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 (DYNAMO) 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 (0.10 N and 80.50 E) to measure atmospheric and oceanic conditions between October 4 and October 30, 2011 (Leg 2 of the DYNAMO research cruise) and November 11th and December 4th, 2011 (Leg 3). Throughout the campaign, background marine atmospheric conditions were generally observed with average particle number concentration of less than 300 cm-3 and ozone (O3) <30 ppbv. 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 almost doubled from 1.8 × 1.1 μg m-3 in October to 3.3 × 2.2 μg m-3 in November/December. In addition, the aerosol chemical composition and optical properties appeared to be more influenced by continental sources. The effect of MJO-associated convection anomalies on aerosols in the remote marine boundary layer (MBL) were 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 population's size, composition, optical properties, and concentration as increased vertical mixing introduced new particles into the MBL, rainout cleared the atmosphere of submicrometer continental aerosol particles while high winds enhanced the concentration sea salt aerosol particles in the local atmosphere. Four stages of MJO-affected aerosol population changes in the remote Indian Ocean are defined.