Search

Abstract. Aerosol particle optical, physical and chemical properties have been previously studied in the United Arab Emirates (UAE), but there is still a gap in the knowledge of particle sources, and in the horizontal and vertical transport of aerosol particles and their precursors in the area. To investigate how aerosol particle and SO2 concentrations at the surface responded to changes in horizontal and vertical transport, we used data from a one-year measurement campaign at a background site where local sources of SO2 where expected to be minimal. The measurement campaign provided a combination of in-situ measurements at the surface, and the boundary layer evolution from vertical and horizontal wind profiles measured by a Doppler lidar. The diurnal structure of the boundary layer in the UAE was very similar from day to day, with deep well-mixed boundary layer during the day transitioning to a shallow nocturnal layer, with the maximum boundary layer height usually being reached around 1400 local time. Both SO2 and nucleation mode aerosol particle concentrations were elevated for surface winds coming from the east or western sectors.We attribute this to oil refineries located on the eastern and western coasts of the UAE. The concentrations of larger cloud condensation nuclei (CCN) sized particles and their activation fraction did not show any clear dependence on wind direction, but the CCN number concentration showed some dependence on wind speed, with higher concentrations coinciding with the weakest surface winds. Peaks in SO2 concentrations were also observed despite low surface wind speeds and wind directions unfavourable for transport. However, winds aloft were much stronger, with wind speeds of 10 m s−1 at 1 km common at night, and with wind directions favourable for transport, and surface-measured concentrations increased rapidly once these particular layers started to be entrained into the growing boundary layer, even if the surface wind direction was from a clean sector. These conditions also displayed higher nucleation mode aerosol particle concentrations, i.e. new particle formation events occurring due to the increase in the gaseous precursor. [ABSTRACT FROM AUTHOR]

Optical properties of surface aerosols at Dome C, Antarctica in 2007-2013 and their potential source areas are presented. Scattering coefficients (ssp) were calculated from measured particle number size distributions with a Mie code and from filter samples using mass scattering efficiencies. Absorption coefficients (sap) were determined with a 3-wavelength Particle Soot Absorption Photometer (PSAP) and corrected for scattering by using two different algorithms. The scattering coefficients were also compared with ssp measured with a nephelometer at the South Pole Station (SPO). The minimum sap was observed in the austral autumn and the maximum in the austral spring, similar to other Antarctic sites. The darkest aerosol, i.e., the lowest single scattering albedo ?o ˜ 0.91 was observed in September and October and the highest ?o ˜ 0.99 in February and March. The uncertainty of the absorption Ångström exponent aap is high. The lowest aap monthly medians were observed in March and the highest in August – October. The equivalent black carbon (eBC) mass concentrations were compared with eBC measured at three other Antarctic sites: the SPO and two coastal sites, Neumayer and Syowa. The maximum monthly median eBC concentrations are almost the same (~3 ± 1 ng m-3 ) at all these sites in October-November. This suggests that there is no significant difference in eBC between the coastal and plateau sites. The seasonal cycle of the eBC mass fraction exhibits a minimum f(eBC) ˜ 0.1% in February-March and a maximum ~4-5% in August-October. Source areas were calculated using 50-day FLEXPART footprints. The highest eBC concentrations and the lowest ?o were associated with air masses coming from South America, Australia and Africa. Vertical simulations that take BC particle removal processes into account show that there would be essentially no BC particles arriving at Dome C from north of latitude 10°S at altitudes < 1600 m. The main biomass-burning regions Africa, Australia and Brazil are more to the south and their smoke plumes have been observed at higher altitudes than that so they can get transported to Antarctica. The seasonal cycle of BC emissions from wildfires and agricultural burning and other fires in South America, Africa and Australia were calculated from data downloaded from the Global Fire Emissions Database (GFED). The maximum total emissions were in August-September but the peak of monthly average eBC concentrations is observed 2 – 3 months later in November not only at Dome C but also at SPO and the coastal stations. The air mass residence-time-weighted BC emissions from South America are approximately an order of magnitude larger than from Africa and Oceania suggesting that South American BC emissions are the largest contributors to eBC at Dome C. At Dome C the maximum and minimum scattering coefficients were observed in austral summer and winter, respectively. At SPO ssp was similar to that observed at Dome C in the austral summer but there was a large difference in winter, suggesting that in winter SPO is more influenced by sea spray emissions than Dome C. The seasonal cycles of ssp at Dome C and at the SPO were compared with the seasonal cycles of secondary and primary marine aerosol emissions. The ssp measured at SPO correlated much better with the sea-spray aerosol emission fluxes in the Southern Ocean than ssp at Dome C. The seasonal cycles of biogenic secondary aerosols were estimated from monthly average phytoplankton biomass concentrations obtained from the CALIOP satellite sensor data. The analysis suggests that a large fraction of the biogenic scattering aerosol observed at Dome C has been formed in the polar zone but it may take a month for the aerosol to be formed, grown and get transported from the sea level to Dome C. [ABSTRACT FROM AUTHOR]