FOX, the authors suggest, is currently appropriate for engines burning conventional jet fuel since engine measurements from tests with conventional fuels are used to calibrate the model. However, it could subsequently be developed to capture the reductions in EI_BC observed as a result of burning synthetic or other alternative fuels which contain fewer PAH compounds.

Aircraft gas turbine engines emit carbonaceous particles that warm Earth’s atmosphere, thereby contributing to climate change. Black carbon (BC) aerosols emitted by aircraft strongly absorb solar radiation and have a long lifetime relative to near-surface BC emissions, leading to a positive radiative forcing (RF). They are also thought to have a significant indirect influence by acting as ice nuclei in the formation of contrails and contributing to aviation-induced cloudiness. BC emitted by aircraft also contributes to the degradation of air quality, both globally and in the vicinity of airports.

The current regulations regarding aircraft particulate matter (PM) emissions were originally concerned with plume visibility during the landing and takeoff (LTO) cycle (i.e., near the ground). The regulation is applied through a limit on the engine smoke number (SN), the observed change in reflectance of a filter after sampling a given mass of exhaust. Current research efforts are focused toward defining a new standard procedure for measurement of BC mass emissions from aircraft at ground level for regulatory purposes, and several studies have measured aircraft engine BC emission indices (EI), that is, mass of BC emitted per kg of fuel burned. These measurements indicate that EI_BC spans almost 4 orders of magnitude (5 × 10⁻⁴ to ∼1 g/kg-fuel) and is a function of engine model and thrust setting. However, EI_BC has not been measured for the vast majority of engines currently in service. EI_BC is typically estimated using the SN for LTO emissions, and no standard method exists for cruise emissions, with researchers typically applying ad hoc estimates of fleet-averaged EI_BC.

...[In this paper] The accuracy of current methods used to estimate aircraft EI_BC are evaluated, a new method appropriate to the data limitations is developed, an updated global estimate of aviation BC emissions is provided and the potential climatic importance of this for aviation is discussed. The purpose of the method developed is not to replace future direct measurements of EI_BC, but to “backfill” the majority of the fleet for which emissions measurements are not available and may never be available given current information, and which will continue to be relevant for several decades.

—Stettler et al.

Reviewing current practice, the authors noted that at the introduction of the smoke number regulations, experiments correlated SN to BC mass concentration (C_BC) in the engine exhaust. Subsequent studies used these correlations to estimate emission rates of BC from aircraft engines.

Currently the most rigorous method to estimate aircraft EI_BC during landing and takeoff is the first-order approximation version 3.0 (FOA3), developed so that emissions from airports could be estimated. The FOA3 method proposes that EI_BC is equal to the product of the mass concentration of BC, C_BC (mg/m³), and the volumetric flow rate (Q)—that is, the volume of exhaust gas per kg of fuel burned (m³/kg-fuel).

Q is estimated by assuming an air-to-fuel ratio (AFR); however, engine AFRs are proprietary and in order to apply this methodology to all engines in the fleet, representative AFRs corresponding to four certification thrust settings are defined for use by FOA3. Up to now, the authors of the paper note, a comprehensive assessment of the CBC and Q components has not been published.

EI_BC at cruise has been measured using a chase plane in the plume of the leading plane; results suggest that EI_BC at cruise is ∼0.01 g/kg-fuel, although these measurements are representative of engines operating at reduced power (∼20% of maximum engine fuel flow).

The FOA3 technique, developed to estimate BC mass emissions normalized by fuel burn [EI_BC] from SN, underestimates EI_BC by >90% in 35% of directly measured cases (R² = −0.10), according to the team.

As there are no plans to measure BC emissions from all existing certified engines—which will be in service for several decades—it is necessary to estimate EI_BC for existing aircraft on the ground and at cruise. An alternative method, called FOX, that is independent of the SN is developed to estimate BC emissions. Estimates of EI_BC at ground level are significantly improved (R² = 0.68), whereas estimates at cruise are within 30% of measurements.

—Stettler et al.

Using the FOX method, the team founds that ∼9% of total fuel consumption and ∼14% of total BC emissions occur during the LTO cycle, reflecting the higher EI_BC during high-thrust engine operations (i.e., takeoff and climb). The fleet average EI_BC for LTO is ∼70% higher than that for cruise.

An earlier but recent study has estimated the direct radiative forcing due to aviation BC to be ∼0.0034 W/m². Applying a linear scaling to account for the increased estimate of BC emissions from the FOX method, Stettler et al. estimated the direct RF due to aviation BC to be ∼0.0095 W/m², equivalent to ∼1/3 of the RF due to CO₂ (0.0280 W/m²) and ∼70% of the net RF due to aviation NO_x emissions (0.0138 W/m²). If indirect RF due to aircraft BC (i.e., contrails and induced cirrus cloudiness) were taken into account, the impact of the updated BC emissions inventory is likely to be even more significant, the authors added. However, they also noted that there remains significant uncertainty in aviation direct BC RF estimates even given a specific BC emissions estimate.

...the FOX method provides an updated estimate of global aviation BC emissions. While it has been optimized to estimate EI_BC during the LTO and shown to estimate EI_BC at cruise with better accuracy than existing methods, further measurements of EI_BC at cruise for flights that are representative of real operations (i.e., greater than ∼20% fuel flow) would be extremely valuable and could be used to validate the FOX method and the updated global inventory of aviation BC emissions.

—Stettler et al.

Resources

• Marc E.J. Stettler, Adam M. Boies, Andreas Petzold and Steven R.H. Barrett (2013) Global Civil Aviation Black Carbon Emissions. Environmental Science & Technology doi: 10.1021/es401356v