• Alex Savin
• March 29, 2022
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Satellite Measurements of NOx Emission From Shipping

The shipping industry is a big polluter and is often endangering nature’s flora and fauna. One of the biggest aspects of its pollution is NO2 / NOx emission.

Nitrogen dioxide is the chemical compound with the formula NO2. It is one of several nitrogen oxides. NO2 is an intermediate in the industrial synthesis of nitric acid, with millions of tons produced each year. This reddish-brown toxic gas has a characteristic sharp, biting odor and is a prominent air pollutant. Nitrogen dioxide is a paramagnetic bent molecule with C2v point group symmetry.

The environmental impact of shipping includes greenhouse gas emissions and oil pollution. Carbon dioxide emissions from shipping are currently estimated at 4 to 5 percent of the global total and assessed by the International Maritime Organisation (IMO) to rise by up to 72 percent by 2020 if no action is taken. Exhaust emissions from ships are considered a significant source of air pollution, with 18 to 30 percent of all nitrogen oxide and 9 percent of sulfur oxide pollution. The 15 biggest ships emit as much sulfur oxide pollution as all cars combined. “By 2010, up to 40 percent of air pollution over land could come from ships.” Sulfur in the air creates acid rain, which damages crops and buildings. When inhaled, the sulfur is known to cause respiratory problems and even increase the risk of a heart attack.

NOx Sea Emission

Modifications of existing clouds by the exhaust of ships are well-known but poorly studied atmospheric effects which could contribute to climate change. The perturbation of a cloud layer by ship-generated aerosol changes the cloud reflectivity and is identified by long curves in satellite images, known as ship tracks. As ship tracks indicate pollution of a very clean marine environment and affect the radiation budget below and above the cloud, it is important to investigate their radiative and climatic effects. Satellite data from MODIS on Terra are used to examine a scene from 10 February 2003 where ship tracks were detected close to the North American West Coast. The cloud optical and microphysical properties are derived using a semi-analytical retrieval technique combined with a look-up-table approach. 

Ship-track pixels are distinguished from the unperturbed cloud pixels, and the optical properties of the former are compared to those of the latter. A significant change in the droplet number concentration, the effective radius, and the optical thickness are found within the ship tracks compared to the unaffected cloud. A significant increase in liquid water could not be confirmed. The resulting cloud properties calculate the radiation budget below and above the cloud. Assuming a mean solar zenith angle, the mean surface radiation below the ship track is decreased by 43.25 Wm/2, and 40.73 Wm/2 increases the mean reflectance at TOA. For the selected scene, the ship emission decreases the solar radiation at the surface by 2.10 Wm/2. It increases the backscattered solar radiation at the top of the atmosphere (TOA) by 2.00 Wm/2. Increased backscattered radiation is partly compensated by decreasing the thermal radiation by 0.43 Wm/2. The resulting net effect at TOA increases 1.57 Wm/2, corresponding to a negative radiative forcing and a cooling. 2006 research.

Nitrogen oxides (NOxNO+NO2) play an important role in tropospheric chemistry, particularly catalytic ozone production. 

Lightning provides:

• A natural source of nitrogen oxides.
• Dominating the display in the tropical upper troposphere.
• Strongly impacting tropospheric ozone and the atmosphere’s oxidizing capacity.

Recent estimates of lightning-produced NOx (NOx) are 5 Tg [N] per year, with high uncertainties in one order of magnitude.

The Global Ozone Monitoring Experiment (GOME) onboard the ESA satellite ERS-2 allows the retrieval of tropospheric column densities of NO2 on a global scale. Here we present the GOME NO2 measurement directly over a large convective system over the Gulf of Mexico. Simultaneously, cloud-to-ground (C.G.) flashes are counted by the U.S. National Lightning Detection Network (NLDNTM) and extrapolated to include intra-cloud (I.C.)+CG flashes based on a climatological I.C.: C.G. ratio derived from NASA’s space-based lightning sensors. A series of 14 GOME pixels shows largely enhanced column densities over thick and high clouds, coinciding with strong lightning activity. The enhancements can not be explained by the transport of anthropogenic NOx and must be due to the new production of NOx. A quantitative analysis, accounting in particular for the visibility of LNOx from satellite, yields an LNOx output of 90 (32-240) moles of NOx, or 1.3 (0.4-3.4) kg [N], per flash. This corresponds to a global NOx production of 1.7 (0.6-4.7)Tg [N]/yr if extrapolated.

Emissions from fossil fuel combustion and biomass burning reduce local air quality and affect global tropospheric chemistry. Nitrogen oxides are emitted by all combustion processes and play a key part in the photochemically induced catalytic production of ozone, resulting in summer smog and increased tropospheric ozone levels globally. The release of nitrogen oxide also results in nitric acid deposition and—at least locally—increases radiative forcing effects due to the absorption of downward propagating visible light. Nitrogen oxide concentrations in many industrialized countries are expected to decrease, but rapid economic development has the potential to increase the emissions of nitrogen oxides in parts of Asia significantly. Here we present the tropospheric column amounts of nitrogen dioxide retrieved from two satellite instruments, GOME and SCIAMACHY, from 1996 to 2004. We find substantial reductions in nitrogen dioxide concentrations over some areas of Europe and the USA, but a highly significant increase of about 50 percent—with an accelerating annual growth rate—over the industrial areas of China, more than current bottom-up inventories suggest.

As the study shows, the global impact of shipping on atmospheric chemistry and radiative forcing and the associated uncertainties have been quantified using ten state-of-the-art atmospheric chemistry models and a pre-defined set of emission data. The analysis is performed for present-day conditions (2000) and two future ship emission scenarios. In one system, ship emissions stabilize at 2000 levels; in the other, ship emissions increase with a constant annual growth rate of 2.2% up to 2030 (termed the “Constant Growth Scenario” (CGS)). Most other anthropogenic emissions follow the IPCC (Intergovernmental Panel on Climate Change) SRES (Special Report on Emission Scenarios) A2 scenario, while biomass burning and natural emissions remain at the year 2000 levels. An intercomparison of the model results with observations over the Northern Hemisphere (25°–60° N) oceanic regions in the lower troposphere showed that the models are capable of reproducing ozone (O3) and nitrogen oxides (NOx=NO+NO2) reasonably well. In contrast, sulfur dioxide (SO2) in the marine boundary layer is significantly underestimated. The most pronounced changes in annual mean tropospheric NO2 and sulfate columns are simulated over the Baltic and North Seas. 

Other significant changes occur over the North Atlantic, the Gulf of Mexico, and along the main shipping lane from Europe to Asia, across the Red and Arabian Seas. Maximum contributions from shipping to annual mean near-surface O3 are found over the North Atlantic (5–6 ppbv in 2000; up to 8 ppbv in 2030). Ship contributions to tropospheric O3 columns over the North Atlantic and Indian Oceans reach 1 D.U. in 2000 and 1.8 DU in 2030. Tropospheric O3 forcings due to shipping are 9.8±2.0 mW/m2 in 2000 and 13.6±2.3 mW/m2 in 2030. Increasing O3, ship NOx simultaneously enhances hydroxyl radicals over the remote ocean, reducing the global methane lifetime by 0.13 yr in 2000 and up to 0.17 yr in 2030, introducing a negative radiative forcing. The models show future increases in NOx and O3 burden, which scale almost linearly with increases in NOx emission totals. Increasing emissions from shipping would significantly counteract the benefits derived from reducing SO2 emissions from all other anthropogenic sources under the A2 scenario over the continents, for example, in Europe. Globally, shipping contributes 3% to increases in O3 burden between 2000 and 2030 and 4.5% to increases in sulfate under A2/CGS. However, if future ground-based emissions follow a more stringent scenario, the relative importance of ship emissions will increase. Inter-model differences in the simulated O3 contributions from ships are significantly smaller than estimated uncertainties stemming from the ship emission inventory, mainly the ship emission totals, the distribution of the global emissions, and the neglect of ship plume dispersion.

Long-term satellite measurements of nitrogen dioxide in the troposphere are combined with a continental scale air quality model to verify and improve available estimates of multi-annual changes in nitrogen oxides (NOx) emissions in Europe and the Mediterranean area between 1996 and 2005. As a result, a measurement-based data set of NOx emissions on a 1° by 1° grid and averaged over the summer months is elaborated.

The results are compared with NOx emission data based on the EMEP emission inventory. Our data agree with the EMEP estimates, suggesting a general decline in NOx emissions in Western and Central European countries (France, Germany, Great Britain, and Poland). Over Southern Europe and for shipping emissions, neutral to positive trends are found both for the inverted and bottom-up emissions. In contrast, considerable differences between both data sets are found in some other countries. In particular, significant negative trends instead of the positive ones in the “bottom-up” inventory are located in the Balkan countries, Russia and Turkey. The NOx emission trends derived from satellite measurements demonstrate larger spatial heterogeneity than those calculated with the EMEP data, especially in Russia and Ukraine.

The obtained estimates of the decadal trends in NOx emission for Great Britain are consistent with independent data from the U.K. Automatic Urban and Rural Network (AURN). It is also found that using our emission estimates yields better agreement of model calculations with near-surface ozone measurements of the European EMEP network.

Conclusion

NO2 is one of the serious problems when it comes to shipping pollution. It has to be approached seriously and, if possible, lowered as much as possible.