Model analysis of trace gas measurements and pollution impact during INDOEX

An analysis of acetone (CH3COCH3) and acetonitrile (CH3CN) measurements, performed during the Indian Ocean Experiment (INDOEX), using a chemistry general circulation model is presented. A comparison with measurements indicates that the model simulates realistic CO and acetone distributions, except toward the Indian west coast near the surface. The latter may be related to a sea breeze circulation at the Indian west coast, which is not resolved by the model. A comparison of the measured and modeled correlation between CO and acetone indicates the presence of a background marine acetone source. A model sensitivity study suggests a global marine emission strength of 15-20 Tg acetone yr(-1), which is a significant contribution to the estimated global acetone source of 56 (37-80) Tg acetone yr(-1). The comparison of measured and modeled CO-acetonitrile correlation from surface measurements indicates that a model sink of acetonitrile in the marine boundary layer is missing. A model sensitivity study suggests that this could be dry deposition (deposition velocity estimate: 0.01-0.05 cm s(-1)) on the ocean surface. A comparison of measured and modeled tropospheric acetonitrile indicates that the model overestimates the free tropospheric acetonitrile mixing ratios up to a factor of 3, which points to a missing free tropospheric sink of acetonitrile in the model. A possible explanation may be stratospheric loss and subsequent stratosphere-troposphere exchange, which was not included in the model.