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Inland ships are an important source of NOx , especially for cities along busy waterways. The amount and effect of such emissions depend on the traffic density and NOx emission rates of individual vessels. Ship emission rates are typically derived using in situ land measurements in relation to NOx emission factors (e.g. the number of pollutants emitted by ships per unit of burnt fuel). In this study, a different approach is taken, and NOx emission rates are obtained (in gs-1). Within the EU LIFE project, CLean INland SHipping (CLINSH), a new approach to calculating the NOx emission rates from data of in situ measurement stations has been developed and is presented in this study. Peaks (i.e. elevated concentrations) of NOx were assigned to the corresponding source ships, using the AIS (automated identification system) signals they transmit. Each ship passage was simulated, using a Gaussian puff model, in order to derive the emission rate of the respective source ship. In total, over 32 900 ship passages have been monitored over the course of 4 years. The emission rates of NOx were investigated with respect to ship speed, ship size, and direction of travel. Comparisons of the onshore-derived emission rates and those on board for selected CLINSH ships show good agreement. The derived emission rates are of a similar magnitude to emission factors from previous studies. Most ships comply with existing limits due to grandfathering. The emission rates (in gs-1) can be directly used to investigate the effect of ship traffic on air quality, as the absolute emitted number of pollutants per unit of time is known. In contrast, for relative emission factors (in gkg-1 fuel), further knowledge about the fuel consumption of the individual ships is needed to calculate the number of pollutants emitted per unit of time. [ABSTRACT FROM AUTHOR]

Inland ships are an important source of NOx, especially for cities along busy waterways. The amount and effect of these emissions depends on the traffic density and the NOx emission rates of the individual vessels. Monitoring of ship emissions is usually carried out using in situ instruments on land and often relative NOx emission factors, e.g. the amount of emitted pollutants per amount of burnt fuel is reported, but in this study, NOx emission rates in g s-1 are investigated. Within the EU Life project Clean Inland Shipping (CLINSH), a new approach to calculate NOx emission rates from data of in situ measurement stations has been developed and is presented in this study. Peaks (i.e. elevated concentrations) of NOx were assigned to the corresponding source ships and each ship passage was simulated using a Gaussian-puff-model in order to derive the emission rate. In total over 32900 ship passages have been monitored over the course of 4 years. The emission rates of NOx were investigated with respect to ship speed, ship size and direction of travel. Individual comparisons of the on-shore emission rates and those made on-board of selected CLINSH ships show good agreement. Also the emission rates are of similar magnitude as emission factors from previous studies. In contrast to relative emission factors (in grams per kilogram fuel), the emission rates (in grams per second) do not need further knowledge about the fuel consumption of the ship and can therefore be used directly to investigate the effect of ship traffic on air quality. [ABSTRACT FROM AUTHOR]

Ships are an important source of SO 2 and NO x , which are key parameters of air quality. Monitoring of ship emissions is usually carried out using in situ instruments on land, which depend on favourable wind conditions to transport the emitted substances to the measurement site. Remote sensing techniques such as long-path differential optical absorption spectroscopy (LP-DOAS) measurements can supplement those measurements, especially in unfavourable meteorological conditions. In this study 1 year of LP-DOAS measurements made across the river Elbe close to Hamburg (Germany) have been evaluated. Peaks (i.e. elevated concentrations) in the NO 2 and SO 2 time series were assigned to passing ships, and a method to derive emission rates of SO 2 , NO 2 and NO x from those measurements using a Gaussian plume model is presented. A total of 7402 individual ship passages have been monitored, and their respective NO x , SO 2 and NO 2 emission rates have been derived. The emission rates, coupled with the knowledge of the ship type, ship size and ship speed, have been analysed. Emission rates are compared to emission factors from previous studies and show good agreement. In contrast to emission factors (in grams per kilogram fuel), the derived emission rates (in grams per second) do not need further knowledge about the fuel consumption of the ship. To our knowledge this is the first time emission rates of air pollutants from individual ships have been derived from LP-DOAS measurements. [ABSTRACT FROM AUTHOR]

Ships are an important source of SO2 and NOx, which are key parameters of air quality. Monitoring of ship emissions is usually carried out using in situ instruments on land, which depend on favourable wind conditions to transport the emitted substances to the measurement site. Remote sensing techniques such as long path DOAS (LP-DOAS) measurements can supplement those measurements, especially in unfavourable meteorological conditions. In this study one year of LP-DOAS measurements made across the river Elbe close to Hamburg (Germany) have been evaluated. Peaks (i.e. elevated concentrations) in the NO2 and SO2 time series were assigned to passing ships and a method to derive emission rates of SO2, NO2 and NOx from those measurements using a Gaussian plume model is presented. 7402 individual ship passages have been monitored and their respective NOx, SO2 and NO2 emission rates have been derived. The emission rates, coupled with the knowledge of the ship type, ship size and ship speed have been analysed. Emission rates are compared to emission factors from previous studies and show good agreement. In contrast to emission factors (in gram per kilogram fuel) the derived emission rates (in gram per second) do not need further knowledge about the fuel consumption of the ship. To our knowledge this is the first time emission rates of air pollutants from individual ships have been derived from LP-DOAS measurements. [ABSTRACT FROM AUTHOR]

Measurements from the Global Ozone Monitoring Experiment (GOME) are used to study the chemical evolution of the stratosphere during the unusual 2002 winter in the Southern Hemisphere. The results show that chlorine activation as indicated by OClO columns was similar to previous years in the vortex until the major warming on 26 September 2002 after which it decreased rapidly. Similarly, NO2 columns were only slightly larger than in previous years before the warming, indicating strong denoxification and probably also denitrification. After the warming, very large NO2 columns were observed for a few days, which then decreased again as the vortex reestablished itself until the final warming. Ozone columns were much larger than in any previous year from September onward, mainly as a result of the unusual dynamical situation. Analysis of the global long-term time series of GOME measurements since 1996 provides a unique opportunity to set the austral winter 2002 into perspective. The GOME data reveal the large difference in variability of chlorine activation between the two hemispheres, whereas denoxification shows surprisingly little variation from year to year in both hemispheres. However, NO2 depletion in the Southern Hemisphere is usually sustained for about one month longer in the Antarctic stratosphere as a result of the stable vortex. Compared to the observations in the Northern Hemisphere, the austral winter 2002 was still stable and cold and had a high potential for chemical ozone destruction. [ABSTRACT FROM AUTHOR]

Ship emissions are an important source of trace gases and particles like CO, CO2, NOx (NO + NO2), SO2, VOCs, PM and black carbon. To analyze ship emissions a bunch of different instruments have been set up in Wedel close to the port of Hamburg which is the busiest in Germany and number three in Europe. This activity is part of the project MeSmarT (Measurements of shipping emissions in the troposphere, www.mesmart.de). Three of the instruments used in this study are DOAS-type (Differential Optical Absorption Spectroscopy): two different Open Path-DOAS and one MAX-DOAS. Two commercial in-situ measurement systems, meteorological instruments and an AIS-decoder are complementing the measurement site. One of the Long Path-DOAS systems is a newly developed prototype specifically designed to measure ship emissions and is currently tested in Wedel, while the other one serves as a reference system. The light path of all the DOAS systems cross the Elbe river and thus allows to detect the plumes of passing ships. The prototype system has been set up in October 2018, while the reference Open Path system was set up in April 2018. MAX-DOAS and in-situ systems are already running for several years.This study focuses on the time series of NO2 and SO2. Differences between the instruments are discussed in detail, taking into account meteorological conditions, photochemistry and radiative transfer. SO2/NO2 ratios are investigated focusing on the question, whether this information can be used to monitor the sulfur content of shipping fuel. [ABSTRACT FROM AUTHOR]

This study describes a novel application of an "onion-peeling" approach to multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements of shipping emissions aiming at investigating the strong horizontal inhomogeneities in NO2 over a shipping lane. To monitor ship emissions on the main shipping route towards the port of Hamburg, a two-channel (UV and visible) MAX-DOAS instrument was deployed on the island Neuwerk in the German Bight, 6–7 km south of the main shipping lane. Utilizing the fact that the effective light path length in the atmosphere depends systematically on wavelength, simultaneous measurements and DOAS retrievals in the UV and visible spectral ranges are used to probe air masses at different horizontal distances to the instrument to estimate two-dimensional pollutant distributions. Two case studies have been selected to demonstrate the ability to derive the approximate plume positions in the observed area. A situation with northerly wind shows high NO2 concentrations close to the measurement site and low values in the north of the shipping lane. The opposite situation with southerly wind, unfavorable for the on-site in situ instrumentation, demonstrates the ability to detect enhanced NO2 concentrations several kilometers away from the instrument. Using a Gaussian plume model, in-plume NO2 volume mixing ratios can be derived from the MAX-DOAS measurements. For validation, a comparison to airborne imaging DOAS measurements during the NOSE campaign in July 2013 is performed, showing good agreement between the approximate plume position derived from the onion-peeling MAX-DOAS and the airborne measurements as well as between the derived in-plume NO2 volume mixing ratios (VMRs). [ABSTRACT FROM AUTHOR]

This study describes a novel application of an onion peeling like approach to MAX-DOAS measurements of shipping emissions aiming at investigating the strong horizontal inhomogeneities in N2O over a shipping lane. To monitor ship emissions on the main shipping route towards the port of Hamburg, a two-channel (UV and visible) MAX-DOAS instrument was deployed on the island Neuwerk in the German Bight, 6-7km south of the main shipping lane. Utilizing the fact that the effective light path length in the atmosphere depends systematically on wavelength, simultaneous measurements and DOAS retrievals in the UV and visible spectral range are used to probe air masses at different horizontal distances to the instrument to estimate two-dimensional pollutant distributions. Two case-studies have been selected to demonstrate the ability to derive the approximate plume positions in the observed area. A situation with northerly wind shows high N2O concentrations close to the measurement site and low values in the north of the shipping lane. The opposite situation with southerly wind, unfavorable for the on-site in situ instrumentation, demonstrates the ability to detect enhanced N2O concentrations several kilometers away from the instrument. To validate the approach, a comparison to air-borne imaging DOAS measurements during the NOSE campaign in July 2013 is performed, showing good agreement between the approximate plume position derived from the onion peeling MAX-DOAS and the air-borne measurements. Combining synergistically information about the plume width from the air-borne measurements and about the vertical plume extent from MAX-DOAS, yields N2O concentrations in the plume from both measurements which agree very well. [ABSTRACT FROM AUTHOR]

Airborne imaging differential optical absorption spectroscopy (DOAS), ground-based stationary DOAS, and car DOAS measurements were conducted during the S5P-VAL-DE-Ruhr campaign in September 2020. The campaign area is located in the Rhine-Ruhr region of North Rhine-Westphalia, western Germany, which is a pollution hotspot in Europe comprising urban and large industrial sources. The DOAS measurements are used to validate spaceborne NO 2 tropospheric vertical column density (VCD) data products from the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI). Seven flights were performed with the airborne imaging DOAS instrument for measurements of atmospheric pollution (AirMAP), providing measurements that were used to create continuous maps of NO 2 in the layer below the aircraft. These flights cover many S5P ground pixels within an area of 30 km × 35 km and were accompanied by ground-based stationary measurements and three mobile car DOAS instruments. Stationary measurements were conducted by two Pandora, two Zenith-DOAS, and two MAX-DOAS instruments. Ground-based stationary and car DOAS measurements are used to evaluate the AirMAP tropospheric NO 2 VCDs and show high Pearson correlation coefficients of 0.88 and 0.89 and slopes of 0.90 ± 0.09 and 0.89 ± 0.02 for the stationary and car DOAS, respectively. Having a spatial resolution of about 100 m × 30 m, the AirMAP tropospheric NO 2 VCD data create a link between the ground-based and the TROPOMI measurements with a nadir resolution of 3.5 km × 5.5 km and are therefore well suited to validate the TROPOMI tropospheric NO 2 VCD. The observations on the 7 flight days show strong NO 2 variability, which is dependent on the three target areas, the day of the week, and the meteorological conditions. The AirMAP campaign data set is compared to the TROPOMI NO 2 operational offline (OFFL) V01.03.02 data product, the reprocessed NO 2 data using the V02.03.01 of the official level-2 processor provided by the Product Algorithm Laboratory (PAL), and several scientific TROPOMI NO 2 data products. The AirMAP and TROPOMI OFFL V01.03.02 data are highly correlated (r=0.87) but show an underestimation of the TROPOMI data with a slope of 0.38 ± 0.02 and a median relative difference of - 9 %. With the modifications in the NO 2 retrieval implemented in the PAL V02.03.01 product, the slope and median relative difference increased to 0.83 ± 0.06 and + 20 %. However, the modifications resulted in larger scatter and the correlation decreased significantly to r=0.72. The results can be improved by not applying a cloud correction for the TROPOMI data in conditions with high aerosol load and when cloud pressures are retrieved close to the surface. The influence of spatially more highly resolved a priori NO 2 vertical profiles and surface reflectivity are investigated using scientific TROPOMI tropospheric NO 2 VCD data products. The comparison of the AirMAP campaign data set to the scientific data products shows that the choice of surface reflectivity database has a minor impact on the tropospheric NO 2 VCD retrieval in the campaign region and season. In comparison, the replacement of the a priori NO 2 profile in combination with the improvements in the retrieval of the PAL V02.03.01 product regarding cloud heights can further increase the tropospheric NO 2 VCDs. This study demonstrates that the underestimation of the TROPOMI tropospheric NO 2 VCD product with respect to the validation data set has been and can be further significantly improved. [ABSTRACT FROM AUTHOR]

Airborne imaging differential optical absorption spectroscopy (DOAS), ground-based stationary and car DOAS measurements were conducted during the S5P-VAL-DE-Ruhr campaign in September 2020. The campaign area is located in the Rhine-Ruhr region of North Rhine-Westphalia,Western Germany, which is a pollution hotspot in Europe comprising urban and large industrial emitters. The measurements are used to validate space-borne NO2 tropospheric vertical column density data products from the Sentinel-5 Precursor (S5P) TROPOspheric Monitoring Instrument (TROPOMI). Seven flights were performed with the airborne imaging DOAS instrument formeasurements of atmospheric pollution (AirMAP), providing measurements which were used to create continuous maps of NO2 in the layer below the aircraft. These flights cover many S5P ground pixels within an area of 30 kmx 35 km and were accompanied by ground-based stationary measurements and three mobile car DOAS instruments. Stationary measurements were conducted by two Pandora, two zenith-sky and two MAX-DOAS instruments distributed over three target areas. Ground-based stationary and car DOAS measurements are used to evaluate the AirMAP tropospheric NO2 vertical column densities and show high Pearson correlation coefficients of 0.87 and 0.89 and slopes of 0.93±0.09 and 0.98±0.02 for the stationary and car DOAS, respectively. Having a spatial resolution of about 100mx 30m, the AirMAP tropospheric NO2 vertical column density (VCD) data creates a link between the ground-based and the TROPOMI measurements with a resolution of 3.5 kmx 5.5 km and is therefore well suited to validate the TROPOMI tropospheric NO2 VCD. The measurements on the seven flight days show strong NO2 variability, which is dependent on the different target areas, the weekday, and the meteorological conditions. The AirMAP campaign dataset is compared to the TROPOMI NO2 operational off-line (OFFL) V01.03.02 data product, the reprocessed NO2 data, using the V02.03.01 of the official L2 processor, provided by the Product Algorithm Laboratory (PAL), and several scientific TROPOMI NO2 data products. The TROPOMI data products and the AirMAP data are highly correlated with correlation coefficients between 0.72 and 0.87, and slopes of 0.38±0.02 to 1.02±0.07. On average, TROPOMI tropospheric NO2 VCDs are lower than the AirMAP NO2 results. The slope increased from 0.38±0.02 for the operational OFFL V01.03.02 product to 0.83±0.06 after the improvements in the retrieval of the PAL V02.03.01 product were implemented. Different auxiliary data, such as spatially higher resolved a priori NO2 vertical profiles, surface reflectivity and the cloud treatment, are investigated using scientific TROPOMI tropospheric NO2 VCD data products to evaluate their impact on the operational TROPOMI NO2 VCD data product. The comparison of the AirMAP campaign dataset to the scientific data products shows that the choice of surface reflectivity data base has a minor impact on the tropospheric NO2 VCD retrieval in the campaign region and season. In comparison, the replacement of the a priori NO2 profile in combination with the improvements in the retrieval of the PAL V02.03.01 product regarding cloud heights has a major impact on the tropospheric NO2 VCD retrieval and increases the slope from 0.88±0.06 to 1.00±0.07. This study demonstrates that the underestimation of the TROPOMI tropospheric NO2 VCD product with respect to the validation dataset has been and can be further significantly improved. [ABSTRACT FROM AUTHOR]

This paper reports on ground-based validation of the atmospheric OClO data record produced in the framework of EUMETSAT's Satellite Application Facility on Atmospheric Chemistry Monitoring (AC SAF) using the GOME2-A and -B instruments over the 2007-2016 and 2013-2016 periods, respectively. OClO slant column densities are compared to correlative measurements collected from 9 NDACC Zenith-Scattered-Light DOAS (ZSL-DOAS) instruments distributed in both the Arctic and Antarctic. Sensitivity tests are performed on the ground-based data to estimate the impact of the different OClO DOAS analysis settings. On this basis, we infer systematic uncertainties of about 25 % between the different ground-based data analysis, reaching total uncertainties ranging from about 26 % to 33 % for the different stations. Time-series at the different sites show good agreement between satellite and ground-based data, both for the inter-annual variability and the overall OClO seasonal behavior. GOME-2A results are found to be nosier than those of GOME-2B, especially after 2011, probably due to instrumental degradation effects. Daily linear regression analysis for OClO activated periods yield correlation coefficients of 0.8 for GOME-2A and 0.87 for GOME-2B, with slopes of 0.64 and 0.72, respectively. Biases are within 8 x 1013 molec/cm2 with some differences between GOME-2A and GOME- 2B, depending on the station. Overall, considering all the stations, a median bias of about -2.2 x 1013 molec/cm2 is found for both GOME-2 instruments. [ABSTRACT FROM AUTHOR]

A novel imaging-DOAS (differential optical absorption spectroscopy) instrument IMPACT (Imaging MaPper for AtmospheriC observaTions) is presented combining full-azimuthal pointing (360 ∘) with a large vertical coverage (∼41 ∘). Complete panoramic scans are acquired at a temporal resolution of ∼15 min, enabling the retrieval of NO2 vertical profiles over the entire panorama around the measurement site. IMPACT showed excellent agreement (correlation >99 %) with coincident multiaxis DOAS (MAX-DOAS) measurements during the Second Cabauw Intercomparison of Nitrogen Dioxide measuring Instruments (CINDI-2) campaign. The temporal variability of NO2 slant columns within a typical MAX-DOAS vertical scanning sequence could be resolved and was as large as 20 % in a case study under good viewing conditions. The variation of corresponding profiles and surface concentrations was even larger (40 %). This variability is missed when retrieving trace gas profiles based on standard MAX-DOAS measurements. The azimuthal distribution of NO2 around the measurement site showed inhomogeneities (relative differences) up to 120 % (on average 35 %) on short timescales (individual panoramic scans). This is more than expected for the semirural location. We explain this behavior by the transport of pollution. Exploiting the instrument's advantages, the plume's trajectory during a prominent transport event could be reconstructed. Finally, the potential for retrieving information about the aerosol phase function from O4 slant columns along multiple almucantar scans of IMPACT is demonstrated, with promising results for future studies. [ABSTRACT FROM AUTHOR]

A novel imaging-DOAS instrument (IMPACT) is presented combining full-azimuthal pointing (360°) with a large vertical coverage (~40°). Complete panoramic scans are acquired at a temporal resolution of ~15 minutes enabling the retrieval of NO2 vertical profiles over the entire panorama around the measurement site. IMPACT showed excellent agreement (correlation >99%) with coinciding MAX-DOAS measurements during the CINDI-2 campaign. The temporal variability of NO2 slant columns within a typical MAX-DOAS vertical scanning sequence could be resolved and was as large as 20% in a case study under good viewing conditions. The variation of corresponding profiles and surface concentrations were even larger (40%). This variability is missed when retrieving trace gas profiles based on standard MAX-DOAS measurements. The azimuthal distribution of NO2 around the measurement site showed inhomogeneities (relative differences) up to 120% (on average 35%) on short time scales (individual panoramic scans). This is more than expected taking into account the semi-rural location. One reason for this are transport events. Exploiting the instrument's advantages, the plume's trajectory during a prominent transport event could be reconstructed. Furthermore, the potential of retrieving information about the aerosol phase function from O4 slant columns along multiple almucantar scans of IMPACT is demonstrated, with promising results for future studies. [ABSTRACT FROM AUTHOR]

We present a new MAX-DOAS profiling algorithm for aerosols and trace gases, BOREAS, which utilizes an iterative solution method including Tikhonov regularization and the optimal estimation technique. The aerosol profile retrieval is based on a novel approach in which the absorption depth of O4 is directly used in order to retrieve extinction coefficient profiles instead of the commonly used perturbation theory method. The retrieval of trace gases is done with the frequently used optimal estimation method but significant improvements are presented on how to deal with wrongly weighted a priori constraints and for scenarios in which the a priori profile is inaccurate. Performance tests are separated into two parts. First, we address the general sensitivity of the retrieval to the example of synthetic data calculated with the radiative transfer model SCIATRAN. In the second part of the study, we demonstrate BOREAS profiling accuracy by validating the results with the help of ancillary measurements carried out during the CINDI-2 campaign in Cabauw, the Netherlands, in 2016. The synthetic sensitivity tests indicate that the regularization between measurement and a priori constraints is insufficient when knowledge of the true state of the atmosphere is poor. We demonstrate a priori pre-scaling and extensive regularization tests as a tool for the optimization of retrieved profiles. The comparison of retrieval results with in situ, ceilometer, NO2 lidar, sonde and long-path DOAS measurements during the CINDI-2 campaign always shows high correlations with coefficients greater than 0.75. The largest differences can be found in the morning hours, when the planetary boundary layer is not yet fully developed and the concentration of trace gases and aerosol, as a result of a low night-time boundary layer having formed, is focused in a shallow, near-surface layer. [ABSTRACT FROM AUTHOR]

We present a new MAX-DOAS profiling algorithm for aerosols and trace gases, BOREAS, which utilizes an iterative solution method including Tikhonov regularization and the optimal estimation technique. Performance tests are separated into two parts. First, we address the general sensitivity of the retrieval on the example of synthetic data calculated with the radiative transfer model SCIATRAN. In a second part of the study, we demonstrate BOREAS profiling accuracy by validating results with the help of ancillary measurements carried out during the CINDI-2 campaign in Cabauw, the Netherlands in 2016. The synthetic sensitivity tests indicate, that the regularization between measurement and a priori constraints is insufficient when knowledge of the true state of the atmosphere is poor. We demonstrate a priori pre-scaling and extensive regularization tests as a tool for the optimization of retrieved profiles. The comparison of retrieval results with in situ, ceilometer, NO2 LIDAR, sonde and long path DOAS measurements during the CINDI-2 campaign always shows high correlations with coefficients greater than 0.79. The largest differences can be found in the morning hours, when the planetary boundary layer is not yet fully developed and the concentration of trace gases and aerosol, as a result of a low night-time boundary layer having formed, is focused in a shallow, surface near layer. [ABSTRACT FROM AUTHOR]

The differential optical absorption spectroscopy (DOAS) method is a well-known remote sensing technique that is nowadays widely used for measurements of atmospheric trace gases, creating the need for harmonization and characterization efforts. In this study, an intercomparison exercise of DOAS retrieval codes from 17 international groups is presented, focusing on NO2 slant columns. The study is based on data collected by one instrument during the Multi-Axis DOAS Comparison campaign for Aerosols and Trace gases (MAD-CAT) in Mainz, Germany, in summer 2013. As data from the same instrument are used by all groups, the results are free of biases due to instrumental differences, which is in contrast to previous intercomparison exercises. While in general an excellent correlation of NO2 slant columns between groups of >99.98% (noon reference fits) and >99.2% (sequential reference fits) for all elevation angles is found, differences between individual retrievals are as large as 8% for NO2 slant columns and 100% for rms residuals in small elevation angles above the horizon. Comprehensive sensitivity studies revealed that absolute slant column differences result predominantly from the choice of the reference spectrum while relative differences originate from the numerical approach for solving the DOAS equation as well as the treatment of the slit function. Furthermore, differences in the implementation of the intensity offset correction were found to produce disagreements for measurements close to sunrise (8-10% for NO2, 80% for rms residual). The largest effect of ≈ 8% difference in NO2 was found to arise from the reference treatment; in particular for fits using a sequential reference. In terms of rms fit residual, the reference treatment has only a minor impact. In contrast, the wavelength calibration as well as the intensity offset correction were found to have the largest impact (up to 80 %) on rms residual while having only a minor impact on retrieved NO2 slant columns. [ABSTRACT FROM AUTHOR]

The Differential Optical Absorption Spectroscopy (DOAS) method is a well-known remote sensing technique that is nowadays widely used for measurements of atmospheric trace gases, creating the need for harmonization and characterization efforts. In this study, an intercomparison exercise of DOAS retrieval codes from 17 international groups is presented focusing on NO2 slant columns. The study is based on data collected by one instrument during the Multi-Axis DOAS Comparison campaign for Aerosols and Trace gases (MAD-CAT) in Mainz, Germany, in summer 2013. As data from the same instrument is used by all groups, the results are free of biases due to instrumental differences, which is in contrast to previous intercomparison exercises. While in general an excellent correlation of NO2 slant columns between groups of > 99.98% (noon reference fits), and > 99.2% (sequential reference fits) for all elevation angles is found, differences between individual retrievals are as large as 8% for NO2 slant columns and 100% for RMS residuals. Two kinds of disagreements were identified: (1) Absolute slant column differences were found to result predominantly from the choice of the reference spectrum. (2) Relative differences were found to originate from the numerical approach for solving the DOAS equation as well as the treatment of the slit function. Differences in the implementations of the intensity offset correction lead to disagreements for retrievals close to sunrise (8-10% for NO2, 80% for RMS residual). Apart from this, the largest effect of ≈ 8% difference in NO2 was found to arise from the reference treatment, in particular for fits using a sequential reference. In terms of RMS fit residual, the reference treatment has only a minor impact. In contrast, the wavelength calibration as well as the intensity offset correction were found to have the largest impact (up to 80%) on RMS residual while having only a minor impact on retrieved NO2 slant columns. [ABSTRACT FROM AUTHOR]

Slant column (SC) densities of nitrogen dioxide (NO 2 ), formaldehyde (HCHO), glyoxal (CHOCHO) and oxygen dimer (O 4 ) were successfully retrieved for the first time in Athens, by using spectral measurements from a ground-based multi-azimuth Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) system. The data span the period from October 2012 to March 2014 and measurements were conducted at NOA's (National Observatory of Athens) station in Penteli (38.0°N, 23.9°E, 527 m a.s.l.) at eight azimuth angles and eight off-axis elevation angles. The SC NO2 , SC HCHO and SC CHOCHO measurements at +1 ο elevation angle, pointing towards the urban area, range from 0.6 to 24·10 16 , 0.8–9.6·10 16 and 0.3–5.2·10 15 molec cm −2 (mean daily values throughout the whole period), respectively. Seasonal modulation characterised by summertime maximum and wintertime minimum was observed for HCHO and CHOCHO, while for NO 2 the maximum values were recorded during winter. Changes in the diurnal variability of all trace gases with season and day of the week are investigated suggesting a strong link to primary anthropogenic sources for NO 2 and a weaker one, compared to photochemistry, for HCHO and CHOCHO. In addition, the impact of the reduced anthropogenic emissions during weekends on the measured SC values was quantified and 30%–50% lower SC NO2 values were found during weekends. The contribution of local urban emissions to the overall recorded amounts of the selected species was assessed. Using meteorological data from NOA's station in Penteli, the impact of the local circulation patterns on the SC levels was estimated, and a strong relation between western wind direction, which is related to the industrial area, and enhanced SC measurements was found. [ABSTRACT FROM AUTHOR]

This paper reports on ground-based validation of the atmospheric OClO data record produced within the framework of EUMETSAT's Satellite Application Facility on Atmospheric Chemistry Monitoring (AC SAF) using the Global Ozone Monitoring Experiment (GOME)-2A and GOME-2B instrument measurements, covering the 2007–2016 and 2013–2016 periods, respectively. OClO slant column densities are compared to correlative measurements collected from nine Zenith-Scattered-Light Differential Optical Absorption Spectroscopy (ZSL-DOAS) instruments from the Network for the Detection of Atmospheric Composition Change (NDACC) distributed in both the Arctic and Antarctic. Sensitivity tests are performed on the ground-based data to estimate the impact of the different OClO DOAS analysis settings. On this basis, we infer systematic uncertainties of about 25 % (i.e., about 3.75×1013 molec. cm -2) between the different ground-based data analyses, reaching total uncertainties ranging from about 26 % to 33 % for the different stations (i.e., around 4 to 5×1013 molec. cm -2). Time series at the different sites show good agreement between satellite and ground-based data for both the inter-annual variability and the overall OClO seasonal behavior. GOME-2A results are found to be noisier than those of GOME-2B, especially after 2011, probably due to instrumental degradation effects. Daily linear regression analysis for OClO-activated periods yield correlation coefficients of 0.8 for GOME-2A and 0.87 for GOME-2B, with slopes with respect to the ground-based data ensemble of 0.64 and 0.72, respectively. Satellite minus ground-based offsets are within 8×1013 molec. cm -2 , with some differences between GOME-2A and GOME-2B depending on the station. Overall, considering all the stations, a median offset of about -2.2×1013 molec. cm -2 is found for both GOME-2 instruments. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides a comprehensive overview of the actual knowledge on the atmospheric pollution sources, transport, transformation and levels in the East Mediterranean. It focuses both on the background atmosphere and on the similarities and differences between the urban areas that exhibited important urbanization the past years: the two megacities Istanbul, Cairo and the Athens extended area. Ground-based observations are combined with satellite data and atmospheric modeling. The overall evaluation pointed out that long and regional range transport of natural and anthropogenic pollution sources have about similar importance with local sources for the background air pollution levels in the area. [Copyright &y& Elsevier]

A field campaign was conducted in Ny-Ǻlesund (78°54''N, 11°53''E), Svalbard (Norway) during April and May 2005. An Atmospheric Mercury (Hg) Depletion Event (AMDE) was observed from the morning of April 24 until the evening of April 27. Transport of already Hg and ozone (O3) depleted air masses could explain this observed depletion. Due to a snowfall event during the AMDE, surface snow Hg concentrations increased two fold. Hg deposition took place over a short period of time corresponding to 3–4 days. More than 80% of the deposited Hg was estimated to be reemitted back to the atmosphere in the days following the event. During the campaign, we observed night and day variations in surface snow Hg concentrations, which may be the result of gaseous elemental mercury (GEM) oxidation to divalent Hg at the snow/air interface by daylight surface snow chemistry. Finally, a decrease in the reactive Hg (HgR) fraction of total Hg (HgT) in the surface snow was observed during spring. We postulate that the transformation of HgR to a more stable form may occur in Arctic snow during spring. [Copyright &y& Elsevier]