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MODIS Data Product Non-Technical Description - MOD 04

The surface of the Earth is encased in a bubble of air – air that we rely upon for life and which directly affects the world’s climates. We rely not only on the presence of air, but the quality of the air as well. The quality of air is determined by its aerosol composition – a gaseous suspension of fine, solid or liquid particles, such as water, carbon, and dust. The presence of too much aerosol can worsen the air quality, such as when nitrogen oxides (by-products of burning) react with other chemicals in the atmosphere to produce ground-level ozone and other products that make up smog over densely populated areas. Many natural aerosols come from the earth's oceans (such as those produced by phytoplankton and via air-sea exchange), but land surfaces produce the majority of aerosols, especially anthropogenic ones (those produced non-naturally, such as by industry).

Having data on aerosol properties and their distribution throughout the atmosphere will help scientists to understand how aerosols affect a number of climatological phenomena, such as the radiation budget (the balance of incoming and outgoing radiation) and cloud albedo (the percentage of incoming solar energy that an object reflects back into space). Understanding aerosols also helps to explain certain temperature trends because different sizes and colors of aerosol particles interact with the light from the sun in different ways. Dark-colored particles, called black carbon aerosols, absorb a lot of radiation and heat the atmosphere; light-colored particles reflect sunlight and cool the Earth. Information on aerosols will also enhance the study of many biogeochemical cycles, such as phytoplankton concentration (see sidebar). To fully understand these processes, the aerosol characteristics (composition, size, distribution, and total content) have to be determined on a global scale.

Atmospheric scientists Yoram Kaufman and Didier Tanre developed the MODIS Aerosol Product (MOD04) to track the content, concentration, and other aspects of aerosols. They use the MODIS instruments onboard Aqua and Terra to gather data about the aerosol properties of the atmosphere globally over ocean and almost globally over land. To determine the different aerosol properties of the atmosphere, Kaufman and Tanre observe how radiation reflects off of the oceans and land, and the difference between levels of radiation going into the atmosphere and coming out. Observing the varied radiation wavelengths can tell researchers what kinds of molecules are in the air, their concentration, and other characteristics.

These aerosol measurements are a necessary component of climate and energy budget models because of their capacity for radiative forcing. Radiative forcing is a change in the balance between incoming solar radiation and outgoing infrared (heat) radiation. Without any radiative forcing, radiation coming in would be approximately equal to the radiation emitted from the Earth. The addition of greenhouse gases traps and increases the fraction of infrared radiation, reradiating it back toward the surface and creating a warming influence. Adding more greenhouse gasses to the atmosphere makes our planet retain more radiation, which is what scientists refer to as the greenhouse effect.

One of the greatest sources of uncertainties in climate modeling is due to aerosols. Factoring in the role of aerosols may help explain the fact that global temperatures have not increased as greatly as our climate models have predicted given the increasing amounts of greenhouse gases produced in the last century. “Radiative forcing by aerosols may explain the difference between the observed and modeled temperature trends,” wrote Kaufman and Tanre in their MOD04 Algorithm Theoretical Basis Document (ATBD). If temperatures haven't increased as much models have predicted, it may be because aerosols are influencing how much energy is being reflected and absorbed.

With the data that the two MODIS instruments provide, Kaufman and Tanre will be able to monitor the sources, transports, and sinks of specific aerosol types, the interaction of aerosols with water vapor and clouds (both monitored by MODIS), and radiative forcing. Additionally, the aerosol product will help other MODIS scientists account for the "interference" that aerosols can create in their products, such as measurements of surface reflectance over land.

The data have to be processed by two algorithms, depending on whether the data was gathered over land or ocean. Land and ocean data have two separate algorithms because they have very different reflectance properties – whereas oceans are relatively uniform in color, texture, and aerosol content, land surfaces are highly varied, produce a wider variety of aerosol particles, and are much more difficult to obtain useful data from because of the their variability.

Sidebar:
Collectively, phytoplankton have a large impact on the global climate and air quality in that they are a major source of natural aerosols. Like their land-based plant cousins, phytoplankton take in carbon dioxide from the atmosphere and, through the process of photosynthesis, release oxygen into the air. Large concentrations of these organisms, sustained over long periods of time, could significantly lower atmospheric carbon dioxide levels and, in turn, lower average temperatures. Because their population density is tied to the amount of carbon dioxide they can pull out of the atmosphere, it is very important to understand just how much of this chemical is in the atmosphere.


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