Carbon-, Oxygen-, and Size-Resolved Model to Simulate the Microphysics, Chemistry, and Thermodynamics of Wildfire Organic Aerosol

Wildfires are an important source of organic aerosol (OA) emissions to the atmosphere that can vary substantially with the fuel type, burn conditions, and ambient conditions. Additionally, atmospheric mixing and photochemical oxidation are expected to alter the size, mass, and composition of wildfire OA. Yet, there are large uncertainties when it comes to understanding the physicochemical evolution of wildfire OA and its consequent impacts on air quality, climate, and human health.

In this work, we will develop a state-of-the-science OA model that combines the two-dimensional statistical oxidation model (SOM) with the TwO-Moment Aerosol Sectional (TOMAS) model.The SOM uses a two-dimensional carbon-oxygen grid to track the gas- and particle-phase chemistry, gas/particle partitioning, and properties of gas- and particle-phase organic precursors and products. The TOMAS model uses two moments, that of number and mass, of the aerosol size distribution to model processes of nucleation, condensation, and coagulation. This updated model, resolved in dimensions of carbon number, oxygen number, and size, will simulate the microphysics, chemistry, and thermodynamics of BBOA and include the following processes: (a) semi-volatile and reactive POA, (b) SOA formation from semi-volatile, intermediate-volatility and volatile organic compounds, (c) multi-phase, multi-generational aging that includes functionalization and fragmentation reactions, (d) low-volatility SOA formation from autoxidation and oligomerization reactions, (e) influence of vapor wall losses encountered in laboratory SOA formation experiments, and (f) phase state of OA.

The model was evaluated against environmental chamber and flow reactor experiments performed at the Fire Laboratory in Missoula, MT as part of the FLAME3/4 and FIREX campaigns. Ongoing work is focussed on identifying the most important precursors, pathways, and experimental artifacts that control the size, mass, and composition of wildfire OA. Insight from this work will be used to develop models and parameterizations for regional and global air quality and climate models.