Progress of chloride depletion and nitrate enrichment in individual field-collected sea salt particles was monitored as a function of time using experimental advances recently developed in our laboratory. Results presented here were obtained from aerosol samples collected at the William’s Tower in Houston where our newly developed time-resolved aerosol collector had been deployed during Texas 2000 Air Quality Studies. Using this instrument, the aerosol samples were taken sequentially with 10 minutes time resolution over the entire month of the field campaign. The aerosol samples were taken onto grid-supported ultra-thin carbon films, which are best for the single particle SEM/EDX analysis used in our work.

The presentation is focused on the results obtained just in one short period between midnight of 08/17/00 and early morning of 08/18/00. Near 150 individual aerosol samples were collected and stored from that period of time. Chemical composition of aerosols in these samples was determined later in our laboratory using automated SEM/EDX single particle analysis. For this, 25 samples with near one-hour intervals were chosen for the analysis. In each tested sample 1500-2000 individual particles were analyzed for their elemental composition including detection of low-Z elements: C, N and O. The analysis revealed that for the chosen period of time sodium-containing particles comprised 50 to 70% by number of the coarse particle mode (0.7 < Dp < 2.5 micron) and 10 to 30% by number of the fine particle mode (0.2 < Dp < 0.7 micron). During the night of 08/17/00 these particles were almost pure sea salt particles, while during the day they were effectively converted mainly to sodium nitrate particles and, to a lesser extent, sodium sulfate particles. During the evening this process was stopped and the sea salt particles appeared again in the night samples of 08/18/00. Using our time-resolved collection approach coupled with the automated SEM/EDX analysis we followed in great detail the processes of both compositional and morphology changes in the sea salt particles during the sea salt/sodium nitrate conversion. The dynamics of this process fits very well with the meteorology and gas trace observations obtained from other research groups, as will be discussed.