Aerosol-Cloud Interactions in the Kilauea Volcanic Plume

Aerosol-cloud interactions (ACI) are a primary topic of study in the cloud physics community. Sulfate aerosols originate from both anthropogenic sources, such as power plants, and natural sources, such as volcanic eruptions. While ACI from anthropogenic sulfate sources are of particular interest due to their estimated negative radiative forcing of climate, there are large uncertainties in the magnitude of ACI. To simplify the study of sulfate ACI, effusive volcanoes, especially those located in remote oceanic regions like Kilauea in Hawaii, can be used as natural laboratories. In this environment SO₂ is emitted directly into the marine boundary layer where it may affect otherwise pristine clouds. ACI due to SO₂ can then be deduced by sampling air masses (and clouds) with and without the presence of volcanic SO₂.

In this work we investigate the impact of sulfur aerosols on marine cloud characteristics downwind of the Kilauea Volcano on the island of Hawaii.  In the Hawaiian Islands volcanic emissions of SO₂ that oxidize to H₂SO₄ creates a volcanic acid fog known as ‘Vog.’ We combine MODIS Level 2 cloud properties, OMI aerosol observations and model results from a regional Vog prediction model to identify when clouds are located outside of and within the Vog plume. Through this method we are able to compare cloud properties (effective radius, optical depth, LWP, number concentration) in and out of the Vog plume. Individual MODIS cases are chosen based on several criteria: a) Hawaii Island must be present in the Level 2 Granule, b) Clouds must be present downwind (SW of the island chain), c) sun glint must be minimal in the region SW downwind, and d) presence of a confirmed aerosol plume via OMI or other satellite. Figure 1 shows an example case (June 3, 2015) where MODIS and OMI data show a clear Vog plume and clouds down wind of Hawaii Island.  Vog model results identify the forecasted location of the plume for a particular case (day and time closest to MODIS overpass).  MODIS data are then sorted into “plume” and “non-plume” categories. We present statistical summaries of cloud characteristics in and out of the plume for a variety of cases. By using the Kilauea SO₂ plume and otherwise pristine Hawaiian clouds as a natural laboratory we are able to isolate volcanic sulfate ACI and use our results as a proxy for ACI produced by anthropogenic SO₂.  These results can then be compared to constrained model simulations of the same cases to evaluate the simulations of the (a) Vog itself and (b) ACI in this environment. This will increase our confidence in simulating the Vog for assessing regional air quality, and help constrain sulfate ACI to better constrain climate forcing.

This work lays the ground work for the proposed Kilauea Aerosol and Microphysics Project (KAMP) field campaign to make in situ measurements of the Kilauea SO₂ plume and nearby clouds. In addition to ACI effects, sulfur chemistry related to the production of CCN from SO₂ and resulting air pollution events will be investigated.

Figure 1: a) OMI SO₂ Column (DU), 6/3/2015, from http://so2.gsfc.nasa.gov/ , b) MODIS quick look image of cloud top effective radius (um) on June 3, 2015 from MODIS MYD06 products at http://ladsweb.nascom.nasa.gov.