PNNL

Abstract

Wildfires emit solid-state strongly absorptive brown carbon (solid S-BrC, commonly known as tar ball), critical to Earth’s radiation budget and climate. However, their light absorption properties are highly variable and typically not accounted for in climate models. Here, we show from a Pacific Northwest wildfire in September 2017, greater than 90% (by number) of particles were solid S-BrC with a mean refractive index of 1.49+0.059i at 550 nm. Model sensitivity studies of solid S-BrC regional absorption aerosol optical depth (AAOD) show that RI variation may result in a ~200% difference in AAOD. Contrary to common assumptions that solid S-BrC are hydrophobic, our results show that ~50% of solid S-BrC particles from this sample uptake water above 97% relative humidity. We hypothesize these results from a hygroscopic organic coating, potentially facilitating solid S-BrC as nuclei for cloud droplets. This water uptake also results in a lensing enhancement at 550 nm by a factor of 2. Furthermore, the organic coating on solid S-BrC can lead to even higher lensing enhancements than water. Our study suggests that incorporating the interaction of solid S-BrC and water should improve the prediction of Earth’s radiation budget.