The increase in aviation related activities has led to concern about the emissions from aircraft operations and their impact on local air quality (Unal et al., 2005; Woody et al., 2011), global climate (Lee et al., 2009; Brasseur et al., 2016), and public health (Levy et al., 2012; Brunelle-Yeung et al., 2014). The primary products of conventional jet fuel combustion in an aircraft engine are NOx, unburned hydrocarbon (UHC), CO, SOx, CO2, H2O, and soot aerosol or soot particulate matter (PM). As the aircraft engine exhaust plume expands, mixes with ambient air, and cools, volatile species present in the gas phase at the engine exit plane undergo gas-to-particle conversion and begin to condense onto existing soot particles and form new particles (Onasch et al., 2009; Lobo et al., 2012; Timko et al., 2013). The black carbon component of the PM is referred to as non-volatile particulate matter (nvPM), while the volatile component consists of sulfates, nitrates, and organic compounds (Onasch et al., 2009). The composition of the volatile PM in the expanding aircraft engine exhaust plume varies greatly and depends on a number of factors such as fuel composition, ambient meteorological conditions, and plume age (Lobo et al., 2007, 2012, 2015a; Timko et al., 2013).

The commercial aviation sector has been focused on developing and implementing sustainable alternative jet fuels for use by airlines to diversify fuel supplies and mitigate the impacts of aircraft engine emissions. The American Society for Testing and Materials (ASTM) and other fuel specification bodies have established a standard specification for the manufacture of aviation turbine fuel consisting of conventional and synthetic blending components under ASTM D7566 (ASTM International, 2016). The pure alternative fuels have low to negligible amounts of aromatic, naphthalene, and sulfur content when compared to conventional jet fuel. Studies have shown that nvPM and sulfur oxide emissions are dramatically reduced during alternative fuel combustion in aircraft engines (Timko et al., 2010; Lobo et al., 2011, 2015b, 2016; Beyersdorf et al., 2014; Moore et al., 2015). The nvPM at the engine exit plane is hydrophobic, but as the nvPM evolves in the expanding plume, its aging results in enhanced hydrophilicity (Weingartner et al., 1997; Zhang et al., 2008).

Investigation of atmospheric pollution, and in particular atmospheric visibility, has shown that aerosol optical properties are affected by size, composition, and hygroscopic growth of particles (Tang et al., 1981; Horvath, 1995; Kim et al., 2006; Meier et al., 2009). In urban environments, emissions from vehicles including soot, sulfates, and nitrates have been found to be the main contributors to visibility degradation (Ferron et al., 2005; Kim et al., 2006).

Hygroscopicity tandem differential mobility analysis (HTDMA) systems have been widely used to measure the hygroscopic growth properties of PM in the subsaturated regime in different environments (Massling et al., 2007; Swietlicki et al., 2008; Park et al., 2009b; Wu et al., 2013). HTDMA measurements of PM emissions from jet engine combustors (Gysel et al., 2003; Popovicheva et al., 2008) have also been performed. However, the application of a HTDMA system to measure the hygroscopic properties of PM emissions in evolving aircraft engine exhaust plumes from the combustion of different fuels has not been previously performed.

For field measurements, where ambient temperature and humidity cannot be controlled, the HTDMA system must be fairly rugged, stable, and versatile. The Missouri University of Science and Technology (MST) has developed a HTDMA system to quantify the hygroscopic properties of PM emitted from aircraft engines. The HTDMA system was automated to operate such that it could determine the hygroscopic properties for an aerosol in approximately 45 s. This is critical when conducting aircraft engine emission tests which can be quite expensive, and where the expanding exhaust plumes are subject to perturbations in wind speed and wind direction. This paper reports the results of lab-based experiments to evaluate the performance of the MST HTDMA system and in-field measurements of PM emissions in exhaust plumes from the combustion of conventional and alternative fuels in a CFM56-2C1 engine during the Alternative Aviation Fuels EXperiment (AAFEX) II field campaign.

The MST HTDMA system consists of two differential mobility analyzers (DMAs), a humidifier (HUM), and a condensation particle counter (CPC), similar to other systems (McMurry and Stolzenburg, 1989). Figure 1 presents the schematic of the MST HTDMA system. The polydisperse aerosol was first preconditioned by passing through an ice bath (IB-0) to remove excess water vapor as much as reasonably possible and returning it to room temperature with a saturation ratio of ∼0.15. The aerosol was then brought to charge equilibrium by passing it through a bipolar charger (BC), which can contain 500 to 2000 µCi of polonium-210 prior to entering the first DMA (DMA1). The DMAs used in the HTDMA system were custom designed and have been used in previous studies to classify aerosols based on electrical mobility (Schmid, 2000). The DMAs were of cylindrical geometry and had the following dimensions: effective inner length of 72.77 cm, a sample flow annulus with an inner diameter of 5.07 cm, and an outer diameter of 8.88 cm. The polydisperse aerosol flow rate (Qp) was set to 3 L min-1 and the sheath flow rate (Qs) was adjusted to 15 L min-1 using mass flow meters (Aalborg Instruments GFM 371) which were calibrated periodically. In DMA1, the polydisperse aerosol was classified by size, and monodisperse particles with a “dry” size (Xd) selected. The excess flow in the DMA was recirculated as Qs1, after passing through a second ice bath (IB-1) and a high-efficiency particulate air (HEPA) filter to further ensure that the sample remained dry and had not prematurely deliquesced to a solution droplet. DMA1 was set at a fixed voltage, permitting the selection of a monodisperse aerosol. The monodisperse sample flow (Qm1) out of DMA1 entered the humidifier (HUM) section of the HTDMA system, where it is referred to as the polydisperse flow, Qp2. The HUM brought the aerosol sample to a controlled, precisely known saturation ratio (SR), typically 0.91 to 0.99, which caused the particles to deliquesce to a new equilibrium “wet” diameter (Xw). Valves V2 and V3 were used to direct the aerosol flow Qp2 to either pass through HUM (wet mode) or to bypass it (dry mode). Valves V4 and V5 were used to achieve the same function for the sheath air flow (Qs2). The third ice bath (IB-2) in the Qs2 loop removed the water vapor from Qs2 and minimized any unwanted vapors co-emitted from the combustion process. The second DMA (DMA2) in conjunction with a CPC (TSI 3022) measured Xw. The MST HTDMA system was designed to provide only one SR condition and to hold that value regardless of variations in ambient temperature and humidity or sampling duration. The water bath that encased HUM/DMA2 was maintained at a fixed temperature by a refrigerated water re-circulator that controlled the water temperature around the HUM/DMA2 to 16±0.1 ∘C. This water passed alongside the Qp2 and Qs2 lines (not shown in figure). Thus, the dew point achieved in HUM was well below room temperature. The water flow rate through the water bath surrounding HUM/DMA2 was approximately 5 L min-1.

The SR values in flows Qp2 and Qs2 were brought to near unity at 16 ∘C by passing the aerosol through stainless-steel tubes lined with wet cloth. The flow Qp2 passed through four such tubes (11 mm ID × 762 mm L), thus having a total length of 3048 mm and a residence time of 5.8 s. The flow Qs2 passed through eight similar tubes, thus having a total length of 6096 mm and a residence time of 2.3 s. Theoretical studies have shown that the lengths of wet-walled tubing should be sufficient to bring the Qp2 and Qs2 to very near SR of 1 (Fitzgerald et al., 1981). Just before entering DMA2, the SR of Qs2 was measured by a dew point hygrometer (DPH) (Vaisala HMP247). The flow Qd, in parallel with the CPC, reduced the lag time (LT2) between when voltage was imposed on DMA2 and when particles selected by that voltage reached the CPC.

During routine operation, to maximize the data acquisition frequency, the HTDMA system was computer controlled by a LabVIEW program (LV). When the program was initiated, it (1) set the desired voltage (HV1) in DMA1 causing it to deliver dry particles of diameter Xd, (2) waited long enough for this monodisperse aerosol to travel from the outlet of DMA1 through the HUM and into DMA2, (3) set the high voltage in DMA2 (HV2) to some fraction of that in DMA1 (typically 0.1 × HV1), and (4) caused HV2 to step through 104 increments such that the final value was a multiple of HV1 (typically 10 × HV1). During the stepwise voltage increase of HV2 (the logarithm of the voltage was linear with time.), LV recorded (at 1 Hz) values of HV1, HV2, Qs1, Qs2, Qd, P1, P2, SR, CPC concentration, and elapsed time (dt). The operator provided the general region (in time) where the peak in CPC readings occurred as input, and LV fitted a quadratic function to the CPC concentration time series. The quadratic function was differentiated and the value of dt at the maximum was obtained (dtmax). Based on calibrations performed previously, LV computed the lag time (LT2) between when a certain diameter of droplet was selected by DMA2 and when it arrived at the CPC. This lag time has been found to be a function of Qs2 and Qp2. LV found the value of the high voltage on the central rod of DMA2 at that time. It then computed the wet diameter (Xw) of the solution droplet (using the operating equation of the DMA2) and finally computed the hygroscopic properties. LV was developed such that the hygroscopic properties could be determined on more than one Xd. LV changes the particle diameter produced by DMA1 before the end of the voltage sweep on DMA2. The new particle diameter selected did not arrive at DMA2 while the current HV2 voltage sweep was running but did arrive immediately after that sweep had been completed. DMA2 then immediately started the sweep on this new wet diameter. Thus, the time taken to flush the tubing and the HUM is minimized. This reduced the time for performing HV2 sweeps on 12 different dry diameters to ∼9 min.

Periodically, experiments were performed where a challenge aerosol of a pure inorganic salt (sodium chloride, NaCl, ammonium sulfate, (NH4)2SO4, potassium iodide, KI, or potassium chloride, KCl) was used to validate/update the calibration of DPH (as described in Suda and Peters, 2013). During an automated stepwise increase of HV2, the diameters Xd and Xw were precisely determined. The calculated saturation ratios (SR-calc) were obtained from knowledge of the dry diameter Xd, the wet diameter Xw, and the fact that the particles were a pure chemical of known properties. The SR-calc values were computed and compared to the value reported by the dew point hygrometer (SR-DPH). A calibration for the DPH was thus obtained. Typically, a value of 0.85 to 0.99 is obtained for SR-calc.

In the MST HTDMA system, the SR is measured in the growth region by performing experiments (as recommended by Johnson et al., 2008). The SR is a function of not only the water vapor–air mixing ratio but also a function of gas temperature. Even though the mixing ratio will not change as Qs2 travels from the region of the DPH to the middle of DMA2, the temperature may, resulting in a potential change in SR. Thus, it is better to self-calibrate the HTDMA system using this method. Furthermore, it is generally known that reliable measurements of SR from commercial instruments become very hard to obtain the closer one gets to SR of 1.

All HTDMA systems described in the literature are designed to provide precise values for the hygroscopic growth factor. Furthermore, almost all of these systems have the ability to vary the SR, thus requiring separate thermostating for the HUM and for DMA2 (Suda and Peters, 2013; Woods et al., 2013; Shi et al., 2012; Fors et al., 2010; Park et al., 2009a; Massling et al., 2011; Hu et al., 2010; Biskos et al., 2006; Lopez-Yglesias et al., 2014). Others (Johnson et al., 2008; Cubison et al., 2005) utilize controlled mixing of humid and dry air to achieve the desired humidity. Some systems include water baths (Hennig et al., 2005; Weingartner et al., 2002), temperature-controlled cabinets (Cocker et al., 2001), and passive, insulated regions (Virkkula et al., 1999; Johnson et al., 2008).

Although these designs offer very good precision and the ability to vary the SR, they may not be well suited for field measurements, since most of them involve two separate volumes that must have their temperatures maintained very precisely. It is the temperature difference between these two volumes that is the critically important parameter. The MST HTDMA system was designed to be less susceptible to ambient temperature fluctuations. This was achieved by encasing both the HUM and DMA2 in the same thermostated container (volume ∼ 14 L). Other systems have also immersed DMA2 and the HUM in a water bath (Cubison et al., 2005; Hennig et al., 2005; Weingartner et al., 2002) to minimize the temperature gradients. In the MST HTDMA system, temperature drifts are not critical, since the temperature difference between the HUM and the DMA2 (and the exposure time of the Qp2 and Qs2 in HUM) is what determines the SR, and that remains constant (zero temperature difference).

Suda and Peters (2013) discussed the problem of DMA offset, whereby the diameter as measured by DMA1 may be slightly different from the diameter as measured by DMA2, even if they both sample the same aerosol simultaneously. This situation was avoided in the MST HTDMA system by performing a self-calibration. To accomplish this, an inorganic challenge aerosol (e.g., (NH4)2SO4)) was delivered to DMA1, and LV directed DMA1 to deliver sample particles with a given diameter Xd. The HUM was bypassed and LV initiated a voltage sweep on DMA2, which yielded a diameter Xwswp. This was repeated for a series of Xd values ranging from 10 to 160 nm, establishing a calibration curve between Xd and Xwswp, with Xd taken as the true diameter. Within LV, this calibration was utilized to synchronize the two DMAs. Since DMA1 was static during a voltage sweep and its Xd involves no error from uncertainties in the lag time (LT2), DMA1 was chosen as the reference.

The MST HTDMA system was deployed as part of the Alternative Aviation Fuels EXperiment (AAFEX II) campaign conducted during 20 March–2 April 2011 at the NASA Dryden Aircraft Operations Facility (DAOF), Palmdale, CA, USA. The NASA DC-8 aircraft equipped with CFM56-2C1 engines was utilized as the emissions source. The aircraft was parked in an open-air run-up facility with no other aircraft or emission sources in the vicinity of the test site. Detailed descriptions of the test site and experimental setup have been previously reported (Timko et al., 2013; Moore et al., 2015). The main objective of the campaign was to investigate the gaseous and PM emissions characteristics of the CFM56-2C1 engine burning conventional and alternative fuels as a function of engine thrust conditions at several sampling locations in the exhaust plume. PM emissions data were acquired for a typical cycle which consisted of the following engine thrust conditions: 4 %, 7 %, 30 %, 65 %, 85 %, and 100 % rated thrust. Two test cycles were run for each fuel – one stepping up from 4 % to 100 % rated thrust and the other stepping down from 100 % to 4 % rated thrust. Five fuels were used during the campaign: (1) JP-8 (the military equivalent of conventional Jet A/JetA-1), (2) tallow-based hydroprocessed esters and fatty acids (HEFA), (3) coal-derived Sasol Fischer–Tropsch (FT), (4) a blend of HEFA and JP-8, and (5) FT doped with tetrahydrothiophene (THT) to boost the sulfur content of the fuel. A summary of selected fuel properties is provided in Table 1. Chemical and physical analyses of the HEFA and FT fuels have been reported elsewhere (Corporan et al., 2011).

The emissions from the CFM56-2C1 engine were measured at several distances (1, 30, and 143 m) from the engine exit plane to study the PM characteristics as the exhaust plume cooled and mixed with ambient air. Only data acquired at the 143 m location are presented and discussed here to investigate the hygroscopic properties of the evolving plume.

A 5.08 cm aluminum tube (∼1.3 m above the concrete apron) positioned downwind from engine no. 3 on the starboard side of the aircraft was used to extract exhaust plume samples at the 143 m location. The exhaust was transported through the tube to a small trailer approximately 18 m away which housed the MST HTDMA system to measure hygroscopic properties. The exhaust gas flow rate through the 0.052 m ID × 18 m L tubing was well over 100 L min-1. Also housed in the trailer was a Cambustion DMS500 (Reavell et al., 2002; Hagen et al., 2009) which measured the real-time particle size distributions, and a LI-COR 840A nondispersive infrared (NDIR) detector that measured exhaust CO2 concentration. Ambient meteorological conditions such as temperature, pressure, and relative humidity were also monitored and recorded throughout the campaign. The exhaust samples at 4 % and 7 % engine thrust conditions were impacted by the ambient conditions, specifically, wind speed and wind direction. However, the CO2 measurements during the 7 % trust periods were approximately twice the background level, indicating that the exhaust plume was being sampled.

The DMS500 measured total PM size distributions. The nvPM size distributions were obtained by passing the sample through a thermal denuder. The thermal denuder consisted of a coil of stainless-steel tubing (0.457 cm ID) housed in a temperature-controlled aluminium box heated to 300 ∘C, followed by a cooling section. It is similar in design to that used by Saleh et al. (2011) and has been used in a previous study (Rye et al., 2012). Laboratory evaluations have demonstrated that H2SO4 droplets of diameter 10–100 nm are almost completely evaporated in the thermal denuder.

The total and nvPM number-based size distributions were converted to number-based emission index (EIn) distributions to account for varying amounts of dilution for each plume, and are presented for selected fuels at the 100 % thrust condition shown in Fig. 7. The total PM size distributions are bimodal with a strong nucleation mode (<20 nm) and an accumulation mode. These observations are consistent with those reported for PM emissions measured downwind of several different aircraft engine types (Lobo et al., 2007, 2012, 2015a). The enhancement of the nucleation mode in measurements made downwind of the engine exit plane is due to gas-to-particle conversion in the exhaust plume driven by fuel composition, ambient conditions, and degree of mixing. Timko et al. (2013) found that the driving force for gas-to-particle conversion in the expanding exhaust plume was the ratio of particle precursors (both organic and sulfate) to soot.

The sulfur in the fuel is oxidized to SO2, a portion of which then undergoes oxidation to SO3 and subsequently to sulfuric acid (H2SO4) in the exhaust plume (Miake-Lye et al., 1998; Schumann et al., 2002). The H2SO4 either homogenously nucleates to form pure H2SO4 droplets or condenses onto existing soot particles to form hybrid particles that have significant water-soluble components (Gysel et al., 2003; Wyslouzil et al., 1994).

The data acquired with the MST HTDMA system was used to calculate GF and κ of these particles as a function of fuel type, engine thrust condition, and dry particle diameter. The HTDMA was operated with a SR of 0.91. Figure 8 shows GF and κ as a function of Xd for particles generated at different engine thrust conditions and different fuels. The uncertainty in GF was 9 % for particles with diameter ∼10 nm and 3 % for the larger diameters (26 nm). The uncertainty in κ was 7 % and 2 % for particles with diameter ∼10 and ∼26 nm, respectively.

Gysel et al. (2007) state that H2SO4 is expected to retain water at 5 %–10 % relative humidity, corresponding to a growth factor of ∼1.15, and took this into account when calculating the mixed particle growth factor in their data. This procedure was similarly followed for the current dataset. Thus, the measured Xd values were scaled by a factor of 0.869.

For a given engine thrust condition, both GF and κ increased with increasing fuel sulfur content. GF and κ were also observed to be dependent on particle diameter, with the highest GF and κ for particles ∼ 10 nm, and decreasing for large particle diameters. This increase in GF and κ corresponds to the nucleation mode in the size distributions (Fig. 7), which was composed of particles or droplets formed by the homogeneous nucleation of low equilibrium vapor pressure species, such as H2SO4 and other water-soluble organic compounds. The GF and κ were also found to increase with increasing engine thrust condition for a given Xd, with the largest values observed at the 100 % engine thrust condition.

Gysel et al. (2003) reported GF of particles from a jet engine combustor burning three different fuels with 50, 410, and 1270 ppm of sulfur at two inlet temperature operating conditions: 566 and 766 K. These data are in good agreement with the current study for very low sulfur (HEFA and FT) fuels, conventional JP-8, and the sulfur-enhanced FT (FT plus THT), respectively.

A robust, mobile HTDMA system has been developed for field measurements that involve (1) particle sources that are very expensive to operate, (2) exhaust plumes influenced by wind speed and direction, and (3) varying meteorological conditions. The GF exhibited by particles of four inorganic salts was studied and found to be in good agreement with theory and with other experimental data reported in the literature. The fixed SR provided by the HTDMA system during laboratory evaluation (typically ∼0.98) was found to be quite constant over long periods of time, even when the ambient temperature varied considerably, making the MST HTDMA system suitable for field experiments. The HTDMA was demonstrated to perform a scan to determine GF and κ for one dry diameter in approximately 45 s. It performed scans over as many as 12 dry diameters sequentially in ∼9 min. The HTDMA system provided parameterization for hygroscopic properties for aircraft engine exhaust plumes in terms of GF and κ during the AAFEX II field campaign. It was observed that GF and κ (1) increased with fuel sulfur content, (2) increased with increasing engine thrust condition, and (3) decreased with increasing dry particle diameter.