Herein we report on the first successful airborne deployment of
the CHemical Analysis of AeRosol ONline (CHARON) particle inlet which allowed us to measure the chemical
composition of atmospheric submicrometer particles in real time using a
state-of-the-art proton-transfer-reaction time-of-flight mass spectrometry
(PTR-ToF-MS) analyzer. The data were collected aboard the NASA DC-8 Airborne
Science Laboratory on 26 June 2018 over California in the frame of NASA's
Student Airborne Research Program (SARP). We show exemplary data collected
when the airplane (i) shortly encountered a fresh (
Proton-transfer-reaction mass spectrometry (PTR-MS) is a well-established
technique for online and real-time detection of organic trace gases in the
Earth's atmosphere (de Gouw and Warneke, 2007; Yuan et al., 2017; and
references therein). One of the main advantages of PTR-MS is the rapidness
at which the air can be analyzed. Measurement frequencies of 1 to 10 Hz make
PTR-MS instruments ideally suited for deployment on fast moving platforms
such as airplanes. The first airborne deployment of a PTR-MS analyzer dates
back to 1998 (Crutzen et al., 2000). Nowadays, most in situ tropospheric
chemistry payloads of research aircraft include a PTR-MS instrument. Our
group has been flying PTR-MS analyzers on NASA airplanes (DC-8, P-3B, C-130)
since 2008. We developed and deployed the first airborne
proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS)
instrument (Müller et al., 2014). More recently, we conceived and built
the CHARON (CHemical Analysis of AeRosol ONline) particle inlet which enables PTR-MS analyzers to also
measure and chemically characterize submicrometer particles in real time
(Eichler et al., 2015). In ground-based studies, we have successfully used
the CHARON inlet in combination with a PTR-TOF 8000 analyzer for measuring
(i) particle mass concentrations of total organics, ammonium and nitrate;
(ii) aerosol bulk average
The data presented herein were collected on a flight that the NASA DC-8
carried out in California on 26 June 2018 (DC-8 flight number 1271). The
flight track is shown in the Supplement (Fig. S1). We note that, while SARP
provided an excellent opportunity for test-flying the CHARON inlet, the
flight plan was neither designed for best characterizing the instrument's
performance nor for addressing specific science questions related to the
chemical composition and transformation of organic particles in the
atmosphere. Herein we present data from three selected flight segments.
During one flight segment, the NASA DC-8 shortly penetrated the smoke plume
emanating from the Lions Fire (Wikipedia, 2018) in the Sierra Nevada (see photograph shown in Fig. S2). The fire was burning in
red fir and brush, in an area of heavy blowdown from a 2011 wind event
(InciWeb, 2018). The airplane overflew the fire in the plume direction and
dropped into the plume at a safe distance, 14.3 km downwind of the source.
This converts into a plume travel time of
In the San Joaquin Valley, the NASA DC-8 flew race track patterns in the boundary layer over the greater Bakersfield area which include oil and gas extraction areas to the N and NW of the city and agricultural areas to its S and SE. The first race track pattern included a missed approach (down to 50 m a.g.l.) at the Meadows Field Airport, which is located approximately 5 km NW of downtown Bakersfield.
Ambient air was sampled into the DC-8 through a forward facing shrouded
solid diffuser inlet known in the literature as the UH/LARGE inlet. The
inlet and its characteristics have been described in detail by McNaughton et
al. (2007). The authors of that study found that the inlet transmits
particles with unit efficiency up to particle sizes in the micrometer (
The CHARON particle inlet for PTR-ToF-MS instruments was described in detail
elsewhere (Eichler et al., 2015) and we will only give a brief description
here. The CHARON inlet consists of a gas-phase denuder, an aerodynamic lens
and a particle vaporizer. Sampling air is first passed through an activated
carbon monolith denuder, which removes gaseous organics and transmits
particles larger than 50 nm. The aerodynamic lens efficiently enriches
particles in the 150 to 1000 nm range in the instrument subsampling flow,
with typical enrichment factors ranging from 35 to 45. Particles in the 60
to 150 nm range are less efficiently concentrated in the subsampling flow.
The enrichment factor is determined in an external calibration with
size-selected particles that are counted using a condensation particle counter (CPC). Due to a misadjustment, the enrichment factor was only 12 (8 for the Lions Fire plume
encounter due to the reduced sampling pressure) for the study presented
herein. The diameter of the critical orifice at the entrance of the
aerodynamic lens was increased by 10 % to account for the lower sampling
pressure from the pressure-controlled inlet. Particles were vaporized at a
temperature of 150
The airborne PTR-ToF-MS instrument used for this study was described in
detail by Müller et al. (2014). Since the time of publication of this
method paper, the instrument has been upgraded with an ion funnel at the end
of the drift tube (ION BOOSTER, Ionicon Analytik, Innsbruck, Austria) and a
hexapole interface (ION GUIDE, Ionicon Analytik, Innsbruck Austria) to the
orthogonal acceleration region of the time-of-flight analyzer. These
changes have resulted in a 15 to 20 fold increase in sensitivity (Schiller,
2018). In addition, geometrical changes were made for improving the mass
resolving power (
The PTR-TOF data analyzer (Müller et al., 2013) was used
for the initial data analysis (mass axis calibration, iterative residual
peak analysis and quantification, and assignment of molecular sum formula to
Carbon monoxide (CO) was measured at 10 Hz by a fast commercial
In this section, we will demonstrate that the CHARON PTR-ToF-MS instrument is fast and sensitive enough for measuring and chemically characterizing pollution plumes from a jet research aircraft.
Figure 1 includes two graphs showing the data that were obtained when the
NASA DC-8 shortly penetrated the smoke plume emanating from the Lions Fire
in the Sierra Nevada over California. Figure 1a shows the 1 Hz
averaged time trace of carbon monoxide (CO). The DC-8 shortly penetrated the
fresh smoke plume three times, resulting in three CO peaks of 15, 15 and 3 s
width and maximum mixing ratios exceeding 800, 600 and 300 ppb,
respectively. For safety reasons, the DC-8 only skimmed the plume edge which
explains why the observed CO enhancements were below 1 ppm. Figure 1b shows
the 1 Hz time traces of the 10 CHARON PTR-ToF-MS signals (in raw counts per
seconds, cps) with the highest mass loadings. The 90 s signal average recorded
immediately before the plume penetrations was subtracted from each of the
signals to yield the excess signal in the plume. Apart from this background
subtraction, no other signal processing has been applied. As explained in
the Methods section, the instrument suffered from an elevated background during
this flight which explains the relatively high noise in some of the signals
(e.g.,
The 1 Hz time series of the
Figure 2 shows times series of a set of processed data. The black and the
red traces in Fig. 2a show the uncorrected and corrected excess total
organic mass concentrations, respectively. The fragmentation correction (see Methods section) increases the total organic mass concentration by
The 1 Hz time series of excess mass concentrations of
The average mass spectrum depicted in Fig. S4 shows that most of the analyte
ion signals were observed in the
Figure 2b shows the 1 Hz time trace of the total mass concentrations associated with monoaromatic and polyaromatic ions, respectively, as well as ions generated from polycyclic aromatic hydrocarbons (PAHs). The reader is cautioned to observe the definitions, assumptions and limitations of our classification method given in Sect. 2.5. Monoaromatics (mostly composed of methoxyphenols and substituted methoxyphenols) account for 14 %–18 % of the total organic mass concentration, 8 %–13 % of the total mass is associated with polyaromatics (hydroxy- and dihydroxynaphthalene being the most abundant species), and PAHs account for 4.7 % of the total mass. The carbon atom and aromaticity equivalent distributions are shown in Fig. S5.
The association the
Figure 3 shows the CHARON PTR-ToF-MS data that were obtained when the NASA
DC-8 passed 0.85 km downwind of a petroleum refinery located in Kern County,
SE of Bakersfield. The insert shows that the predominant wind direction was
SSW. The airplane was flying at an altitude of 630 m a.g.l. Three atypical
organo-nitrogen ions,
Spatial plot showing the concentration enhancements of
Figure 4 shows the geographic distributions of ammonium, nitrates and total
organic mass concentrations as measured during the late morning (Fig. 4a, c, e) and early afternoon (Fig. 4b, d, f) hours, respectively, in the
boundary layer of the San Joaquin Valley. The inserts in Fig. 4a and b show that the prevalent wind direction shifted from SW to NW between
the morning and afternoon measurements. Individual points represent 10 s
data averages. In the morning, elevated levels of ammonium and nitrate were
observed over the city of Bakersfield and in the oil region, NE of
Bakersfield. The enhancement over the oil region may be caused by local
emissions or the wind pushing urban pollution towards the Sierra Nevada
foothills. Ammonium and nitrate were typically observed in the
stoichiometric ratio of ammonium nitrate. Only over the oil region, a higher
ammonium-to-nitrate ratio was observed, suggesting the additional presence
of sulfates. The concentration of total organics peaked in the same two regions, reaching a
maximum of only 2.2
Mass concentrations of ammonium
During the afternoon measurements, the wind had shifted and the air
temperature had increased by 2–3
Figure 5 shows the vertical profile data (100 m bins, shading indicates
lower and upper quartiles) obtained during a missed approach at the Meadows
Field Airport, which is located approximately 5 km NW of downtown
Bakersfield. The data were taken between 10:48 and 10:55 local time, 17:48 and 17:55 UTC, when the planetary boundary layer extended to an altitude of 800 m a.g.l.
Above the boundary layer, only organic aerosol was observed, with a maximum
mass concentration of 0.40
Vertical profiles (in 100 m bins, shading indicates lower and upper quartiles) of ammonium, nitrate and total organics as observed during a missed approach at Meadows Field Airport, which is located approximately 5 km NW of downtown Bakersfield, California. The data were taken in the period between 10:48 and 10:55 local time, 17:48 and 17:55 UTC.
We have successfully carried out the first test deployment of a CHARON
PTR-ToF-MS instrument on a jet research aircraft. Most importantly, the data
recorded during a test flight indicate that the instrument measures fast
enough to be deployed on a jet research aircraft. The data obtained
during short encounters of 3 to 15 s duration with particle plumes emanating from
a small wildfire and from a refinery, respectively, demonstrate the
feasibility of airborne point or small area source emission measurements.
Further improvements are, however, warranted to eliminate or reduce the
observed signal tailing (1/e decay time between 6 and 20 s). Recent test
measurements by the instrument manufacturer with a yet undisclosed treatment
of all wetted stainless steel surfaces indicate that the response time can
be reduced by at least a factor of 2 compared to the data shown in this work
(Piel et al., 2019). Nonetheless, the work presented herein serves
as proof of concept that CHARON can indeed be flown and generate useful
data. The recorded high time resolution data allowed us to generate highly
spatially resolved maps (1–2 km in the horizontal, 100 m in the vertical) of
atmospheric particle chemical constituents. Exemplary data shown in this
work include (i) highly time-resolved mass concentrations of ammonium, nitrate
and total organics; (ii) highly time-resolved mass concentrations of classes of
organic compounds (CH vs. CHO vs. CHN vs. CHNO compounds; aliphatic vs. monoaromatic vs. polyaromatic compounds); (iii) bulk aerosol average
The capability of the CHARON PTR-ToF-MS instrument to chemically characterize submicrometer atmospheric particles in a quantitative manner, at the near-molecular level and in real time brings a new and unprecedented measurement capability to the airborne atmospheric science community.
All data are available from the corresponding author upon request.
The supplement related to this article is available online at:
FP, MM, TM and AW designed the instrument adaptations for airborne use. FP and MM prepared the instrument for the test flight. TM and FP installed the instrument on the DC-8. FP and AW carried out the measurements. FP and MM reduced and analyzed the data. FP, MM and AW interpreted the data and conceived the paper. FP and AW wrote the paper, with comments from MM. SP provided the CO data.
Felix Piel and Markus Müller both work for Ionicon Analytik, which is commercializing CHARON PTR-ToF-MS instruments. Armin Wisthaler and Markus Müller both profit from a license agreement (CHARON inlet) between the University of Innsbruck and Ionicon Analytik.
Special thanks go to Barry Lefer, manager of NASA's Tropospheric Composition Program (TCP), who supported and encouraged us to test-fly our CHARON PTR-ToF-MS system. We also thank Melissa Yang Martin, Emily Schaller, Adam Webster, David Van Gilst and Steven R. Schill from the National Suborbital Research Center (NSRC) for hosting us on the NASA DC-8 during the SARP-2018 flights and for providing logistical and technical support. We further acknowledge instrumental and technical support by Bruce Anderson and Eddie Winstead from the NASA Langley Aerosol Research Group (LARGE) and by Charles (Chuck) Brock, Matt Richardson and Christina Williamson from the NOAA ESRL Chemical Sciences Division. Don Blake from UC Irvine is acknowledged for logistical support. We also thank the pilots and crew of the NASA DC-8. Our collaboration with NASA has profited from long-year financial support by the Austrian Federal Ministry for Transport, Innovation and Technology (BMVIT) through the Austrian Space Applications Programme (ASAP) of the Austrian Research Promotion Agency (FFG).
FP has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement no. 674911.
This paper was edited by Bin Yuan and reviewed by two anonymous referees.