Stochastic Methods for Setting Effective Aviation NOₓ Policies

Abstract

Nitrogen Oxides (NOₓ) from aviation emissions are well known to have detrimental effects on air quality and the climate. Presently, they are regulated to preserve local air quality around airports. As part of the regulation process, aircraft engines are placed on a test stand with NOₓ levels measured at different thrust settings meant to mimic the aircraft’s emissions during landing and take-off. These are then constrained as a function of the engine’s overall pressure ratio (OPR) and rated thrust, with the allowed NOₓ emissions increasing with OPR. Despite increases in the stringency of this regulation, recent research suggests this regulation is insufficient for protecting surface air quality degradation from NOₓ emissions at cruise. Moreover, at high OPRs, NOₓ emissions increase substantially for relatively small reductions in fuel burn. In light of this, a new metric representative of cruise emissions is being investigated. This work considers effective methods to define this new regulation given a wide range of uncertainties in the tradeoff between NOₓ and CO₂ emissions at high OPRs. First, an estimate for the combined climate and air quality cost of NOₓ from aviation cruise emissions is estimated as ∼$95,000/tonne using a 2019 flight inventory. Then, cruise limits are proposed informed by the combined impact of NOₓ and CO₂ at cruise and with a similar slope to the current LTO standard. Finally, a Monte Carlo simulation is run, sampling NOₓ and CO₂ social costs for a series of hypothetical aircraft designed using the open-source Transportation Aircraft System OPTimization (TASOPT) model. This work takes a worst-case scenario approach, where the only response engine manufacturers can make to stricter standards is to reduce OPR and sacrifice fuel efficiency. Each aircraft’s emissions are evaluated during cruise to determine the probability of increasing environmental harm under different policy scenarios given these uncertainties. The combined cost of NOₓ and CO₂ are compared to the baseline engines that meet current regulations for each scenario. Results show defining a cruise metric informed by the weighted combined cost of CO₂ and NOₓ could reduce total environmental cost at cruise by 15 – 43% while carrying a 6 – 7.4% risk of increasing total environmental cost for wide-body aircraft engines in the most stringent scenario. Less stringent scenarios showed similar risks of increasing harm for smaller potential environmental savings. In all cases, the risks associated with the proposed limits are driven by low-likelihood extremes in the uncertainty distributions of NOₓ and CO₂, further suggesting the benefit of an environmentally conscious standard.