Preprints
https://doi.org/10.5194/acp-2021-243
https://doi.org/10.5194/acp-2021-243

  17 Jun 2021

17 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal ACP.

Aerosol particle characteristics measured in the United Arab Emirates and their response to mixing in the boundary layer

Jutta Kesti1, John Backman1, Ewan James O'Connor1, Anne Hirsikko1, Eija Asmi1, Minna Aurela1, Ulla Makkonen1, Maria Filioglou2, Mika Komppula2, Hannele Korhonen1, and Heikki Lihavainen1,3 Jutta Kesti et al.
  • 1Finnish Meteorological Institute, Helsinki, 00560, Finland
  • 2Finnish Meteorological Institute, Kuopio, 70211, Finland
  • 3Svalbard Integrated Arctic Earth Observing System, Longyearbyen, 9170, Norway

Abstract. Aerosol particles play an important in role in the microphysics of clouds and hence on their likelihood to precipitate. In the changing climate already dry areas such as the United Arab Emirates (UAE) are predicted to become even drier. Comprehensive observations of the daily and seasonal variation in aerosol particle properties in such locations are required reducing the uncertainty in such predictions. We analyse observations from a one-year measurement campaign at a background location in the United Arab Emirates to investigate the properties of aerosol particles in this region, study the impact of boundary layer mixing on background aerosol particle properties measured at the surface and study the temporal evolution of the aerosol particle cloud formation potential in the region. We used in-situ aerosol particle measurements to characterise the aerosol particle composition, size, number and cloud condensation nuclei (CCN) properties, in-situ SO2 measurements as an anthropogenic signature and a long-range scanning Doppler lidar to provide vertical profiles of the horizontal wind and turbulent properties to monitor the evolution of the boundary layer. Anthropogenic sulphate dominated the aerosol particle mass composition in this location. There was a clear diurnal cycle in the surface wind direction, which had a strong impact on aerosol particle total number concentration, SO2 concentration and black carbon mass concentration. Local sources were the predominant source of black carbon, as concentrations clearly depended on the presence of turbulent mixing, with much higher values during calm nights. The measured concentrations of SO2, instead, were highly dependent on the surface wind direction as well as on the depth of the boundary layer when entrainment from the advected elevated layers occurred. The wind direction at the surface or of the elevated layer suggests that the cities of Dubai, Abu Dhabi and other coastal conurbations were the remote sources of SO2. We observed new aerosol particle formation events almost every day (on four days out of five on average). Calm nights had the highest CCN number concentrations and lowest κ values and activation fractions. We did not observe any clear dependence of CCN number concentration and κ parameter on the height of the daytime boundary layer, whereas the activation fraction did show a slight increase with increasing boundary layer height, due to the change in the shape of the aerosol particle size distribution where the relative portion of larger aerosol particles increased with increasing boundary layer height. We believe that this indicates that size is more important than chemistry for aerosol particle CCN activation at this site. The combination of instrumentation used in this campaign enabled us to identify periods when anthropogenic pollution from remote sources that had been transported in elevated layers was present, and had been mixed down to the surface in the growing boundary layer.

Jutta Kesti et al.

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-2021-243', Anonymous Referee #2, 04 Aug 2021
  • RC2: 'Comment on acp-2021-243', Anonymous Referee #1, 05 Sep 2021
  • AC1: 'Comment on acp-2021-243', Jutta Kesti, 15 Oct 2021

Jutta Kesti et al.

Jutta Kesti et al.

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Short summary
In this study we combined aerosol particle measurements at the surface with a scanning Doppler lidar providing vertical profiles of the atmosphere to study the effect of different boundary layer conditions on aerosol particle properties in the understudied Arabian Peninsula region. The instrumentation used in this study enabled us to identify periods when pollution from remote sources was mixed down to the surface and initiated new particle formation in the growing boundary layer.
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