Investigating aerosol vertical distribution using CALIPSO time series over the Middle East and North Africa (MENA), Europe, and India: A BFAST-based gradual and abrupt change detection

Brakhasi, Foad; Hajeb, Mohammad; Mielonen, Tero; Matkan, Aliakbar; Verbesselt, Jan


Monitoring spatio-temporal changes of aerosols is necessary to better understand atmospheric processes. Here, the vertical distribution of aerosols and how it has changed from 2006 to 2017 is studied using time series data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite. Aerosol centroid altitude, a variable representing the aerosol vertical extent, is estimated using the aerosol flags of the Vertical Feature Mask (VFM) product of CALIOP. The case study includes four overpasses, with various aerosol types and densities, covering the Middle East and North Africa (MENA), and India regions characterized by frequent dust storms and smoke emissions, respectively, and Europe, a pioneer in establishing air quality policies. The values of aerosol centroid altitude are examined to determine the effects of climate (space) and season (time) on aerosol vertical distribution. For this purpose, 10-year mean values of aerosol centroid altitude were calculated over climate regions extracted from the Food and Agriculture Organization (FAO) climate map as well as over each season, and the resultant values were then statistically analyzed. The results indicate that environmental heat, steam from climate or season, leads to aerosols increasing in altitude. The time series was imported in a version of the Breaks For Additive Season and Trend (BFAST) algorithm, so-called BFAST01, allowing the magnitude of gradual changes along with the time and type of the major abrupt change to be determined. The strongest positive trends in aerosol centroid altitude were recorded in the desert regions. Positive trends were also observed in the mid-latitudes steppes and the North Sea. India was another region with a relatively positive trend which could be mainly caused by the increasing anthropogenic activities and smoke emission. The strongest negative trend detected in Europe is a result of strict environmental regulation policies and efforts for air quality improvement. The results show that a small fraction of granules (12 out of 4036) mainly in the northern part of overpass A, over Europe, the mid part of overpasses B and C, over the Middle East, and southern part of overpass D, over India, experienced structural changes. The reversal changes type 8 (50% of the abrupt changes) and interruption type 6 (33%) are the dominant types, mostly occurred around 2014 and 2012, respectively. Changing environmental factors such as temperature, wind speed, and anthropogenic activities are the main drivers of these changes. The study demonstrates the applicability of BFAST01 and the usability of CALIOP VFM time series data to monitor the vertical structure of the atmosphere.