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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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.

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.

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.

Algorithm theoretical baseline for formaldehyde retrievals from S5P TROPOMI and from the QA4ECV project
Smedt, Isabelle De; Theys, Nicolas ; Yu, Huan ; Danckaert, Thomas ; Lerot, Christophe ; Compernolle, Steven ; Roozendael, Michel Van; Richter, Andreas ; Hilboll, Andreas ; Peters, Enno ; Pedergnana, Mattia ; Loyola, Diego ; Beirle, Steffen ; Wagner, Thomas ; Eskes, Henk ; Geffen, Jos Van; Folkert Boersma, Klaas ; Veefkind, Pepijn - \ 2018
Atmospheric Measurement Techniques 11 (2018)4. - ISSN 1867-1381 - p. 2395 - 2426.
On board the Copernicus Sentinel-5 Precursor (S5P) platform, the TROPOspheric Monitoring Instrument (TROPOMI) is a double-channel, nadir-viewing grating spectrometer measuring solar back-scattered earthshine radiances in the ultraviolet, visible, near-infrared, and shortwave infrared with global daily coverage. In the ultraviolet range, its spectral resolution and radiometric performance are equivalent to those of its predecessor OMI, but its horizontal resolution at true nadir is improved by an order of magnitude. This paper introduces the formaldehyde (HCHO) tropospheric vertical column retrieval algorithm implemented in the S5P operational processor and comprehensively describes its various retrieval steps. Furthermore, algorithmic improvements developed in the framework of the EU FP7-project QA4ECV are described for future updates of the processor. Detailed error estimates are discussed in the light of Copernicus user requirements and needs for validation are highlighted. Finally, verification results based on the application of the algorithm to OMI measurements are presented, demonstrating the performances expected for TROPOMI.
QA4ECV total and tropospheric HCHO column data from OMI
Smedt, Isabelle De; Geffen, Jos Van; Richter, Andreas ; Beirle, Steffen ; Yu, Huan ; Vlietinck, Jonas ; Roozendael, Michel Van; A, Ronald van der; Lorente Delgado, A. ; Scanlon, Tracy ; Compernolle, Steven ; Wagner, Thomas ; Boersma, K.F. - \ 2017
Structural uncertainty in air mass factor calculation for NO2 and HCHO satellite retrievals
Lorente Delgado, Alba ; Folkert Boersma, K. ; Yu, Huan ; Dörner, Steffen ; Hilboll, Andreas ; Richter, Andreas ; Liu, Mengyao ; Lamsal, Lok N. ; Barkley, Michael ; Smedt, Isabelle De; Roozendael, Michel Van; Wang, Yang ; Wagner, Thomas ; Beirle, Steffen ; Lin, Jin Tai ; Krotkov, Nickolay ; Stammes, Piet ; Wang, Ping ; Eskes, Henk J. ; Krol, Maarten - \ 2017
Atmospheric Measurement Techniques 10 (2017)3. - ISSN 1867-1381 - p. 759 - 782.
Air mass factor (AMF) calculation is the largest source of uncertainty in NO2 and HCHO satellite retrievals in situations with enhanced trace gas concentrations in the lower troposphere. Structural uncertainty arises when different retrieval methodologies are applied within the scientific community to the same satellite observations. Here, we address the issue of AMF structural uncertainty via a detailed comparison of AMF calculation methods that are structurally different between seven retrieval groups for measurements from the Ozone Monitoring Instrument (OMI). We estimate the escalation of structural uncertainty in every sub-step of the AMF calculation process. This goes beyond the algorithm uncertainty estimates provided in state-of-the-art retrievals, which address the theoretical propagation of uncertainties for one particular retrieval algorithm only. We find that top-of-atmosphere reflectances simulated by four radiative transfer models (RTMs) (DAK, McArtim, SCIATRAN and VLIDORT) agree within 1.5 %. We find that different retrieval groups agree well in the calculations of altitude resolved AMFs from different RTMs (to within 3 %), and in the tropospheric AMFs (to within 6 %) as long as identical ancillary data (surface albedo, terrain height, cloud parameters and trace gas profile) and cloud and aerosol correction procedures are being used. Structural uncertainty increases sharply when retrieval groups use their preference for ancillary data, cloud and aerosol correction. On average, we estimate the AMF structural uncertainty to be 42 % over polluted regions and 31 % over unpolluted regions, mostly driven by substantial differences in the a priori trace gas profiles, surface albedo and cloud parameters. Sensitivity studies for one particular algorithm indicate that different cloud correction approaches result in substantial AMF differences in polluted conditions (5 to 40 % depending on cloud fraction and cloud pressure, and 11 % on average) even for low cloud fractions (<0.2) and the choice of aerosol correction introduces an average uncertainty of 50 % for situations with high pollution and high aerosol loading. Our work shows that structural uncertainty in AMF calculations is significant and that it is mainly caused by the assumptions and choices made to represent the state of the atmosphere. In order to decide which approach and which ancillary data are best for AMF calculations, we call for well-designed validation exercises focusing on polluted conditions in which AMF structural uncertainty has the highest impact on NO2 and HCHO retrievals.
Substantial Underestimation of Post-Harvest Burning Emissions in the North China Plain Revealed by Multi-Species Space Observations
Stavrakou, T. ; Müller, J.F. ; Bauwens, M. ; Smedt, I. De; Lerot, C. ; Roozendael, M. Van; Coheur, P.F. ; Clerbaux, C. ; Boersma, K.F. ; Song, Y. - \ 2016
Scientific Reports 6 (2016). - ISSN 2045-2322

The large-scale burning of crop residues in the North China Plain (NCP), one of the most densely populated world regions, was recently recognized to cause severe air pollution and harmful health effects. A reliable quantification of the magnitude of these fires is needed to assess regional air quality. Here, we use an eight-year record (2005-2012) of formaldehyde measurements from space to constrain the emissions of volatile organic compounds (VOCs) in this region. Using inverse modelling, we derive that satellite-based post-harvest burning fluxes are, on average, at least a factor of 2 higher than state-of-the-art bottom-up statistical estimates, although with significant interannual variability. Crop burning is calculated to cause important increases in surface ozone (+7%) and fine aerosol concentrations (+18%) in the North China Plain in June. The impact of crop fires is also found in satellite observations of other species, glyoxal, nitrogen dioxide and methanol, and we show that those measurements validate the magnitude of the top-down fluxes. Our study indicates that the top-down crop burning fluxes of VOCs in June exceed by almost a factor of 2 the combined emissions from other anthropogenic activities in this region, underscoring the need for targeted actions towards changes in agricultural management practices.

Improved spectral fitting of nitrogen dioxide from OMI in the 405–465 nm window
Geffen, J.H.G.M. van; Boersma, K.F. ; Roozendael, M. van; Hendrick, F. ; Mahieu, E. ; Smedt, I. de; Sneep, M. ; Veefkind, J.P. - \ 2015
Atmospheric Measurement Techniques 8 (2015). - ISSN 1867-1381 - p. 1685 - 1699.
An improved nitrogen dioxide (NO2) slant column density retrieval for the Ozone Monitoring Instrument (OMI) in the 405–465 nm spectral region is presented. Since the launch of OMI on board NASA's EOS-Aura satellite in 2004, differential optical absorption spectroscopy (DOAS) retrievals of NO2 slant column densities have been the starting point for the KNMI DOMINO and NASA SP NO2 vertical column data as well as the OMI NO2 data of some other institutes. However, recent intercomparisons between NO2 retrievals from OMI and other UV/Vis and limb spectrometers, as well as ground-based measurements, suggest that OMI stratospheric NO2 is biased high. This study revises and, for the first time, fully documents the OMI NO2 retrieval in detail. The representation of the OMI slit function to convolve high-resolution reference spectra onto the relevant spectral grid is improved. The window used for the wavelength calibration is optimised, leading to much-reduced fitting errors. Ozone and water vapour spectra used in the fit are updated, reflecting the recently improved knowledge of their absorption cross section in the literature. The improved spectral fit also accounts for absorption by the O2–O2 collision complex and by liquid water over clear-water areas. The main changes in the improved spectral fitting result from the updates related to the wavelength calibration: the RMS error of the fit is reduced by 23% and the NO2 slant column by 0.85 × 1015 molec cm-2, independent of latitude, solar zenith angle and NO2 value. Including O2–O2 and liquid water absorption and updating the O3 and water vapour cross-section spectra further reduces NO2 slant columns on average by 0.35 × 1015 molec cm-2, accompanied by a further 9% reduction in the RMS error of the fit. The improved OMI NO2 slant columns are consistent with independent NO2 retrievals from other instruments to within a range that can be explained by photochemically driven diurnal increases in stratospheric NO2 and by small differences in fitting window and approach. The revisions indicate that current OMI NO2 slant columns suffered mostly from an additive positive offset, which is removed by the improved wavelength calibration and representation of the OMI slit function. It is therefore anticipated that the improved NO2 slant columns are most important to retrievals of spatially homogeneous stratospheric NO2 rather than to heterogeneous tropospheric NO2.
Multi-model ensemble simulations of troposheric NO2 compared with GOME retrievals for the year 2000
Noije, T.P.C. van; Eskes, H.J. ; Dentener, F.J. ; Stevenson, D.S. ; Ellingsen, K. ; Schultz, M.G. ; Wild, O. ; Amann, M. ; Atherton, C.S. ; Bergmann, D. ; Bey, I. ; Boersma, K.F. ; Butler, T. ; Cofala, J. ; Drevet, J. ; Fiore, A.M. ; Gauss, M. ; Hauglustaine, D.A. ; Horowitz, L.W. ; Isaksen, I.S.A. ; Krol, M.C. ; Lamarque, J.F. ; Lawrence, M.G. ; Martin, R.V. ; Montanaro, V. ; Muller, J.F. ; Pitari, G. ; Prather, M.J. ; Pyle, J.A. ; Richter, A. ; Rodriguez, J.M. ; Savage, N.H. ; Strahan, S.E. ; Sudo, K. ; Szopa, S. ; Roozendael, M. van - \ 2006
Atmospheric Chemistry and Physics 6 (2006)10. - ISSN 1680-7316 - p. 2943 - 2979.
chemical-transport model - ozone monitoring experiment - radiative-transfer model - aircraft mozaic data - satellite-observations - nitrogen-dioxide - nonmethane hydrocarbons - surface reflectivity - global distributions - 3-dimensional model
We present a systematic comparison of tropospheric NO2 from 17 global atmospheric chemistry models with three state-of-the-art retrievals from the Global Ozone Monitoring Experiment (GOME) for the year 2000. The models used constant anthropogenic emissions from IIASA/EDGAR3.2 and monthly emissions from biomass burning based on the 1997¿2002 average carbon emissions from the Global Fire Emissions Database (GFED). Model output is analyzed at 10:30 local time, close to the overpass time of the ERS-2 satellite, and collocated with the measurements to account for sampling biases due to incomplete spatiotemporal coverage of the instrument. We assessed the importance of different contributions to the sampling bias: correlations on seasonal time scale give rise to a positive bias of 30¿50% in the retrieved annual means over regions dominated by emissions from biomass burning. Over the industrial regions of the eastern United States, Europe and eastern China the retrieved annual means have a negative bias with significant contributions (between ¿25% and +10% of the NO2 column) resulting from correlations on time scales from a day to a month. We present global maps of modeled and retrieved annual mean NO2 column densities, together with the corresponding ensemble means and standard deviations for models and retrievals. The spatial correlation between the individual models and retrievals are high, typically in the range 0.81¿0.93 after smoothing the data to a common resolution. On average the models underestimate the retrievals in industrial regions, especially over eastern China and over the Highveld region of South Africa, and overestimate the retrievals in regions dominated by biomass burning during the dry season. The discrepancy over South America south of the Amazon disappears when we use the GFED emissions specific to the year 2000. The seasonal cycle is analyzed in detail for eight different continental regions. Over regions dominated by biomass burning, the timing of the seasonal cycle is generally well reproduced by the models. However, over Central Africa south of the Equator the models peak one to two months earlier than the retrievals. We further evaluate a recent proposal to reduce the NOx emission factors for savanna fires by 40% and find that this leads to an improvement of the amplitude of the seasonal cycle over the biomass burning regions of Northern and Central Africa. In these regions the models tend to underestimate the retrievals during the wet season, suggesting that the soil emissions are higher than assumed in the models. In general, the discrepancies between models and retrievals cannot be explained by a priori profile assumptions made in the retrievals, neither by diurnal variations in anthropogenic emissions, which lead to a marginal reduction of the NO2 abundance at 10:30 local time (by 2.5¿4.1% over Europe). Overall, there are significant differences among the various models and, in particular, among the three retrievals. The discrepancies among the retrievals (10¿50% in the annual mean over polluted regions) indicate that the previously estimated retrieval uncertainties have a large systematic component. Our findings imply that top-down estimations of NOx emissions from satellite retrievals of tropospheric NO2 are strongly dependent on the choice of model and retrieval.
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