The aerosol end-of-the-pier show

Dust, sea salt, soot, bacteria, and pollutant particles all add to the mix of atmospheric aerosols that can seed cloud formation as water and ice condense on these tiny particles and ultimately lead to precipitation. Understanding future climate change might hinge on the analysis of all such contributing atmospheric aerosols. Now, infrared spectroscopy and mass spectrometry have revealed important clues as to the role of aerosols in affecting climate patterns.

The hunt for aerosols is helping researchers draw new global maps that overlay rainfall and snowfall patterns, atmospheric warming and cooling, and a host of other climate phenomena. So important are aerosols, that the Intergovernmental Panel on Climate Change (IPCC) in 2007 specifically listed the effect of aerosols on cloud formation as the largest source of uncertainty in present-day climate models.

Atmospheric chemist Lynn Russell of the Scripps Institution of Oceanography at University of California San Diego and her team have spent the last year sampling the air from the end of the Scripps Pier. This has allowed them to create an almost real-time record of the multitude of aerosol particles - from sea salt crystals to vehicle exhaust particulates - floating in the air at any given time.

When they hook these data up to wind speed and direction measurements they can get a clear picture of the origins of the particles and can then map their route around Southern California.

Russell and her colleagues have used infra-red spectroscopy and mass spectrometry as complementary analytical techniques at the end-of-the-pier to characterize short- and long-term aerosol trends. These are then combined with particle profiles made by the team's growing number of collaborators. They are making some surprising discoveries about local climate but also pushing the boundaries on understanding global climate change.

For instance, Las Vegas has often been blamed for pollution in Southern California. However, the researchers suspected that aerosol journeys of one or two thousand kilometres or more could be explained by shorter commutes between the different counties in Southern California.

When the researchers put all their data together and colourised the mapped information, they see some striking patterns representing the presence of different airborne chemicals 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 carbon-containing organic compounds from vehicular and industrial emissions. 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.

A complete analysis suggests that Las Vegas might not be the source of all the airborne pollution after all. Russell's pollution map suggests that organic human-made aerosols might be simply blowing towards Nevada from San Bernardino and Riverside then blowing back again towards San Diego as the winds change direction. "We were really surprised," said Russell. "We did not expect to have such consistent winds for the selected study days."

The need for so many different data sources and a range of collaborators with different expertise is obvious. "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," explains Russell. "There are so many parameters, there's no one instrument or even one person who can do all of it at once."

Russell's long-time collaborator Kim Prather of UCSD and Scripps explains the importance of understanding aerosols: "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."

Aerosols have a much wider influence. Diesel exhaust, industrial emissions, and the smoke from burning wood and brush eject tonnes of sooty black carbon, which can form so-called "brown clouds" of photochemical smog. This pollution has a dual heating and cooling effect as the particles absorb heat and make the air warmer at the altitudes to which they tend to rise but deflect sunlight back into space cooling the land below.

The Arctic Circle is particularly sensitive to changing aerosol mixes. 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. IR spectroscopic analysis of organic aerosol particles collected in Barrow, Alaska, suggest that the presence of the Arctic brown cloud is consistent with dust and biomass burning taking place in Siberia.