Aviation-induced aerosol-phase perturbations

2.4.2.2.1 Sulfates (SO42-)

Limiting factors for the formation of sulfates is the availability of OH and H2O2 in the troposphere, and as O3 is a source gas for OH creation a link between the relation in formation of these secondary pollutants from aviation is observed (Unger et al., 2006a; Unger et al.,

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2008). Aviation-induced sulfates have been shown to predominately form in the Northern Hemisphere (Figure 2.10) (Righi et al., 2013; Barrett et al., 2012; Unger, 2011). With peaks in sulfate concentrations of ~10 ng m^-3 occuring in the northern mid-latitude (~30°N) and peaks correlating with aviation cruise height (~250 hPa) of ~5–10 ng m^-3 (Righi et al., 2013).

Literature shows agreement with percentage increases in aviation-induced sulfates, with relative changes peaking about cruise height (~250 hPa) and in the northern high-latitudes (~60°N–90°N), and associated relative changes ranging between ~5–15% (Righi et al., 2013;

Unger, 2011).

Studies investigating the impacts of both aviation-induced sulfates and nitrates, highlighting the interdependencies between sulfate and nitrate formation, their precursor emissions and atmospheric species which need to be considered (Righi et al., 2013; Unger, 2011; Unger et al., 2013; Fiore et al., 2012; Unger et al., 2010). Unger (2011) reported that aviation-induced ammonium sulfate form partially at the expense of ammonium nitrate in the high northern altitudes and latitudes. The sulfate/nitrate formation mechanism competes for available ammonia in the atmosphere, as these aerosols are closely linked to O3 photochemistry as they are formed from the SO2, NH3 and NOX precursors, and the availability of atmospheric oxidants, OH and H2O2 (Unger, 2011; Unger et al., 2010).

Figure 2.10: Sulfate multi-year average (1996–2005) zonal means: (a) absolute and (b) percentage differences from Righi et al. (2013), and (c) percentage difference from Unger (2011).

From literature investigating the impact of aviation-borne emissions on sulfate formation, it is observed that the distribution of aviation-induced sulfates in the Northern Hemisphere formed coincides with the region where the majority of aviation emissions are released (Unger, 2011;

Unger et al., 2013; Olivié et al., 2012; Righi et al., 2013; Barrett et al., 2012).

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In addition to the formation of aviation-induced sulfates from the release of aviation-borne SO2 emissions, sulfates can also be induced from increases in aviation-induced OH from aviation-borne NOX, a process which yields OH as well; with the resultant OH being able to participate in the oxidation of SO2 from other anthropogenic or natural sources to form sulfate (Barrett et al., 2010) (Section 2.3.5).

2.4.2.2.2 Nitrate (NO3-)

Ammonium nitrate forms when the sulfate aerosol is fully neutralised or when there is excess ammonia in the atmosphere (Forster et al., 2007), with decreases in sulfate precursors benefiting the formation of ammonium nitrate (Bellouin et al., 2011). This is due to the coupling between sulfates and nitrates and their competition for ammonia available in the atmosphere (Unger, 2011).

Referring to Figure 2.11(a) over the northern mid-latitude there is a mean decrease in nitrate concentrations of ~1 ng m^-3, equating to a 20% relative decrease in atmospheric nitrate over this region (Righi et al., 2013). Breaking down perturbations in aviation-induced nitrates Figure 2.11(a) goes on to show that zonal mean concentrations increase by up to ~10 ng m^-3 20°–

70°N, with the increases in zonal mean nitrate concentrations in the northern mid-latitudes of between 1–2 ng m^-3 below cruise altitude. Above cruise altitude decreases in zonal mean nitrate concentrations of between ~–5 to –2 ng m^-3 between ~30°N–90°N (Righi et al., 2013).

Figure 2.11: Nitrate multi-year average (1996–2005) zonal means: (a) absolute and (b) percentage differences from Righi et al. (2013), and (c) percentage difference from Unger (2011).

Figure 2.11(b) and Figure 2.11(c) show the relative changes in aviation-induced nitrates from Righi et al. (2013) and Unger (2011). These figures show relative increases in nitrate aerosols

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below cruise phase of flight ranging between 0–5%, and decreases at cruise altitude and above.

Barrett et al. (2012) found aviation increased surface-layer nitrate concentrations by up to ~50 ng m^-3 over South Canada and by up to ~130 ng m^-3 over eastern China and central Europe.

As seen in the recent decrease in anthropogenic SO2 emissions, nitrates becomes a more important aerosol specie, and may continue to be so in the future (Bellouin et al., 2011; Bauer et al., 2007). Bauer et al. (2007) through investigating reduced future SO2 emissions simulates reductions in sulfate and increases in nitrates. This is as relatively smaller sulfate concentrations leads to favourable reactions of ammonia with nitric acid to form ammonium nitrate; instead of reacting with sulfates. And with projected increases in future nitrate precursor emissions and a decline in the formation of ammonium sulfate formed, the importance of nitrates in the future climate is further emphasised (Bauer et al., 2007). Unger (2011) demonstrated the interplay between the formation of ammonium sulfates and ammonium nitrates, when investigating the use of desulfurised jet fuel.

Through simulating a desulfurised aviation fuel case increases in aviation-induced nitrates of between 2–10% are estimated the NH mid-latitude up to an altitude of 200 mb (corresponding with cruise-level) with decreases of between –10 to –20% in the NH high-latitude above 500 mb. In comparison to standard jet fuel (Jet A/Jet A-1), increases in aviation-induced nitrates of between 2–10% are estimated in the NH mid-latitude up a latitude of ~480 mb, with decreases in aviation-induced nitrates of up to –55.6% in the NH high-latitude above ~440 mb (Figure concentrations about altitudes relating to aviation’s cruise altitude (Righi et al., 2013; Wei et al., 2001). Wei et al. (2001) using NASA aviation emissions for 1992 find a peak difference in BC concentrations of ~1.5 ng m^-3 between 10–12 km (267–200 hPa) for 1992 (Figure 2.12(c)). Righi et al. (2013) estimate peaks at the surface (~30°N) of between 0.2–0.5 ng m^-3, and at altitudes relating to a cruise altitude (~250 hPa) of between 0.1–0.2 ng m^-3 in the Northern Hemisphere mid-latitudes. When considering the relative differences in BC induced by aviation emissions

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these are again seen to dominate in the Northern Hemisphere peaking at 4–5% between 250–

300 hPa.

Barrett et al. (2012) investigated aviation-induced surface-layer BC concentrations. From their work they find that the main regions which return increases in BC concentrations of up to ~0.5 ng m^-3 over central Europe and the eastern US seaboard, increases of up to ~0.4 ng m^-3 over the western US seaboard, and increases of up to ~0.2 ng m^-3 over eastern China in line with total column integrated aviation BC emissions (Lamarque et al., 2009). These peaks in surface layer BC concentrations correlate with aviation fuelburn emissions (Lamarque et al., 2010b;

Lamarque et al., 2009).

Figure 2.12: Black carbon multi-year average (1996–2005) zonal means: (a) absolute and (b) percentage differences from Righi et al. (2013), and (c) absolute difference for 2001 from Wei et al. (2001).

Aviation emitted BC is estimated to represent ~0.01% of total anthropogenic BC emissions from fossil fuel sources (Balkanski et al., 2010). The size of BC particles produced by aviation is much smaller than that from other emitters of BC (Balkanski et al., 2010). This is of importance as even though the total mass emitted by aviation may be small in comparison to other BC emitters the total number of particles actually emitted by aviation could represent more than 30% of the total particle numbers over a large part of the Northern Hemisphere free troposphere (Balkanski et al., 2010).

2.4.2.2.4 Organic carbon (OC)

Akin to BC aerosols OC is formed from the incomplete complete combustion (Kim et al., 2012), but unlike BC scatter light efficiently (Jacobson, 2005).

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When these initially hydrophobic particles age and or other chemicals such as H2SO4 condense upon them they have the potential to act as cloud condensation nuclei (CCN) (Jacobson, 2005).

Surface layer OC perturbations follow a similar distribution in surface-layer aviation-induced BC found by Barrett et al. (2012), explained through the relationship of BC and OC emissions to aviation fuelburn (Eyers et al., 2004). Barrett et al. (2012) demonstrated that peak surface-layer aviation-induced OC perturbations occur in western Europe with peaks in concentrations of ~5 ng m^-3.

Aviation-induced OC perturbations are an aspect of aviation-emissions induced changes that have not received much attention to date (Olivié et al., 2012; Righi et al., 2013; Lee et al., 2010). This can be attributed to aviation-borne emissions species currently including in aviation emissions inventories used (Balkanski et al., 2010; Barrett et al., 2012; Unger, 2011;

Olsen et al., 2013a; Eyers et al., 2004).