Quantitative time-resolved monitoring of nitrate formation in sea salt particles using a CCSEM/EDX single particle analysis

Progress of the nitrate formation in individual sea salt particles was detected as a function of time using aerosol samples collected during the TexAQS 2000 experiment. We demonstrate that the time-resolved collection approach coupled with the automated EDX single particle analysis made it possible to follow in detail the time evolution of sea salt particles within a diverse aerosol mixture. Using a custom built Time-Resolved Aerosol Collector (TRAC), particulate samples were taken sequentially on grid-supported 50 nm carbon films with a time resolution of 10 min between two consecutive samples. The samples were analyzed in the laboratory using Computer Controlled Scanning Electron Microscopy with Energy-Dispersed analysis of X-rays (CCSEM/EDX). Between midnight of 08/16/00 and the early morning of 08/17/00, a steady, particularly sea salt rich aerosol was observed at the measurement site, which later showed the effects of atmospheric processing. During the night of 08/17/00 the sea salt particles were almost unprocessed, having elemental composition close to that of seawater. By 12 noon,the evolving atmosphere was able to completely convert them, predominantly to sodium nitrate particles. During the next night, this process had nearly stopped and fairly virgin sea salt particles appeared again.