Aqueous chemistry of brown carbon formation: characterizing organic carbon in fog and interstitial aerosol
For some scientists it’s not necessarily a disadvantage to work downwind of Los Angeles. Lelia Hawkins is an assistant professor of chemistry at Harvey Mudd College, situated downwind of downtown L.A., near the San Gabriel Mountains. Historically, the school has experienced high levels of photochemical smog – a subject Hawkins is investigating. She notes the Los Angeles basin also experiences frequent thick marine layer clouds and fog, especially during the fall and winter months. The phenomenon has made her curious about how the moisture droplets in the air interact with other small particles that make up so-called “brown carbon” particles present in the greater L.A. atmosphere. Brown carbon is a catch-all term coined to describe the collection of light-absorbing organic matter other than soot (black carbon). Among other sources, it comes from atmospheric aerosols produced by tar-like materials from combustion; very small particles that contain living organisms or that were released from living organisms; as well as soil humics and humic-like substances. (Humics are major components of the natural organic materials in soil and water. They make up much of the characteristic brown color of decaying plant debris and contribute to the brown or black color in surface soils.) “Brown carbon compounds are ubiquitous in fog water and may play a significant role in climate change,” Hawkins said. “If fog processing proves to be an important additional source of brown carbon, this will have important implications on many levels. First, the additional particulate matter produced during fog events will aid in understanding and improving climate models, and potentially Los Angeles air quality. Second, this will motivate future work exploring aqueous [water-based] chemistry pathways by which atmospheric pollutants transform into various compounds.” Specifically Hawkins and her students will take atmospheric samples to determine the origin of the brown carbon compounds, as well as simulate their interaction with fog in a laboratory chamber. She hopes to come up with enough evidence to prompt a larger, more permanent study of how fog and human-caused pollution relate to regional warming and other conditions in the Los Angeles basin.