Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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European NOx emissions in WRF-Chem derived from OMI: Impacts on summertime surface ozone
Visser, Auke J. ; Folkert Boersma, K. ; Ganzeveld, Laurens N. ; Krol, Maarten C. - \ 2019
Atmospheric Chemistry and Physics 19 (2019)18. - ISSN 1680-7316 - p. 11821 - 11841.

Ozone (O3) is a secondary air pollutant that negatively affects human and ecosystem health. Ozone simulations with regional air quality models suffer from unexplained biases over Europe, and uncertainties in the emissions of ozone precursor group nitrogen oxides (NOx= NO+NO) contribute to these biases. The goal of this study is to use NO2 column observations from the Ozone Monitoring Instrument (OMI) satellite sensor to infer top-down NOx emissions in the regional Weather Research and Forecasting model with coupled chemistry (WRF-Chem) and to evaluate the impact on simulated surface O3 with in situ observations. We first perform a simulation for July 2015 over Europe and evaluate its performance against in situ observations from the AirBase network. The spatial distribution of mean ozone concentrations is reproduced satisfactorily. However, the simulated maximum daily 8h ozone concentration (MDA8 O3) is underestimated (mean bias error of -14.2μgm-3), and its spread is too low. We subsequently derive satellite-constrained surface NOx emissions using a mass balance approach based on the relative difference between OMI and WRF-Chem NO2 columns. The method accounts for feedbacks through OH, NO2's dominant daytime oxidant. Our optimized European NOx emissions amount to 0.50TgN (for July 2015), which is 0.18TgN higher than the bottom-up emissions (which lacked agricultural soil NOx emissions). Much of the increases occur across Europe, in regions where agricultural soil NOx emissions dominate. Our best estimate of soil NOx emissions in July 2015 is 0.1TgN, much higher than the bottom-up 0.02TgN natural soil NOx emissions from the Model of Emissions of Gases and Aerosols from Nature (MEGAN). A simulation with satellite-updated NOx emissions reduces the systematic bias between WRF-Chem and OMI NO2 (slopeCombining double low line0.98, r2Combining double low line0.84) and reduces the low bias against independent surface NO2 measurements by 1.1μgm-3 (-56%). Following these NOx emission changes, daytime ozone is strongly affected, since NOx emission changes particularly affect daytime ozone formation. Monthly averaged simulated daytime ozone increases by 6.0μgm-3, and increases of >10μgm-3 are seen in regions with large emission increases. With respect to the initial simulation, MDA8 O3 has an improved spatial distribution, expressed by an increase in r2 from 0.40 to 0.53, and a decrease of the mean bias by 7.4μgm-3 (48%). Overall, our results highlight the dependence of surface ozone on its precursor NOx and demonstrate that simulations of surface ozone benefit from constraining surface NOx emissions by satellite NO2 column observations.

Space watchdog for Earth’s atmosphere
Boersma, K.F. - \ 2019
Towards a traceable climate service: Assessment of quality and usability of essential climate variables
Zeng, Yijian ; Su, Zhongbo ; Barmpadimos, Iakovos ; Perrels, Adriaan ; Poli, Paul ; Boersma, K.F. ; Frey, Anna ; Ma, Xiaogang ; Bruin, Karianne de; Goosen, Hasse ; John, Viju O. ; Roebeling, Rob ; Schulz, Jörg ; Timmermans, Wim - \ 2019
Remote Sensing 11 (2019)10. - ISSN 2072-4292
Climate data record (CDRs) - Climate services - Essential climate variables (ECVs) - Quality assurance - Traceability - Usability assessment

Climate services are becoming the backbone to translate climate knowledge, data & information into climate-informed decision-making at all levels, from public administrations to business operators. It is essential to assess the technical and scientific quality of the provided climate data and information products, including their value to users, to establish the relation of trust between providers of climate data and information and various downstream users. The climate data and information products (i.e., from satellite, in-situ and reanalysis) shall be fully traceable, adequately documented and uncertainty quantified and can provide sufficient guidance for users to address their specific needs and feedbacks. This paper discusses details on how to apply the quality assurance framework to deliver timely assessments of the quality and usability of Essential Climate Variable (ECV) products. It identifies an overarching structure for the quality assessment of single product ECVs (i.e., consists of only one single variable), multi-product ECVs (i.e., more than one single parameter), thematic products (i.e., water, energy and carbon cycles), as well as the usability assessment. To support a traceable climate service, other than rigorously evaluating the technical and scientific quality of ECV products, which represent the upstream of climate services, how the uncertainty propagates into the resulting benefit (utility) for the users of the climate service needs to be detailed.

Trends and trend reversal detection in 2 decades of tropospheric NO2 satellite observations
Georgoulias, Aristeidis K. ; Der, Ronald A.J. Van; Stammes, Piet ; Folkert Boersma, K. ; Eskes, Henk J. - \ 2019
Atmospheric Chemistry and Physics 19 (2019)9. - ISSN 1680-7316 - p. 6269 - 6294.

In this work, a 21-year global dataset from four different satellite sensors with a mid-morning overpass (GOME/ERS-2, SCIAMACHY/ENVISAT, GOME- 2/Metop-A, and GOME-2/Metop-B) is compiled to study the long-term tropospheric NO2 patterns and trends. The Global Ozone Monitoring Experiment (GOME) and GOME-2 data are corrected relative to the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) data to produce a self-consistent dataset that covers the period April 1996September 2017. The highest tropospheric NO2 concentrations are seen over urban, industrialized, and highly populated areas and over ship tracks in the oceans. Tropospheric NO2 has generally decreased during the last 2 decades over the industrialized and highly populated regions of the western world (a total decrease of the order of 49% over the US, the Netherlands, and the UK; 36% over Italy and Japan; and 32% over Germany and France) and increased over developing regions (a total increase of 160% over China and 33% over India). It is suggested here that linear trends cannot be used efficiently worldwide for such long periods. Tropospheric NO2 is very sensitive to socioeconomic changes (e.g., environmental protection policies, economic recession, warfare, etc.) which may cause either short-term changes or even a reversal of the trends. The application of a method capable of detecting the year when a reversal of trends happened shows that tropospheric NO2 concentrations switched from positive to negative trends and vice versa over several regions around the globe. A country-level analysis revealed clusters of countries that exhibit similar positive-to-negative or negative-topositive trend reversals, while 29 out of a total of 64 examined megacities and large urban agglomerations experienced a trend reversal at some point within the last 2 decades.

From photon paths to pollution plumes: better radiative transfer calculations to monitor NOx emissions with OMI and TROPOMI
Lorente Delgado, Alba - \ 2019
Wageningen University. Promotor(en): M.C. Krol, co-promotor(en): K.F. Boersma. - Wageningen : Wageningen University - ISBN 9789463439329 - 195
Adverse results of the economic crisis : A study on the emergence of enhanced formaldehyde (HCHO) levels seen from satellites over Greek urban sites
Zyrichidou, I. ; Balis, D. ; Koukouli, M.E. ; Drosoglou, T. ; Bais, A. ; Gratsea, M. ; Gerasopoulos, E. ; Liora, N. ; Poupkou, A. ; Giannaros, C. ; Melas, D. ; Smedt, I. De; Roozendael, M. Van; A, R.J. van der; Boersma, K.F. ; Valks, P. ; Richter, A. - \ 2019
Atmospheric Research 224 (2019). - ISSN 0169-8095 - p. 42 - 51.
Biomass burning - GOME-2 instrument - HCHO - NO - Urban air pollution

In order to study the years of the outbreak of the financial crisis in Greece, we use a continuous eight-year record (2008–2015) of formaldehyde, HCHO, columns retrieved from the GOME-2/MetOp-A and GOME-2/MetOp-B satellite instruments over two urban Greek regions, Thessaloniki and Athens. A statistical linear regression analysis that was applied to the GOME-2/MetOp-A HCHO time series over both cities revealed positive annual changes. On seasonal basis, the wintertime HCHO change per annum was found to be 7.43 ± 2.26% and 6.13 ± 2.12% over Thessaloniki and Athens, respectively. A corresponding seasonal time series analysis was also applied to tropospheric nitrogen dioxide, NO 2 , levels. The tropospheric NO 2 winter change per annum is shown to be negative, at −3.0 ± 0.13% and −3.94 ± 0.14% over Thessaloniki and Athens, respectively. The satellite HCHO and NO 2 observations levels are comparable to collocated ground-based MAX-DOAS measurements. Furthermore, the Comprehensive Air Quality Model with extensions chemical transport model, CAMx CTM, as well as the driving emission inventory in CAMx are used for further investigation of the seasonality of the HCHO concentrations and emissions. The CTM model analysis of the annual HCHO cycle and the winter season anti-correlation between HCHO total columns and surface temperature, mostly over Thessaloniki (R = –0.3), point to the fact that the winter HCHO enhancements, likely connected with enhanced anthropogenic activities, are not being captured by the model. The results of this study indicate a possible enhanced anthropogenic activity during the winter season that was strengthened through the investigation on the origins of the wintertime increase in HCHO columns since the beginning of the economic crisis in the country.

On the Legal Categorisation of New Plant Breeding Technologies: Insights from Communication Science and Ways Forward
Poortvliet, P.M. ; Purnhagen, K. ; Boersma, R.J. ; Gremmen, H.G.J. - \ 2019
European Journal of Risk Regulation 10 (2019)1. - ISSN 1867-299X - p. 180 - 186.
In July 2018 the Court of Justice of the European Union (CJEU) ruled that organisms obtained from most New Plant Breeding Technologies (NPBT) fulfil the requirements of the GMO definition of Directive 2001/18. Practically, organisms created with NPBT have since been legally treated as GMOs. While we do not seek to contest the judgment in itself, in the present contribution we draw attention to the effects of such a categorisation from the perspective of communication science. Extrapolating from communication research conducted in adjacent technology domains, we will argue that by putting organisms obtained from NPBT semantically in the same basket as GMOs may carry a serious risk – transferring analogous communication problems that GMOs encountered in the past, to organisms obtained from NPBT, while they may not address similar risks. Possible consequences such as these can hardly be considered at the stage of legal interpretation (such as with the CJEU). Rather, as discussion now unfolds whether and how to change the legal definition, insights from communication science and risk perception research on the effect of such a definition
should be taken into account.
Balance of Emission and Dynamical Controls on Ozone During the Korea-United States Air Quality Campaign From Multiconstituent Satellite Data Assimilation
Miyazaki, K. ; Sekiya, T. ; Fu, D. ; Bowman, K.W. ; Kulawik, S.S. ; Sudo, K. ; Walker, T. ; Kanaya, Y. ; Takigawa, M. ; Ogochi, K. ; Eskes, H. ; Boersma, K.F. ; Thompson, A.M. ; Gaubert, B. ; Barre, J. ; Emmons, L.K. - \ 2019
Journal of Geophysical Research: Atmospheres 124 (2019)1. - ISSN 2169-897X - p. 387 - 413.
air quality - Asia - data assimilation - emission - ozone - satellite

Global multiconstituent concentration and emission fields obtained from the assimilation of the satellite retrievals of ozone, CO, NO2, HNO3, and SO2 from the Ozone Monitoring Instrument (OMI), Global Ozone Monitoring Experiment 2, Measurements of Pollution in the Troposphere, Microwave Limb Sounder, and Atmospheric Infrared Sounder (AIRS)/OMI are used to understand the processes controlling air pollution during the Korea-United States Air Quality (KORUS-AQ) campaign. Estimated emissions in South Korea were 0.42 Tg N for NOx and 1.1 Tg CO for CO, which were 40% and 83% higher, respectively, than the a priori bottom-up inventories, and increased mean ozone concentration by up to 7.5 ± 1.6 ppbv. The observed boundary layer ozone exceeded 90 ppbv over Seoul under stagnant phases, whereas it was approximately 60 ppbv during dynamical conditions given equivalent emissions. Chemical reanalysis showed that mean ozone concentration was persistently higher over Seoul (75.10 ± 7.6 ppbv) than the broader KORUS-AQ domain (70.5 ± 9.2 ppbv) at 700 hPa. Large bias reductions (>75%) in the free tropospheric OH show that multiple-species assimilation is critical for balanced tropospheric chemistry analysis and emissions. The assimilation performance was dependent on the particular phase. While the evaluation of data assimilation fields shows an improved agreement with aircraft measurements in ozone (to less than 5 ppbv biases), CO, NO2, SO2, PAN, and OH profiles, lower tropospheric ozone analysis error was largest at stagnant conditions, whereas the model errors were mostly removed by data assimilation under dynamic weather conditions. Assimilation of new AIRS/OMI ozone profiles allowed for additional error reductions, especially under dynamic weather conditions. Our results show the important balance of dynamics and emissions both on pollution and the chemical assimilation system performance.

High-Resolution Mapping of Nitrogen Dioxide With TROPOMI : First Results and Validation Over the Canadian Oil Sands
Griffin, Debora ; Zhao, Xiaoyi ; McLinden, Chris A. ; Boersma, Folkert ; Bourassa, Adam ; Dammers, Enrico ; Degenstein, Doug ; Eskes, Henk ; Fehr, Lukas ; Fioletov, Vitali ; Hayden, Katherine ; Kharol, Shailesh K. ; Li, Shao Meng ; Makar, Paul ; Martin, Randall V. ; Mihele, Cristian ; Mittermeier, Richard L. ; Krotkov, Nickolay ; Sneep, Maarten ; Lamsal, Lok N. ; Linden, Mark ter; Geffen, Jos van; Veefkind, Pepijn ; Wolde, Mengistu - \ 2019
Geophysical Research Letters 46 (2019)2. - ISSN 0094-8276 - p. 1049 - 1060.
nitrogen dioxide - OMI - TROPOMI

TROPOspheric Monitoring Instrument (TROPOMI), on-board the Sentinel-5 Precurser satellite, is a nadir-viewing spectrometer measuring reflected sunlight in the ultraviolet, visible, near-infrared, and shortwave infrared. From these spectra several important air quality and climate-related atmospheric constituents are retrieved, including nitrogen dioxide (NO2) at unprecedented spatial resolution from a satellite platform. We present the first retrievals of TROPOMI NO2 over the Canadian Oil Sands, contrasting them with observations from the Ozone Monitoring Instrument satellite instrument, and demonstrate TROPOMI's ability to resolve individual plumes and highlight its potential for deriving emissions from individual mining facilities. Further, the first TROPOMI NO2 validation is presented, consisting of aircraft and surface in situ NO2 observations, and ground-based remote-sensing measurements between March and May 2018. Our comparisons show that the TROPOMI NO2 vertical column densities are highly correlated with the aircraft and surface in situ NO2 observations, and the ground-based remote-sensing measurements with a low bias (15–30 %); this bias can be reduced by improved air mass factors.

Improved aerosol correction for OMI tropospheric NO2 retrieval over East Asia : Constraint from CALIOP aerosol vertical profile
Liu, Mengyao ; Lin, Jintai ; Folkert Boersma, K. ; Pinardi, Gaia ; Wang, Yang ; Chimot, Julien ; Wagner, Thomas ; Xie, Pinhua ; Eskes, Henk ; Roozendael, Michel Van; Hendrick, François ; Wang, Pucai ; Wang, Ting ; Yan, Yingying ; Chen, Lulu ; Ni, Ruijing - \ 2019
Atmospheric Measurement Techniques 12 (2019)1. - ISSN 1867-1381 - p. 1 - 21.

Satellite retrieval of vertical column densities (VCDs) of tropospheric nitrogen dioxide (NO2) is critical for NOx pollution and impact evaluation. For regions with high aerosol loadings, the retrieval accuracy is greatly affected by whether aerosol optical effects are treated implicitly (as additional effective clouds) or explicitly, among other factors. Our previous POMINO algorithm explicitly accounts for aerosol effects to improve the retrieval, especially in polluted situations over China, by using aerosol information from GEOS-Chem simulations with further monthly constraints by MODIS/Aqua aerosol optical depth (AOD) data. Here we present a major algorithm update, POMINO v1.1, by constructing a monthly climatological dataset of aerosol extinction profiles, based on level 2 CALIOP/CALIPSO data over 2007-2015, to better constrain the modeled aerosol vertical profiles. We find that GEOS-Chem captures the month-to-month variation in CALIOP aerosol layer height (ALH) but with a systematic underestimate by about 300-600 m (season and location dependent), due to a too strong negative vertical gradient of extinction above 1 km. Correcting the model aerosol extinction profiles results in small changes in retrieved cloud fraction, increases in cloud-top pressure (within 2 %-6 % in most cases), and increases in tropospheric NO2 VCD by 4 %-16 % over China on a monthly basis in 2012. The improved NO2 VCDs (in POMINO v1.1) are more consistent with independent ground-based MAX-DOAS observations (R2=0.80, NMB =-3.4 %, for 162 pixels in 49 days) than POMINO (R2=0.80, NMB =-9.6 %), DOMINO v2 (R2=0.68, NMB =-2.1 %), and QA4ECV (R2=0.75, NMB =-22.0 %) are. Especially on haze days, R2 reaches 0.76 for POMINO v1.1, much higher than that for POMINO (0.68), DOMINO v2 (0.38), and QA4ECV (0.34). Furthermore, the increase in cloud pressure likely reveals a more realistic vertical relationship between cloud and aerosol layers, with aerosols situated above the clouds in certain months span id=page2 instead of always below the clouds. The POMINO v1.1 algorithm is a core step towards our next public release of the data product (POMINO v2), and it will also be applied to the recently launched S5P-TROPOMI sensor.

Improving algorithms and uncertainty estimates for satellite NO2 retrievals : Results from the quality assurance for the essential climate variables (QA4ECV) project
Boersma, K.F. ; Eskes, Henk J. ; Richter, Andreas ; Smedt, Isabelle De; Lorente, Alba ; Beirle, Steffen ; Geffen, Jos H.G.M. van; Zara, Marina ; Peters, Enno ; Roozendael, Michel Van; Wagner, Thomas ; Maasakkers, Joannes D. ; A, Ronald J. van der; Nightingale, Joanne ; Rudder, Anne De; Irie, Hitoshi ; Pinardi, Gaia ; Lambert, Jean Christopher ; Compernolle, Steven C. - \ 2018
Atmospheric Measurement Techniques 11 (2018)12. - ISSN 1867-1381 - p. 6651 - 6678.

Global observations of tropospheric nitrogen dioxide (NO2) columns have been shown to be feasible from space, but consistent multi-sensor records do not yet exist, nor are they covered by planned activities at the international level. Harmonised, multi-decadal records of NO2 columns and their associated uncertainties can provide crucial information on how the emissions and concentrations of nitrogen oxides evolve over time. Here we describe the development of a new, community best-practice NO2 retrieval algorithm based on a synthesis of existing approaches. Detailed comparisons of these approaches led us to implement an enhanced spectral fitting method for NO2, a 1°  ×  1° TM5-MP data assimilation scheme to estimate the stratospheric background and improve air mass factor calculations. Guided by the needs expressed by data users, producers, and WMO GCOS guidelines, we incorporated detailed per-pixel uncertainty information in the data product, along with easily traceable information on the relevant quality aspects of the retrieval. We applied the improved QA4ECV NO2 algorithm to the most current level-1 data sets to produce a complete 22-year data record that includes GOME (1995–2003), SCIAMACHY (2002–2012), GOME-2(A) (2007 onwards) and OMI (2004 onwards). The QA4ECV NO2 spectral fitting recommendations and TM5-MP stratospheric column and air mass factor approach are currently also applied to S5P-TROPOMI. The uncertainties in the QA4ECV tropospheric NO2 columns amount to typically 40 % over polluted scenes. The first validation results of the QA4ECV OMI NO2 columns and their uncertainties over Tai'an, China, in June 2006 suggest a small bias (−2 %) and better precision than suggested by uncertainty propagation. We conclude that our improved QA4ECV NO2 long-term data record is providing valuable information to quantitatively constrain emissions, deposition, and trends in nitrogen oxides on a global scale.

The elephant in the room: How a technology’s name affects its interpretation
Boersma, Reginald ; Poortvliet, P.M. ; Gremmen, Bart - \ 2018
Public Understanding of Science 28 (2018)2. - ISSN 0963-6625 - p. 218 - 233.
attitudes - genomics - learning - nanotechnology - public education

In this work, using experiments, we investigate the role of the name of a technology on the informed evaluation of that technology. We argue that a name can influence interpretations by activating cognitive structures. Using genomics-accelerated breeding as a case, we show that the name ‘genomics’ makes people evaluate related information as similar to genetic modification. Replacing the name ‘genomics’ with ‘natural crossing’ causes evaluations similar to those for traditional breeding. The results show that a name can have a strong influence on public attitudes, and we call for more consideration in choosing a name for a technology.

Widespread reduction in sun-induced fluorescence from the Amazon during the 2015/2016 El Niño
Koren, Gerbrand ; Schaik, Erik van; Araújo, Alessandro C. ; Boersma, K.F. ; Gärtner, Antje ; Killaars, Lars ; Kooreman, Maurits L. ; Kruijt, Bart ; Laan-Luijkx, Ingrid T. van der; Randow, Celso von; Smith, Naomi E. ; Peters, Wouter - \ 2018
Philosophical Transactions of the Royal Society B. Biological sciences 373 (2018)1760. - ISSN 0962-8436 - 10 p.
Amazon rainforest - drought response - El Niño-Southern Oscillation - gross primary production - sun-induced fluorescence - tropical terrestrial carbon cycle

The tropical carbon balance dominates year-to-year variations in the CO2 exchange with the atmosphere through photosynthesis, respiration and fires. Because of its high correlation with gross primary productivity (GPP), observations of sun-induced fluorescence (SIF) are of great interest. We developed a new remotely sensed SIF product with improved signal-to-noise in the tropics, and use it here to quantify the impact of the 2015/2016 El Niño Amazon drought. We find that SIF was strongly suppressed over areas with anomalously high temperatures and decreased levels of water in the soil. SIF went below its climatological range starting from the end of the 2015 dry season (October) and returned to normal levels by February 2016 when atmospheric conditions returned to normal, but well before the end of anomalously low precipitation that persisted through June 2016. Impacts were not uniform across the Amazon basin, with the eastern part experiencing much larger (10-15%) SIF reductions than the western part of the basin (2-5%). We estimate the integrated loss of GPP relative to eight previous years to be 0.34-0.48 PgC in the three-month period October-November-December 2015.This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Top-down NOx emissions of european cities based on the downwind plume of modelled and space-borne tropospheric NO2 columns
Verstraeten, Willem W. ; Boersma, Klaas Folkert ; Douros, John ; Williams, Jason E. ; Eskes, Henk ; Liu, Fei ; Beirle, Steffen ; Delcloo, Andy - \ 2018
Sensors 18 (2018)9. - ISSN 1424-8220
LOTOS-EUROS CTM - OMI data - Surface NO emissions - Tropospheric NO column

Top-down estimates of surface NOX emissions were derived for 23 European cities based on the downwind plume decay of tropospheric nitrogen dioxide (NO2) columns from the LOTOS-EUROS (Long Term Ozone Simulation-European Ozone Simulation) chemistry transport model (CTM) and from Ozone Monitoring Instrument (OMI) satellite retrievals, averaged for the summertime period (April–September) during 2013. Here we show that the top-down NOX emissions derived from LOTOS-EUROS for European urban areas agree well with the bottom-up NOX emissions from the MACC-III inventory data (R2 = 0.88) driving the CTM demonstrating the potential of this method. OMI top-down NOX emissions over the 23 European cities are generally lower compared with the MACC-III emissions and their correlation is slightly lower (R2 = 0.79). The uncertainty on the derived NO2 lifetimes and NOX emissions are on average ~55% for OMI and ~63% for LOTOS-EUROS data. The downwind NO2 plume method applied on both LOTOS-EUROS and OMI tropospheric NO2 columns allows to estimate NOX emissions from urban areas, demonstrating that this is a useful method for real-time updates of urban NOX emissions with reasonable accuracy.

Quality assurance framework development based on six new ECV data products to enhance user confidence for climate applications
Nightingale, Joanne ; Boersma, Klaas Folkert ; Muller, Jan Peter ; Compernolle, Steven ; Lambert, Jean Christopher ; Blessing, Simon ; Giering, Ralf ; Gobron, Nadine ; Smedt, Isabelle De ; Coheur, Pierre ; George, Maya ; Schulz, Jörg ; Wood, Alexander - \ 2018
Remote Sensing 10 (2018)8. - ISSN 2072-4292
Climate applications - Climate data records - CO - Earth observation satellites - Essential climate variables - FAPAR - HCHO - LAI - NO2 - Quality assurance - Surface albedo - Traceability - User requirements

Data from Earth observation (EO) satellites are increasingly used to monitor the environment, understand variability and change, inform evaluations of climate model forecasts, and manage natural resources. Policymakers are progressively relying on the information derived from these datasets to make decisions on mitigating and adapting to climate change. These decisions should be evidence based, which requires confidence in derived products, as well as the reference measurements used to calibrate, validate, or inform product development. In support of the European Union's Earth Observation Programmes Copernicus Climate Change Service (C3S), the Quality Assurance for Essential Climate Variables (QA4ECV) project fulfilled a gap in the delivery of climate quality satellite-derived datasets, by prototyping a generic system for the implementation and evaluation of quality assurance (QA) measures for satellite-derived ECV climate data record products. The project demonstrated the QA system on six new long-term, climate quality ECV data records for surface albedo, leaf area index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR), nitrogen dioxide (NO2), formaldehyde (HCHO), and carbon monoxide (CO). The provision of standardised QA information provides data users with evidence-based confidence in the products and enables judgement on the fitness-for-purpose of various ECV data products and their specific applications.

The importance of surface reflectance anisotropy for cloud and NO2 retrievals from GOME-2 and OMI
Lorente, Alba ; Boersma, K.F. ; Stammes, Piet ; Tilstra, L.G. ; Richter, Andreas ; Yu, Huan ; Kharbouche, Said ; Muller, Jan Peter - \ 2018
Atmospheric Measurement Techniques 11 (2018)7. - ISSN 1867-1381 - p. 4509 - 4529.

The angular distribution of the light reflected by the Earth's surface influences top-of-atmosphere (TOA) reflectance values. This surface reflectance anisotropy has implications for UV/Vis satellite retrievals of albedo, clouds, and trace gases such as nitrogen dioxide (NO2). These retrievals routinely assume the surface to reflect light isotropically. Here we show that cloud fractions retrieved from GOME-2A and OMI with the FRESCO and OMCLDO2 algorithms have an east-west bias of 10% to 50 %, which are highest over vegetation and forested areas, and that this bias originates from the assumption of isotropic surface reflection. To interpret the across-track bias with the DAK radiative transfer model, we implement the bidirectional reflectance distribution function (BRDF) from the Ross-Li semi-empirical model. Testing our implementation against state-of-the-art RTMs LIDORT and SCIATRAN, we find that simulated TOA reflectance generally agrees to within 1 %. We replace the assumption of isotropic surface reflection in the equations used to retrieve cloud fractions over forested scenes with scattering kernels and corresponding BRDF parameters from a daily high-resolution database derived from 16 years' worth of MODIS measurements. By doing this, the east-west bias in the simulated cloud fractions largely vanishes. We conclude that across-track biases in cloud fractions can be explained by cloud algorithms that do not adequately account for the effects of surface reflectance anisotropy. The implications for NO2 air mass factor (AMF) calculations are substantial. Under moderately polluted NO2 and backwardscattering conditions, clear-sky AMFs are up to 20% higher and cloud radiance fractions up to 40% lower if surface anisotropic reflection is accounted for. The combined effect of these changes is that NO2 total AMFs increase by up to 30% for backward-scattering geometries (and decrease by up to 35% for forward-scattering geometries), which is stronger than the effect of either contribution alone. In an unpolluted troposphere, surface BRDF effects on cloud fraction counteract (and largely cancel) the effect on the clearsky AMF. Our results emphasise that surface reflectance anisotropy needs to be taken into account in a coherent manner for more realistic and accurate retrievals of clouds and NO2 from UV/Vis satellite sensors. These improvements will be beneficial for current sensors, in particular for the recently launched TROPOMI instrument with a high spatial resolution.

Improved slant column density retrieval of nitrogen dioxide and formaldehyde for OMI and GOME-2A from QA4ECV : Intercomparison, uncertainty characterisation, and trends
Zara, Marina ; Boersma, K.F. ; Smedt, Isabelle De; Richter, Andreas ; Peters, Enno ; Geffen, Jos H.G.M. van; Beirle, Steffen ; Wagner, Thomas ; Roozendael, Michel Van; Marchenko, Sergey ; Lamsal, Lok N. ; Eskes, Henk J. - \ 2018
Atmospheric Measurement Techniques 11 (2018)7. - ISSN 1867-1381 - p. 4033 - 4058.

Nitrogen dioxide (NO2/ and formaldehyde (HCHO) column data from satellite instruments are used for air quality and climate studies. Both NO2 and HCHO have been identified as precursors to the ozone (O3/ and aerosol essential climate variables, and it is essential to quantify and characterise their uncertainties. Here we present an intercomparison of NO2 and HCHO slant column density (SCD) retrievals from four different research groups (BIRA-IASB, IUP Bremen, and KNMI as part of the Quality Assurance for Essential Climate Variables (QA4ECV) project consortium, and NASA) and from the OMI and GOME-2A instruments. Our evaluation is motivated by recent improvements in differential optical absorption spectroscopy (DOAS) fitting techniques and by the desire to provide a fully traceable uncertainty budget for the climate data record generated within QA4ECV. The improved NO2 and HCHO SCD values are in close agreement but with substantial differences in the reported uncertainties between groups and instruments. To check the DOAS uncertainties, we use an independent estimate based on the spatial variability of the SCDs within a remote region. For NO2, we find the smallest uncertainties from the new QA4ECV retrieval (0.8×1015 molec. cm-2 for both instruments over their mission lifetimes). Relative to earlier approaches, the QA4ECV NO2 retrieval shows better agreement between DOAS and statistical uncertainty estimates, suggesting that the improved QA4ECV NO2 retrieval has reduced but not altogether eliminated systematic errors in the fitting approach. For HCHO, we reach similar conclusions (QA4ECV uncertainties of 8-12×1015 molec. cm-2), but the closeness between the DOAS and statistical uncertainty estimates suggests that HCHO uncertainties are indeed dominated by random noise from the satellite's level 1 data. We find that SCD uncertainties are smallest for high top-of-atmosphere reflectance levels with high measurement signal-to-noise ratios. From 2005 to 2015, OMI NO2 SCD uncertainties increase by 1-2%year-1, which is related to detector degradation and stripes, but OMI HCHO SCD uncertainties are remarkably stable (increase <1%year-1) and this is related to the use of Earth radiance reference spectra which reduces stripes. For GOME-2A, NO2 and HCHO SCD uncertainties increased by 7-9 and 11-15%year-1 respectively up until September 2009, when heating of the instrument markedly reduced further throughput loss, stabilising the degradation of SCD uncertainty to <3%year-1 for 2009-2015. Our work suggests that the NO2 SCD uncertainty largely consists of a random component (∼65% of the total uncertainty) as a result of the propagation of measurement noise but also of a substantial systematic component (∼35% of the total uncertainty) mainly from "stripe effects". Averaging over multiple pixels in space and/or time can significantly reduce the SCD uncertainties. This suggests that trend detection in OMI, GOME-2 NO2, and HCHO time series is not limited by the spectral fitting but rather by the adequacy of assumptions on the atmospheric state in the later air mass factor (AMF) calculation step.

Comparing OMI-based and EPA AQS in situ NO2 trends: towards understanding surface NOx emission changes
Zhang, R. ; Wang, Y. ; Smeltzer, C. ; Qu, Hang ; Koshak, W. ; Boersma, K.F. - \ 2018
Atmospheric Measurement Techniques 11 (2018)7. - ISSN 1867-1381 - p. 3955 - 3967.
With the improved spatial resolution of the Ozone Monitoring Instrument (OMI) over earlier instruments and more than 10 years of service, tropospheric NO2 retrievals from OMI have led to many influential studies on the relationships between socioeconomic activities and NOx emissions. Previous studies have shown that the OMI NO2 data show different relative trends compared to in situ measurements. However, the sources of the discrepancies need further investigations. This study focuses on how to appropriately compare relative trends derived from OMI and in situ measurements. We retrieve OMI tropospheric NO2 vertical column densities (VCDs) and obtain the NO2 seasonal trends over the United States, which are compared with coincident in situ surface NO2 measurements from the Air Quality System (AQS) network. The Mann–Kendall method with Sen's slope estimator is applied to derive the NO2 seasonal and annual trends for four regions at coincident sites during 2005–2014. The OMI-based NO2 seasonal relative decreasing trends are generally biased low compared to the in situ trends by up to 3.7%yr−1, except for the underestimation in the US Midwest and Northeast during December, January, and February (DJF). We improve the OMI retrievals for trend analysis by removing the ocean trend, using the Moderate Resolution Imaging Spectroradiometer (MODIS) albedo data in air mass factor (AMF) calculation. We apply a lightning flash filter to exclude lightning-affected data to make proper comparisons. These data processing procedures result in close agreement (within 0.3%yr−1) between in situ and OMI-based NO2 regional annual relative trends. The remaining discrepancies may result from inherent difference between trends of NO2 tropospheric VCDs and surface concentrations, different spatial sampling of the measurements, chemical nonlinearity, and tropospheric NO2 profile changes. We recommend that future studies apply these procedures (ocean trend removal and MODIS albedo update) to ensure the quality of satellite-based NO2 trend analysis and apply the lightning filter in studying surface NOx emission changes using satellite observations and in comparison with the trends derived from in situ NO2 measurements. With these data processing procedures, we derive OMI-based NO2 regional annual relative trends using all available data for the US West (−2.0%±0.3yr−1), Midwest (−1.8%±0.4yr−1), Northeast (−3.1%±0.5yr−1), and South (−0.9%±0.3yr−1). The OMI-based annual mean trend over the contiguous United States is −1.5%±0.2yr−1. It is a factor of 2 lower than that of the AQS in situ data (−3.9%±0.4yr−1); the difference is mainly due to the fact that the locations of AQS sites are concentrated in urban and suburban regions
Unexpected slowdown of US pollutant emission reduction in the past decade
Jiang, Zhe ; McDonald, Brian C. ; Worden, Helen ; Worden, John R. ; Miyazaki, Kazuyuki ; Qu, Zhen ; Henze, Daven K. ; Jones, Dylan B.A. ; Arellano, Avelino F. ; Fischer, Emily V. ; Zhu, Liye ; Folkert Boersma, K. - \ 2018
Proceedings of the National Academy of Sciences of the United States of America 115 (2018)20. - ISSN 0027-8424 - p. 5099 - 5104.
Decadal scale variation - Emission regulations - Nitrogen oxides

Ground and satellite observations show that air pollution regulations in the United States (US) have resulted in substantial reductions in emissions and corresponding improvements in air quality over the last several decades. However, large uncertainties remain in evaluating how recent regulations affect different emission sectors and pollutant trends. Here we show a significant slowdown in decreasing US emissions of nitrogen oxides (NOx) and carbon monoxide (CO) for 2011–2015 using satellite and surface measurements. This observed slowdown in emission reductions is significantly different from the trend expected using US Environmental Protection Agency (EPA) bottom-up inventories and impedes compliance with local and federal agency air-quality goals. We find that the difference between observations and EPA’s NOx emission estimates could be explained by: (i) growing relative contributions of industrial, area, and off-road sources, (ii) decreasing relative contributions of on-road gasoline, and (iii) slower than expected decreases in on-road diesel emissions.

Naming is framing: the public understanding of scientific names
Boersma, Reginald - \ 2018
Wageningen University. Promotor(en): B. Gremmen, co-promotor(en): P.M. Poortvliet. - Wageningen : Wageningen University - ISBN 9789463432948 - 195
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