Observations of H2SO4 and MSA during PEM-Tropics-A

Results are presented of measurements of the concentration of gas phase H2S O4 and methane sulfonic acid (MSA) performed aboard the NASA P3-b aircraft during the Pacific Exploratory Mission (PEM) Tropics study using the select ed ion chemical ionization mass spectrometry (SICIMS) technique. During a n ighttime portion of one flight the [H2SO4] was found to increase with decre asing relative humidity (RH). When compared to laboratory measurements of H 2SO4 vapor pressure as a function of RH and particle neutralization (NH4+ a nd SO42- ionic composition) and model predictions using a liquid drop hydra te model, these measurements indicate that the particles from which the H2S O4 is evaporating are relatively unneutralized, a result which is in good a greement with filter measurements. Overall, the same increase in the gas ph ase [MSA] with decreasing RH or decreasing [NH4+] (obtained from filter mea surements) was also observed, indicating a hi,oh volatility of MSA at low R H values or particle neutralization. When gas phase MSA values are compared to methane sulfonate (MS) values obtained from filter measurements, it was found that MSA was totally volatilized at low RH values, while MSA resides mainly in the particulate form at high RH values. Combining the gas phase and filter measurements, the boundary layer MS/(MS + SO42-) ratio showed a distinct increase with decreasing temperature and suggests that little or n o MS or MSA is produced in the boundary layer at temperatures above 300 K.