Preprints
https://doi.org/10.5194/acp-2022-811
https://doi.org/10.5194/acp-2022-811
10 Jan 2023
 | 10 Jan 2023
Status: a revised version of this preprint is currently under review for the journal ACP.

How horizontal transport and turbulent mixing impacts aerosol particle and precursor concentrations at a background site in the UAE

Jutta Kesti, Ewan James O'Connor, Anne Hirsikko, John Backman, Heikki Lihavainen, Hannele Korhonen, and Eija Asmi

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.

Jutta Kesti, Ewan James O'Connor, Anne Hirsikko, John Backman, Heikki Lihavainen, Hannele Korhonen, and Eija Asmi

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-811', Anonymous Referee #1, 21 Feb 2023
    • AC1: 'Reply on RC1', John Backman, 31 Jan 2024
  • RC2: 'Comment on acp-2022-811', Anonymous Referee #2, 08 Mar 2023
    • AC2: 'Reply on RC2', John Backman, 31 Jan 2024
Jutta Kesti, Ewan James O'Connor, Anne Hirsikko, John Backman, Heikki Lihavainen, Hannele Korhonen, and Eija Asmi
Jutta Kesti, Ewan James O'Connor, Anne Hirsikko, John Backman, Heikki Lihavainen, Hannele Korhonen, and Eija Asmi

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Short summary
The study combines aerosol particle measurements at the surface and vertical profiling of the atmosphere with a scanning Doppler lidar to investigate how transport together with boundary layer evolution can affect particle and SO2 concentrations at the surface in the Arabian Peninsula region. The instrumentation enabled us to see elevated nucleation mode particle and SO2 concentrations at the surface when air masses transported from polluted areas are mixed in the boundary layer.
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