Physical and chemical properties of the aerosol within the southeastern Pacific marine boundary layer

During the Stratus 2003 and 2004 cruises, a Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) was used to measure the size‐resolved concentration, hygroscopicity, and volatility of the submicron aerosol below the stratocumulus deck of the southeastern Pacific. The number size distributions were typically bimodal, although a nucleation event in 2003 skewed the relative importance of the two modes, while in 2004, influence from anthropogenic sources near the coast of Chile reduced the separation between the two modes. The average total aerosol number concentration was 398 ± 145 cm−3 and 301 ± 100 cm−3 for the 2003 and 2004 cruises, respectively. Measured size‐ and temperature‐dependent volatility suggested Aitken and accumulation mode particles were composed of refractory cores and acidic sulfate solutions having NH4/SO4 molar ratios of roughly 0.56 and 0.23, respectively. Cloud condensation nuclei (CCN) concentrations at the peak in‐cloud supersaturation and over a range in supersaturation were calculated using the collected data. The data were also used to quantify the number to volume concentration ratio of the accumulation mode aerosol.