Aerosol optical depth trend over the Middle East

Klaus Klingmüller,Jos Lelieveld,Swen Metzger,Georgiy L Stenchikov,Andrea Pozzer

doi:10.5194/acp-16-5063-2016

Abstract

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.

Highlights

The Middle East and the adjacent Mediterranean region have been identified as a hot-spot of climate change (Hoerling et al, 2012; Hemming et al, 2010; Lelieveld et al, 2012; Giorgi and Lionello, 2008)
Using multiple linear regression we show that aerosol optical depth (AOD) trends and interannual variability can be attributed to soil moisture, precipitation and surface winds, being the main factors controlling the dust cycle
7 Conclusions Using the combined Dark Target/Deep Blue AOD products of the recently available moderate-resolution imaging spectroradiometer (MODIS) collection 6, our study corroborates the positive AOD trend over the Middle East during the 10 year period from 2001 to 2010, which has been reported previously based on other satellite products

Summary

Introduction

The Middle East and the adjacent Mediterranean region have been identified as a hot-spot of climate change (Hoerling et al, 2012; Hemming et al, 2010; Lelieveld et al, 2012; Giorgi and Lionello, 2008). Various studies show a strong increase in the AOD over the Middle East during the last decade These studies were performed with numerical models (Pozzer et al, 2015) and with different remotely sensed observational data, e.g., from SeaWiFS (Hsu et al, 2012; IPCC, 2014), MODIS, MISR and AERONET (de Meij et al, 2012; de Meij and Lelieveld, 2011; Stevens and Schwartz, 2012). Different from previous studies, in this work we analyse trends using the most recent (as of 2015) data collection 6 of the Moderate-resolution Imaging Spectroradiometer (MODIS) (Hubanks et al, 2015; Levy et al, 2013; MODIS MOD08 M3, 2015) which includes AOD data based on refined retrieval algorithms (Sayer et al, 2014), in particular the expanded Deep Blue algorithm The latter is of special importance for our study because it is suited for retrievals over bright surfaces such as the deserts and semi-deserts covering large parts of the Middle East.

Aerosol trends

Precipitation

Soil moisture

Surface wind

Multiple linear regression analysis

Conclusions