Atmospheric aerosol properties over the equatorial Indian Ocean and the impact of the Madden-Julian Oscillation

The chemical, physical, and optical properties of sub- and supermicrometer aerosols over the equatorial Indian Ocean were measured on board the R/V Revelle during the fall 2011 Dynamics of the Madden-Julian Oscillation field campaign. During this time, both the retreating of the Asian monsoon and two Madden-Julian Oscillation (MJO) events were observed. The R/V Revelle was on station (0.1 degrees N and 80.5 degrees E) to measure atmospheric and oceanic conditions between 4 October and 30 October 2011 (Leg 2) and 11 November and 4 December 2011 (Leg 3). Throughout the campaign, background marine atmospheric conditions were usually observed. As the Asian monsoon season retreated over the boreal fall and the general wind direction changed from southerly to northerly transporting, respectively, clean marine and polluted continental air masses, the average submicrometer aerosol mass nearly doubled from Leg 2 to Leg 3 and the aerosol appeared to be more influenced by continental sources. The effect of MJO-associated convection anomalies on aerosols in the remote marine boundary layer (MBL) was measured during November when a complete MJO convection wave moved over the equatorial Indian Ocean and during October when a partial MJO event was observed. MJO-associated convection strongly affected the local aerosol as increased vertical mixing introduced new particles into the MBL, rainout cleared the atmosphere of submicrometer aerosol particles, and high winds enhanced the concentration of sea salt aerosol particles in the local atmosphere. Four stages of MJO-affected aerosol population changes in the remote Indian Ocean are defined.