Aviation impact on air quality present day and mid-century simulated in the Community Atmosphere Model (CAM)
Introduction
Aviation emits gases and particles that are a concern for air quality and human health. Aviation emits nitrogen oxides (NOx = NO + NO2), volatile organic compounds (VOCs), sulfur oxides (SOx), soot, and other particles, which have an impact on the concentrations of ozone, and fine particles at the surface (Brasseur et al., 1996; IPCC, 1999; Lee et al., 2010; Cameron et al., 2017). Additionally, between 1989 and 2009, aviation has grown at a rate of 4.4% per year and has generally outpaced economic growth (ICAO, 2010). Given the potential impacts that aviation may have on air quality and human health, and the rapid growth of the aviation sector, it is important to assess the current and future impact that aviation may have. Recently a study by Cameron et al. (2017) used five global atmospheric computer models to evaluate the effects of global aircraft emissions on surface air quality by calculating changes in ozone and small particles. This study evaluated the global air quality effects due to all aviation emissions (i.e. both LTO and cruise emissions) and reported a near-surface annual changes in ozone of 0.3–1.9%, and near-surface annual changes in PM2.5 of −1.9–1.2%, globally.
Most earlier studies only considered the effects of landing and take-off emissions (LTO) on air quality (Herndon et al., 2004, 2008; Schurmann et al., 2007). While aviation does have a localized effect on air quality near airports due to landing and take-off emissions, recent modeling studies suggest that aviation non-LTO emissions (i.e. cruise altitude emissions), emitted in the upper troposphere and lower stratosphere (UTLS), do have an impact on surface air quality by increasing the concentrations of O3 and particulate matter of 2.5 μm and smaller (PM2.5) at the surface (Barrett et al., 2010a; Jacobson et al., 2013; Lee et al., 2013).
Barrett et al. (2010a) first examined the impacts of aviation cruise emissions on surface air quality and put them in terms of human mortality. Using the Goddard Earth Observing System model with Chemistry (GEOS-Chem), a three dimensional chemical transport model (CTM), the study suggested that aviation cruise emissions, while contributing ∼ 1% of air quality related mortalities, comprises 80% of aviation emission deaths. The study found that secondary H2SO4 
HNO3 NH3 aerosols are mainly responsible for premature mortalities and that aviation cruise emissions are responsible for ∼8000 premature mortalities per year.
Lee et al. (2013) then used the Community Atmosphere Model version 3 (CAM3) CTM to evaluate the effect of aviation cruise emissions on surface air quality. That study found that aviation non-LTO emissions increased surface O3 regionally by up 1–2 ppbv in January and ∼0.5 ppbv in July and surface PM2.5 by ∼0.5% (less than 0.2 μg/m3). Additionally, while most perturbations were not statistically significant, the statistically significant perturbations were less than 1% of the background. It is noted that background concentration of any pollutant within the context of this paper refers to atmospheric concentration of that pollutant in the absence of aviation emissions. A mortality estimate was not done for this study due to uncertainties with health impacts of such low PM2.5 increases.
In a similar study, Jacobson et al. (2013), using the Gas, Aerosol, Transport, Radiation, General-Circulation, Mesoscale, and Ocean Model (GATOR-GCMOM) chemical response model (CRM), found that aviation cruise emissions increased surface ozone by ∼0.4%. Additionally, surface PAN was increased by ∼0.1%. It was estimated that ∼620 deaths per year were attributed to aviation cruise emissions, with about half due to ozone and half due to PM2.5.
To further examine the impact of aviation non-LTO emissions on surface air quality we used the latest version of CAM5 (Community Atmosphere Model version 5), the atmospheric component model for the Community Earth System Model (CESM). Moreover, we did examine the future impact of aviation non-landing and take-off (non-LTO) emissions on surface air quality for the year 2050 and for two different future scenarios.
A major update in CAM5 relative to its previous versions is the inclusion of a modal aerosol module (MAM), which provides a more accurate representation of aerosols. The use of modal aerosol model allows the prediction of aerosol mass and the total number in each mode as opposed to aerosol bulk model that prescribes a fixed size distribution for aerosols and this allow a more representative simulation of aerosols Surface Area Density (SAD). Aerosols surface area density has relevance for heterogeneous reactions occurring on the surface of aerosol particles since these reactions do not directly relate to the aerosol mass but rather depend on the amount of tropospheric SAD. SAD depends not only on aerosol mass but also on their size distribution. As such, the use of modal aerosol model in CAM5 helps to better simulate the reactions that have relevance for calculations of aviation-induced changes in aerosols.
This paper goes beyond the model intercomparison study by also providing the first evaluation of the mid-century (2050) impact of aviation non-LTO emissions on surface air quality. The mid-century aviation impact was analyzed assuming two different jet fuels, a standard scenario assuming use of fossil fuels, and a scenario assuming the use of biofuels, with the assumption of 50% less soot and no sulfur emissions compared to the fossil fuel scenario.
The rest of this paper is organized as follows. Section 2 describes the model and simulation set up. Results of the present day and mid-century simulations are presented in section 3 and concluding remarks are presented in section 4.
Section snippets
Model description and simulation set-up
Simulations were run with CAM5. The model includes tropospheric and stratospheric chemistry, with 133 species and 330 photochemical reactions described in Lamarque et al. (2012). A major update over previous versions of CAM is the modal aerosol module (MAM) for aerosol treatment (Liu et al., 2012).
MAM was developed with two versions, one with seven lognormal modes (MAM7) and one with three lognormal modes (MAM3) (Liu et al., 2012). For this study, MAM7 was used which represents Aitken,
Effects on NOx and O3
Aviation NOx emissions increase the concentrations of O3 in the UTLS. As O3 is transported towards the surface, it converts surface NOx to HNO3. Fig. 1 shows the surface NOx difference between the simulations with and without aviation emissions for January and July. For the mid-century simulations, only Scenario 1 is shown for NOx and O3 (Fig. 2, below) because the aviation emissions that affect NOx and O3 concentrations at the surface are the same between the two mid-century fuel scenarios. As
Conclusions
The present day and mid-century global impact of aviation non-LTO emissions on surface air quality were evaluated with CAM5. The mid-century (2050) simulations had two fuel scenarios, a fossil fuel (SC1) which assumed some technological advancement, and a biofuel (Alt), which differed from the fossil fuel in that it had 50% less soot and no sulfur emissions. Aviation emissions increase surface O3 concentrations mainly in the NH. The impact is larger in January, when the background O3
Acknowledgements
The authors would like to thank the Federal Aviation Administration, Aviation Climate Change Research Initiative (ACCRI) for support under Contract #: 10-C-NE-UI amendment 001 and The Partnership for AiR Transportation Noise and Emissions Reduction (PARTNER). The opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of ACCRI, PARTNER, or the FAA. We would like to acknowledge high-performance computing
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