Sulphate aerosol size distributions at Mumbai , India , during the INDOEX - FFP ( 1998 )

Sulphate size distributions were measured at the coastal station of Mumbai (formerly Bombay) through 1998, during the Indian ocean experiment (INDOEX) "rst "eld phase (FFP), to "ll current gaps in size-resolved aerosol chemical composition data. The paper examines meteorological, seasonal and source-contribution e!ects on sulphate aerosol and discusses potential e!ects of sulphate on regional climate. Sulphate size-distributions were largely trimodal with a condensation mode (mass median aerodynamic diameter or MMAD 0.6 m), a droplet mode (MMAD 1.9}2.4 m) and a coarse mode (MMAD 5 m). Condensation mode sulphate mass-fractions were highest in winter, consistent with the high meteorological potential for gas-to-particle conversion along with low relative humidity (RH). The droplet mode concentrations and MMADs were larger in the pre-monsoon and winter than in monsoon, implying sulphate predominance in larger sized particles within this mode. In these seasons the high RH, and consequently greater aerosol water in the droplet mode, would favour aerosol-phase partitioning and reactions of SO . Coarse mode sulphate concentrations were lowest in the monsoon, when continental contribution to sulphate was low and washout was e$cient. In winter and pre-monsoon, coarse mode sulphate concentrations were somewhat higher, likely from SO gas-to-particle conversion. Low daytime sulphate concentrations with a large coarse fraction, along with largely onshore winds, indicated marine aerosol predominance. High nighttime sulphate concentrations and a coincident large "ne fraction indicated contributions from anthropogenic/industrial sources or from gas-to-particle conversion. Monthly mean sulphate concentrations increased with increasing SO concentrations, RH and easterly wind direction, indicating the importance of gas-toparticle conversion and industrial sources located to the east. Atmospheric chemistry e!ects on sulphate size distributions in Mumbai, indicated by this data, must be further examined. 2001 Elsevier Science Ltd. All rights reserved.