These two satellite images show how aerosols can obscure the land and sea beneath, blocking incoming sunlight. On the top, aerosols over northeastern India and Bangladesh partially obscure the Ganges River and then are swept out over the Bay of Bengal. Notice how the high-altitude air over the Himalayas, near the top of the image, is clearer. On the bottom, smoke from dozens of fires (left side of image) in China swirls down along valleys and then out over Bo Hai Bay (upper right) on its way towards Korea and the Pacific Ocean.
Credits: Images courtesy of Jacques Descloitres, MODIS Land Rapid Response Team, NASA GSFC. Text from UCAR’s page on aerosols here.
From Ascribe Newswire
COLLEGE PARK, Md, March 12 — A University of Maryland-led team has compiled the first decades-long database of aerosol measurements over land, making possible new research into how air pollution changes affect climate change.
Using this new database, the researchers show that clear sky visibility over land has decreased globally over the past 30 years, indicative of increases in aerosols, or airborne pollution. Their findings are published in the March 13 issue of Science.
“Creation of this database is a big step forward for researching long-term changes in air pollution and correlating these with climate change,” said Kaicun Wang, assistant research scientist in the University of Maryland’s department of geography and lead author of the paper. “And it is the first time we have gotten global long-term aerosol information over land to go with information already available on aerosol measurements over the world’s oceans.”
Wang, together with Shunlin Liang, a University of Maryland professor of geography, and Robert Dickinson, a professor of geological science at the University of Texas, Austin, created a database that includes visibility measurements taken from 1973 – 2007 at 3,250 meteorological stations all over the world and released by the National Climatic Data Center (NCDC). Visibility was the distance a meteorological observer could see clearly from the measurement source. The more aerosols present in the air, the shorter the visibility distance.
According to the researchers, the visibility data were compared to available satellite data (2000-2007), and found to be comparable as an indicator of aerosol concentration in the air. Thus, they conclude, the visibility data provide a valid source from which scientists can study correlations between air pollution and climate change.
Aerosols, Greenhouse Gases and Climate Change
Aerosols are solid particles or liquid droplets suspended in air. They include soot, dust and sulfur dioxide particles, and are what we commonly think of when we talk about air pollution. Aerosols come, for example, from the combustion of fossil fuels, industrial processes, and biomass burning of tropical rainforests. They can be hazardous to both human health and the environment.
Aerosol particles affect the Earth’s surface temperature by either reflecting light back into space, thus reducing solar radiation at Earth’s surface, or absorbing solar radiation, thus heating the atmosphere. The variable cooling and heating effects of aerosols also modify properties of cloud cover and rainfall.
Unlike aerosol particles, carbon dioxide and other greenhouse gases are transparent and have no effect on visibility. Sunlight passes right through them, just as it does through the oxygen and nitrogen that are the main constituents of our atmosphere. Though present in the atmosphere in relatively small amounts, greenhouse gases cause global warming because these “trace” gases trap solar energy absorbed at the earth’s surface and prevent it from being radiated as heat back into space.
While the climate warming impacts of increased greenhouse gases are clear, the effects of increased aerosols are not. Studies of the long-term effects of aerosols on climate change have been largely inconclusive up to now due to limited over-land aerosol measurements, according to Wang and his team. However, with this database researchers now can compare temperature, rainfall and cloud cover data from the past 35 years with the aerosol measurements in the new database.
Global Dimming
According to the authors, a preliminary analysis of the database measurements shows a steady increase in aerosols over the period from 1973 to 2007. Increased aerosols in the atmosphere block solar radiation from the earth’s surface, and have thus caused a net “global dimming.” The only region that does not show an increase in aerosols is Europe, which has actually experienced a “global brightening,” the authors say.
The largest known source of increased aerosols is increased burning of fossil fuels. And a major product of fossil fuel combustion is sulfur dioxide. Thus, the team notes, that their finding of a steady increase in aerosols in recent decades, also suggests an increase in sulfate aerosols. This differs from studies recently cited by the Intergovernmental Panel Climate Change showing global emissions of sulfate aerosol decreased between 1980 and 2000.
I lived in Mumbai for 18 months. There a lot of the air pollution is dust and dirt swept up from the ground by wind, as happens when you don’t have ground paved over by concrete and tarmac.
Right now that air pollution is blocking a little bit of sunlight and where the roads are fully developed yet the earth absorbed heat. In the future, when roads and sidewalks are up to modern standards, we’ll see a reduction in natural air pollution that will allow more sunlight to penetrate the atmosphere. This will heat up all those new roads and pavements. Mumbai will get warmer and the media will blame it on global warming instead of urban development and clean air.
The effect of brown cloud pollution on heating depends on several variables. The altitude of the cloud, the color of the cloud compared to the surface it is masking, and the angle of incidence of the incoming light.
Here in Colorado the Denver basin was notorious for a thick brown cloud during the winter with temperature inversions. That low level pollution layer could be seen to move up and down the local drainage basin (Platte River valley) each day in a diurnal cycle). During the night hours the brown cloud would slide down slope into the Greeley area, then as the sun came up and heating began the air flow would reverse and carry the brown cloud back up slope into the Denver Basin. As long as the heating was not sufficient to break the temperature inversion the brown cloud layer would cause significant dimming in the core city area. It was very obvious as you drove out of the basin and out from under the pall of the brown cloud that the local solar isolation went up. At some sun angles the top of that inversion layer could be very bright and reflect a large amount of light off the top of the cloud layer. Pilots reported the top of the cloud being so bright they could not see the ground below it, as if they were looking at a reflection off of a layer of snow. As I understand it the upper layer of the brown cloud would heat due to absorption of the suns energy but the lower layers of the cloud were cooled due to lack of sunlight, so it could intensify a temperature inversion under some condtions and “cap off” the basin, capturing all the local pollution in the basin.
In the 1970’s they came to the conclusion that a large contributor to the brown cloud in the winter, was pulverized sand used on snow covered roads as they thawed and dried out after a snow storm. They drastically cut back on sand applications for that reason in the winter, and that significantly improved the local brown cloud development in post snow storm conditions.
If the brown cloud was masking snow covered terrain the total albedo went down as sun light that in clear skies would have been reflected off the snow cover was now absorbed in the brown clouds upper layer. In the summer time the brown cloud might be lighter color than the dark green of trees and grass, so it is hard to categorically state if particulates warm or cool the local climate until you consider these issues.
In the summer time during forest fire season we can have significant particulate from distant fires. At times we actually can smell the smoky smell of burning wood from fires 100’s of miles to the west of the Metro area during the fire season.
http://query.nytimes.com/gst/fullpage.html?res=9A02E1D91E3CF932A15757C0A96E958260
http://www.raqc.org/presentations/azbcs080800.PDF
Larry
Larry
For the first time, a large study shows the deadly effects of chronic exposure to ozone, one of the most widespread pollutants in the world and a key component of smog, according to a study in today’s New England Journal of Medicine.
Doctors have long known that ground-level ozone — which is formed when sunlight interacts with pollution from tailpipes and coal-burning power plants — can make asthma worse. This study, which followed nearly 450,000 Americans in 96 metropolitan areas for two decades, also shows that ozone increases deaths from respiratory diseases.
Bangladesh truly has nasty air. Well, I guess we have to trade something off for cheap Nikes.