Aristeidis K. Georgoulias

Spatiotemporal variability and contribution of different aerosol types to the aerosol optical depth over the Eastern Mediterranean

This study characterizes the spatiotemporal variability and relative contribution of different types of aerosols to the Aerosol Optical Depth (AOD) over the Eastern Mediterranean as derived from MODIS Terra (3/2000-12/2012) and Aqua (7/2002-12/2012) satellite instruments. For this purpose, a 0.1° × 0.1° gridded MODIS dataset was compiled and validated against sunphotometric observations from the AErosol RObotic NETwork (AERONET). The high spatial resolution and long temporal coverage of the dataset allows for the determination of local hot spots like megacities, medium sized cities, industrial zones, and power plant complexes, seasonal variabilities, and decadal averages. The average AOD at 550 nm (AOD550) for the entire region is ~ 0.22 ± 0.19 with maximum values in summer and seasonal variabilities that can be attributed to precipitation, photochemical production of secondary organic aerosols, transport of pollution and smoke from biomass burning in Central and Eastern Europe, and transport of dust from the Sahara Desert and the Middle East. The MODIS data were analyzed together with data from other satellite sensors, reanalysis projects and a chemistry-aerosol-transport model using an optimized algorithm tailored for the region and capable of estimating the contribution of different aerosol types to the total AOD550. The spatial and temporal variability of anthropogenic, dust and fine mode natural aerosols over land and anthropogenic, dust and marine aerosols over the sea is examined. The relative contribution of the different aerosol types to the total AOD550 exhibits a low/high seasonal variability over land/sea areas, respectively. Overall, anthropogenic aerosols, dust and fine mode natural aerosols account for ~ 51 %, ~ 34 % and ~ 15 % of the total AOD550 over land, while, anthropogenic aerosols, dust and marine aerosols account ~ 40 %, ~ 34 % and ~ 26 % of the total AOD550 over the sea, based on MODIS Terra and Aqua observations.

Trends of Shortwave and Longwave Surface Radiation in Europe: Spatiotemporal Analysis and Comparison of Satellite and Ground-Based Observations

A detailed investigation of the shortwave (SW) and longwave (LW) up-welling and down-welling surface radiation trends over Europe is presented here. For the purposes of this work, satellite observations from the International Satellite Cloud Climatology Project (ISCCP) for the period 1984–2009 have been spatiotemporally analyzed at a ~280 × 280 km2 resolution. A Fourier-based harmonic analysis technique has been used for the calculation of the trend also allowing for the assessment of its statistical significance. The results are compared to trends calculated from ground-based observations from several World Radiation Data Center (WRDC) stations. The stations have been categorized taking into account their position and the special characteristics of the surrounding region (rural/urban, high/low elevation, population, etc.). The variability of the SW and LW radiation within selected ISCCP grid cells is investigated with the use of ground-based observations. Observed trends and their significance depends on the area of study. Estimated trend for SW radiation over Europe as derived from ISCCP data for the period 1984–2009 is about −0.6 W/m2 and for LW radiation is about 2.47 W/m2.

A 3-D Evaluation of the MACC Reanalysis Dust Product Over Europe Using CALIOP/CALIPSO Satellite Observations

This work focuses on the evaluation of the MACC (Monitoring atmospheric composition and climate) reanalysis dust product over Europe. Europe receives significant amounts of dust on an annual basis primarily from the large neighboring area sources (Sahara Desert, Arabian Peninsula) and from smaller local sources. Dust affects a number of processes in the atmosphere modulating weather and climate and exerts an impact on human health and the economy. Hence, the ability of simulating adequately the amount of dust and its optical properties is essential. For the evaluation of the MACC reanalysis, pure dust satellite-based retrievals from CALIOP/CALIPSO are utilized for the period 2007–2012. Specifically, the CALIOP/CALIPSO data used here come from an optimized retrieval scheme that was originally developed within the framework of the LIVAS (Lidar Climatology of Vertical Aerosol Structure for Space-Based LIDAR Simulation Studies) project. The natural aerosol extinction coefficients from MACC and the dust optical depth patterns at 550 nm are validated against dust extinction coefficients and dust optical depth patterns at 532 nm from CALIOP/CALIPSO. Our results highlight the important role that space-based lidars may play in the improvement of the MACC aerosol product.

A new method for deriving trace gas emission inventories from satellite observations: The case of SO2 over China

A method is developed that allows the construction of spatial emission inventories. The method is applied for anthropogenic SO2 over China (0.25°×0.25°). The Enhancement Ratio Method (ERM) allows for the calculation of SO2 emissions using relationships between gridded satellite measurements of SO2 and NO2 at low wind speeds, and satellite-based NOx emission estimates. Here, we derive SO2 emissions for five years (2007-2011). A large decrease of emissions during 2007-2009 and a modest increase between 2010 and 2011 is observed. The evolution of emissions over time calculated here is in general agreement with bottom-up inventories, although differences exist, not only between the current inventory and other inventories but also among the bottom up inventories themselves. The ERM-derived emissions are consistent, spatially and temporally, with existing inventories.

Towards Improved Air Quality Monitoring Using Publicly Available Sky Images.

Air pollution causes nearly half a million premature deaths each year in Europe. Despite air quality directives that demand compliance with air pollution value limits, many urban populations continue being exposed to air pollution levels that exceed by far the guidelines. Unfortunately, official air quality sensors are sparse, limiting the accuracy of the provided air quality information. In this chapter, we explore the possibility of extending the number of air quality measurements that are fed into existing air quality monitoring systems by exploiting techniques that estimate air quality based on sky-depicting images. We first describe a comprehensive data collection mechanism and the results of an empirical study on the availability of sky images in social image sharing platforms and on webcam sites. In addition, we present a methodology for automatically detecting and extracting the sky part of the images leveraging deep learning models for concept detection and localization. Finally, we present an air quality estimation model that operates on statistics computed from the pixel color values of the detected sky regions.

Spatial and seasonal variations of aerosols over China from two decades of multi-satellite observations. Part I: ATSR (1995–2011) and MODIS C6.1 (2000–2017)

Aerosol optical depth (AOD) patterns and interannual and seasonal variations over China are discussed based on the AOD retrieved from the Along-Track Scanning Radiometer (ATSR-2, 1995–2002), the Advanced ATSR (AATSR, 2002–2012) (together ATSR) and the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite (2000–2017). The AOD products used were the ATSR Dual View (ADV) v2.31 AOD and the MODIS/Terra Collection 6.1 (C6.1) merged dark target (DT) and deep blue (DB) AOD product. Together these datasets provide an AOD time series for 23 years, from 1995 to 2017. The difference between the AOD values retrieved from ATSR-2 and AATSR is small, as shown by pixel-by-pixel and monthly aggregate comparisons as well as validation results. This allows for the combination of the ATSR-2 and AATSR AOD time series into one dataset without offset correction. ADV and MODIS AOD validation results show similar high correlations with the Aerosol Robotic Network (AERONET) AOD (0.88 and 0.92, respectively), while the corresponding bias is positive for MODIS (0.06) and negative for ADV (− 0.07). Validation of the AOD products in similar conditions, when ATSR and MODIS/Terra overpasses are within 90 min of each other and when both ADV and MODIS retrieve AOD around AERONET locations, show that ADV performs better than MODIS in autumn, while MODIS performs slightly better in spring and summer. In winter, both ADV and MODIS underestimate the AERONET AOD. Similar AOD patterns are observed by ADV and MODIS in annual and seasonal aggregates as well as in time series. ADV–MODIS difference maps show that MODIS AOD is generally higher than that from ADV. Both ADV and MODIS show similar seasonal AOD behavior. The AOD maxima shift from spring in the south to summer along the eastern coast further north. The agreement between sensors regarding year-to-year AOD changes is quite good. During the period from 1995 to 2006 AOD increased in the southeast (SE) of China. Between 2006 and 2011 AOD did not change much, showing minor minima in 2008–2009. From 2011 onward AOD decreased in the SE of China. Similar patterns exist in year-toyear ADV and MODIS annual AOD tendencies in the overlapping period. However, regional differences between the ATSR and MODIS AODs are quite large. The consistency Published by Copernicus Publications on behalf of the European Geosciences Union. 11390 L. Sogacheva et al.: Spatial and seasonal variations of aerosols over China – Part 1 between ATSR and MODIS with regards to the AOD tendencies in the overlapping period is rather strong in summer, autumn and overall for the yearly average; however, in winter and spring, when there is a difference in coverage between the two instruments, the agreement between ATSR and MODIS is lower. AOD tendencies in China during the 1995–2017 period will be discussed in more detail in Part 2 (a following paper: Sogacheva et al., 2018), where a method to combine AOD time series from ADV and MODIS is introduced, and combined AOD time series are analyzed.

Summer Total Cloud Cover Weekly Variability over Europe: Sign of Aerosols’ Indirect Effect?

The Day of Week (DoW) variability of total cloud cover (TCC) and aerosol optical depth (AOD550) and its statistical significance over six European regions is investigated using 10 years of summertime satellite observations from TERRA and AQUA MODIS. In general, positive (higher values during midweek) weekly cycles in both AOD550 and TCC appear over Central Europe (CE), while a strong negative weekly cycle appears over the Iberian Peninsula and NE Europe (NEE). A very similar TCC and AOD550 weekly variability appears over CE, SW Europe (SWE) and NEE and a rather similar one over Central Mediterranean (CM). Possible scenarios that could explain the common weekly variability of aerosols and cloud cover through the aerosol indirect effects are discussed. An effort is made to verify our results and the proposed aerosol-cloud interaction scenarios with the use of independent satellite TCC data from the ISCCP and ground-based precipitation data from the ECA&D project.

Weekly cycles in meteorological variables over large-scales: fact or myth?

Several studies have found weekly cycles of meteorological variables appearing over large domains, which can hardly be related only to urban effects, while other studies fail to reproduce these with statistical significance. In addition to the lack of unequivocal evidence for these cycles, their possible physical causes have been controversially discussed. We present a short summary of the main results about this topic published during the recent two decades. Also, a brief summary of the suggested reasons, especially focusing on the possible aerosol-cloud-radiation interaction as a result of the weekly cycles of anthropogenic activities, is presented.