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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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