Characterizing Oxidized North American Fire Emissions and Their Aqueous/Multiphase Atmospheric Transformations through the FIREX Campaign

Aqueous multiphase chemistry in aging fire plumes can alter the behavior and climate-relevant properties of atmospheric aerosols. Evidence suggests that multiphase chemistry in clouds, fogs and wet aerosols generates organosulfates, imidazoles, polyols and carboxylic acids, which may be processed to form light-absorbing oligomers (brown carbon). Such chemistry is a sink for reactive gases (e.g., isoprene epoxydiols, glyoxal, peroxides, HO₂, NH₃) and a source of organic aerosol (OA). Although the multiphase chemistry of oxidized fire emissions in pyrocumulus clouds has a high potential to form light-absorbing and –scattering OA, the aqueous chemistry of only a few fire plume organics (phenols and glycolaldehyde) has been studied. Most oxidized organic constituents of fire plumes remain unidentified and their chemistry unexamined. While laboratory and field studies support the formation of brown carbon in reactions involving ammonia (NH₃) or amines in wet aerosols and evaporating droplets, the mechanisms of formation during atmospheric processing of fire plumes are largely unknown.

We will measure oxidized gas- and particle-phase emissions, poorly characterized to date, at the Fire Science Laboratory (FSL) as part of FIREX. We will conduct controlled multiphase chemistry experiments at UNC with the complex mixtures of gases collected during FSL burns, to better understand the atmospheric transformation of fire emissions, their aerosol formation and optical properties. We will apply our expertise in aqueous multiphase (heterogeneous) organic chemistry, organic chemical characterization, synthesis, and kinetics to achieve the following specific aims:

(1) Identify oxidized gaseous and particulate organics at the molecular level during planned FSL burns by on- and off-line high-resolution mass spectrometry techniques.

(2) Study SOA formation through cloud processing in pyrocumulus by scrubbing gaseous fire emissions into water (using mist chambers) and conducting aqueous oxidation and droplet evaporation experiments with and without added NH₄⁺/NH₃.

(3) Characterize brown carbon from Aims 1 (fresh emissions) and 2 (aged emissions by aqueous-phase chemistry) and determine the chemical composition.

(4) Test hypotheses developed through the work with FSL mixtures by conducting similar experiments with synthetic standards of newly identified single compounds.