Articles | Volume 16, issue 8
Atmos. Chem. Phys., 16, 5063–5073, 2016
Atmos. Chem. Phys., 16, 5063–5073, 2016

Research article 22 Apr 2016

Research article | 22 Apr 2016

Aerosol optical depth trend over the Middle East

Klaus Klingmüller1, Andrea Pozzer1, Swen Metzger2, Georgiy L. Stenchikov3, and Jos Lelieveld1,2 Klaus Klingmüller et al.
  • 1Max Planck Institute for Chemistry, P.O. Box 3060, 55020 Mainz, Germany
  • 2The Cyprus Institute, P.O. Box 27456, 1645 Nicosia, Cyprus
  • 3King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia

Abstract. We use the combined Dark Target/Deep Blue aerosol optical depth (AOD) satellite product of the moderate-resolution imaging spectroradiometer (MODIS) collection 6 to study trends over the Middle East between 2000 and 2015. Our analysis corroborates a previously identified positive AOD trend over large parts of the Middle East during the period 2001 to 2012.

We relate the annual AOD to precipitation, soil moisture and surface winds to identify regions where these attributes are directly related to the AOD over Saudi Arabia, Iraq and Iran. Regarding precipitation and soil moisture, a relatively small area in and surrounding Iraq turns out to be of prime importance for the AOD over these countries. Regarding surface wind speed, the African Red Sea coastal area is relevant for the Saudi Arabian AOD.

Using multiple linear regression we show that AOD trends and interannual variability can be attributed to soil moisture, precipitation and surface winds, being the main factors controlling the dust cycle. Our results confirm the dust driven AOD trends and variability, supported by a decreasing MODIS-derived Ångström exponent and a decreasing AERONET-derived fine mode fraction that accompany the AOD increase over Saudi Arabia. The positive AOD trend relates to a negative soil moisture trend. As a lower soil moisture translates into enhanced dust emissions, it is not needed to assume growing anthropogenic aerosol and aerosol precursor emissions to explain the observations. Instead, our results suggest that increasing temperature and decreasing relative humidity in the last decade have promoted soil drying, leading to increased dust emissions and AOD; consequently an AOD increase is expected due to climate change.

Short summary
During the last decade, the Middle East experienced the strongest increase in atmospheric aerosol concentrations worldwide. Based on satellite observations, the present study corroborates this trend and reveals correlations with soil moisture and precipitation in and surrounding the Fertile Crescent. This suggests that the increasing drought conditions in this region have enhanced dust emissions, a tendency which is expected to be intensified by climate change.
Final-revised paper