, which further modify the Earth's energy balance.
Aerosols are a fundamental constituent of the atmosphere which we breathe in every day, and in most cases if there is no active air quality warning for a given location they are not a cause for concern. In cases where the aerosol loading is very high, such as severe dust storms or smog, exposure to aerosols can have negative effects on the heart and lungs. Some agencies, such as the United States Environmental Protection Agency (EPA), use aerosol information to issue air quality warnings. In these contexts the term 'particulate matter' is often used to refer to surface-level aerosol loading.
Volcanic ash aerosols can be dangerous to air traffic, and so are also monitored by Volcanic Ash Advisory Centers (VAACS) in the USA and elsewhere.
Deep Blue and Dark Target are different algorithms with different strengths and weaknesses. This means that in some situations one may be more reliable than the other. This paper goes into some details of the differences between the MODIS Deep Blue and Dark Target data sets.
Some particular strengths of Deep Blue are that it can provide aerosol information over bright land surfaces, such as deserts, while Dark Target does not. Additionally, Deep Blue can be applied to sensors like SeaWiFS, which did not make measurements at certain wavelengths in the shortwave infrared spectral region required by Dark Target.
Deep Blue is an algorithm which has been applied to measurements taken by several satellite sensors to monitor aerosols in the Earth's atmosphere. A major advantage of Deep Blue compared to many prior techniques is that it extends the spatial coverage of aerosol data sets to include bright land surfaces such as deserts.
The Aerosol Robotic Network (AERONET) is a network comprised of several hundred sun photometers, spread throughout the world. They are used for a variety of applications related to aerosol characterization. You can visit the AERONET webpage here.
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