Studies of Atmospheric Brown Carbon Chemistry in Support of the FIREX Campaign

NOAA’s “Fire Influence on Regional and Global Environments Experiment” (FIREX) is focused on understanding and predicting the impact of North American fires on the atmosphere. Brown Carbon (BrC) is an important type of light-absorbing organic aerosol predominately produced through biomass burning. The mechanisms and rates of its atmospheric transformations that affect its impact on the atmosphere are, however, largely unexplored. This project will examine the chemical composition and atmospheric chemistry of BrC formed in the FIREX studies. Biomass burning is the major source of “primary BrC” emissions while “secondary BrC” is produced through atmospheric multiphase reactions between the gas- and the condensed-phase species present in smoke. BrC is recognized as an important component in the atmosphere that affects climate forcing both directly through absorption of solar and terrestrial radiation and through indirect effects on cloud formation and microphysics. Furthermore, long-range transport and deposition of BrC plays a substantial role in carbon and nitrogen cycling between atmosphere, land, and water.

The existing evidence suggests that even small amounts of strongly absorbing BrC chromophores may have a pronounced effect on the overall optical properties of organic aerosol. Although the chemical composition of BrC is not well-characterized, several classes of compounds have been proposed to contribute to its light absorption properties. Specifically, nitrogen-containing nitroaromatic compounds (e.g. nitrophenols) and reduced nitrogen species (e.g. imidazoles and other N-heterocyclic compounds) have been identified as BrC chromophores. Less is known about the molecular composition of light-absorbing humic-like substances and oligomers produced through condensation reactions. It has been also proposed that supramolecular aggregates and complexes of organic molecules with transition metals may be responsible for the observed optical properties of BrC. However, it is unclear whether typical BrC is composed of a few strong chromophores or is a mixture of a large number of weak chromophores. Furthermore, little is known about the effect of photochemical aging during atmospheric transport of BrC on its light absorption.

The proposed work will characterize the chemical composition of BrC formed in FIREX studies; identify key chromophores, their light-absorbing properties and concentrations; and examine their transformation upon atmospheric aging. This information is critically important to obtaining predictive understanding of the regional (e.g. Western US) and global impact of BrC on the atmosphere. The proposed research will address the following questions aligned with the FIREX objectives: 1) What is the chemical composition, molecular identity and light-absorption properties of BrC chromophores associated with emissions of aerosols from North American fires? 2) What are the chemical transformations of BrC chromophores during atmospheric aging? 3) What is the impact of nitrogen-containing compounds on the optical properties of BrC? 4) What BrC chromophores are common across different emission sources, and what BrC chromophores are source-specific?