Introduction
Concentrated animal feeding operations (CAFOs) emit many volatile organic
compounds (VOCs) into the atmosphere, including carboxylic acids, alcohols,
carbonyls, phenolic compounds, sulfur- and nitrogen-containing compounds
(Hobbs et al., 2004; Filipy et al., 2006; Sun et al., 2008; Ni et al., 2012).
These VOCs can contribute to the formation of ozone (Howard et al., 2010a, b;
Gentner et al., 2014) and fine particles (Sintermann et al., 2014; Perraud et
al., 2015), both affecting regional air quality. Many VOCs from CAFOs are
also responsible for the unpleasant odor problems nearby or downwind of these
facilities (McGinn et al., 2003; Rabaud et al., 2003; Parker et al., 2010;
Woodbury et al., 2015). Some VOCs (e.g., phenolic species) (US EPA, 2017)
from CAFOs are harmful to human health.
There are a large number of potential VOC sources inside a CAFO, potentially
including animal exhalation, animal waste in animal pens, flushing lanes,
lagoons, silage storage piles and silos, and feed mixtures in feed lanes and
bunks (Alanis et al., 2008; Chung et al., 2010). Early studies mainly focused
on VOC emissions from animal waste (e.g., slurry and manure) under laboratory
conditions (Hobbs et al., 1997, 1998, 2004). Ngwabie et al. (2007, 2008)
reported that VOC concentrations in dairy, sheep and pig CAFOs were the highest
during animal waste removal and feeding, indicating that large emissions were
related to these activities. Recent studies found that VOC concentrations in
dairy farms were significantly higher near silage and piles of animal feed
(i.e., total mixed rations) than near other places (animal pens, lagoons and
flush lanes), suggesting that feed-related sources dominate VOC emissions
(Alanis et al., 2008; Chung et al., 2010). Enhancements of some VOCs (e.g.,
acetone) in animal sheds are also related to animal exhalation (Shaw et al.,
2007; Ngwabie et al., 2008; Sintermann et al., 2014). However, the
contributions of different sources to individual VOC emissions from a
facility are not accurately known (Ngwabie et al., 2008). This poor
understanding of VOC sources hinders the development of management practices
that reduce VOC emissions in animal feeding facilities (Ngwabie et al.,
2007). Thus, a comprehensive characterization of VOC sources and their
relative importance within a CAFO is needed.
Many studies of VOCs from animal feeding operations have been conducted with
offline analytical methods (Filipy et al., 2006; Alanis et al., 2008; Sun
et al., 2008; Chung et al., 2010; Ni et al., 2012). VOCs were collected on
filters, or in canisters and cartridges and were quantified in the
laboratory using various methods (see reviews in
Ni et al., 2012). These offline methods are labor-intensive, which limits
the number of VOC samples. Online fast measurement techniques (mainly
proton-transfer reaction mass spectrometers, PTR-MS) allow for more detailed
investigation of CAFO facilities (Shaw et al., 2007; Ngwabie et al.,
2008; Sintermann et al., 2014). The previous online measurements usually
used a single stationary sampling inlet either inside a stall or at a fence
line, which does not provide spatial distribution information for VOCs in
the facilities.
In this study, we deployed a high time-resolution instrument on board a
mobile laboratory driven on public roads and a NOAA WP-3D research aircraft
to measure VOCs downwind of CAFO facilities. We will use this data set to
characterize chemical compositions of VOC emissions and explore different
sources within the facilities that contribute to VOC emissions.
Experiments
Mobile laboratory measurements were conducted near Greeley in northeastern
Colorado, USA. Six different CAFOs were studied, including two dairy farms,
two beef feed yards, one sheep feed yard and one egg-laying chicken farm
(Table S1 in the Supplement). Among the six CAFOs, emissions of NH3, N2O and
CH4 in four facilities (the two dairy farms, one beef cattle feed yard
and the sheep feed yard) have been measured previously using an instrumented
van (Eilerman et al., 2016). We added a new VOC instrument to the
payload, and performed mobile measurements in wintertime (February, 2016)
for the six CAFOs by sampling at their downwind flanks 1–2 times for each
facility. Duplicated measurements at the same facilities agreed well.
VOCs were measured using a hydronium ion time-of-flight chemical-ionization
mass spectrometer (H3O+ ToF-CIMS) instrument on the mobile
laboratory. Here, we provide a brief description of the instrument
(see details in Yuan et al., 2016). VOCs are ionized by
H3O+ ions in a drift tube, similar to a PTR-MS (de Gouw and
Warneke, 2007). The protonated product ions are detected using a
high-resolution time-of-flight (ToF) analyzer (Tofwerk AG) (m/Δm=4000–6000). A number of VOC species were calibrated using either
gravimetrically prepared gas cylinders or permeation tubes (see
details in Yuan et al., 2016). VOC background signals in the instrument were
determined by passing ambient air through a catalytic converter. The
detection limits are compound-dependent and range between 10 and 100 ppt for
most VOC species at a time resolution of 1 s. Besides VOCs, two inorganic
species, NH3 and H2S, were measured at m/z 18.034 (NH4+)
and m/z 34.995 (H3S+) using the H3O+ ToF-CIMS, respectively
(Li et al., 2014; Müller et al., 2014).
In addition to the H3O+ ToF-CIMS, a cavity ring-down spectrometer instrument (Picarro G1301m) measuring methane (CH4) and carbon
dioxide (CO2) and an off-axis integrated cavity output spectrometer (Los Gatos Research) measuring nitrous oxide (N2O) along with
carbon monoxide (CO) were deployed during the mobile laboratory
measurements. Measurements of ambient temperature, relative humidity, wind
direction, wind speed and vehicle location were performed using
meteorological sensors (R.M. Young 85004 and AirMax 300WX) and a GPS compass
system (ComNav G2B). A summary of meteorological conditions during the
mobile laboratory measurements is shown in Table S2.
(a, b) Drive track of mobile laboratory color- and
size-coded by NH3 (a) and ethanol (b) concentrations
around a beef feed yard (beef no. 1). The prevailing wind is shown by wind
barbs (light blue flags) in the map. (c) Time series of NH3,
CH4, CO2, N2O, ethanol, acetic acid and acetone measured
downwind of the beef feed yard. Numbers (1–4) in panels (a) and
(c) are used to allow for alignment of the mobile laboratory locations
on the map with the corresponding time series in panel (c).
Measurements of agricultural plumes were also performed using the NOAA WP-3D
research aircraft in March–April 2015 during the Shale Oil and Natural Gas
Nexus (SONGNEX) campaign. Data from three flights (28 March, 29 March, 13 April)
over northeastern Colorado are used in this study. VOCs were measured
using the same H3O+ ToF-CIMS instrument as mobile measurements
(Yuan et al., 2016). Another chemical ionization mass spectrometer
(CIMS) was used to detect NH3 during SONGNEX
(Nowak et al., 2007). Due to background issues
and lower concentrations, NH3 signals were not retrievable from
H3O+ ToF-CIMS during the SONGNEX campaign.
Results and discussions
Spatial distributions from mobile laboratory measurements
Figure 1 shows measured concentrations of NH3, CH4, CO2,
N2O, ethanol (C2H5OH), acetic acid (CH3COOH) and acetone
(CH3COCH3) around a beef feed yard (beef no. 1). The
concentrations of the seven species were enhanced and highly variable
downwind of the facility. Different time variations for the seven species
were clearly observed. NH3 concentrations peaked at around 13:46 local
time (LT) and the peak location was northwest of this facility, directly
downwind of the animal pens. This is consistent with fresh waste of animals
(urine and feces) as the main source of NH3 within a CAFO facility
(Hristov et al., 2011). CO2 and CH4 are emitted
from animal respiration and eructation of the cattle (Shaw et al., 2007;
Sintermann et al., 2014; Owen and Silver, 2015). CO2 (R=0.77) and
CH4 (R=0.77) correlated well with NH3 between 13:42 and 13:46 LT when
NH3 was high. These observations reflect the fact that animals and
their fresh waste may be largely co-located in the animal pens. But, waste
cleaning time/practices in the facility were unknown, owing to no access to
the facility. Previous mobile and aircraft measurements have also observed
enhancements of NH3 and CH4 concentrations downwind of animal pens
in cattle feedlots (Miller et al., 2015; Hacker et al., 2016). The time
variations of two VOCs, acetic acid and acetone, followed reasonably well
with both NH3 and CO2, suggesting that animals and their waste
contributed to the enhancements of the two VOCs. Based on previous studies,
the emissions from animal respiration should dominate over waste for
acetone, and vice versa for acetic acid (Ngwabie et al., 2008; Sintermann
et al., 2014). The similar time variations of NH3, CO2, acetone
and acetic acids (and less clearly for CH4) imply that the co-located
emissions from animals and their waste may not be separated based on the
variations observed (over a short time span of several minutes) while
measuring along the downwind flanks.
(a, b) Drive track of mobile laboratory color- and
size-coded by NH3 (a) and ethanol (b) concentrations
downwind of a dairy farm (dairy no. 1). The prevailing wind is shown by wind
barbs (light blue flags) in the map. (c) Time series of NH3,
CH4, CO2, N2O, ethanol, acetic acid and acetone measured
downwind of the dairy farm. Numbers (1–4) in panels (a) and
(c) are used to allow for alignment of the mobile laboratory locations
on the map with the corresponding time series in panel (c).
A single, narrow high-concentration spike (up to 1 ppm) of ethanol was
observed around 13:44 LT (Fig. 1). The hotspot of ethanol was located
downwind of a feed mill at the west side of the feed yard, indicating that
the feed mill and its related activities can emit large amounts of ethanol.
Distillers grains, a fermented by-product from ethanol production, are
commonly used as an ingredient of feed in beef cattle feed yards (Raabe,
2012)
(although not known specifically here). Therefore, it is not surprising to
observe large emissions of ethanol in the feed mill area. High concentrations
of many other VOC species (e.g., acetic acid and acetone) were observed in the
feed mill plume, whereas NH3, CH4 and N2O were not enhanced.
Combustion sources, possibly due to equipment operation in the feed mill
area, are likely responsible for the enhancement of CO2 and CO at
∼ 13:44 LT. VOC emissions from these combustion plumes are negligible
(see details in the Supplement, Fig. S1).
Measurements downwind of a dairy farm (dairy no. 1) are shown in Fig. 2.
The highest concentrations of NH3 and CO2 were observed downwind
of the animal pens, similar to the beef feed yard shown in Fig. 1.
Interestingly, several high concentration peaks of ethanol were observed
along the drive track of the mobile laboratory. These peaks were in close
proximity or downwind of the feed lanes (white lines on the satellite image,
Fig. 2b). As shown for beef no. 1, feed mills can be an important source
of ethanol and other VOCs. Different from the usage of distillers grains in
beef cattle feed yards, silage is more commonly used as fodder for dairy
cattle (Raabe, 2012). Previous studies showed that ethanol is the
most abundant VOC species emitted from feed silage (Hafner et
al., 2013). It is expected that VOCs will continue evaporating from the feed
mixtures after the feed is delivered to the feed lanes. Time variations of
acetic acid (and acetone) correlated more closely with ethanol (R=0.72)
than with NH3 (R=-0.30) and CO2 (R=-0.14), which differs from
the beef feed yard. This suggests that the three VOCs were mainly from
emissions of feed lanes, rather than animals and their waste in this dairy
farm.
The fractional contributions of different VOC classes to the total
VOC concentrations (a), odor activity values (b), OH
reactivity (c) and NO3 reactivity (d) for the six
investigated CAFO sites. The mean wind speeds during the measurements of the
CAFO sites are shown in panel (e). The mean values for the five
parameters from each CAFO are shown at the top of each panel.
In addition to animals and their waste (referred to as animal+waste
hereafter) and feed storage and handling (referred to as feed
storage+handling hereafter), we identified another important VOC source
from the other dairy farm studied (dairy no. 2, Fig. S2). High
concentrations of ethanol, acetone, dimethyl sulfide (DMS, C2H6S)
and CH4 were observed downwind of three milking parlors. Acetic acid
was only moderately elevated, whereas NH3 was not enhanced. Compared to
feed storage+handling, emission compositions from the milking parlors are
different. The emissions from milking parlors might result from several
sources, including animal exhalation and milking-related activities. It is
worth noting that we did not distinctly observe emissions from the milking
parlor in the dairy farm no. 1, which were potentially mixed with emissions
from feed prior to sampling.
The measurements downwind of three other CAFO sites (beef no. 2, the sheep
feed yard and the chicken house) are investigated in a similar way, as shown
in Figs. 1–2 (and Figs. S2–S3). From this analysis, we identify three main
VOC emission sources in animal feeding facilities, namely animal+waste,
feed storage+handling and milking parlors. These measurements suggest that
combustion sources are not important for VOC emissions in these facilities.
VOC chemical compositions of different CAFOs
The enhancements of VOCs downwind of each CAFO were integrated to determine
the averaged VOC compositions for each facility (Fig. 3). The measured VOC
species are divided into six different groups, namely carboxylic acids,
alcohols, carbonyls, phenolic species, nitrogen- and sulfur-containing
species. The averages of the sum of measured VOC concentrations downwind of
the sites are in the range of 22–139 ppb, with higher concentrations at
dairy farm no. 1 and the two beef feed yards (wind speeds during the
measurements were similar except at the chicken house, Fig. 3e and Table S1).
As demonstrated in Fig. 3a, alcohols (55–87 %, mole fractions) and
carboxylic acids (4–32 %) represent major classes of VOC from these CAFOs.
Other VOC classes account for 8–21 % of VOC concentrations in total.
As discussed in Sect. 1, VOC emissions from CAFOs can contribute to
unpleasant odor problems and ozone formation. We utilize odor activity value
(OAV) and the OH and NO3 reactivity to evaluate the relative
contribution of each VOC class to the two environmental effects,
respectively. The dimensionless OAV is estimated from VOC concentrations
divided by the species' single compound odor thresholds (SCOT)
(OAVi=Ci / SCOTi) (Feilberg et al., 2010; Parker et al.,
2010; Woodbury et al., 2015). The reported SCOT values in the literature are
highly variable and we use the geometric means of literature values for each
compound, as compiled in Parker et al. (2010). The
averaged total OAVs from all measured VOCs are in the range of 0.8–5.5 at
different sites (Fig. 3b). Sulfur-containing species contribute the
largest fractions (51–91 %) to total OAVs at different sites, followed by
phenolic species (6–37 %) and carboxylic acids (3–11 %). The relative
contributions of different species to OAV agree well with previous estimates
based on measurements from a beef feed yard (Woodbury et al.,
2015).
OH reactivity (OHR) and NO3 reactivity (NO3R) are determined as the
products of VOC concentrations and the respective reaction rate constants of
VOCs with the two oxidants (Atkinson et al., 2006) (OHRi=Ci×kOH,i; NO3Ri=Ci×kNO3,i). The averaged OH
reactivities range between 1 and 10 s-1, which is comparable or lower than
the typical OHR observed in urban areas (a few s-1 to 50 s-1) (Yang et al., 2016). Alcohols are the largest contributors
(40–75 %) to OHR at the sites, although the fractions from
carbonyls, phenolic and sulfur-containing species are also significant
(Fig. 3c). These results are generally consistent with the finding that
ethanol accounts for the majority of ozone formation potential of VOC
emissions from a dairy farm (Howard et al., 2008). The
averaged NO3 reactivities range from 0.02 to 0.26 s-1, which are
remarkably higher than in urban areas (usually < 0.01 s-1)
(Tsai et al., 2014; Brown et al., 2016). In contrast to the OH
reactivity, phenolic species account for the largest fractions (66–90 %)
of the NO3 reactivity for all of the sites, with remaining
contributions primarily from sulfur- and nitrogen-containing species (Fig. 3d).
We note that OAV, OH and NO3 reactivity are measured along the
fence line and they decrease rapidly with downwind distance and dilution
(see example in Sect. 3.4 for aircraft measurement results associated with
a factor of ∼ 10 lower concentrations than those from mobile
laboratory).
Relative importance of different sources for VOC emissions
As shown in Sect. 3.1, ethanol was primarily emitted from feed
storage+handling (and milking parlors), whereas NH3 and CO2 were
attributed to emissions from animals and their waste. This suggests that
these species can be used as tracers to separate the emissions from sources.
However, there are two issues that need to be considered: (1) emissions of
animal exhalation and waste are largely co-located in the animal pens. As
CO2 is also emitted from combustion sources (see details in the Supplement) and
animal exhalation is only important for a few species (e.g., acetone),
NH3 will be used as a tracer for the emissions from animals and their
waste. It is worth mentioning that long-term measurements in CAFO facilities
could permit separation of the two co-located sources (see
example in Sintermann et al., 2014). (2) There is some ethanol attributable
to animal+waste emissions that needs to be accounted for. Ethanol
concentrations solely from feed emissions ([C2H5OH]Feed) can be
calculated by subtracting the contribution of ethanol by animal+waste from
measured ethanol concentrations (see details in the Supplement, Fig. S4).
The relative contributions of feed storage+handling,
animal+waste and milking parlors (only for dairy farm no. 2) to emissions
of different VOC species and total VOC for the investigated CAFO sites. DMDS:
dimethyl disulfide; DMA: dimethylamine; TMA: trimethylamine.
After correcting ethanol for animal+waste emissions, the contributions of
emissions from feed storage+handling and animal+waste to measured VOC
enhancements at each individual site can be determined using multivariate
linear fits to [C2H5OH]Feed and NH3 concentration ([NH3]).
[VOC]=ERC2H5OH×[C2H5OH]Feed+ERNH3×NH3+bg
Here, [VOC] and [bg] are measured concentrations of the VOC
species and the background concentration outside the CAFO plumes,
respectively. ERC2H5OH and ERNH3 are the emission
ratios of the VOC species relative to ethanol and NH3 from the
emissions of feed storage+handling and animal+waste, respectively. Along
with [bg], the emission ratios are determined from the multivariate linear
fits.
Based on the fitted parameters from Eq. (1) (and Eq. S3 for dairy farm no. 2),
the relative contributions of different sources to the enhancements of
various VOC species can be calculated for the investigated sites (Fig. 4).
In general, large differences in fractional contributions to VOC
enhancements exist among both different VOC species and different animal
types. The main findings from Fig. 4 are as follows:
Phenol, cresols, butanediones and many nitrogen-containing species are
primarily associated with animal+waste emissions for the investigated
sites.
Both feed storage+handling and animal+waste account for significant
fractions of emissions of many oxygenated VOCs and sulfur-containing
species.
Based on the results from the dairy farm no. 2, emissions from milking
parlors contribute significantly to the enhancements of a limited number of
VOC species, including ethanol (23 ± 1 %), acetone (35 ± 3 %),
acetaldehyde (31 ± 3 %), methanol (18 ± 3 %), MEK (14 ± 2 %) and DMS (14 ± 2 %).
Feed storage+handling plays important roles in the emissions of many VOC
species from the chicken farm. Based on a news report on the facility, a
manure belt system is used to manage manure in this facility. The manure belt
system catches the excreta from chicken to transport manure to a separate
location for storage. The chicken houses with manure belts usually lead to
substantially lower emissions (e.g., NH3) from animal waste (Wood et al.,
2015). It is consistent with significantly lower NH3 concentrations
(0–175 ppb, Fig. S3) at this site compared to ruminant feed yards measured
in this study (0–1000 ppb), although wind speed was 36–60 % higher
during measurements of the chicken house (7.5 m s-1) than others
(4.7–5.5 m s-1). It is also possible that emissions of NH3 and
VOCs were treated when in-house air was ventilated out (Wang et al., 2010).
We further determine the contributions of emissions from feed
storage+handling, animal+waste and milking parlors to the total VOC
concentrations (first columns in Fig. 4, also Fig. S5). Feed
storage+handling emissions account for 35–41, 23–30, 13 and
41 % of the summed total VOC concentrations for the investigated dairy
farms, beef feed yards, the sheep feed yard and the chicken farm in this
study, respectively. The fractional contributions from the sources to odor
activity value, OH reactivity and NO3 reactivity are also calculated
(Fig. S5). The contributions from feed storage+handling emissions to the
three parameters are generally comparable or slightly smaller than those
contributions to the total VOC concentrations.
In addition to the information on relative contributions from different
sources, the multivariate fit analysis also provides the emissions ratios of
VOCs to NH3 for animal+waste and emission ratios of VOCs to ethanol
for both feed storage+handling and milking parlors (see Tables S3–S8).
These emission ratios represent chemical “fingerprints” of the emissions
from various sources. The emission ratios are summed up for the VOC classes
and the fractions of each VOC class in different source emissions are
determined (Fig. 5). Overall, VOC emissions from both feed
storage+handling and milking parlors are dominated by alcohols, whereas
the contributions of carboxylic acids and other VOC classes are
significantly larger for animal+waste emissions. The VOC compositional
fractions shown in Fig. 3a for each site are the weighted average of the
fractions for different sources in Fig. 5.
The relative contributions of each VOC class to emissions from feed
storage+handling (a), animal+waste (b) and milking
parlors (c).
We acknowledge that there are some limitations in separating different
sources inside each facility using measurements from the mobile laboratory,
which may introduce some uncertainties to the results.
In this study, the relative fractions of different sources to VOC
emissions are determined based on snapshots of measurements when the mobile
laboratory passed by the CAFO sites. The relative fractions may change over
time, and may be related to operation activities within the facilities, such
as feed-mixing activities in feed mill area. Nevertheless, some encouraging
evidence was observed: the determined relative fractions from different
sources are reasonably similar between the two beef feed yards. The
agreements between the two dairy farms are not as good as for the two beef
feed yards. The abovementioned observations of the emissions from milking
parlors and potential differences in feed ingredients for dairy cattle,
which are reflected by the discrepancies in VOC compositions emitted from
feed storage+handling (Fig. 5a), could be the reasons.
VOCs from various sources in a CAFO site are mostly emitted at the
surface. VOCs were measured on the van at a single level near-ground
(∼ 3 m). However, the plumes from CAFOs become deeper as they
are transported downwind, and VOC concentrations are vertically diluted by
background air due to turbulent mixing. As shown in Figs. 1–2 (and Figs. S2–S3),
the feed mills and milking parlors at the sites studied in this work
are located nearby public roads, and their contributions may be somewhat
overestimated as a result.
Time series of NH3 and various VOC species of two agricultural
plumes measured from NOAA WP-3D on 13 April 2015 during the SONGNEX
campaign.
(a) Scatterplot of acetic acid versus NH3 from the
three SONGNEX flights in northeastern Colorado. The two black lines and
gray-filled areas indicate emission ratios of acetic acid to NH3 from
beef feed yards and dairy farms determined from the mobile laboratory
measurements, respectively. (b) Comparison of enhancement ratios of
VOCs to NH3 between mobile laboratory and aircraft measurements in
northeastern Colorado.
Aircraft measurements
Time series of NH3 and several VOCs inside two agricultural plumes in
northeastern Colorado measured on 13 April 2015 from the NOAA WP-3D during
the SONGNEX campaign are shown in Fig. 6. Large enhancements of NH3
were observed in the two agricultural plumes, although the peak
concentrations were a factor of ∼ 10 lower than those from
mobile laboratory measurements. VOC species, including acetic acid,
propionic acid and ethanol, were also clearly elevated in the two plumes. As
the aircraft was further away from the CAFOs, the emissions from different
sources inside CAFO facilities have been well mixed prior to sampling. Thus,
separation of the VOCs sources from different parts of the operations
occurring within the facilities is not possible using aircraft measurements.
Figure 7a shows scatter plots of acetic acid versus NH3 from the
three flights over northeastern Colorado during the SONGNEX campaign. The
correlation between acetic acid and NH3 is strong for all of the three
flights (R=0.81–0.87). Two different enhancement ratios of acetic acid to
NH3 were observed from aircraft measurements, which are close to the
determined emission ratios from beef feed yards (30.2 ± 5.5 × 10-3 ppb ppb-1) and dairy farms
(6.4 ± 0.6 × 10-3 ppb ppb-1) from mobile laboratory measurements, respectively. It implies that
the enhancement ratios of acetic acid to NH3 may be used as an
indicator for emissions from different animal types. The relative
contributions to NH3 enhancements between dairy and beef cattle can be
estimated based on data in Fig. 7a. The fractional contributions to
NH3 enhancements from beef cattle are estimated in the range of
0.71–0.98 based on the three SONGNEX flights (28 March: 0.98 ± 0.01;
29 March: 0.71 ± 0.11; 13 April: 0.96 ± 0.02). Combining the three
SONGNEX flights in northeastern Colorado, beef cattle contribute 90 ± 4 %
of measured NH3 enhancements on these flights. This evidence
suggests that beef cattle are more important for NH3 emission from
CAFOs in northeastern Colorado.
The enhancement ratios of other VOC species relative to NH3 are also
calculated from aircraft measurements and they are compared with those from
mobile laboratory measurements in Fig. 7b. The determined enhancement
ratios of carboxylic acids and alcohols compare well between aircraft and
mobile laboratory measurements. The enhancement ratios of acetone and
acetaldehyde to NH3 are more scattered in aircraft measurements, as the
agricultural plumes contributed only small enhancements of these species
over a high background. The enhancement ratios of phenol, cresol, CH3SH
and DMS to NH3 from aircraft measurements are lower than those from
mobile laboratory measurements. The measured concentrations of these species
from aircraft measurements were low. The signals of these species were only
slightly higher than instrument noise levels at higher concentrations range
of NH3 (∼ 150 ppb; see example in Fig. 6), but not
detectable in many plumes with lower NH3 concentrations.