Smoking Clouds over the Western United States: Impact of Wildfire Emissions

Small continental cumulus clouds impacted by smoke from U.S. wildfires were sampled during the airborne Western wildfire Experiment for Cloud chemistry, Aerosol absorption and Nitrogen (WE-CAN). Cloud fields sampled on seven flights were found to have high peak droplet concentrations, up to 3000 cm^-3, an order or magnitude greater than ~170-370 cm^-3 measured by others in remote continental clouds with similar liquid water contents and temperatures. Clouds impacted by smoke had very small droplets, typically about 6 microns mean diameter.

About 65% of smoke particles larger than 0.1 µm diameter activated in clouds with modest liquid water contents of ~0.1-0.3 g m^-3. Aerosol hygroscopicity was consistently low, with Kappa values typically <0.2. This is consistent with PM₁ composition and electron microscopy and X-ray analysis that showed submicron particles were mostly organic carbon, with about 5% refractory black carbon. Inorganic species accounted for only about 10% of submicron mass, yet most smoke particles were relatively large (number mode diameter of ~0.2 µm), so they were still able to act as cloud condensation nuclei.

Measurements of ice nucleating particles within cloud droplet and ice residuals were consistent with an enhanced source from smoke plumes entering clouds. These results demonstrate the need for modeling studies of smoke impacts on both liquid and ice phase processes in clouds. The net impact of smoke on the radiative properties of the cloud fields will be complex, since black and brown carbon in smoke will enhance absorption for a warming effect, while the enhanced droplet numbers will reflect more light for a cooling effect. Collision/coalescence and therefore precipitation will also be inhibited in the smoke impacted clouds, which may likewise influence cloud lifetime and ice phase development.