Abstract
Abstract. Measurements of the submicron fraction of the atmospheric aerosol in the marine boundary layer were performed from January to March 2007 (Southern Hemisphere summer) onboard the French research vessel Marion Dufresne in the Southern Atlantic and Indian Ocean (20° S–60° S, 70° W–60° E). We used an Aerodyne High-Resolution-Time-of-Flight AMS to characterize the chemical composition and to measure species-resolved size distributions of non-refractory aerosol components in the submicron range. Within the "standard" AMS compounds (ammonium, chloride, nitrate, sulfate, organics) "sulfate" is the dominant species in the marine boundary layer with concentrations ranging between 50 ng m−3 and 3 μg m−3. Furthermore, what is seen as "sulfate" by the AMS is likely comprised mostly of sulfuric acid. Another sulfur containing species that is produced in marine environments is methanesulfonic acid (MSA). There have been previously measurements of MSA using an Aerodyne AMS. However, due to the use of an instrument equipped with a quadrupole detector with unit mass resolution it was not possible to physically separate MSA from other contributions to the same m/z. In order to identify MSA within the HR-ToF-AMS raw data and to extract mass concentrations for MSA from the field measurements the standard high-resolution MSA fragmentation patterns for the measurement conditions during the ship campaign (e.g. vaporizer temperature) needed to be determined. To identify characteristic air masses and their source regions backwards trajectories were used and averaged concentrations for AMS standard compounds were calculated for each air mass type. Sulfate mass size distributions were measured for these periods showing a distinct difference between oceanic air masses and those from African outflow. While the peak in the mass distribution was roughly at 250 nm (vacuum aerodynamic diameter) in marine air masses, it was shifted to 470 nm in African outflow air. Correlations between the mass concentrations of sulfate, organics and MSA show a narrow correlation for MSA with sulfate/sulfuric acid coming from the ocean, but not with continental sulfate.
Highlights
Atmospheric research has become increasingly important in recent years, many processes taking place in the atmosphere are still barely understood (IPCC, 2007)
Due to the large differences in the exact masses of the species found at m/z 79 they can be separated at the mass resolution of m/ m=2500 at which the HR-Time-of-Flight Aerosol Mass Spectrometer (ToF-AMS) was operated during the ship measurement campaign
Our estimate for RIEMSA is different from the value used by Phinney et al (2006), who used the same RIE for this species as they used for sulfate (RIEMSA=RIESO4 =1.15)
Summary
Atmospheric research has become increasingly important in recent years, many processes taking place in the atmosphere are still barely understood (IPCC, 2007). While the focus of this campaign was mainly the outflow from Asia, Topping et al (2004) measured several marine influenced air masses that showed a strong variability and the highest AMS “sulfate” mass loadings during that period Another type of laser ablation aerosol mass spectrometer (ATOFMS) was used for on-line characterization of individual aerosol particles during the 2002 North Atlantic Marine Boundary Layer Experiment (NAMBLEX) at the remote marine site at Mace Head (Ireland), but Dall’Osto et al (2004) focused their analysis on sea salt, dust and carbonaceous particles rather than sulfate. As the instrument was deployed on a ship crossing the Southern Atlantic Ocean, our measurements cover a large spatial area of the Southern Hemisphere
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