Assessing a New Complex Sulfur Chemistry Scheme for the Univ. Of Hawai‘i Vog Model

The Vog Measurement and Prediction Project (VMAP), established in 2010, provides daily forecasts across the state of Hawai‘i for near ground sulfur dioxide (SO₂) and sulfate aerosol (SO₄) concentrations in order to mitigate the impacts of exposure to poor air quality resulting from the ongoing eruption of Kīlauea.  Forecasts are produced from the University of Hawai‘i Vog Model - a custom application of the Hybrid Single-Particle Lagrangian Integrated Trajectory model. Meteorological fields from the Weather Research and Forecast model provided input to the Vog model.  SO₂ emissions from the volcano were estimated by Hawaiian Volcano Observatory and are represented as vertical line sources representative of plumes during NE trade wind conditions. A constant conversion rate of SO₂ to SO₄ was specified, originally at 10% per hour.  An early validation study of the VMAP forecasts found that the Vog Model had difficulty forecasting high-end concentrations most hazardous to public health.  Specifically, SO₂ forecasts were a challenge given the rapid conversion rate in the model - meaning most instances when SO₂ concentrations exceeded 0.1 parts per million went unforecasted at sites downwind of Kīlauea.  A more complex chemistry scheme was developed based on chemical kinetic theory and observations collected during the summer of 2015 to better represent the conversion of SO₂ to SO₄.  Evaluation of the new scheme displayed improved probability of detection for SO₂ and PM_2.5 pollution events over the existing scheme.  Longer forecast windows also increased the probability of detection of the new scheme faster than the existing scheme.