The H3O+ ToF-CIMS and its operation during the SONGNEX campaign have been previously reported (Yuan et al., 2016a). The basic operational principle is the same as other PTR-MS instruments. H3O+ ions are generated from water vapor in a hollow cathode discharge ion source. The H3O+ reagent ions are then mixed with ambient air, containing VOCs, in a drift tube section. The proton from H3O+ is transferred to VOCs with sufficiently high proton affinity, and the resulting ionized VOCs are transferred to a mass analyzer (in the H3O+ ToF-CIMS, VOC ions are guided through a set of radio-frequency-only segmented quadrupoles to a time-of-flight unit). The hollow cathode, drift tube, pressure-controlled inlet, background, and calibration components are custom built; the ion transmission and time-of-flight analyzer were produced by Aerodyne Research Inc./Tofwerk. The H3O+ ToF-CIMS has a drift tube E/N (electric field strength divided by number density) of about 120 × 10-17 Td (V cm2). Data are presented in this work at 1 s time resolution unless otherwise noted.

During field operation, the instrument background was determined for 90 s every 20–40 min by flowing ambient air through a catalyst, and a 10-component gas standard was added for 90 s every 1–2 h (single-point calibration) to record instrument stability and sensitivity. In addition to the in-flight single-point calibrations, multi-point calibrations were performed at the beginning and end of each flight. A small amount of trichlorobenzene (C6H3Cl3) was continuously introduced into the instrument as a mass calibrant. Data were analyzed using Tofware high-resolution peak-fitting software (Aerodyne Research Inc./Tofwerk). Data were then corrected for humidity-dependent sensitivities and background-subtracted. In addition to the in-flight calibrations, laboratory calibrations of a larger number of species were performed using standard cylinders and permeation tubes. More details on the data quality assurance procedures related to instrument operation, background subtraction, humidity dependence, and calibration are included in Yuan et al. (2016a).

The H3O+ ToF-CIMS instrument has a mass resolution of approximately 3900–5900 m/Δm over the mass range discussed in this work (m/z 12–200), which is sufficient to determine the elemental formulas of most detected ion masses. Evaluation of data quality related to high-resolution peak fitting is discussed in the Supplement (Sect. S1). Also included in the Supplement are an extensive set of high-resolution mass spectra showing isobaric contributions to nominal masses, which will be useful to operators of both unit-mass- and high-resolution PTR-MS instruments (Fig. S5).

In this work we report signal intensity using normalized counts per second (ncps) and VOC mixing ratio (ppbv) for calibrated species. Normalized counts per second is the instrument signal relative to 106 H3O+ ion counts, corrected for humidity effects, and background-subtracted.

VOC sensitivities were determined by (1) direct calibration, where available; (2) calculated using proton-transfer rate constants, either known or calculated based on polarizability and dipole moment (Sekimoto et al., 2017); or (3) an average sensitivity determined from the calibrated and calculated sensitivities. Accuracy is within 15 % for directly calibrated compounds and generally within a factor of 2 for calculated sensitivities. Some ion masses, such as m/z 45.992 NO2+ (Sect. 3.4.4) and m/z 81.070 C6H8H+ (Sect. 3.4.3), have an ambiguous interpretation, and these are discussed in terms of instrument signal (ncps) and not mixing ratio.

Measured 1 s detection limits (for a signal-to-noise ratio of 3) range from approximately 40 pptv (aromatics) to 400 pptv (methanol). Some mixing ratios reported in this work are smaller than 40 pptv. In a few cases, the signal-to-noise ratio is less than 3, but variability is still discernable, and these species are discussed mostly for the absence of significant enhancement (e.g., styrene, cresol). Other species are presented as an average over a longer period of time: for example, average boundary layer enhancements presented in Fig. 5 and time series in Fig. 13. As averaging time increases, the limit of detection decreases: a typical aromatic compound with 1 s detection limit of 40 pptv (sensitivity of 500 count ppbv-1 s-1, background of 5 counts per second) has a 10 s detection limit of about 9 pptv (a calculation is included in the Supplement, Sect. S2).

In the WP-3D SONGNEX payload, the primary strengths of the H3O+ ToF-CIMS include measurement of small acids and carbonyls not detected by other instruments, and a much higher measurement rate of aromatics and cyclic alkanes, which were also measured by the whole air sampler, which typically collected a sample for 5 s once every 3 min (Lerner et al., 2017). Aromatics and compounds with heteroatoms are generally detected sensitively (aromatics: measured average of 180 ncps ppb-1; polar compounds: 200 ncps ppb-1) with a few exceptions of compounds that can dissociate by dehydration, such as small alcohols and aldehydes (de Gouw and Warneke, 2007). Cyclopentane and cyclohexane cannot be detected, but alkyl-substituted cyclic alkanes are detected, at approximately 5 % of the sensitivity of aromatics. Alkenes containing four or more carbon atoms are detected sensitively (estimated average of 300 ncps ppb-1), although larger alkenes can fragment at the high E/N conditions used in our instrument (Gueneron et al., 2015).

PTR-MS is notably limited in its measurement of alkanes: saturated alkanes smaller than hexane have too low a proton affinity to be detected, and C6 and larger branched- and straight-chain alkanes are detected with very low sensitivity, reacting with H3O+ at a rate 1 or 2 orders of magnitude slower than aromatics (Arnold et al., 1998). Additionally, the larger alkanes fragment extensively (Gueneron et al., 2015). This behavior makes alkanes difficult to measure sensitively and selectively. This is particularly limiting in measurements of emissions from oil and gas operations, where alkanes are typically dominant in terms of mixing ratio. NO2+ cannot be converted to a mixing ratio in a meaningful way and was excluded from the concentration comparison. For hydrocarbons, an average sensitivity was applied (Sect. 2.2.1).

During SONGNEX, fast measurement of methane and ethane was provided by cavity ring-down and direct-absorption spectroscopy (respectively), and speciated C2–C8 alkanes were measured by whole-air sampling/GC-MS. Data from a number of other chemical instruments that were deployed during the SONGNEX mission are used in this analysis. Descriptions of these instruments are given in Table 1. Further information can be found at https://esrl.noaa.gov/csd/groups/csd7/measurements/2015songnex/P3/datainfo.html.

Alkyl-substituted aromatic species are commonly measured by PTR-MS, and the interpretation of these ion masses is well established (de Gouw and Warneke, 2007; Blake et al., 2009). Aromatics can be important to the photochemical production of ozone in oil- and gas-producing regions, and can have high yields of secondary organic aerosol (SOA; Henze et al., 2008; Edwards et al., 2014). Quantifying the distribution of these species emitted from oil and gas sources is important to modeling work and to distinguishing between oil and gas and urban sources.

Figure 8 summarizes the distribution of C7–C10 alkyl-substituted aromatics relative to benzene measured during SONGNEX and compares the findings with other sources. This figure compares (1) measurements from all flights, (2) the average aromatics enhancement over the Permian during the 23 April flight, (3) fresh emissions sampled in a plume from a point source (a natural gas processing plant; 32.49∘ N, 101.35∘ W) on the 6 and 9 April Permian flights, (4) aromatics in the headspace of west Texas crude oil, (5) a literature survey of oil and gas sources, and (6) a literature survey of urban sources. The profiles of the average for all SONGNEX flights, and of aromatics measured in the Permian, are similar to the profile typically measured in oil- and gas-producing regions. The Permian profile is very similar to the composition of west Texas crude oil, and there is significant diversity in aromatics profiles between different oil and gas regions and sources. The average oil and gas profile is clearly differentiated from urban emissions (vehicular sources) by roughly equal enhancements of toluene and benzene, followed by a steady decrease in enhancement with increasing carbon number. The average oil and gas as well as urban composition shown in Fig. 8d were calculated by averaging all oil and gas sources (panels a and b) and all literature profiles shown in panel (c) for the urban profile.

The H3O+ ToF-CIMS also allowed for the investigation of more highly alkyl-substituted aromatics with 4 double-bond equivalents and less saturated aromatics, including PAHs and styrenes. During the 23 April Permian flight, we found that benzene and C7–C9 alkyl-substituted aromatics were by far the most abundant aromatic species, accounting for 95 % of total aromatic signal. These species were observed in both broad enhancements and in localized plumes from point sources. No PAHs with significant enhancement (above estimated 1 s detection limit of 30–40 pptv) were observed. Over all flights, styrene (m/z 105.070 C8H8H+) was consistently enhanced by up to 1.1 ppbv in plumes from point sources and up to 60 pptv during one leg of the 6 April flight, where it was not correlated with other aromatics.

Cycloalkanes are an important component of crude oil (Smith, 1968; National Research Council, 1985; Drozd et al., 2015) and have been detected by GC in significant concentrations in the atmosphere over oil- and gas-producing regions (Simpson et al., 2010; Gilman et al., 2013; Edwards et al., 2014). Although cycloalkanes are measured by PTR-MS with lower sensitivity than aromatics, the resulting product ions are still detectable. We find that cycloalkane product ions are not specific for cycloalkanes but may still be useful for characterizing the VOC composition of air masses and may be specific for cycloalkanes in relatively unaged air masses.

Several previous laboratory and field experiments have explored the PTR-MS response to cycloalkanes. PTR-MS is somewhat less sensitive to cyclic alkanes than to aromatics and oxygenates (Midey et al., 2003; Gueneron et al., 2015). At the E/N conditions in our instrument, cycloalkanes experience significant fragmentation, creating important product ions at m/z 69.070 C5H9+, 83.086 C6H11+, 85.101 C6H13+, 97.101 C7H13+, 111.117 C8H15+, and 125.132 C9H17+ (Midey et al., 2003; Warneke et al., 2003; Yuan et al., 2014; Gueneron et al., 2015). Cyclic alkanes have been measured with PTR-MS in crude oil using a GC interface (Yuan et al., 2014) and in ambient air in the Uintah Basin (Warneke et al., 2014); based on these measurements we expect that m/z 69.070 is produced generally by C6–C9 cycloalkanes; m/z 83.086 mainly from methylcyclopentane, cyclohexane, and methylcyclohexane; m/z 85.101 mainly by methylcyclopentane; m/z 97.101 mainly by methylcyclohexane; and m/z 111.117 and 125.132 by C9 and C10 cycloalkanes, respectively. These masses were clearly enhanced in H3O+ ToF-CIMS measurements over the Permian and other basins. Cumulatively, they account for 50 % of the hydrocarbon concentration (ppbv) measured by H3O+ ToF-CIMS.

Chemically specific measurements of methylcyclohexane and cyclohexane were made by iWAS/GC-MS. The comparison between the H3O+ ToF-CIMS and iWAS/GC-MS methylcyclohexane measurements shows that the H3O+ ToF-CIMS measurement is on average 3 times higher, and the correlation coefficient R2 is 0.71. The ratio of m/z 97.101 to iWAS methylcyclohexane is higher in the more aged air mass (Fig. 9a). This suggests that m/z 97.101 includes a significant contribution from a secondary VOC that fragments to produce C7H13+. Some possibilities include an aldehyde or alcohol with formula m/z 115.112 C7H14OH+ (e.g., heptanal or methylcyclohexanol) or a larger molecule (e.g., m/z 129.127 C8H16OH+ losing CH3OH).

Comparisons of methylcyclohexane between other GC and PTR-MS instruments previously deployed in the Uintah Basin, Utah, indicate that this behavior is not an H3O+ ToF-CIMS-specific issue and is not unique to the Permian Basin. In the winter of 2012, when photochemistry was less active (daily ozone formation of 16 ppbv and ozone below 51 ppbv), quadrupole PTR-MS m/z 97 measurements agreed with the methylcyclohexane measurement from a GC-MS instrument within 23 % (Warneke et al., 2014). However, in the winter of 2013, when photochemistry was much more active (daily ozone formation of 39 ppbv and 49 ozone exceedances) (Edwards et al., 2014), the relationship between PTR-ToF-MS m/z 97.101 C7H13+ and GC with flame ionization detector (GC-FID) methylcyclohexane was dependent on photochemical exposure (Fig. 9b).

The relatively high correlations of other cycloalkane indicator masses (m/z 83.086, m/z 69.070, m/z 111.117, etc.) with acetaldehyde suggest that they too experience interference from similar secondary products (Fig. 7). The secondary compounds could be cycloalkane oxidation products, or other alcohols or aldehydes, which fragment by losing a water molecule – a common PTR-MS fragmentation mechanism (Yuan et al., 2016a).

The H3O+ ToF-CIMS measurements during SONGNEX offer new insights into some VOC ion masses commonly measured by PTR-MS.

It is clear from the comparison to iWAS/GC-MS measurement of isoprene (Fig. 10) that m/z 69.070 C5H9+ is not isoprene, which is the dominant contributor in many air masses (de Gouw and Warneke, 2007; Blake et al., 2009). We interpret it as the sum of a cycloalkane fragment and a secondary compound from the oxidation of oil and gas precursor emissions; several other studies have also suggested non-biogenic interpretations of this mass (Yuan et al., 2014; Gueneron et al., 2015). Similarly, m/z  137.132 C10H16H+, usually interpreted as monoterpenes, was enhanced far above the monoterpene species (α- and β-pinene) detected by iWAS/GC-MS. m/z 137.132 C10H16H+ was also observed using a PTR-ToF-MS instrument in the Uintah Basin, UT, in winter of 2013 (Fig. S7). The Uintah observations indicate that this unknown species is emitted in several oil fields and is likely not an instrument artifact. Possible alternative sources of this VOC ion mass are (1) another isomer of C10H16 but not a monoterpene; (2) a monoterpene species not detected by iWAS/GC-MS; or (3) another species, such as an alcohol or aldehyde, which fragments to m/z 137.132. Each possibility is discussed in more detail below.

Other C10H16 isomer. Other, non-terpene isomers of C10H16 are found in fossil fuels. In particular, the presence of “diamondoids”, characterized by cage-like structures, is well known (Dahl et al., 1999; Araujo et al., 2012). The smallest diamondoid molecule, adamantane (tricyclo[3,3,1,1(3,7)]decane, C10H16), has a tetrahedral structure formed by three cyclohexane rings (Fig. 10). Adamantane can have concentrations from < 1 to 100 % of typical benzene concentrations in crude oil (Verma and Tombe, 2002; Araujo et al., 2012). In the Permian atmospheric measurement, m/z 137.132 C10H16H+ was up to 20 % of the concentration of benzene (assuming similar sensitivity to monoterpenes). Alkyl-substituted adamantanes are relatively abundant (Stout and Douglas, 2004; Wang et al., 2006). We detected enhancement of m/z 151.148 C11H18H+, but larger molecules were below the detection limit of the H3O+ ToF-CIMS during this flight. The measured headspace of west Texas crude oil showed relative enhancements of n(CH2)-substituted C10H16 that are more consistent with the adamantane series than with terpenes, where we would expect to see enhancements at every n(C5H8), but the evidence is not conclusive (Fig. S8). Adamantane has a longer retention time than the GC elution time used for the iWAS samples, so iWAS/GC-MS was not able to confirm or exclude the presence of this molecule.

The H3O+ ToF-CIMS also measured other hydrocarbon signals that could not be tied to a single VOC, and these hydrocarbon masses comprise a significant fraction of the total hydrocarbon signal (Fig. 5). These other important hydrocarbon ion masses include m/z 41.039 C3H4H+, m/z 43.054 C3H6H+, m/z 57.070 C4H6H+, and m/z 71.086 C5H10H+. Unit-mass-resolution PTR-MS has not been able to investigate these ions because of strong interference from isobaric masses, such as C2H2OH+ at m/z 43.018. Several studies have reported that these masses are non-specific and are produced by alkenes, as well as fragmentation of alkanes, alkenes, aldehydes, and alcohols (Buhr et al., 2002; Jobson et al., 2005; Gueneron et al., 2015). Laboratory tests with the H3O+ ToF-CIMS confirmed that multiple alkane and alkene VOCs present during the 23 April flight (as measured by iWAS/GC-MS) fragment to these masses. These ion masses had significant intensity in both the southwestern (less aged) and eastern (photochemically enhanced) areas of the flight, suggesting contributions from fragments of both primary and secondary species.

Our interpretation of the most abundant photochemically produced ion masses is consistent with interpretations often presented in the literature: m/z 45.034 C2H4OH+ is specific for acetaldehyde, m/z 59.049 C3H6OH+ for acetone, m/z 73.065 for MEK, m/z 61.028 C2H4O2H+ for acetic acid, and m/z 75.044 C3H6O2H+ for propionic acid. Some of these compounds might also have direct sources from industrial solvent use, but during the 23 April flight the photochemical source was dominant, because they were only found enhanced over background in the areas where PAN and other photochemical products were also elevated (Sect. 3.2).

The carbonyl species with more than three carbon atoms have both ketone and aldehyde isomers, and both isomers could be products of alkane oxidation. For example, oxidation of propane in the presence of NOx is expected to yield approximately 26 % propanal and 74 % acetone (Calvert et al., 2008). The ratio of propanal to acetone has been measured to be 0.17 (Uintah Basin, 2012, Edwards et al., 2013), 0.19 (Uintah Basin, 2014), and 0.22 (Denver–Julesburg Basin, 2011; Gilman et al., 2013) in oil- and gas-producing regions, compared to 0.06 in an urban area (Los Angeles; Borbon et al., 2013). However, almost all of the signal at H3O+ ToF-CIMS m/z  59.049 C3H6OH+ can be attributed to acetone. The PTR-MS reaction of H3O+ with propanal is dissociative, and the sensitivity of the H3O+ ToF-CIMS to propanal at m/z 59.049 is only 3 % that of acetone. Acetone therefore dominates the signal at this mass, and we assume similar behavior for higher-mass carbonyls.

Abundances of larger masses in the 1-double-bond-equivalent, 1-oxygen-homologous series decrease rapidly with carbon number; these are most likely also carbonyls derived from alkanes. m/z 43.018 C2H2OH+ was identified as a fragment of acetic acid from laboratory tests.

Glycolaldehyde (m/z 61.028 C2H4O2H+) and hydroxyacetone (m/z 75.044 C3H6O2H+) could be interferences with acetic acid and propionic acid, respectively. However, these compounds have been mainly reported in environments affected by biogenic emissions and biomass burning, and acetic acid has been shown to be the dominant contributor to m/z 61.028 C2H4O2H+ in several environments (Karl et al., 2007; Fu et al., 2008; Haase et al., 2012).

Aromatic oxidation products are of particular interest, as they have been shown to be an important source of radicals that drive ozone formation in oil- and gas-producing regions (Edwards et al., 2014). From laboratory and chamber studies, expected aromatic oxidation products include various diketones, phenols and nitrophenols, benzaldehyde-type compounds (from toluene and larger aromatics), and furanones (Wagner et al., 2003; Bloss et al., 2005; Wyche et al., 2009; Yuan et al., 2016b). PTR-MS can detect phenols, benzaldehydes, and furanones (Müller et al., 2012); detection of some dicarbonyls, such as glyoxal and methylglyoxal, may be difficult due to fragmentation, strong humidity dependence, and interference from other species (Pang et al., 2014; Stönner et al., 2017).

By far the most abundant aromatic oxidation product detected was m/z 107.049 C7H6OH+, benzaldehyde (max 360 pptv). Phenol (C6H6O), cresol (C7H8O), and methylfuranone (C5H6O2) were detected at only an estimated 30–40 pptv maximum enhancement. Cresols, diketones, and furanones are expected to have a much higher yield at the observed NOx concentrations (average of 1.1 ppb) (Smith et al., 1998; Müller et al., 2012) but are also much more reactive (Bierbach et al., 1994; Atkinson and Arey, 2003).

Müller et al. (2012) reported several unidentified masses resulting from chamber oxidation of trimethylbenzenes: m/z 87.044 C4H6O2H+ and m/z 101.060 C5H8O2H+. These masses were also detected during the 23 April SONGNEX flight, and they were quite significant relative to other oxygenates – m/z 87.044 at approximately 10 % of the signal intensity of acetic acid. We classify these as aromatic oxidation products but do not have enough information to suggest a structure.

The H3O+ ToF-CIMS detected several potential cycloalkane oxidation products as shown in Fig. 11. The 2-double-bond-equivalent, 1-oxygen-homologous series (CnH2n-2OH+) does not decrease monotonically with carbon number: the species with four carbons (m/z 71.049 C4H6OH+) and six carbons (m/z 99.080 C6H10OH+) are the most abundant (Fig. 11a). In addition, m/z 71.049 C4H6OH+ has a somewhat different spatial distribution than other oxygenates, including much higher intensity in the central part and sections of the northern part of the field, and may have a primary source (Sect. 3.4.4). The enhancement of the C6 oxygenate points to cycloalkane precursors. Similar ion masses were measured by the PTR-ToF-MS instrument deployed in the Uintah Basin, UT, in 2013 (Fig. 11b).

The smallest cycloalkane that exists in significant amounts in fossil fuels is cyclopentane (C5), but we might expect the C6 products to be more abundant because C6 precursors can be more abundant than C5 in oil fields (Simpson et al., 2010; Gilman et al., 2013). Additionally, OH reacts faster with substituted cycloalkanes (starting at C6, methylcyclopentane) than unsubstituted cycloalkanes, although this probably has a smaller effect on ambient composition than the emissions composition (Calvert et al., 2008). Figure 11 also shows the cycloalkane precursors in the Uintah Basin, measured by GC techniques (Edwards et al., 2013, 2014). The suggested oxygenated products have a similar distribution. Data are not available from iWAS/GC-MS to show a similar precursor distribution for the Permian flight.

The C6 cycloalkane oxidation product (C6H10OH+) could be a ketone (methylcyclopentanone or cyclohexanone) or cyclopentyl aldehyde. Aldehyde ions often fragment in PTR-MS by loss of H2O (Buhr et al., 2002). The fragmentary product of C6H10OH+ dehydration, m/z 81.070 C6H8H+, was significantly enhanced and correlated with photochemical species. Similar ions (m/z 95.086 C7H10H+, m/z 109.101 C8H12H+, etc.) also correlated with photochemical species and may also be fragments of cycloalkyl aldehydes. These ion masses were also seen by the PTR-ToF-MS in Utah in 2013 and showed behavior consistent with photochemical species (Fig. S7).

It has been previously reported that PTR-MS detects PAN at the protonated parent mass m/z 122.008 C2H3NO5H+, at m/z 77.023 C2H4O3H+, and at m/z 45.992 NO2+. Similarly, peroxypropionyl nitrate (PPN) is detected at m/z 91.039 C3H6O3H+ (Hansel and Wisthaler, 2000; de Gouw and Warneke, 2007; Kaser et al., 2013). However, m/z 77.023 may include contributions from another species, such as acetone water cluster or peroxyacetic acid, and m/z 45.992 is not expected to be universally specific to PAN (de Gouw and Warneke, 2007; Kaser et al., 2013).

During the April 23 SONGNEX flight over the Permian, there was no peak detected at m/z 122.008 C2H3NO5H+. m/z 77.023 C2H4O3H+ and 91.039 C3H6O3H+ were detected at moderate and low intensities (4 and 2 ncps average enhancement, respectively), and m/z 45.992 NO2+ was one of the most abundant ions detected (28 ncps average enhancement, similar to the average enhancement of benzene). M/z 77.023, m/z 91.039, and m/z 45.992 are compared to several independent measurements of reactive nitrogen: PAN, PPN, NOy, NOy–NOx, and ethyl and propyl alkyl nitrates in Fig. S9. When looking at all the SONGNEX flights, the slope of m/z 45.992 NO2+ vs. NOy–NOx (which represents all NOx oxidation products, including PAN) and the slope of m/z 77.023 vs. PAN vary significantly between different environments. Figure 12 shows the relationship between m/z 77.023 and PAN. Comparison of m/z 45.992 NO2+ with PAN is included in the supplemental information (Fig. S10). The slope of m/z 45.992 vs. NOz depends on the composition of NOz (Fig. S11a). The range of slopes probably depends on the atmospheric variability of NOz, not instrument conditions: the slope of m/z 77.023 vs. PAN is not dependent on instrument conditions such as drift tube humidity (Fig. S11b), which is consistent with behavior reported by Hansel and Wisthaler (2000).

These measurements suggest that these product ions cannot be attributed to PAN-type compounds only. Product ions at m/z 45.992 NO2+ almost certainly derive from a number of NOz species, with a range of response factors, and there are probably at least two species that contribute to m/z 77.023 C2H4O3H+.

An especially interesting observation is the broad enhancement of several nitrogen-containing organic species during the Permian flights. The most clearly enhanced ion, m/z 70.065 C4H7NH+, has signal intensity approximately 15 % that of m/z 93.070 C7H8H+, toluene (Fig. 13).

C4H7N has several possible isomers, each with two degrees of unsaturation: a cyclic alkene structure (pyrroline) and several non-cyclic structures (C4 nitriles). Using a mass spectral context, we suggest a cyclic structure for this compound. Time series of the homologous series (CnH2n-1NH+) including C4H7NH+ are shown in Fig. 13. The species having one, two, and three carbon atoms are not enhanced above background, whereas species containing four or more carbon atoms are enhanced and correlated with aromatic compounds (Figs. 13, S12). A cyclic structure is the most likely explanation for this pattern. A similar argument can be made for the homologous series (CnH2n-3NH+) having three degrees of unsaturation (pyrroles). The series with two (CnH2n+1NH+, pyrrolidines) and four (CnH2n-5NH+, pyridines) degrees of unsaturation were not significantly enhanced. Notably, m/z 84.081 C5H9NH+ had a different distribution than other cyclic nitrogen compounds during the 23 April flight, with high mixing ratios in the eastern and western parts of the field, and overall correlated better with the photochemically produced species. We speculate that this mass may include a fragment of a photochemical product, analogous to the interference with cycloalkanes discussed in Sect. 3.2.2. Pyrroline (m/z 70.065) was enhanced by up to 200 pptv, and its concentration was typically comparable to C8 and C9 aromatics. Pyrrole (m/z 68.050) had a maximum enhancement of 40 pptv.

Organic nitrogen species detected over the Permian may have originated from the crude oil. American shale oils can contain upwards of 2 % nitrogen by weight, and a number of aromatic organic nitrogen species have been quantified in crude oil (Morandi and Jensen, 1966; Holmes and Thompson, 1983; Mushrush et al., 1999; Williams and Chishti, 2001). Porphyrins, nitrogen-containing geochemical fossils derived from chlorophyll, are known to exist in crude oil and contain both pyrrole and pyrroline as subunits (Fig. 14); aromatic nitrogen species may also form through chemical processes in the oil reservoir. There are large differences in nitrogen content and speciation between reservoirs, which could explain why these species were detected in some regions and not in others (Baxby et al., 1994; Li et al., 1995; Oldenburg et al., 2007).

The reactivity of pyrroline is not known. We analyzed the relative decrease in C4H7N in the western half of the field as a function of OH exposure (see Sect. 3.1) and estimated the rate constant with OH to be in the range of (1.3–2.0) × 10-11 cm3 molecule s-1 with a best estimate of 1.7 × 10-11 cm3 molecule s-1 (details in the Supplement). Saturated nitriles have much slower reactivity with OH – reported values are around 2 × 10-14 cm3 molecule s-1 (Harris et al., 1981; Atkinson et al., 2006) – so this high OH reactivity is further evidence that C4H7N is not a nitrile. A similar analysis of C4H5N gave a rate constant of 2.5 × 10-11 cm3 molecule s-1, which is much slower than the reported value for pyrrole (1 × 10-10; Wallington, 1986) but faster than butenenitrile (1.4 × 10-11; Grosjean and Williams, 1992). The pyrrole analysis is much less certain than that of pyrroline, due to the overall lower signal and higher noise. The rate constants for pyrroline and especially pyrrole could be significantly underestimated. In previous work, it has been shown that for fast-reacting species the derived rate constant was approximately constant and similar to the rate of the fastest-reacting aromatic used to calculate OH exposure (de Gouw et al., 2005). In our analysis, the fastest-reacting aromatic considered was C10 aromatics, with a rate constant of 2.4 × 10-11 cm3 molecule s-1. This may explain why the derived rate constant for pyrrole was lower than expected.

Heterocycles are highly reactive with nitrate radicals, especially pyrrole (k=4.6 × 10-11 cm3 molecule-1 s-1) (Atkinson et al., 1985; Cabañas et al., 2004). If pyrroline is similarly reactive, then nitrogen heterocycles could potentially dominate VOC nitrate reactivity in oil and gas fields, because most other VOCs measured during SONGNEX (aromatics and aliphatics) have low reaction rates with nitrate radicals (k∼10-18–10-15 cm3 molecule-1 s-1). A comparison of nitrate reactivity for species measured during SONGNEX and a comparison to nitrate loss rates reported in the literature are given in the Supplement (Fig. S13, Sect. S4). We looked for species with two nitrogen atoms, but at our instrument resolution they are extremely difficult to separate from isobaric hydrocarbon species unless they have very high signal intensity.

Measurement of H2S with PTR-MS has been described by Li et al. (2014). The H3O+ ToF-CIMS improves on the instrument described by Li et al. (2014) as the high mass resolution avoids the isobaric background interference from isotopes of methanol and HO2+. H2S is detected at m/z 34.995 H2SH+ and was calibrated directly using a standard cylinder.

A comparison with the Picarro H2S instrument is shown in Fig. 15. There is good quantitative agreement between the two measurements. Compared to the Picarro instrument, the H3O+ ToF-CIMS H2S measurement is more precise, and the 1 s data are simultaneous with other H3O+ ToF-CIMS measurements, allowing easy comparison. The H3O+ ToF-CIMS H2S measurement had a 3σ detection limit of 2.3 ppbv for a 1 s measurement (or 0.8 ppbv over the 6 s measurement period of the Picarro instrument).

H2S had a maximum concentration of 12.6 ppb during the 23 April Permian flight, comparable to butanes. No other sulfur-containing species measurable with the H3O+ ToF-CIMS were enhanced above an estimated 1 s 3σ detection limit of 30 pptv. (A few sulfur-containing species, such as m/z 121.032 C4H8SO2H+, have nonzero intensity but are instrument contaminants).

Methanol was the most abundant VOC detected by H3O+ ToF-CIMS during the 23 April Permian flight. In this section we discuss some possible sources.

Methanol is used by the oil and gas industry. Significant primary emissions, especially from produced water storage infrastructure and storage containers on well pads, have been measured in the Uintah Basin (Warneke et al., 2014; Mansfield et al., 2016). Industry uses of methanol include addition at well heads or further downstream in pipelines to prevent methane hydrate formation (Anderson and Prausnitz, 1986), to inhibit corrosion and scaling, as a lubricant, and as a solvent in other applications (Mansfield et al., 2016).

Globally, the dominant net source of methanol is direct biogenic emission, although there is a substantial and poorly constrained source from secondary production and oceans (Jacob et al., 2005; Millet et al., 2008). Primary biogenic emissions can explain the high mixing ratios over the Haynesville region, but not over the Permian, given the absence of other biogenic VOCs.

Lewis et al. (2005) calculated rates of photochemical production of methanol from a set of VOC precursors, the most important of which were methane, isobutane, isopentane, and acetaldehyde. The magnitude of photochemical methanol production in the Permian was estimated by scaling the Lewis et al. (2005) precursor concentrations to the highest observed Permian VOC concentrations during the 23 April flight. Only 1–2 ppb of methanol would have formed after 2–3 days of aging – a small amount compared to the measured average (6 ppb) and maximum (19 ppb). It should be noted that the Lewis et al. (2005) calculations were in the remote marine boundary layer, and the photochemical production rate in the Permian could be different. Methanol does have a much stronger correlation with photochemical species than with primary aromatic species (Fig. 7), but it also has a relatively long atmospheric lifetime.

m/z 71.049 C4H6OH+ is typically interpreted as the sum of methyl vinyl ketone (MVK) and methacrolein, two carbonyl products of isoprene oxidation (de Gouw and Warneke, 2007), but isoprene was too low to produce the measured amount of C4H6OH+ in the Permian Basin. C4H6OH+ had a spatial distribution that differed from photochemically produced species, including a much larger enhancement in the central part of the flight and several north–south-oriented plumes running the length of the surveyed area (Fig. 2). Additionally, C4H6OH+ was enhanced above what might be expected from the distribution of precursor species and the enhancements of similar oxygenates (Fig. 11). The enhancement in the more aged area of the field suggests that a large part of this ion signal is generated by a photochemical species, but there may be other contributions. Using the H3O+ ToF-CIMS average sensitivity factor for MVK and methacrolein, the maximum boundary layer enhancement of this species was 540 pptv.

The other carbonyl isomer, crotonaldehyde, has been reported in biomass burning emissions (Karl et al., 2007), which were not evident during this flight. It is possible that MVK, methacrolein, or crotonaldehyde could be directly emitted by anthropogenic sources, or photochemically derived from a non-biogenic species. The cyclic isomer, dihydrofuran, is the oxygenated analogue of the nitrogen heterocycle (pyrroline) discussed in Sect. 3.4.1. Dihydrofurans are known to be products of OH oxidation of alkanes (Lim and Ziemann, 2005).

Preliminary iWAS/GC-MS measurements show strong correlation between MVK, methacrolein, and m/z 71.049 C4H6OH+. The appropriate mass for dihydrofuran was not included in the selected-ion-scan window of the iWAS/GC-quadrupole MS. Other evidence is needed to identify the predominant isomer(s) and determine if there is a significant interference with PTR-MS measurements of biogenic MVK and methacrolein in oil- and gas-producing regions.