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Aerosol FAQs (04_L2)

Frequently Asked Questions

Dark Target Aerosol

Q. What are aerosols and why are they important?  What kinds of aerosols are there?

A. Aerosols are small solid particles or liquid droplets suspended in the atmosphere. We are interested in them for reasons including climate change, air quality (health), hazard monitoring, ecology, and more.  The main categories of aerosols include smoke (from industry and wildfires), industrial aerosols and smog (smoke stack emissions), volcanic ash (from eruptions), mineral dust (from deserts and arid land), sea spray, biogenic aerosols (that come from living things, which include include pollen, spores, and microbes as well as organic chemicals such as limonene from plants).

Q. Are aerosols dangerous?

A. 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.

Q. How are aerosols related to climate change and global warming?

A. 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.

Q. What is the difference between the Dark Target Aerosol Ocean and Dark Target Aerosol Land products?

A. Ocean and land products use completely different algorithms. For any dark target retrieval algorithm we must accurately account for surface effects and build correct models of the aerosols.  Due to the complexities of the land surface it is more difficult for the land retrieval to account for surface effects. In addition aerosol models are fixed for the land product based on season and location.  The ocean product selects coarse and fine mode models dynamically.  These factors account for several differences in the two products.

a)  The greater error of the AOD in land product

b)  AOD values for land are available at 0.47, 0.55, 0.67 and 2.1 µm.  AOD values for ocean are available at 0.47, 0.55, 0.67, 0.87, 1.2, 1.6 and 2.1 µm.

c)  In Collections 5, 6, and 6.1 the ocean product contains parameters which report fine and coarse mode AOD separately and the fine mode fraction. Over land we do not have the ability to retrieve these parameters accurately. These parameters are reported over land in Collection 5, but we do not recommend their use. In Collection 6 and 6.1, only fine mode fraction is reported but it is a diagnostic of how the retrieval is operating and not a representative value of the actual fine mode fraction.

Q. Why can’t the Dark Target algorithm retrieve aerosol data over bright surfaces?

A. The dark target retrieval algorithm assumes that aerosols over a surface will brighten the scene.  Where the surface is too bright this assumption breaks down. 

Q: Why do many of the research papers use Dark Target AOD products derived from the green band?  Why not use the blue band or red band?

A: It should be noted that 550 nm is a common wavelength used by the international aerosol community at large. This is because this particular spectral region is fairly accessible from both remote sensing and laboratory measurements, and both fine-mode and coarse-mode aerosol particles are fairly optically active in the mid-visible spectrum.

Deep Blue Aerosol

Q. What is Deep Blue Aerosol? And why is it called Deep Blue?

A. Deep Blue Aerosol 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 aerosol retrieval techniques is that it extends the spatial coverage of aerosol data sets to include bright land surfaces such as deserts. The development of the Deep Blue algorithm narrowed these gaps in SeaWiFS and MODIS aerosol products by performing retrievals over bright-reflecting surfaces. The Deep Blue algorithm utilizes blue wavelength measurements (see spectrum image, below) from instruments such as SeaWiFS and MODIS, where the surface reflectance over land is much lower than for longer wavelength channels, to infer the properties of aerosols [Hsu et al., 2004].

Q. Why do we need both Deep Blue and Dark Target?

A. 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. [Sayer et al., 2014] details some of the differences between MODIS Deep Blue and MODIS 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.

Q. What geophysical data quantities does Deep Blue provide?

A. The main scientific geophysical data product is the aerosol optical depth (AOD), also called aerosol optical thickness (AOT), at a wavelength of 550 nm. This quantity has been validated against AERONET and other data sources and has generally understood error characteristics. Each of our retrievals also has a quality assurance (QA) flag associated with it, which can be used to filter out retrievals which may potentially be of a lower quality (e.g. from sub-pixel cloud contamination). For most scientific applications, it is strongly recommended that users apply these QA flags before scientific use of the data. For ease of use, each of our files also includes a data set including only those retrievals passing these QA tests. This is referred to as the 'best estimate' data set within the file metadata, and it is expected that most users will do their analyses based on this data. Note that Level-3 Aerosol data are only comprised of such 'best estimate' retrieval pixels. Depending on the sensor and underlying surface/atmospheric conditions, certain other data products, such as the Ångström exponent, are also provided.

Q. To which instruments has the Deep Blue algorithm been applied?

A. The Deep Blue algorithm has been applied to MODIS Terra, MODIS Aqua, SeaWiFS, and VIIRS.

Q. What is AERONET?

A. 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.