Volcanic Aerosol Effects from Hawaii's Kilauea Volcano

Aerosol indirect effects involve complex feedback systems that alter cloud properties, making it difficult to estimate the magnitude of aerosol-cloud interactions (ACI) in climate forcing models. Studies have shown the effectiveness of using natural laboratories, such as effusive volcanoes, to study the impact of sulfur aerosols on clouds and climate. Additionally, these studies can be used as an analog for the effects of anthropogenic sulfur sources, such as power plants, to improve our understanding of human-induced climate change.

Kilauea Volcano on the island of Hawaii offers a favorable setting (e.g. relatively homogenous northeast trades and low background aerosol concentrations) to examine ACI due to its remote location in the central North Pacific. The moisture conditions affecting Kilauea allow sulfur dioxide (SO₂) emissions to quickly oxidize to sulfuric acid aerosol (H₂SO₄), forming volcanic smog also referred to as ‘vog.’ Vog generally affects the air quality of nearby communities, but under certain weather patterns can be a hazard throughout the entire island chain. For this reason, a regional Vog model forecast has been made available to the public through the Vog Measurement and Prediction (VMAP) project.

In this study, we investigate the impact of sulfur aerosols on marine cloud characteristics downwind of the Kilauea Volcano. We combine MODIS Level 2 collection 6 cloud properties—cloud droplet effective radius (CER), cloud optical thickness (COT), cloud water path (CWP), cloud fraction (CF), cloud top pressure (CTP), cloud top temperature (CTT)—and VMAP Vog model results to compare cloud properties within polluted (in-plume) and pristine (out of plume) clouds. MODIS cases are chosen during Hawaii’s summer season based on the following criteria: a) Hawaii Island must be present in the Level 2 Granule, b) clouds must be present downwind (SW) of the islands but not entirely covering the region, c) sun glint must be minimal in the downwind area, and d) the volcanic aerosol plume must be present as determined via MODIS Level 1B Granule images.

Preliminary results using eight individual cases from June and July 2015 show lower CER, higher COT, and higher CWP for polluted clouds as expected by the Twomey effect. However, CF, CTP, and CTT show overall little difference between polluted and pristine clouds. Figure 1 shows an example case (July 9, 2015) where MODIS and Vog model data are used to compare cloud properties between in-plume and out of plume clouds. In this case, the volcanic aerosol plume appears to correlate with higher CF, lower CTP, and lower CTT, which would be expected by the cloud lifetime effect. Additional MODIS cases are needed to provide a more complete database for further investigation of ACI in the Kilauea volcanic plume. We have expanded the study to include data, totally 189 cases, from June, July, and August for the years 2011 - 2016. This allows for a more robust statistical analysis of the impact of the Kilauea aerosol plume on the microphysical properties of trade wind cumulus.