How Aerosols Contribute To Climate Change

As imaged by Lynn Russell, a professor of atmospheric chemistry at Scripps Institution of Oceanography at UC San Diego, and her team, air blown by winds between San Diego and Las Vegas gives the road to Sin City a distinctive look.

The team has sampled air from the tip of the Scripps Pier since last year, creating a near real-time record of what kinds of particles — from sea salt to car exhaust — are floating around at any given time. Add data about wind speed and direction and the scientists can tell where particles came from and can map their pathways around Southern California.

When Russell and her students put it all together, the atmosphere of greater San Diego comes alive in colors representing the presence of different airborne chemical compounds in aerosol form. One streak of deep red draws a distinct line from the pier that sometimes extends all the way to Las Vegas. The red denotes organic mass, a carbon-based component of vehicular and industrial emissions that pops up on Russell’s readouts frequently. Plot the streak on a road atlas and it reveals the daily life of pollution in Southern California. For one stretch of time, it neatly traced Interstate 15 all the way past the California-Nevada border.

“We were really surprised,” said Russell. “We did not expect to have such consistent winds for the selected study days.”

The hunt for various types of aerosols is helping Russell draw new kinds of global maps, ones that depict what organic compounds—whether natural or from sources such as Southern California traffic and industries—could do to affect rainfall, snowfall, atmospheric warming and cooling, and a host of other climate phenomena. Russell is part of an effort that involves several researchers at Scripps and UCSD and around the world. Collectively they are attempting to address a human-caused phenomenon in the Earth system scarcely considered before the last decade.

Aerosol research is considered one of the most critical frontiers of climate change science, much of which is devoted to the creation of accurate projections of future climate. These projections are generated by computer models — simulations of phenomena such as warming patterns, sea level fluctuations, or drought trends. The raw data for the models can come from historical records of climate basics like temperature and precipitation, but scientists often must rely on incomplete data and best guesses to represent more complex phenomena. The more such uncertainty goes into a model, the greater its margin of error becomes, making it less reliable as a guide for forecasts and adaptive actions.

Among these complex phenomena, the actions of aerosols are what some researchers consider the field’s holy grail, representing the biggest barrier to producing accurate representations of climate. In fact, the Intergovernmental Panel on Climate Change in 2007 specifically listed the effect of aerosols on cloud formation as the largest source of uncertainty in present-day climate models.

Bits of dust, sea salt, the remnants of burned wood, and even living things like bacteria all add to the mix of aerosols that create the skeletons on which clouds form. Around these particles, water and ice condense and cluster into cloud masses. The size and number of each of these droplets determine whether the clouds can produce rain or snow.

The aerosols are also influencing climate in other ways. Diesel exhaust, industrial emissions, and the smoke from burning wood and brush eject myriad bits of black carbon, usually in the form of soot, into the sky and form so-called “brown clouds” of smog. This haze has a dual heating and cooling effect. The particles absorb heat and make the air warmer at the altitudes to which they tend to rise but they also deflect sunlight back into space. This shading effect cools the planet at ground level.

The Arctic Circle is one of the places in the world most sensitive to changes in the mix of aerosols. Since the beginnings of the Industrial Revolution, scientists and explorers have noted the presence of the Arctic haze, a swirl of pollution that appears when sunlight returns after a winter of darkness. The presence of smog over a mostly uninhabited region leads many scientists to believe it is the reason the Arctic is experiencing the most rapid climate-related changes in the world. The haze now lingers for a longer period of time every year. It may be contributing to the forces now causing a meltdown of Arctic ice, a release of methane once stored in permafrost, and a host of ecological changes affecting the spectrum of organisms from mosquitoes to polar bears.

Russell has taken part in two recent analyses of polar air to understand where its imported aerosols come from and how the chemical components of those aerosols could be affecting temperature and cloud formation. From a research vessel in the Norwegian Sea and via continuous measurements from a ground station in Barrow, Alaska, Russell’s team is analyzing particles likely to have been blown to the Arctic from Europe and Asia. Her group has just compiled a full season of air samples fed through intake valves onto filters collected at Barrow.

With it, she believes she has proven what colleagues have previously theorized about where the particles are coming from. She is especially interested in organic particles—aerosols containing carbon supplied either by natural sources such as ocean or land plants or by human sources. Work in her group has shown that organics in the spring haze carry a signature consistent with dust and biomass burning taking place most likely in Siberia. The chemical signature changes in other seasons, revealing itself in infrared spectroscopy readings to be the product of aerosols from natural sources.

The aerosols could be influencing how much snowfall the Arctic gets and keeps. Human-produced aerosols are thought to stifle precipitation in some areas but may provide the impetus for torrential rain in others depending on their chemical make-up. Even if the Asian aerosols are not affecting precipitation, however, Russell said they appear to cool the Arctic atmosphere by deflecting light into space. At the same time, there is strong evidence that they are accelerating ice melt in the Arctic by darkening and heating ice once they fall to the ground the way a dark sweater makes its wearer hotter on a sunny day than does a white sweater.

Russell has been part of another collaborative effort launched in 2008, the International Polar Year, that created chemical profiles of relatively untainted air off the Norwegian west coast, which is only occasionally tinged by European smog. She has also teamed with collaborators at Scripps, NOAA and other universities to profile aerosols around Houston, Texas, and Mexico City.

In the latter two projects, she has provided evidence that agriculture adds more to the aerosol mix in an oil town like Houston than previously thought and that organic particles in Mexico City, rising from the smoke of street vendors and exhaust of cars driving on gas formulated differently than in the United States, glom on to dust in a different manner than American pollution to create aerosols with distinct chemical structures. Figuring out what they do locally and regionally is the next step.

Russell collaborates with a number of other faculty at UCSD whose research also focuses on aerosols, such as Kim Prather, an atmospheric chemistry professor with joint appointments at Scripps and the UCSD Department of Chemistry and Biochemistry. Russell and Prather are comparing their results form Mexico City in an effort to better understand the sources of aerosols in the atmosphere.

“We are trying to understand the major sources of aerosols in our atmosphere and how they affect the overall temperature of our planet; as opposed to greenhouse gases which we know are warming, aerosols can cool or warm depending on their composition and where they are located in the atmosphere," said Prather. Like Russell, Prather also studies long-range transport of aerosols from terrestrial and marine sources.  Prather and Russell have worked together on several other projects and recently helped form the Aerosol Chemistry and Climate Institute, a collaboration between Scripps and the Department of Energy’s Pacific Northwest National Laboratory.

For her most comprehensive study, Russell need only to make her shortest journey to the end of Scripps Pier. It is possible that aerosol journeys of a thousand miles or more might be explained by shorter commutes between Southern California counties. Complete analysis of the Interstate 15 data suggests Vegas might not be a source of dirtiness after all. Using data collected over longer time periods, Russell’s pollution map of local counties now suggests organic human-made aerosols might just be blowing toward Nevada from San Bernardino and Riverside then back toward San Diego as winds shift. Russell employs a suite of complementary measurements at the pier to characterize short- and long-term aerosol trends. Those are combined with particle profiles made by Prather’s group and collaborators whose numbers are growing out of necessity.

“Understanding the big picture is the only way we’re going to be able to reduce the uncertainty associated with aerosol particles and their effects on climate,” said Russell. “There are so many parameters, there’s no one instrument or even one person who can do all of it at once.”