This award supported work towards improving the accuracy of global climate models by characterizing the formation and atmospheric transformation of carbon compounds in particulate matter, including short-lived climate forcers (SLCFs) that contribute to atmospheric heating by absorbing solar radiation. These components are also detrimental to air quality, a related but different environmental problem from climate change.

Specifically, this work focused on light absorbing carbon compounds that are not soot, but are instead organic carbon.  In some locations, BrC is estimated to contribute more to atmospheric heating than black carbon (soot). Unlike soot, BrC can form secondarily in the atmosphere in gas and condensed phase (aerosol) reactions and in clouds and fog. The chemical nature of these secondary (transformed) compounds is still very poorly understood since multiple precursors can produce similarly absorptive material; chemically similar systems, however, can produce aerosol ranging from non-absorbing to highly absorbing.

Atmospheric measurements: a major component of this work targeted measurements of ambient particulate matter in an urban setting, the northeastern edge of Los Angeles County. Specifically, our work included measurements of the chemical composition and optical properties of particulate matter using aerosol mass spectrometry and UV/visible absorption spectroscopy, respectively. We deployed these key instruments along with supporting instruments to measure particle number concentration, meteorological parameters, and low-cost sensors to conduct a full characterization over continuous periods of time spanning weeks to months. Our findings indicate that light absorption by the water-soluble portion of this particulate matter is associated most closely with the most processed (most oxidized) organic material. This may mean that in addition to the absorption by soot, secondary reactions are capable of changing the absorption properties of this material.

Laboratory studies: complementing our measurements of atmospheric components were a series of targeted laboratory studies designed to study transformations of known brown carbon-forming compounds in cloud water under a variety of conditions. Specifically, we wanted to test the importance of acidity of cloud water and the wetting/drying process to see how these variables change the chemical and optical properties of the resulting brown carbon. We focused on a well-studied system, ammonium sulfate and methylglyoxal, and discovered a previously unreported class of products, pyrazine-based chromophores, which explained unidentified products in previous studies and shed light on the way these systems can produce important chromophores. Since the reactants are commonly observed in the atmosphere, the important contribution of this work is the finding that these chromophores form as the major product species in some analytical methods and they should be included as possible products in cloud-climate models where this chemistry is parameterized. They may also serve as indicators of aqueous phase transformations of particulate matter.

Broader impacts: In addition to the important advances in understanding the intersection of air quality and climate change, research funded by this award was conducted entirely by the PI and her team of undergraduate students from the period 2016-2023. Undergraduate students were trained on research-grade instrumentation in the field of atmospheric chemistry. These students led the research projects described here, presented their findings at national meetings among graduate students and PIs in atmospheric chemistry, and contributed to writing manuscripts; these have prepared them for graduate work in this and related fields. Several of these students have either completed or are in the process of completing PhD programs in this field. Six HMC students completed their senior theses on this award and many other HMC students benefitted from this award by exposure to aerosol chemistry and aerosol science in course work and in summer research.

Last Modified: 07/30/2023
Modified by: Lelia N Hawkins