On a soggy morning this past April, Daniel Rosenfeld moved briskly
through an overcast parking lot into one of many monolithic brick
structures that dot NASAs Goddard Space Flight Center. When he
arrived at the conference room, he switched on a laptop and began
bringing up satellite images of what looked like an apocalyptic vision
of the East Coast. From Massachusetts to North Carolina, swirling red,
orange and yellow clouds floated above a blue landscape.
Peering over his glasses and intently at the screen, he pointed to an especially fiery spot on the map
near the border of Pennsylvania and New Jersey. "The atmosphere
here is fully polluted," he said. "Those colored streaks are
mostly due to man-made aerosols."
The dense concentration of powerplants, factories, trucks, and automobiles on the U.S. east coast continuously emit soot and other particulate pollutants into the sky. These small particles suspended in the airaerosolsoften end up interacting with clouds, forming more, smaller droplets than those found in an unpolluted cloud. In this satellite image the yellow clouds scattered over the northeast are polluted clouds with small water droplets. The pink clouds over Canada have larger droplets, and are relatively clean. Because the aerosols prevent cloud water droplets from growing large enough to precipitate, this type of pollution can reduce rainfall. (Image by Daniel Rosenfeld, Hebrew University of Israel)
He went on to explain that aerosols in this instance have little to do with household cleaning products. For atmospheric scientists such as him, aerosols are defined as any microscopic particle suspended in the atmosphere. Not only do they make our sunsets particularly vivid and our air particularly gritty on a hot summers day, but they also create clouds in the sky. While most atmospheric aerosols originate from natural sources such as the sea spray and volcanoes, he said nearly all the color enhanced yellow and orange clouds in his image have been affected by aerosols created by factories, power plants and cars up and down the East Coast.
For the past six years, Rosenfeld and a team of scientists from the Hebrew University of Israel used NASA satellites and remote sensing techniques to track how man-made aerosols change clouds. They recently discovered that aerosol particles from factories and power plants increase the number of droplets in clouds they pollute. In doing so, the pollutants create brighter clouds that retain their water and do not produce rain. These results put to rest a decades-long debate about human-generated pollutants and biomass burning as well as verify that our manufacturing processes and our need for energy are changing the global climate and local weather systems.
What Are Aerosols? contains background information about aerosols and their effect on climate.
|Studying Ship Tracks|
Throughout most of the twentieth century, the scientific community was only certain about a few basic facts regarding cloud formation. They knew that in order to form, clouds require water vapor as well as tiny microscopic particles (aerosols) from the surface of the Earth. When water evaporates into the atmosphere, it spreads evenly throughout the surrounding air often to the point where the relative humidity is greater than 100 percent. Aerosols that dissolve easily in water, such as ammonium sulfate and sea salt, give the excess water molecules something to cling to. These aerosols act as the nuclei, or "seeds," around which cloud droplets take shape, and together these droplets form clouds. Were it not for particles in our atmosphere, the sky would almost always be clear and the air around us thick and humid (King et al., 1995).
Many researchers suspected that as the concentration of these particles increases, the properties of the cloud could change. However, they remained unclear on exactly what these changes would be, the effects aerosols could have on rainfall, and most importantly where aerosol particles come from. Many believed that humans produce a large number through the burning of fossil fuels and plants (biomass).
In the late 1980s and early 1990s, a series of investigations on the exhaust from ships smokestacks answered some of these questions. Not significant sources of pollution themselves, ships burn fossil fuels and release their exhaust in the form of sulfur dioxide, a gas that leads to the formation of sulfate aerosols in the atmosphere. The exhaust produces clouds that are relatively low in elevation and resemble larger versions of airplane contrails. Unlike contrails, these "ship tracks" are ideal for study since they remain in the air for many hours and are surrounded by relatively pristine marine air (King et al., 1993).
Initially, the ship tracks were used to see if an increasing number of aerosols from pollutants would make the clouds brighter. The scientists believed the additional aerosols from the ships would give the water vapor more nuclei to cling to, so that a greater number of smaller drops would form in the cloud. These smaller drops, in turn, would make the cloud more reflective to sunlight. The same phenomenon can be seen when ice cubes are crushed. As the ice is broken up, the once smooth surface is shattered into many tiny surfaces at varying angles. These tiny surfaces reflect incoming light in all directions and cause the crushed ice to appear white and opaque. Water droplets do not contain all these ridged surfaces, but the researchers were fairly sure their fragmentation would have a similar effect.
Through satellite observation and in situ aircraft
measurements, scientists not only showed that their hypothesis was
correct, but they also came across an effect that no one had foreseen.
In addition to making the clouds more reflective, the aerosols were
causing them to retain water and to stop drizzling. The cloud seeding
by the ships exhaust made the droplets so small that they could no
longer easily merge together to reach the size needed for gravity to
pull them to the ground. Since no drizzle came out of the seeded clouds,
the cloud water just kept building up (King et al. 1995).
The formation of water droplets in clouds is typically governed by the number of aerosol particles present to act as nuclei. If no aerosols are present, water molecules are unlikely to combine and form droplets, resulting in very humid but cloudless air.
But to take these findings to the next level and prove that man-made sources of pollution make clouds brighter and less likely to precipitate would require testing a wide range of factories and power plants on land. Many deemed it too difficult to measure the effects of aerosols over land given the current level of technology. The wind currents and convection over land are tumultuous, and the clouds are much thicker. In nearly any given industrialized area there are so many things that cause pollution that its hard to isolate just one source. In the eastern United States for instance, the number of aerosols spewed out by power plants and factories merge together in the sky to create one big, thick soup of pollution.
|The puffy streaks of clouds cutting through the center of this image are ship tracksaerosols emitted in the ships exhaust cause many small water droplets to form, creating exceptionally bright white clouds. (Image by Mark Gray, MODIS Atmosphere Science Team)|
While TRMM does have an instrument called a visible and infrared sensor that retrieves the same type of data as AVHRR, it also contains two additional instruments for viewing what occurs inside the clouds. The first, known as the precipitation radar, does exactly what its name implies it bombards the clouds with radio waves and then receives the signals that bounce back. The radar is rigged so that the only signals that bounce back are from droplets that are at least the size of raindrops. By retrieving data from this instrument, scientists can tell whether a cloud is precipitating or not. The other instrument vital to Rosenfelds research is known as the passive microwave imager. This instrument picks up solely on one frequency of microwave radiation that cloud water and ice absorb and emit (Rosenfeld, 1999). "So the passive microwave is something that is sensitive to the total amount of water in the cloud with a little sensitivity to the size of the particles," Rosenfeld explained. In short, when low microwave readings show up on this instrument for clouds floating above a landmass, it means that the clouds as a whole contain much water.
Rosenfeld gathered TRMM satellite data over many of the same factories and power plants he examined with AVHRR. Using the visible and infrared sensor instrument data, he compiled estimates of effective radius and cloud top temperature on both polluted and unpolluted clouds. The results were nearly identical to the readings he derived from the AVHRR data. Rosenfeld then looked at the precipitation radar and microwave imaging data to see what was going on inside the clouds. Sure enough, the clouds in the path of the aerosols were not precipitating. Like the ship tracks, the clouds contained significantly more water than unaffected clouds, and they were on average brighter from above (Rosenfeld, 2000). Again, he ran the same tests on forest and brush fires and came up with an identical conclusion (Rosenfeld, 1998).
As a follow up to these experiments, Rosenfeld approached the engineers at these factories that were producing the pollution tracks and asked them how many aerosols their plants produced. The results were what he expected. The plant in Port Augusta, for instance, was putting out nearly 43 kilograms per hour of ash particles that were all the right size for breaking up the water in clouds. "And they were using electrostatic precipitators, which capture most of the ash," said Rosenfeld.
He also found that the type and number of cloud condensation nuclei coming from these smokestacks was much more varied and potent than the ship tracks. Ship tracks generally burn diesel fuel that throws sulfur dioxide into the air, which becomes sulfate aerosols through chemical reactions. On the other hand, factories put out not only sulfur dioxide, but also a large number of smoke and ash particles. These react immediately with the clouds and moisture in the air, so that the suppression of the cloud precipitation starts at the stack and then become more pronounced as the sulfur dioxide converts to sulfate aerosols.
|Simultaneous measurements of rainfall and cloud properties by satellite demonstrate that pollution can inhibit rainfall. The graph above shows cloud droplet radius versus temperature (equivalent to altitude) in three regions of the image above. Regions number 1 and 3 are in clean air (pink clouds), while region 2 (downwind of Adelaide, Australia) is polluted (yellow clouds). The cloud droplets in the unpolluted areas were above 14µm, and TRMM's Precipitation Radar showed that the clouds were precipitating (white overlay). In region 2, however, the water droplets remained small even at high altitudes, and no rain was falling. (Image by Daniel Rosenfeld, Hebrew University of Israel)|
In terms of global climate change, Rosenfelds work as well as the ship track studies could go a long way towards explaining why the Southern Hemisphere of the Earth has warmed up more than the Northern Hemisphere in recent decades. When fossil fuels are burned, carbon dioxide, sulfur dioxide, and other pollutants are released. Most Earth scientists believe that carbon dioxide is causing heat from solar radiation to become trapped in the Earths atmosphere (King et al., 1993). As Rosenfelds research established, aerosols from these same smokestacks create brighter clouds, which reflect sunlight. Any light that is reflected cannot reach the ground and heat the surface of the Earth. "Of course, reflecting more solar energy back to space means less warming of the system," said Rosenfeld. Over the short term, many scientists think these two sources of pollution have the ability to balance each other out in areas where there is a lot of pollution such as the industrial nations of the Northern Hemisphere.
This is not to say we should increase our burning of fossil fuels to try to stop global warming. As many reading this article have probably guessed, acid rain is the result of sulfate aerosols. Also, carbon dioxide remains in the atmosphere for a long time and can circulate around the globe, while aerosols fall to the Earth after a relatively short time when the clouds precipitate. So we can only receive the "benefits" of aerosols if we create a lot of pollution. Over the long term, most scientists believe that the warming effect of human-produced greenhouse gases will be greater than the cooling effect of aerosols (King et al., 1993).
But before these hypotheses can be accepted or denied, more tests and more experiments will have to be run. Scientists are still a ways off from being able to use satellites to discern exactly how many different types of pollutants a given cloud contains or how great an effect a given amount of pollution has on clouds. "Our research is not the end of the story. Much work is left to be done to understand the exact impact we are having on our climate," said Rosenfeld.
Graham, S., 1999: Tropical Rainfall Measuring Mission Fact Sheet, Earth Observatory, NASAs Goddard Space Flight Center, Greenbelt, MD. http://earthobservatory.nasa.gov/Library/TRMM/
Hastings, D., 1998: Advanced Very High Resolution Radiometer (AVHRR) Overview, NOAA National Data Centers Web site, Washington, DC. http://www.ngdc.noaa.gov/seg/globsys/avhrr.shtml
King, M.D., L.F. Radke, and P. V. Hobbs, 1993: Optical Properties of Marine Stratocumulus Clouds Modified by Ships, Journal of Geophysical Research, 98(D2), pp. 2729-2739.
King, M.D., S. Tsay, and S. Platnick, 1995: In Situ Observations of the Indirect Effects of Aerosols on Clouds. Aerosol Forcing of Climate, ed. by R. J. Charlson and J. Heintzenberg, John Wiley & Sons, New York, pp. 228-246.
Rosenfeld, D., 2000: Suppression of Rain and Snow by Urban and Industrial Pollution, Science, 287, pp. 1793-1796.
Rosenfeld, D., 1999: TRMM Observed First Direct Evidence of Smoke from Forest Fires Inhibiting Rainfall, Geophysical Research Letters, 26(20), pp. 3105-3107.
Rosenfeld, D., and Lensky, I. M., 1998: Satellite-Based Insights into Precipitation Formation Processes in Continental and Maritime Convective Clouds, Bulletin of the American Meteorological Society, 79(11), pp. 2457-2476.
Toon, O. B., 2000: How Pollution Suppresses Rain, Science, 287, pp. 1763-1764.
|The long term effects of the suppression of rain by pollution are unknown. Unanswered questions include: to what extent does pollution shift rainfall patterns? How do aerosols affect rainfall in heavily polluted areas? and what are the varying effects of different types of aerosols? (Image by Daniel Rosenfeld, Hebrew University of Israel)|