Aerosol pattern changes over the dead sea from west to east - Using high-resolution satellite data

Highlights

• For the first time, high-resolution climatology of the Dead Sea environs aerosols was studied.

• The difference in AOD pollution levels between east and west Dead Sea coasts was estimated on a temporal scale.

• The Normalized AOD Difference (NAODD) metric was used.

• The Dead Sea aerosol levels are asymmetrical with enhanced AOD levels over the west shore (Israel) as compared to the east side (Jordan).

• Trend analysis showed a prominent shift towards increasing pollution over the east side.

Abstract

The area of the Dead Sea, a terminal lake on the border between Israel and Jordan, has gone through extensive environmental changes in recent decades, stemming from several processes including extreme evaporation and land degradation. Our study explored the distribution and the long-term high-resolution climatology of the Dead Sea aerosols, using satellite-retrieved Aerosol Optical Depth (AOD) measurements. We used the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm that provides AOD measurements at a 1 km resolution. MAIAC retrievals for the entire study period (2000–2016) have shown a very good agreement (r = 0.8) with the nearby AERONET station. For the first time, the AOD temporal distribution and its variability, on both Dead Sea coasts, were calculated on a monthly/seasonal basis. Three main aerosol pollution patterns (i.e. high-levels of AOD) emerged as follows: western focalized (75 and 59% of the months examined for Aqua and Terra, respectively), eastern focalized (14 and 24%), and similar levels of pollution on both coasts (11 and 17%, respectively). Using the Normalized AOD Difference (NAODD) metric, we studied how the AOD spatial patterns changed and when. Following the negative trends in most of the months, our analysis has demonstrated a shift towards unexplained increasing pollution levels over the Jordanian eastern side. For March–April, this shift had already occurred, and the potential reasons including changes in synoptic regimes are discussed. Local and global implications are of much regional interest because of the Dead Sea deterioration and the need for urgent environmental policy changes.

Introduction

Situated at a height of 430 m below sea level, the Dead Sea environs represent one of the most unique environments on Earth. The area has gone through dramatic environmental changes in the last few decades. Recent discussions in the scientific literature and world media highlight the continuous degradation of this unique area from all aspects. This includes rapid evaporation that led to a prominent fall in the lake size Alpert et al. (1997); Shafir and Alpert (2011); and Hamdani et al., 2018(; expansion of sinkholes (Kottmeier et al., 2016); landscape degradation (Abou Karaki et al., 2016; Ezersky et al., 2013, 2017; Yechieli et al., 2016; and Abou Karaki and Closson, 2012); industrial development (potash plants, phosphate, and agricultural manure production) and a growing tourist industry along the receding coastline (Abu Jaber, 1998; Yechieli et al., 1998; Krumgalz et al., 2000; Wisniak, 2002; Filin et al., 2014; Khlaifat et al., 2010). Above the local impact (the area's topography and local emissions), the change in the frequency of regional synoptic systems as estimated by Hochman et al. (2018) shifts the seasonal balance towards a longer summer and shorter winter, contributing to an increase in the dominance of Persian Trough. How will these changes impact aerosol climatology above the challenging area of the Dead Sea? This question is still open.

Remote sensing offers continues measurements of the atmosphere, which in turn enables the studying of long-term climatology. The common parameter for such studies is aerosol optical depth (AOD), which measures the amount of radiation that reaches the Earth surface as it goes through the atmosphere (Kaufman et al., 2002). Analysis of recent high resolution aerosol patterns can assist in better understanding the future potential AOD trends. The latter impact the Dead Sea evaporation levels, which is vital due to the recent, massive 40-year drop (1 m per year approximately) of the Dead Sea level and its associated hazards such as sinkholes (Shafir and Alpert, 2011, Kottemeir et al., 2016). In the last seven years, a surge in high-resolution remote sensing has yielded new products. For example the Simplified Aerosol Retrieval Algorithm (SARA)- 500 m developed for MODIS- not part of the product (Bilal et al., 2013); the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi NPP satellite, 0.75 km (IP product at 6 km), “standard algorithm” like deep blue (Jackson et al., 2013; Zhang et al., 2016); and the Sentinel 3 SLSTR, product NRT and NTC, level 2, 4.5 km resolution (Donlon et al., 2012). Among them is also, the Multi-Angle Implementation of Atmospheric Correction (MAIAC) which produces AOD at a 1 × 1 km resolution. This product has already been widely used in numerous studies and is currently a part of the MODIS Collection 6 product. To analyze AOD spatio-temporal variability, we selected MAIAC due to its better performance above arid environments and much larger data coverage. Specifically, when considering both clear and partly cloudy days, it provides more retrievals than the conventional MOD04 at 10 and 3 km and with better performance during low pollution days (Chudnovsky et al., 2013a, 2013b). The increase in data coverage and spatial resolution allows us to capture spatial pattern and local dust sources more accurately (Sever et al., 2017). More so, due to the area's most unique world topography (altitudes of ~1000 m to −430 m within just a few 10 s km) the spatial gradients in land surface features and AOD are extremely high (Kishcha et al., 2016).

With aerosols' enormous effect on the earth climate (Kaufman et al., 2002), the AOD balance between the east and west coasts of the Dead Sea can serve as a significant indicator to the regions’ sources of pollution, alongside the synoptic systems that govern the area at a given time. Previous aerosols studies in the region, described case studies (Levin et al., 2005), other parameters (Alpert et al., 1997; Shafir and Alpert, 2011), and model simulations (Kishcha et al., 2016). Therefore, one of the goals of this study is to explore the distribution and long-term high-resolution climatology of the Dead Sea aerosols, and present for the first time the prevailing aerosol patterns in the region.

In our previous study (Sever et al., 2017), we have demonstrated the strength of MAIAC in retrieving AOD over the bright, arid, and topographically intricate terrain of the Dead Sea coastal regions (Fig. 1, Panel A) as oppose to the standard MODIS retrieval that fails to identify the region's aerosol patterns (Sever et al., 2017). Analyzing one year of data (2013), three typical aerosol patterns emerged: higher levels of pollution (i.e. high levels of AOD) at the western shore, higher levels at the eastern shore, and similar values over both shores (Fig. 6 in Sever et al., 2017). Fig. 1, Panel B, shows extreme examples of such west/east coast pollution. In the current study, we aimed to perform an in-depth analysis of the high-resolution long-term aerosol distribution and climatology, over an extensive period (approximately 16 years of MODIS data), focusing on the difference in AOD values between the two coasts of the Dead Sea (Fig. 1, Panel C).