Constraints on ship NOx emissions in Europe using GEOS-Chem and OMI satellite NO2 observations

We present a top-down ship NOx emission inventory for the Baltic Sea, the North Sea, the Bay of Biscay and the Mediterranean Sea based on satellite-observed tropospheric NO2 columns of the Ozone Monitoring Instrument (OMI) for 2005–2006. We improved the representation of ship emissions in the GEOS-Chem chemistry transport model, and compared simulated NO2 columns to consistent satellite observations. Relative differences between simulated and observed NO2 columns have been used to constrain ship emissions in four European seas (the Baltic Sea, the North Sea, the Bay of Biscay and the Mediterranean Sea) using a mass-balance approach, and accounting for non-linear sensitivities to changing emissions in both model and satellite retrieval. These constraints are applied to 39 % of total top-down European ship NOx emissions, which amount to 0.96 Tg N for 2005, and 1.0 Tg N for 2006 (11–15% lower than the bottom-up EMEP ship emission inventory). Our results indicate that EMEP emissions in the Mediterranean Sea are too high (by 60%) and misplaced by up to 150 km, which can have important consequences for local air quality simulations. In the North Sea ship track, our top-down emissions amount to 0.05 Tg N for 2005 (35% lower than EMEP). Increased top-down emissions were found for the Baltic Sea and the Bay of Biscay ship tracks, with totals in these tracks of 0.05 Tg N (131% higher than EMEP) and 0.08 Tg N for 2005 (128% higher than EMEP), respectively. Our study explicitly accounts for the (non-linear) sensitivity of satellite retrievals to changes in the a priori NO2 profiles, as satellite observations are never fully independent of model information (i.e. assumptions on vertical NO2 profiles). Our study provides for the first time a space-based, top-down ship NOx emission inventory, and can serve as a framework for future studies to constrain ship emissions using satellite NO2 observations in other seas

Worldwide biogenic soil NOx emissions inferred from OMI NO2 observations

Biogenic NOx emissions from soils are a large natural source with substantial uncertainties in global bottom-up estimates (ranging from 4 to 15 Tg N yr-1). We reduce this range in emission estimates, and present a top-down soil NOx emission inventory for 2005 based on retrieved tropospheric NO2 columns from the Ozone Monitoring Instrument (OMI). We use a state-of-science soil NOx emission inventory (Hudman et al., 2012) as a priori in the GEOS-Chem chemistry transport model to identify 11 regions where tropospheric NO2 columns are dominated by soil NOx emissions. Strong correlations between soil NOx emissions and simulated NO2 columns indicate that spatial patterns in simulated NO2 columns in these regions indeed reflect the underlying soil NOx emissions. Subsequently, we use a mass-balance approach to constrain emissions for these 11 regions on all major continents using OMI observed and GEOS-Chem simulated tropospheric NO2 columns. We find that responses of simulated NO2 columns to changing NOx emissions are suppressed over low NOx regions, and account for these non-linearities in our inversion approach. In general, our approach suggests that emissions need to be increased in most regions. Our OMI top-down soil NOx inventory amounts to 10.0 Tg N for 2005 when only constraining the 11 regions, and 12.9 Tg N when extrapolating the constraints globally. Substantial regional differences exist (ranging from -40% to +90%), and globally our top-down inventory is 4–35% higher than the GEOS-Chem a priori (9.6 Tg N yr-1). We evaluate NO2 concentrations simulated with our new OMI top-down inventory against surface NO2 measurements from monitoring stations in Africa, the USA and Europe. Although this comparison is complicated by several factors, we find an encouraging improved agreement when using the OMI top-down inventory compared to using the a priori inventory. To our knowledge, this study provides, for the first time, specific constraints on soil NOx emissions on all major continents using OMI NO2 columns. Our results rule out the low end of reported soil NOx emission estimates, and suggest that global emissions are most likely around 12.9 ± 3.9 Tg N yr-1

Constraints on ship NOx emissions in Europe using GEOS-Chem and OMI satellite NO2 observations

We present a top-down ship NOx emission inventory for the Baltic Sea, the North Sea, the Bay of Biscay and the Mediterranean Sea based on satellite-observed tropospheric NO2 columns of the Ozone Monitoring Instrument (OMI) for 2005–2006. We improved the representation of ship emissions in the GEOS-Chem chemistry transport model, and compared simulated NO2 columns to consistent satellite observations. Relative differences between simulated and observed NO2 columns have been used to constrain ship emissions in four European seas (the Baltic Sea, the North Sea, the Bay of Biscay and the Mediterranean Sea) using a mass-balance approach, and accounting for nonlinear sensitivities to changing emissions in both model and satellite retrieval. These constraints are applied to 39% of total top-down European ship NOx emissions, which amount to 0.96 TgN for 2005, and 1.0 TgN for 2006 (11–15% lower than the bottom-up EMEP ship emission inventory). Our results indicate that EMEP emissions in the Mediterranean Sea are too high (by 60 %) and misplaced by up to 150 km, which can have important consequences for local air quality simulations. In the North Sea ship track, our top-down emissions amount to 0.05 TgN for 2005 (35% lower than EMEP). Increased top-down emissions were found for the Baltic Sea and the Bay of Biscay ship tracks, with totals in these tracks of 0.05 TgN (131% higher than EMEP) and 0.08 TgN for 2005 (128% higher than EMEP), respectively. Our study explicitly accounts for the (non-linear) sensitivity of satellite retrievals to changes in the a priori NO2 profiles, as satellite observations are never fully independent of model information (i.e. assumptions on vertical NO2 profiles). Our study provides for the first time a space-based, top-down ship NOx emission inventory, and can serve as a framework for future studies to constrain ship emissions using satellite NO2 observations in other seas

Validation of OMI Tropospheric NO2 Observations During INTEX-B and Application to Constrain NOx Emissions Over the Eastern United States and Mexico

We compare tropospheric NO2 column measurements from the Ozone Monitoring Instrument (OMI) aboard the EOS Aura satellite with coincident in situ aircraft measurements on vertical spirals over the southern United States, Mexico, and the Gulf of Mexico during the INTEX-B campaign in March 2006. Good correlation with no significant bias (r2=0.67, slope=0.99±0.17, n=12) is found for the ensemble of comparisons when the aircraft could spiral sufficiently low to sample most of the NO2 column. Urban spirals where large extrapolations were needed below the aircraft floor (1000 ft) showed poorer agreement. We use the OMI observations together with a global chemical transport model (GEOS-Chem) to estimate emissions of nitrogen oxides over the eastern United States and Mexico in March 2006. Comparison to EPA's National Emissions Inventory 1999 (NEI99) calls for a decrease in power plant emissions and an increase in on-road vehicle emissions relative to that inventory. The rise in vehicular emissions is offsetting the reduction in power plant and industry emissions. These findings are consistent with independent assessments. Our OMI-derived emission estimates for Mexico are higher by a factor of 2.0±0.5 than bottom-up emissions, similar to a comparison between the recently released Mexican NEI99 inventory and the bottom-up showing that the Mexican NEI99 inventory is 1.6–1.8× higher.Earth and Planetary SciencesEngineering and Applied Science

Transpacific Transport of Ozone Pollution and the Effect of Recent Asian Emission Increases on Air Quality in North America: An Integrated Analysis Using Satellite, Aircraft, Ozonesonde, and Surface Observations

We use an ensemble of aircraft, satellite, sonde, and surface observations for April–May 2006 (NASA/INTEX-B aircraft campaign) to better understand the mechanisms for transpacific ozone pollution and its implications for North American air quality. The observations are interpreted with a global 3-D chemical transport model (GEOS-Chem). OMI NO2 satellite observations constrain Asian anthropogenic NOx emissions and indicate a factor of 2 increase from 2000 to 2006 in China. Satellite observations of CO from AIRS and TES indicate two major events of Asian transpacific pollution during INTEX-B. Correlation between TES CO and ozone observations shows evidence for transpacific ozone pollution. The semi-permanent Pacific High and Aleutian Low cause splitting of transpacific pollution plumes over the Northeast Pacific. The northern branch circulates around the Aleutian Low and has little impact on North America. The southern branch circulates around the Pacific High and some of that air impacts western North America. Both aircraft measurements and model results show sustained ozone production driven by peroxyacetylnitrate (PAN) decomposition in the southern branch, roughly doubling the transpacific influence from ozone produced in the Asian boundary layer. Model simulation of ozone observations at Mt. Bachelor Observatory in Oregon (2.7 km altitude) indicates a mean Asian ozone pollution contribution of 9±3 ppbv to the mean observed concentration of 54 ppbv, reflecting mostly an enhancement in background ozone rather than episodic Asian plumes. Asian pollution enhanced surface ozone concentrations by 5–7 ppbv over western North America in spring 2006. The 2000–2006 rise in Asian anthropogenic emissions increased this influence by 1–2 ppbv.Earth and Planetary SciencesEngineering and Applied Science

Decadal Variabilities in Tropospheric Nitrogen Oxides Over United States, Europe, and China

Global trends in tropospheric nitrogen dioxide (NO2) have changed dramatically in the past decade. Here, we investigate tropospheric NO2 variabilities over United States, Europe, and E. China in 2005–2018 to explore the mechanisms governing the variation of this critical pollutant. We found large uncertainties in the trends of anthropogenic nitrogen oxides (NOx) emissions, for example, the reductions of NOx emissions, derived with different approaches and data sets, are in the range of 35%–50% over the United States and 15%–45% over Europe in 2005–2018. By contrast, the analysis in this work indicates declines of anthropogenic NOx emissions by about 40% and 25% over the United States and Europe, respectively, in 2005–2018, and about 20% over E. China in 2012–2018. However, the shift of major NOx sources from power generation to industrial and transportation sectors has led to noticeable diminishing effects in emission controls. Furthermore, satellite measurements exhibit the influence of NO2 background levels over the United States and Europe, which offset the impacts of anthropogenic emission declines, resulting in flatter trends of tropospheric NO2 over the United States and Europe. Our analysis further reveals underestimation of background NO2 by chemical transport models, which can lead to inaccurate interpretations of satellite measurements. We use surface in-situ NO2 observations to diagnose the satellite-observed NO2 trends and find top-down NOx emissions over urban grids represent the changes in anthropogenic NOx emissions better. This work highlights the importance of comprehensive applications of different analysis approaches to better characterizing atmospheric composition evolution

The Cabauw Intercomparison Campaign for Nitrogen Dioxide Measuring Instruments (CINDI): Design, Execution, and Early Results

From June to July 2009 more than thirty different in-situ and remote sensing instruments from all over the world participated in the Cabauw Intercomparison campaign for Nitrogen Dioxide measuring Instruments (CINDI). The campaign took place at KNMI's Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Its main objectives were to determine the accuracy of state-ofthe- art ground-based measurement techniques for the detection of atmospheric nitrogen dioxide (both in-situ and remote sensing), and to investigate their usability in satellite data validation. The expected outcomes are recommendations regarding the operation and calibration of such instruments, retrieval settings, and observation strategies for the use in ground-based networks for air quality monitoring and satellite data validation. Twenty-four optical spectrometers participated in the campaign, of which twenty-one had the capability to scan different elevation angles consecutively, the so-called Multi-axis DOAS systems, thereby collecting vertical profile information, in particular for nitrogen dioxide and aerosol. Various in-situ samplers and lidar instruments simultaneously characterized the variability of atmospheric trace gases and the physical properties of aerosol particles. A large data set of continuous measurements of these atmospheric constituents has been collected under various meteorological conditions and air pollution levels. Together with the permanent measurement capability at the CESAR site characterizing the meteorological state of the atmosphere, the CINDI campaign provided a comprehensive observational data set of atmospheric constituents in a highly polluted region of the world during summertime. First detailed comparisons performed with the CINDI data show that slant column measurements of NO2, O4 and HCHO with MAX-DOAS agree within 5 to 15%, vertical profiles of NO2 derived from several independent instruments agree within 25% of one another, and MAX-DOAS aerosol optical thickness agrees within 20-30% with AERONET data. For the in-situ NO2 instrument using a molybdenum converter, a bias was found as large as 5 ppbv during day time, when compared to the other in-situ instruments using photolytic converters

Satellite observations of tropospheric nitrogen dioxide : retrieval, interpretation, and modelling

The research questions set out in Chapter 1 that guided the investigation in this thesis are repeated here. The answers to these questions contain the most important conclusions of the various chapters and are given below. 1. How can we retrieve accurate information on total and tropospheric NO2 from the backscatter UV-VIS measurements by the Ozone Monitoring Instrument? 2. Can we develop a concept that relates a vertical distribution of a trace gas to a satellite observed column? 3. What is the error buget for tropospheric NO2 retrievals for an operational backscatter instrument like GOME? 4. How can we use GOME tropospheric NO2 observations to constrain estimates of the tropical lightning NOx production? The first research question is topic of chapter 2. One of the ‘Science questions’ posed in the framework of the EOS-AURA mission is: "Is air quality changing?". A contribution to anwering this question may be provided by accurate and precise measurements of tropospheric NO2 columns from the Ozone Monitoring Instrument that become widely available for scientific purposes. To this end, an algorithm for the retrieval of total and tropospheric NO2 has been developed and tested in Chapter 2. This algorithm has been implemented for the operational retrieval of the standard KNMI/NASA OMI NO2 "product". The OMI algorithm builds on the heritage of GOME tropospheric NO2 retrievals, and contains a number of important improvements over previous algorithms. In principle, one of these improvements is the extended size of the spectral fitting window: sensitivity studies indicated that a significant reduction in the slant column uncertainty can be attained for a 405-465 nm window. The derivation of the stratosferische background is improved by accounting for variability along a zonal band. Application of a low-pass filter approach on GOME data shows significant variability in stratospheric NO2 along locations of the same latitude that would lead to otherwise significant systematic errors. Climatological NO2 profiles can be used for troposferic air mass factor calculations. In situations of urban pollution, climatological NO2 profiles simulated with TM3 do not show large variability in their shape. In situations of biomass burning and outflow of NOx-related pollution over comparatively clean areas, the vertical distribution is quite different with NO2 peaking at higher altitudes. We estimate that OMI NO2 columns will be retrieved with a precision of approximately 5% for unpolluted situations, largely due to the uncertainty in the spectral fitting. For situations with NOx levels in the troposphere that are far above background, we expect to measure tropospheric columns with errors up to 60%, largely due to retrieval assumptions on the state of the atmosphere. In Chapter 3, the second research question is adressed. This chapter discussed the strong height-dependent sensitivity of a satellite instrument to a tracer density is discussed in relation to the averaging kernel. This sensitivity was already topic of Chapter 2, but is here discussed in the context of general retrieval theory as developed by Rodgers. It is shown that the averaging kernel provides a direct interpretation of the satellite retrieved column density to users. For intercomparisons with independent data, such as vertical profiles from models or validation measurements, the dependence on a priori assumptions about the profile shape dissapears when the averaging kernel is used. The third Chapter on the retrieval of tropospheric NO2, is Chapter 4. In this Chapter, an extensive error analysis of tropospheric NO2 retrievals is presented in order to answer the third research question. It is shown that GOME tropospheric NO2 retrievals have errors in the 35-60% range, largely determined by air mass factor errors. The most important errors -in order of importance- are errors due to uncertainty in model parameters such as clouds, surface albedo and a priori profile shape. Apart from the error analysis, a number of retrieval improvements has been suggested in Chapter 4. Most relevant is a new method to estimate the stratospheric background from an assimilation approach. This approach has the advantage of accounting for dynamical features in stratospheric NO2, and reduces the otherwise large systematic error in the estimate of stratopheric NO2. A correction for the temperature-dependence of the NO2 cross-section is demonstrated to remove systematic errors on the order of 10%. Finally, we conclude that a correction for the presence of aerosols needs to be accompanied by aerosol corrections in cloud retrieval schemes. Chapter 5 relies on the previous chapters and focuses on the fourth and last research question. In Chapter 5, columns and their error estimates (Chapter 4) are used in an extensive comparison -through the averaging kernel (Chapter 3)- with modelled lightning NO2 columns in order to test lightning parametrisations in TM3 and to impose top-down constraints on the global lightning NOx production. First, it is shown that tropospheric measurements by GOME are sensitive to NO2 produced by lightning. Tropospheric NO2 columns show a rapid increase with the fifth power of the cloud top height for clouds with tops higher than 6.5 km. This estimate of the cloud height-dependence of LNO2 is consistent with the observed power-law relationship of lightning frequencies and cloud top height. Second, a statistical comparison of simulated LNO2 and observed NO2 columns in the tropical region between 40??S and5??N shows that the TM3 model is well capable of reproducing observed patterns of LNO2. This is true for two different lightning parameterisations in TM3. Moreover, the absolute values of modelled and observed (L)NO2 are in good agreement over tropical continents. However, over tropical oceans, the model appears to overestimate the LNO2 contribution to the total tropospheric column. This model bias is likely due to assumptions on the assumed energy ratio (10:1) between cloud-to-ground and intra-cloud lightning, and on the assumed ratio (10:1) between continent-to-ocean convective intensity. For the scheme based on convective precipitation, there are significant regional differences in rainfall-to-lightning ratios that may also lead to the bias over the tropical ocean. From rescaling the modelled LNOx production between 40??S and 5??N, we arrive at a LNOx production estimate of ??1.0 Tg[N] in the 40??S118 Summary, conclusions, and outlook 5??N region. Based on assumptions for rescaling factors in the rest of the world, the global LNOx production in 1997 is estimated to be in the 1.1-6.4 Tg[N] range

Reductions in nitrogen oxides over Europe driven by environmental policy and economic recession

Fuel combustion is a significant source of numerous air pollutants, which reduce local air quality, and affect global tropospheric chemistry. Satellite observations of nitrogen dioxide, emitted by combustion processes, allow for robust monitoring of atmospheric concentrations at high spatial resolution on continental scales. Here we evaluate changes in tropospheric NO2 concentrations over Europe between 2004 and 2010. We isolate long-term (timescales greater than one year) variability in the daily NO2 observations from the Ozone Monitoring Instrument (OMI) using a spectral analysis. In 2010, we find substantial reductions in NO2 concentrations of at least 20% throughout Europe. These reductions are as much the result of temporary reductions prompted by the 2008–2009 global economic recession, as of European NOx emission controls. Our results demonstrate that realistic concentration pathways of NO2 do not follow simple linear trends, but reflect a compilation of environmental policy and economic activit