Scientific Method / Science & Exploration

Identifying uncertainty in role of aviation NOx emissions

Scientists study the uncertainty in the impact of aviation emissions of …

Climatologists continue to develop and refine predictive models for climate change, attempting to limit uncertainty. A lot of attention is focused on carbon dioxide emissions, but a major source of uncertainty is the role of nitrogen oxides (NOx, the sum of NO and NO2), which promote greenhouse gas formation. In addition to other sources, these are emitted by aircraft engines in the upper atmosphere. A paper that recently appeared in Proceedings of the National Academies of Science studied the complex influence of NOx and describes the processes that need to be studied further to reduce our uncertainty about the impact of NOx emissions.

NOx emissions from combustion contribute on the ground to smog and acid rain, and in the upper atmosphere they act as an indirect greenhouse gas by causing a short-term increase of ozone (O3), an important greenhouse gas. Atmospheric ozone is eliminated through reactions with the hydroxyl (OH) and hydroperoxyl (HO2) radicals, and NO interferes with these reactions by competing for HO2, leading to an increase in ozone. This increase in ozone causes a greenhouse forcing, an increase in the amount of energy retained by the atmosphere.

In the long term, emissions of NOx lead to reduced levels of methane (CH4), another potent greenhouse gas. Due to the complex relationships between chemical species in the atmosphere, the loss of methane also leads to a longer-term decrease in ozone. These decreases lead to negative greenhouse forcing, causing atmospheric cooling.

Currently, our uncertainty lies in the relative amount of negative and positive forcing, which are both large and almost cancel out. Most past studies showed that NOx emissions result in a net positive forcing (and therefore emissions tend to cause global warming), but other studies showed a net negative forcing.

<!--The authors of the paper took multiple recently-published models and calculated the resulting radiative forcing due to 1.0 Tg/yr of NOx emissions (the actual estimated emissions ranges from 0.4-1.3 Tg/yr). Then, taking the resulting values as samples in a normal distribution, they found the following radiative forcing effects: +27.3 ± 9.7 mW/m2 from short-term O3 increase, -16.1 ± 5.6 mW/m2 from long-term CH4 reduction, and -6.6 ± 3.3 mW/m2 from long-term O3 reduction. Taken together, the net forcing is +4.5 ± 4.5 mW/m2.
--> The authors of the paper took multiple recently published models and calculated the resulting radiative forcing due to 1.0 Tg/yr of NOx emissions (estimates of emissions range from 0.4-1.3 Tg/yr). Then, taking the resulting values as samples in a normal distribution, they found the net radiative forcing is +4.5 ± 4.5 mW/m2. That would imply a likely warming effect, although the impact could be quite small.

Using this approach, however, the sources of uncertainty cannot be determined. To get at those, the authors used an approach called factor decomposition to break down each of the sources of forcing into key components, such as the responses of O3 and CH4 due to NOx emissions, the atmospheric concentrations of the species, and the radiative forcing efficiency of each species. Another benefit of this approach is that it depends more on fundamental chemistry than modeling. Assuming each of the terms are not correlated, the resulting net radiative forcing calculated in this manner is +0.6 ± 8.3 mW/m2. This mostly agrees with the results from the set of models, although the uncertainty is larger.

The authors found that factors specifically related to aviation emissions (the response of chemical concentrations to aviation NOx) control the uncertainty in the forcing due to the short-term O3 and long-term CH4 changes. But long-term O3 impacts are influenced by how ozone and methane interact in the atmosphere (which isn't aviation specific). 

Another interesting result in the paper is that the positive greenhouse forcing due to the short-term increase in O3 and the negative forcing due to long-term CH4 reduction appear to be strongly negatively correlated (when one goes up, the other goes down). If this is taken into account in the uncertainty calculations, this correlation drops the uncertainties to 20 percent of the original values.

The authors suggest that background processes that control naturally produced NOx are the primary factors affecting both the uncertainty and the strong negative correlation between positive and negative forcings. For instance, lightning is a large source of NOx (4 Tg/yr based on one study), and the impact of aviation emissions will be influenced by this value. A better understanding of this and other natural processes will reduce the uncertainty when it comes to aviation NOx, as will further studies focusing on the factors identified by the authors of this paper.

PNAS, 2011. DOI: 10.1073/pnas.1101458108  (About DOIs).

Listing image by Image: NASA

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