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Sentinel-5 precursor (S-5p), launched on 13 October 2017, is the first mission of the Copernicus Programme dedicated to the monitoring of air quality, climate, ozone and UV radiation. The S-5p characteristics, such as the fine spatial resolution, introduce many new opportunities and challenges, requiring to carefully assess the quality and validity of the generated data products by comparison with independent measurements and analyses.While routine validation is performed within the ESA Mission Performance Center (MPC) based on a limited number of Fiducial Reference Measurements (FRM), additional validation activities including aerial and ground-based campaigns are conducted in research mode as part of the S-5p Validation Team (S5PVT). The validation activities bring together various teams and instruments to address specific validation requirements and provide a more in-depth, complete insight into the S-5p instrument performance and the fitness for purpose of its data products. The acquired reference data sets allow to address product accuracy and precision, spatial and temporal validation requirements, algorithm parameters (a priori profiles, albedo, etc.) and specific requirements, such as validation of strongly polluted and heterogeneous scenes.Here, we present a series of decentralized activities planned to take place in 2021-2022 (s5pcampaigns.aeronomie.be), which have been identified to address key priorities for S5-p validation.A first set of activities concentrates on the main S-5p UV-Vis tropospheric products (NO2, HCHO and SO2). Airborne deployment, consisting of both in-situ spiral and remote sensing mapping flights, is planned over cities and industrial areas in Romania (Bucharest; Jiu valley), the German Ruhr area (Cologne; Duisburg; Dusseldorf), Berlin, and Belgium (Antwerp (port); Brussels). Airborne operations will be complemented with various deployments on the ground (MAX-DOAS, car-DOAS, sun-photometer, ceilometer, lidar, etc.). The validation activities over Berlin and Bucharest are focused on recurrent airborne observations with hyperspectral imagers in order to have a large number of flights (12 to 18) over a time interval of approximately one year, in order to have a large statistical data set covering variable meteorological and geo-physical conditions, as well as different overpass configurations.A second set of activities will focus on the validation of SWIR data products (CO and CH4). COCCON (COllaborative Carbon Column Observing Network) portable low-resolution EM27/SUN FTIR spectrometers will be deployed for an extended period at different sites in the world in order to obtain a good coverage of geophysical parameters (strong sources, background sites, sites with high humidity, etc.) and different ground scenes, e.g. very high/low albedo sites.Additionally, synergies are created with large field campaigns, such as the Asian Summer Monsoon Chemical and Climate Impact Project (ACCLIP) and the 2021 NET-Sense HyTES Joint European Campaign which will provide airborne measurements of NO2, CO, CH4 columns and vertical profiles, among others.The various airborne and ground-based instrument deployments will produce a key ensemble of independent reference observations. For each product, a core team will coordinate the validation tasks, making use of data collected in all relevant instrumental deployments.

The satellite based monitoring initiative for regional air quality (SAMIRA) initiative was set up to demonstrate the exploitation of existing satellite data for monitoring regional and urban scale air quality. The project was carried out between May 2016 and December 2019 and focused on aerosol optical depth (AOD), particulate matter (PM), nitrogen dioxide (NO2), and sulfur dioxide (SO2). SAMIRA was built around several research tasks: 1. The spinning enhanced visible and infrared imager (SEVIRI) AOD optimal estimation algorithm was improved and geographically extended from Poland to Romania, the Czech Republic and Southern Norway. A near real-time retrieval was implemented and is currently operational. Correlation coefficients of 0.61 and 0.62 were found between SEVIRI AOD and ground-based sun-photometer for Romania and Poland, respectively. 2. A retrieval for ground-level concentrations of PM2.5 was implemented using the SEVIRI AOD in combination with WRF-Chem output. For representative sites a correlation of 0.56 and 0.49 between satellite-based PM2.5 and in situ PM2.5 was found for Poland and the Czech Republic, respectively. 3. An operational algorithm for data fusion was extended to make use of various satellite-based air quality products (NO2, SO2, AOD, PM2.5 and PM10). For the Czech Republic inclusion of satellite data improved mapping of NO2 in rural areas and on an annual basis in urban background areas. It slightly improved mapping of rural and urban background SO2. The use of satellites based AOD or PM2.5 improved mapping results for PM2.5 and PM10. 4. A geostatistical downscaling algorithm for satellite-based air quality products was developed to bridge the gap towards urban-scale applications. Initial testing using synthetic data was followed by applying the algorithm to OMI NO2 data with a direct comparison against high-resolution TROPOMI NO2 as a reference, thus allowing for a quantitative assessment of the algorithm performance and demonstrating significant accuracy improvements after downscaling. We can conclude that SAMIRA demonstrated the added value of using satellite data for regional- and urban-scale air quality monitoring.

Accelerated advances in Earth Observation technologies determine their successful use in various fields, [...]

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns took place in Romania in September 2014 and August 2015. They focused on two sites: the Bucharest urban area and large power plants in the Jiu Valley. The main objectives of the campaigns were to test recently developed airborne observation systems dedicated to air quality studies and to verify their applicability for the validation of space-borne atmospheric missions such as the TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We present the AROMAT campaigns from the perspective of findings related to the validation of tropospheric NO2, SO2, and H2CO. We also quantify the emissions of NOx and SO2 at both measurement sites. We show that tropospheric NO2 vertical column density (VCD) measurements using airborne mapping instruments are well suited for satellite validation in principle. The signal-to-noise ratio of the airborne NO2 measurements is an order of magnitude higher than its space-borne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. However, we show that the temporal variation of the NO2 VCDs during a flight might be a significant source of comparison error. Considering the random error of the TROPOMI tropospheric NO2 VCD (σ), the dynamic range of the NO2 VCDs field extends from detection limit up to 37 σ (2.6×1016 molec. cm−2) and 29 σ (2×1016 molec. cm−2) for Bucharest and the Jiu Valley, respectively. For both areas, we simulate validation exercises applied to the TROPOMI tropospheric NO2 product. These simulations indicate that a comparison error budget closely matching the TROPOMI optimal target accuracy of 25 % can be obtained by adding NO2 and aerosol profile information to the airborne mapping observations, which constrains the investigated accuracy to within 28 %. In addition to NO2, our study also addresses the measurements of SO2 emissions from power plants in the Jiu Valley and an urban hotspot of H2CO in the centre of Bucharest. For these two species, we conclude that the best validation strategy would consist of deploying ground-based measurement systems at well-identified locations.

The Airborne ROmanian Measurements of Aerosols and Trace gases (AROMAT) campaigns took place in Romania in September 2014 and August 2015. They focused on two sites: the Bucharest urban area and the power plants in the Jiu Valley. Their main objectives were to test recently developed airborne observation systems dedicated to air quality studies and to verify the concept of such campaigns in support of the validation of spaceborne atmospheric missions such as TROPOspheric Monitoring Instrument (TROPOMI)/Sentinel-5 Precursor (S5P). We show that tropospheric NO2 vertical column density (VCD) measurements using airborne mapping instruments are valuable for satellite validation. The signal to noise ratio of the airborne NO2 measurements is one order of magnitude higher than its spaceborne counterpart when the airborne measurements are averaged at the TROPOMI pixel scale. A significant source of comparison error appears to be the time variation of the NO2 VCDs during a flight, which we estimated at about 4 x 1015 molec cm-2 in the AROMAT conditions. Considering the random error of the TROPOMI tropospheric NO2 VCD (σ), the dynamic range of the NO2 VCDs field extends from detection limit up to 37σ (2.6 x 1016 molec cm-2) or 29σ (2 x 1016 molec cm-2) for Bucharest and the Jiu Valley, respectively. For the two areas, we simulate validation exercises of the TROPOMI tropospheric NO2 product using airborne measurements. These simulations indicate that we can closely approach the TROPOMI optimal target accuracy of 25 % by adding NO2 and aerosol profile information to the mapping data, which constrains the investigated accuracy within 28 %. In addition to NO2, we also measured significant amounts of SO2 in the Jiu Valley, as well as a hotspot of H2CO in the center of Bucharest. For these two species, we conclude that the best validation strategy would consist in deploying ground-based measurement systems at key locations which the AROMAT observations help identify.

An intensive measurement campaign was performed during September 2014 in southern Romania in two different locations: Magurele, Ilfov County and Turceni, Gorj County. This paper presents one case study with analysis of the aerosol properties from in-situ, passive remote sensing and active remote sensing measurements. A Multiwavelength Raman Lidar (RALI) provided one hour averaged vertical profiles of extinction and backscatter from the 532 nm and 1064 nm channels in Magurele. The UV scanning Lidar (MILI) provided one hour averaged backscattered and extinction vertical profiles for Turceni. Planetary Boundary Layer Height (PBLH) was calculated using the altitude of the maximum negative gradient of the range corrected signal. Mass concentrations for different aerosol species (organics, nitrate, sulphate, ammonium and chloride) were obtained from in-situ measurements using Aerosol Mass Spectrometer located in M�gurele and Aerosol Chemical Speciation Monitor (ACSM) located in Turceni.

Total ozone and other trace gases are measured and reported regularly due to the increased interest started with the ozone hole discovery but the new satellites dedicated to worldwide observations of these species need both short-and long-term well calibrated ground based observation for validation procedures. The ESA/NASA Pandora network established a sophisticated, automatic calibration procedure that utilizes a variety of narrow-line and broadband emission lamps with temperature control for their UV-Vis-NIR spectroradiometers. In this study, we describe additional calibration efforts for ozone retrievals. In this paper we explore the local aerosol field influence on the retrieval of the ozone spectra from PANDORA 2S measurements using collocated lidar and sunphotometer measurements and proposed a methodology to be implemented in the calibration procedure of the instruments.

Particle depolarization ratio retrieved from lidar measurements are commonly used for aerosol typing studies, microphysical inversion, or mass concentration retrievals. The particle depolarization ratio is one of the primary parameters that can differentiate several major aerosol components, but only if the measurements are accurate enough. The uncertainties related to the retrieval of particle depolarization ratios are the main factor in determining the accuracy of the derived parameters in such studies. This paper presents an extended analysis of different depolarization calibration procedures, in order to reduce the related uncertainties. The calibration procedures are specific to each lidar system of the European Aerosol Research Lidar Network – EARLINET with polarising capabilities. The results illustrate a significant improvement of the depolarization lidar products for all the selected lidar stations. The calibrated volume and particle depolarization profiles at 532 nm show values that agree with the theory for all selected atmospheric constituents (several aerosol species, ice particles and molecules in the aerosol free regions).

The present study is focused on microphysics analysis of different cloud types observed by a multi-wavelength Raman lidar. Particle’s content from a wide variety of cloud types, produced in various atmospheric environments, were investigated using data provided by lidar and validated using ECMWF’s ERA Interim reanalysis. The results emphasized the capability of lidar instruments to detect multiple cloud layers, and to discriminate between ice particles and water droplets found within the studied clouds.

During August 2016, a quasi-stationary high-pressure system spreading over Central and North-Eastern Europe, caused weather conditions that allowed for 24/7 observations of aerosol optical properties by using a complex multi-wavelength PollyXT lidar system with Raman, polarization and water vapour capabilities, based at the European Aerosol Research Lidar Network (EARLINET network) urban site in Warsaw, Poland. During 24–30 August 2016, the lidar-derived products (boundary layer height, aerosol optical depth, Angstrom exponent, lidar ratio, depolarization ratio) were analysed in terms of air mass transport (HYSPLIT model), aerosol load (CAMS data) and type (NAAPS model) and confronted with active and passive remote sensing at the ground level (PolandAOD, AERONET, WIOS-AQ networks) and aboard satellites (SEVIRI, MODIS, CATS sensors). Optical properties for less than a day-old fresh biomass burning aerosol, advected into Warsaw’s boundary layer from over Ukraine, were compared with the properties of long-range transported 3–5 day-old aged biomass burning aerosol detected in the free troposphere over Warsaw. Analyses of temporal changes of aerosol properties within the boundary layer, revealed an increase of aerosol optical depth and Angstrom exponent accompanied by an increase of surface PM10 and PM2.5. Intrusions of advected biomass burning particles into the urban boundary layer seem to affect not only the optical properties observed but also the top height of the boundary layer, by moderating its increase.

Particle depolarization ratio retrieved from lidar measurements are commonly used for aerosol typing studies, microphysical inversion, or mass concentration retrievals. The particle depolarization ratio is one of the primary parameters that can differentiate several major aerosol components, but only if the measurements are accurate enough. The uncertainties related to the retrieval of particle depolarization ratios are the main factor in determining the accuracy of the derived parameters in such studies. This paper presents different depolarization calibration procedures used to improve the quality of the depolarization data. The results illustrate a significant improvement of the depolarization lidar products for all the selected lidar stations that had implemented depolarization calibration procedures. The calibrated volume and particle depolarization profiles at 532 nm show values that fall within a range of values that are generally accepted in the literature.

In this study, we used boundary layer heights derived from lidar in Romania to validate the Weather Research Forecast (WRF) model improved by ARIA Technologies SA in the framework of ROMAIR LIFE project. Lidar retrievals were also compared to the retrievals from meteorological data, both modeled (Global Data Assimilation System; GDAS) and measured (microwave radiometry). Both the gradient and the wavelet covariance methods were used to compute the boundary layer height (BLH) from the range corrected lidar signal, and their equivalence was shown. The analysis was performed on 102 datasets, spread over all seasons and 3 years (2009–2011). A good agreement was found for the remote sensors (lidar and microwave radiometer) which are co-located and measure simultaneously. The correlation of the measured boundary layer height and the modelled one was 0.66 for the entire dataset, and 0.73 when considering daytime data, i.e., for a well defined boundary layer. A systematic underestimation of the boundary layer height by the WRF during non-convective periods (nocturne, stable atmosphere) was found.

[1] This paper focuses on optical and microphysical properties of long-range transported biomass burning (BB) aerosols and their variation with atmospheric evolution (ageing), as observed by a multiwavelength Raman lidar, part of EARLINET (European Aerosol LIdar NETwork). Chemical analysis of the atmospheric aerosol was done using a colocated aerosol mass spectrometer (AMS). One relevant optical parameter for the ageing process is the Angstrom exponent. In our study, we find that it decreases from 2 for fresh to 1.4–0.5 for aged smoke particles. The ratio of lidar (extinction-to-backscatter) ratios (LR532/LR355) changes rapidly from values 1 for aged particles. The imaginary part of the refractive index is the most sensitive microphysical parameter. It decreases sharply from 0.05 to less than 0.01 for fresh and aged smoke particles, respectively. Single-scattering albedo (SSA) varies from 0.74 to 0.98 depending on aerosol age and source. The AMS was used to measure the marker ions of wood-burning particles during 2 days of measurements when the meteorological conditions favored the downward mixing of aerosols from lofted layers. Particle size distribution and particle effective radius from both AMS and lidar are similar, i.e., particle effective radii were approximately 0.27 µm for fresh BB aerosol particles. Microphysical aerosol properties from inversion of the lidar data agree with similar studies carried out in different regions on the globe. Our study shows that the Angstrom exponent LR532/LR355 and the imaginary part of the refractive index can be used to clearly distinguish between fresh and aged smoke particles.

This paper presents the results from a coordinated approach for atmospheric investigation, exploring synergies between different techniques. A wide range of instruments have been used during an intensive measurement period both from ground (lidar, sunphotometer, aethalometer and Aerosol Chemical Speciation Monitor) and airborne (aerodynamic particle sizer, the Picarro gas analyzer and the NO2CAPS analyzer) in 2016 over Magurele, 6 km South West of Bucharest.

During an exceptionally warm September of 2016, the unique, stable weather conditions over Poland allowed for an extensive testing of the new algorithm developed to improve the Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) aerosol optical depth (AOD) retrieval. The development was conducted in the frame of the ESA-ESRIN SAMIRA project. The new AOD algorithm aims at providing the aerosol optical depth maps over the territory of Poland with a high temporal resolution of 15 minutes. It was tested on the data set obtained between 11-16 September 2016, during which a day of relatively clean atmospheric background related to an Arctic airmass inflow was surrounded by a few days with well increased aerosol load of different origin. On the clean reference day, for estimating surface reflectance the AOD forecast available on-line via the Copernicus Atmosphere Monitoring Service (CAMS) was used. The obtained AOD maps were validated against AODs available within the Poland-AOD and AERONET networks, and with AOD values obtained from the PollyXT-UW lidar. of the University of Warsaw (UW).

An air-mass modification, on its way from Poland to Romania, observed between 19-21 July 2014 is discussed. The air-mass was investigated using data of two multi-wavelength lidars capable of performing regular elastic, depolarization and Raman measurements in Warsaw, Poland, and in Magurele, Romania. The analysis was focused on evaluating optical properties of aerosol in order to search for similarities and differences in the vertical profiles describing the atmospheric layers above the two stations within given period.

This observational study focused on three-years time-averaged data set (January 2012–2015). An investigation of long-term trends was performed on two different data sets derived from active and passive remote sensing measurements in Magurele, Romania. Measurements of sun photometer aerosol optical depth (AOD) at 500 nm and 340 nm show the mean values of 0.230 ±0 .118 and 0.398 ± 0.185, respectively. The lidar AOD at 532 and 355nm has a mean of 0.271 ±.0.164 and 0.472 ± 0.165 respectively. The highest seasonal mean value was measured by the lidar during the summer of 2014 while the lowest seasonal value was measured by the sunphotometer in February 2012. The origin of atmospheric aerosols has been analyzed using both backtajectories of Hysplit and Circulation Type Classification (CTCs) methods.

Located at a distance more than 3400Km from Iceland, where the eruption of Eyjafjallajokull volcano occurred, Romania was under the influence of volcanic ash transported by middle altitude air masses across Europe. Apart from two clear episodes on April 18 and April 21, 2010 the mix of volcanic ash with Saharan dust was detected by the multiwavelength Raman lidar in Bucharest. Optical properties of aerosol layers for these particular cases showed an increase of the linear particle depolarization, as well as a decrease of the Angstrom exponent, compared with the pure long-range transported volcanic ash. The case of May 5 th , 2010 is analyzed from lidar and ground-level data, accounting for layer dynamics, optical properties and chemical composition. Using high resolution lidar data we could make a clear distinction between ash and dust layers up to their mixing in the PBL. In order to account for similarities and differences, the same analysis was done for May 10 th and 11 th , when several distinct layers were depicted. The signature of ash (sulfate) was identified by Aerosol Mass Spectrometer at ground-level, on May 5 th and May 11 th .

Vertically resolved profiles of optical properties of aerosols were measured using a multi-wavelength lidar system-RALI, set up at the scientific research center in Magurele, Bucharest area (44.35 N latitude, 26.03 E longitude) during 2008. The use of multiple laser wavelengths has enabled us to observe significant variations in backscatter profiles depending on the particle origins. An air mass backward trajectory analysis, using Hysplit-4, was carried out to track the aerosol plumes. Aerosols can serve as valuable tracers of air motion in the planetary boundary layer (PBL). The height of layers in the lower troposphere from lidar signal was calculated using the gradient method- minima of the first derivative. The Richardson number method was used to estimate PBL height from the radio-soundings. We have used pressure, temperature and dew point profiles as well as the wind direction profiles from NOAA (National Oceanic and Atmospheric Administration) data base. The results were consistent with the ones obtained from LIDAR.

A lidar measurements campaign took in Magurele Platform, southwestern part of Bucharest, during on June 25th, 26th and 28th of 2007 and was intended for aerosol loading characteristics over the urban area. An event of long-range Saharan dust transport to Eastern Europe (Romania) observed during this time is presented in here. We have used an elastic backscattering lidar, based on an Nd:YAG laser, at 1064nm sounding wavelength. It can detect in real time aerosols density profiles up to 10 Km high with a spatial resolution of 12 m. Origin of lidar sampled air masses arriving at various heights over Bucharest have been determined by the analytical back-trajectories from NOAA HYSPLIT model. Saharan dust layers reached the southern part of Romania predominantly by cyclonic circulation due to the strong through observed at all the levels from a cyclonic system located in northwestern part of Africa. Analysis of cloud cover and dust load was estimated by the Dust Regional Atmospheric Modeling (Dream model). The dust event presented highlights how the synergy of Lidar data together with 3-D back trajectories analysis and model calculations can improve our ability to determine accurately the source of high aerosol loading.

Daily PM10 concentrations of samples collected at two sites, urban and rural from Romania have been used to estimate the aerosol direct radiative forcing. Using OPAC (Optical Properties of Aerosols and Cloud) model we determined the single scattering albedo, the aerosol optical depth and aerosol up-scatter fraction, aerosol's properties needed to estimate the magnitude and sign of direct aerosol radiative forcing. The surface albedo was assumed 0.2 for the urban site and 0.06 for the rural site for all wavelengths. For aerosol scale height we used 1km in winter and 2 km in the summer to calculate the optical depth of the boundary layer. Statistical analysis of the PM10 concentration for both sites show clear seasonal cycle with maxima in the winter. As a consequence of urban atmospheric pollution the radiative forcing for urban site appears strongly modified in comparison with rural site.

The open-path technique is a widely use method for air pollution monitoring, being capable of detecting trace gases down to ppb concentrations. Using this technique, the DOAS system can measure simultaneously several types of atmospheric molecular compounds having UV absorption spectra, by averaging on a long optical path (hundreds of meters). In this paper, two kinds of comparative measurements by DOAS and in situ detectors (using the point monitoring technique), in high polluted areas from Bucharest and the surroundings, are presented and analyzed. Only point monitoring is a standardized technique and therefore by comparing the two methods, several strengths and weaknesses of the open path technique will be evidentiated. This study is intended to mark out main environmental conditions (e.g. atmospheric fluctuations, topography) responsible for important differences that appear between experimental data acquired with the two techniques.

Today the interest in aerosols is high mainly because of their effect on human health and their role in climate change. They have also a determining effect on visibility and contribute to the soiling of monuments. Observations and model calculations show that the increase in the atmospheric aerosol burden is delaying the global warming expected from the increase in greenhouse gasses (1). The aim of present paper is to analyze the direct effect of the aerosol and to compute the radiative forcing of the aerosol in different sites and meteorological conditions. The calculation of PM 10 levels and radiative forcing must necessarily be based on a description of the emissions of the individual chemical species and how they transform and mix in the atmosphere. We performed a statistical analysis of the emission of the aerosol for one year, amplitude of the concentrations and the temporal variability as is described in Section 1. The optical parameters and the equations used to compute the radiative forcing are presented in Section 2. Discussion of the results and a few concluding remarks are made in Section 3

LIDAR systems have demonstrated their ability to map aerosol variations throughout the atmospheric column and therefore they have has become a central technology in current strategies for tropospheric aerosol research. Its use is complicated, however, by the fact that the lidar signal contains a convolution of two basic optical properties of the aerosol particles: the backscatter coefficient and the extinction coefficient. A quantitative retrieval of either property requires knowledge of their relationship along the laser path which is referred as lidar ratio. If the lidar ratio can not be measured by high spectral resolution lidar, or Raman lidar, then either an assumed value of LR a must be used in the lidar retrieval, leading to very large uncertainties in light extinction, or models can be used for determination of LR a profile. Our research refers to the development of an iterative hybrid regularization technique for elastic backscatter lidar data processing and retrieval of the aerosols optical parameters using the atmospheric model, Mie model and Fernald-Klett, but also Ackermann algorithm for lidar ratio calculation based on relative humidity profile. This study focuses on a numerical investigation about the lidar ratio of tropospheric aerosols characterizing Romanian atmosphere. The model can be also used for other type of atmosphere in order to improve the derivation of aerosols optical parameters from elastic backscatter lidar data when no other information than meteorological data are available.

We correct typographical errors in our article [J. Comput. Phys. 237 (2013) 79–94].