Aerosol radiative forcing from the Eyjafjallajökull volcanic eruptions

Here, we quantify direct shortwave and longwave radiative forcing associated with ash and sulfur dioxide emissions from the Eyjafjallajökull volcanic eruptions of 2010, which gained notoriety for disrupting European air traffic. Several unique features of this event lead us to hypothesize that its net direct aerosol forcing was positive. These features include: 1) a high ratio of ash to SO2 emissions, 2) little sulfur injection into the stratosphere owing to relatively low explosivity, 3) deposition of light-absorbing ash onto snow and sea-ice, and 4) the location (63.6°N) and season (April-May) of the eruptions, which facilitated larger positive radiative forcing over reflective surfaces exposed to strong insolation. To quantify aerosol forcing we apply inversions of the 4-dimensional ash and sulfate fields derived from remote sensing retrievals and Lagrangian dispersion modeling, combined with radiative transfer calculations using the atmosphere, snow, and sea-ice components of the Community Earth System Model (CESM). We derive ranges in radiative forcing associated with uncertainty in ash optical properties, and also provide estimates of changes in global snow melt associated with reduced surface albedo from ash deposition.