The effects of SO2, volcanic ash and sulfate aerosols on photolysis rates and the sulfate chemical production following the volcanic eruptions

Abstract:

Large volcanic eruptions inject substantial amounts of water, ash, and SO₂ directly into the lower stratosphere. The sulfate aerosols are then produced photochemically in the stratosphere. In this chemical mechanism, ozone photolysis is a primary source of OH, which drives the SO₂ oxidation. Ash, SO₂, and sulfate aerosols are optically active and affect the actinic flux and thus the ozone photolysis rates as well as the rate of the sulfate aerosols production. However, their relative contributions change over time. Ash particles strongly absorb photons, but, following the eruption, most of them are quickly removed from the atmosphere by gravitational settling. The SO₂ cloud rises above aerosols layer due to radiative heating. It strongly absorbs UV radiation but is slowly depleted by the oxidation in the course of sulfate aerosols production.

Currently, most of the models with the interactive chemistry predict faster sulfate formation, which results in an earlier peak of sulfate optical depth than in observations. To more accurately calculate the volcanic plume formation we have implemented the SO₂, ash and sulfate aerosols effect on the photolysis rates in the WRF-Chem model and validated results using standalone line-by-line calculations. We show that SO₂ and ash decrease photolysis rates, slow down the SO₂ oxidation and improve the agreement between the simulated and observed sulfate aerosol optical depth.