Annual cycle of Background Aerosol at Troll Station, Antarctica

1. Annual cycle of Background Aerosol at Troll Station, Antarctica Fiebig, M.; Lunder, C.; Solberg. S; Stohl, A.Norwegian Institute for Air Research Hirdman, D.now at: Swedish Meteorological and Hydrological Institute (SMHI), Norrköping, Sweden Ogren, J.A.Earth System Research Laboratory / Global Monitoring Division, National Oceanic and Atmospheric Administration, Boulder (CO), U.S.A.

2. Where is Troll Station Located? Neumeyer,Germany Troll Station,Queen Maud Land 220 km 420 km SANAE IV,South Africa 190 km 2000 km South Pole • Located in Queen Maud Land, Antarctica, • Operated year-round since 2005, atmospheric observatory since February 2007. • 220 km from coastline, 190 km from SANAE IV (South Africa), 420 km from Neumeyer (Germany), 2000 km from South Pole.

3. Troll Station: Setting and Aerosol Parameters Measured

4. Annual Cycle in Baseline Aerosol at Troll • Peaks due to intrusions of marine aerosol. • Annual cycle in baseline aerosol visible in both, scattering coefficient and particle number size distribution.

5. Annual Cycle of Antarctic Baseline Aerosol: A Large Scale Phenomenon (1/3) • Baseline aerosol annual cycle is visible at South Pole as well as Troll…

6. Annual Cycle of Antarctic Baseline Aerosol: A Large Scale Phenomenon (2/3) • Dome C: • from: Järvinen at al. (2013), ACP • … and also at Dome C!

7. Annual Cycle of Antarctic Baseline Aerosol: A Large Scale Phenomenon (3/3) Troll Station,Queen Maud Land 2000 km 3100 km South Pole Dome C 1670 km • The baseline aerosol annual cycle seems to be common to all Central Antarctic stations.

8. Annual Cycles of Scattering Coefficient and Size Distribution: A Common Cause • Compare scattering coefficient measured with scattering coefficient calculated from particle size distribution with Mie-theory (assuming ammonium sulphate) • Agreement within uncertainty! • Annual cycles in optical properties and particle size distribution have common cause.

9. Basaseline Monthly Particle Size Distribution (PSD) Medians by Season summer autumn winter spring • Summer PSD dominated by Aitken-mode (Dp < 100 nm), mode formed up to 10 days before observation. • Winter PSD dominated by accumulation mode, indicating ageing by self-processing. • Spring and autumn represent transition periods.

10. Origin of Troll Baseline Aerosol (1/4): The FLEXPART Answer • Footprint of all 2007 FLEXPART backward plumes corresponding to scattering coefficient values below 20th percentile. • Antarctic baseline air descends from aloft to the surface, no surface contact for 20 days prior to observation other than in Antarctica. • Particles formed within this period must be formed in situ.

11. Origin of Troll Baseline Aerosol (2/4): Investigating Ground-Level Ozone • Also ozone has pronounced annual cycle, origin not fully understood. • Peaks from ozone production events (NOx from snow, VOCs from gas-phase). • For investigating baseline vs. non-baseline air, annual cycle needs to be filtered out.

12. Origin of Troll Baseline Aerosol (3/4): Comparing Baseline & Non-Baseline Ozone • Calculate difference to 25, 50, 75 percentile for every data point, calculate averages of differences, separately for baseline and non-baseline air. • Baseline air corresponds to ozone 56 percentile, non-baseline air to 42 percentile. • Baseline air contains some lower stratispheric air.

13. Origin of Troll Baseline Aerosol (4/4): Field Data on FT Plumes, Analyzed by FLEXPART • On 25 October 2009, the NASA DC-8 observed a plume at 12 km altitude at 86°S. • Gas-phase composition indicates mixture of anthropogenic and biomass burning sources. • Plume was elevated from lower free troposphere by frontal conveyer belt over Southern Ocean 9-10 prior to observation. • Baseline air subsiding over Antarctica is a mixture of upper free troposphere and lower stratosphere (indicated by ozone concentrations). • Baseline air hasn’t seen external particle sources for at least 10 days, but Aitken-mode particles of that age are present in summer. • Air mass internal source of particle volume! • from: Yang at al. (2012), JGR 117, D17309

14. Annual Cycle in Particle Volume and Solar Insolation • Particle volume in Troll baseline air has distinct annual cycle. • Seems to be correlated with solar insolation integrated over 20-day backward plume (assuming no clouds).

15. Particle Volume Production in Antarctic Baseline Air: Insolation Limited • In situ production of particle volume in Antarctic baseline air is an insolation-limited process. • This despite low concentrations in precursor gases. • No obvious effect of precursor gas type, despite large variation in air mass origin. • OC has been observed in Antarctic aerosol, possible precursor gases: marine VOCs, anthropogenic and biomass burning VOCs

16. Conclusions • Annual cycle in Antarctic baseline aerosol properties (microphysical and optical) with common origin. • Baseline aerosol annual cycle is a large scale phenomenon common to whole Central Antarctic. • Antarctic baseline air descends from free troposphere / lower stratosphere, but no in situ sources less than 10 days prior to descend despite variety of origins. • Observe sunlight-limited in situ production of aerosol volume in Antarctic baseline air, strength apparently independent of air mass origin despite huge variations precursor gases and transport pathways. • Further research 1: easy scheme for parameterising SOA production in aerosol models? • Further research 2: benchmark process for separating natural from anthopogenic processes, at least in Southern hemisphere / Antarctica. • more: Fiebig et al. (2013): Annual cycle of Antarctic baseline aerosol: controlled by photooxidation-limited aerosol formation