Atmos. Chem. Phys., 19, 1867–1880, 2019https://doi.org/10.5194/acp-19-1867-2019© Author(s) 2019. This work is distributed underthe Creative Commons Attribution 4.0 License.

Importance of biogenic volatile organic compoundsto acyl peroxy nitrates (APN) production in thesoutheastern US during SOAS 2013Shino Toma1, Steve Bertman1, Christopher Groff2, Fulizi Xiong2, Paul B. Shepson2,3, Paul Romer4, Kaitlin Duffey4,Paul Wooldridge4, Ronald Cohen4, Karsten Baumann5, Eric Edgerton5, Abigail R. Koss6,7,a, Joost de Gouw6,Allen Goldstein8, Weiwei Hu7,9, and Jose L. Jimenez7,9

1Department of Chemistry, Western Michigan University, Kalamazoo, MI, USA2Departments of Chemistry, Purdue University, West Lafayette, IN, USA3Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, West Lafayette, IN, USA4Department of Chemistry, University of California, Berkeley, CA, USA5Atmospheric Research & Analysis, Inc., Cary, NC, USA6NOAA ESRL Chemical Sciences Division, Boulder, CO, USA7Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO, USA8Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, USA9Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO, USAanow at: Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA

Correspondence: Steve Bertman (steven.bertman@wmich.edu)

Received: 25 April 2018 – Discussion started: 10 July 2018Revised: 6 December 2018 – Accepted: 16 December 2018 – Published: 12 February 2019

Abstract. Gas-phase atmospheric concentrations of perox-yacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), andperoxymethacryloyl nitrate (MPAN) were measured on theground using a gas chromatograph electron capture detector(GC-ECD) during the Southern Oxidants and Aerosols Study(SOAS) 2013 campaign (1 June to 15 July 2013) in Centre-ville, Alabama, in order to study biosphere–atmosphere in-teractions. Average levels of PAN, PPN, and MPAN were169, 5, and 9 pptv, respectively, and the sum accounts for anaverage of 16 % of NOy during the daytime (10:00 to 16:00local time). Higher concentrations were seen on average inair that came to the site from the urban NOx sources to thenorth. PAN levels were the lowest observed in ground mea-surements over the past two decades in the southeastern US.A multiple regression analysis indicates that biogenic volatileorganic compounds (VOCs) account for 66 % of PAN for-mation during this study. Comparison of this value with a0-D model simulation of peroxyacetyl radical production in-dicates that at least 50 % of PAN formation is due to iso-prene oxidation. MPAN has a statistical correlation with iso-prene hydroxynitrates (IN). Organic aerosol mass increases

with gas-phase MPAN and IN concentrations, but the massof organic nitrates in particles is largely unrelated to MPAN.

1 Introduction

Acyl peroxy nitrates (carboxylic nitric anhydrides) (APN,RC(O)OONO2), products of the photooxidation of volatileorganic compounds (VOCs) in the presence of nitrogen ox-ides (NOx ≡ NO+NO2), play an important role in the chem-istry of both gases and particles in the troposphere. Measure-ments around the world have shown that APN can comprise15 %–40 % of total gas-phase oxidized nitrogen (NOy) in ru-ral and forested areas, including the eastern United Statesdepending on how aged the air sampled at the site is (e.g.,Trainer et al., 1993; Nouaime et al., 1998). The fraction de-pends on air mass history and conditions, since wet deposi-tion of nitric acid, the other major component, will affect to-tal NOy levels. APN are ubiquitous reservoir species for NOxand radicals in the atmosphere that affect the lifetime of NOx

Published by Copernicus Publications on behalf of the European Geosciences Union.

1868 S. Toma et al.: APN in the southeastern US during SOAS 2013

and enhance tropospheric O3 formation (Carter et al., 1981).In addition, they are significant health hazards for both hu-mans and plants (Finlayson-Pitts and Pitts, 2000; Kleindienstet al., 1990).

The peroxyacyl (PA) radicals that are the immediate pre-cursors of APN can form directly from aldehydes via hydro-gen abstraction or from intermediate fragmentation of largerorganic compounds and hence can have both anthropogenicand biogenic origins (Roberts, 2007). Peroxyacetyl nitrate(PAN) is the simplest and most abundant of the APN, typ-ically accounting for more than three-quarters of the totalAPN concentration. Scission of the thermally weak bond toNO2 regenerates the PA radical (Reaction R1). Permanentloss of PAN occurs when the PA radical is lost (Reaction R2).With sufficient NOx , this occurs via reduction by NO, andgas-phase PAN thermal lifetime, assuming steady-state forPA, depends on the ratio NO/NO2 as Eq. (1). Peroxy radi-cals compete with NO for reaction with PA at low NOx .

PAN PA+NO2 (R1)PA+NO−→ Products (R2)

−d[PAN]

dt= k1[PAN]

1−1

1+(

k2[NO]k−1[NO2]

) (1)

Peroxypropionyl nitrate (PPN) and peroxymethacryloyl ni-trate (MPAN) also are observed in the field (e.g., Nouaimeet al., 1998; Pippin et al., 2001; Roberts et al., 2002).PAN is formed from both anthropogenic and biogenic hy-drocarbon precursors. PPN, on the other hand, is formedprimarily from anthropogenic hydrocarbons (AHCs) (e.g.,propanal, propane, 1-butene), while MPAN is derived frommethacrolein (MACR), an oxidation product of the mostlybiogenic hydrocarbon (BHC) isoprene (e.g., Biesenthal andShepson, 1997; Carter and Atkinson, 1996).

Recent laboratory experiments have suggested that OH re-action with the double bond of MPAN could be involved inthe formation of secondary organic aerosol (SOA) (Chan etal., 2010; Kjaergaard et al., 2012; Lin et al., 2013; Nguyen etal., 2015; Surratt et al., 2010; Worton et al., 2013). This path-way is currently treated in a few models that include isoprene(e.g., Pye et al., 2013, 2015; Jenkin et al., 2015; Wennberget al., 2018), although isoprene is the biogenic non-methanehydrocarbon with the greatest global emission rate (Guentheret al., 1995), the contribution of isoprene photooxidation tosecondary organic aerosol formation may be underestimated.

We measured APN concentration during the SOAS 2013campaign to characterize the systematic behavior and levelsof individual APN species at an urban-impacted forest and toassess the current state of the attribution of PAN formationto biogenic and anthropogenic precursors quantitatively us-ing several statistical methods. Finally, we compared MPANwith another nitrogen compound in the gas phase, total iso-prene hydroxynitrates (IN), and with organic nitrates or total

organic aerosol (OA) in the particle phase to investigate re-lationships that might explain their influence on SOA forma-tion.

2 Experiment

Ground-based measurements were conducted from 1 June to15 July 2013 at the Southeastern Aerosol Research and Char-acterization (SEARCH) Centreville (CTR) site, which is lo-cated in mixed deciduous–evergreen forest in the TalladegaNational Forest near Brent, Alabama (lat: +32◦54′11.81′′,long: −87◦14′59.79′′). The major anthropogenic influenceat this site comes from the cities of Tuscaloosa and Birm-ingham, which are located 50 km northwest and 80 kmnortheast, respectively. The meteorological conditions of theground site have been described in detail in Carlton etal. (2018) and Hidy et al. (2014).

Measurements of APN using similar methods to those de-scribed below were made in Dickson, TN, from 15 Juneto 14 July, 1999 as part of the Southern Oxidants Study(SOS) (Cowling et al., 1998) and are referred to in the text.The site is in an area of mixed deciduous forest and pas-ture land located near Montgomery Bell State Park about60 km west–southwest (upwind) of downtown Nashville, TN(Chen, 2001). While in a different part of the southeast, webelieve that the distance from major urban areas makes thissite a good comparison.

APN were quantified using a custom gas chromatograph(GC) equipped with a Shimadzu GC-Mini-2 63Ni electroncapture detector (ECD) maintained at 55 ◦C (described byNouaime et al., 1998). A polar column (RESTEK, Rtx-200, 15 m× 0.53 mm ID× 1 µm) was kept at 15 ◦C to min-imize thermal decomposition of APN compounds. Heliumwas used as a carrier (8 cm3 min−1) with N2 make-up gas(3 cm3 min−1). Ambient air was drawn through a 1/4 in. ODPFA Teflon tube from 8.2 m height above the ground at 1SLPM and a subsample of this air was drawn through a1 cm3 sample loop at 50 sccm. The residence time was ap-proximately 9 s. The sample loop contents were injected intothe column at 20 min intervals via a six-port Teflon valve(Hamilton). The baseline and sensitivity of the GC-ECDwere checked every day during the campaign using standardaddition of gas streams from liquid standards added to ambi-ent air scrubbed through a charcoal trap at the beginning ofthe sampling line. In this way, the impact of the inlet line wasaccounted for in the calibration. Separate calibrations wereperformed with synthetic PAN, PPN, and MPAN samples indodecane or tridecane maintained at ice water temperature indiffusion cells. The level of analyte in each synthetic com-pound was determined with a chemiluminescence NOx ana-lyzer (Themo Environmental Instruments, Inc., Model 42S)equipped with a Mo converter held at 325 ◦C. The con-verter efficiency was tested by O3 titration of NO to NO2.Calibration of the NOx analyzer was done against a NIST-

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S. Toma et al.: APN in the southeastern US during SOAS 2013 1869

traceable cylinder of 2 ppmv NO in N2 (SCOTT-MARRIN,INC). Based on sensitivity and background measurements,the detection limits (S/N = 2) for PAN, PPN, and MPANwere estimated as 2.5, 3.6, and 3.9 pptv, respectively. Uncer-tainty determined by error propagation, most of which camefrom the chemiluminescence NOx analyzer, was estimated tobe 20 % RSD.

Measurements of other trace gases, such as NOy (totalreactive nitrogen measured by catalytic conversion), NOx ,and O3, wind direction, temperature, and boundary layerheight were made by Atmospheric Research & Analysis,Inc. (ARA) as described by Hidy et al. (2014). Boundarylayer heights were calculated by ARA using lidar back-scatter measurements from a Lufft CHM 15k ceilometer.Total isoprene hydroxynitrate (IN) concentrations were de-termined by Purdue University using a chemical ionizationmass spectrometer (CIMS) with operating conditions de-scribed by Xiong et al. (2015). VOC measurements, includ-ing isobutane, isopentane, MACR, and isoprene, were mea-sured by NOAA ESRL Chemical Sciences Division and Uni-versity of California, Berkeley (Goldstein group) using aGC-MS. Particle-phase organic nitrates (pONs) were mea-sured by University of California, Berkeley (Cohen group)using thermal dissociation laser-induced fluorescence (TD-LIF), described by Rollins et al. (2010), and by Univer-sity of Colorado with a high-resolution time-of-flight aerosolmass spectrometer (HR-ToF-AMS) described by DeCarlo etal. (2006) and Hu et al. (2015). A comparison in Lee etal. (2016) found that the pONs-TD-LIF was generally higherby a factor of ∼ 5 than pONs-HR-ToF-AMS. Both sets ofdata provide a reasonable range of pONs concentration. To-tal OA mass was measured using HR-ToF-AMS.

3 Results

3.1 General behavior of APN in 2013

Figure 1 shows a time series of PAN, PPN, and MPANthroughout the campaign. Data that were below detectionlimit (BDL) are plotted at half of the reported detection limitfor that compound. This was done to distinguish the BDLpoints from missing data due to tests, calibrations, and theperiodic existence of a noise interference that often appearedduring this campaign and could not be eliminated, and to notlose the low concentration information content. Relativelyhigh levels of APN were observed as periodic spikes dur-ing the campaign, but overall APN levels were lower thanmost other measurements in the southeast made over the last20 years (see Sect. 3.2). A local biomass burning event wasobserved on 4 June (Washenfelder et al., 2015), which re-sulted in an unusually high level of PAN of around 1600 pptvand an extreme deviation from the median. Hence, the dataon 4 June were removed from statistical analyses.

Figure 1. Time series of PAN, PPN, MPAN , O3, and NOx , andtemperature during SOAS 2013 campaign. Data that were belowdetection limit (BDL) are included at half of the detection limit.

General descriptive statistics for all daytime data are sum-marized in Table 1. Daytime was defined as 10:00 to 16:00local time (CDT). PAN was consistently the most abun-dant peroxyacyl nitrate compound, the mean daytime lev-els accounting for approximately 90 % of total APN. In Ta-ble 1, “APN” describes the sum of individual PAN, PPN, andMPAN values. The average of the ratio of APN /NOy duringdaytime was 0.16. Peroxyacryloyl nitrate (APAN) was alsoobserved occasionally during the campaign. APAN has beenproposed to arise from 1,3-butadiene, either from anthro-pogenic sources or biomass burning, and from direct emis-sion of acrolein (Roberts et al., 2001; Tanimoto and Aki-moto, 2001). Our data did not show a strong relationshipto biomass burning events, as identified by Washenfelder etal. (2015), although an instrument interference problem lim-ited the amount of reportable APAN data, so no clear conclu-sion can be drawn.

Although sampled air most frequently came from the southduring the SOAS 2013 campaign, air from the north con-tained levels of APN that were twice as large as from south.The averages of PAN, PPN, and MPAN with air from thenorth were 182, 5.3, and 8.4 pptv, respectively, while aver-

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1870 S. Toma et al.: APN in the southeastern US during SOAS 2013Table

1.Summ

aryofselective

SouthernO

xidantsStudy

(SOS)data

since1990.M

ean±

standarddeviation.

Year

19901990

19921994

19951999

19992013 2

Elberton,G

AR

OSE

,AL

RO

SE,A

LN

ewH

endersonville,TN

Youth

Inc.,TN

Dickson,T

NC

orneliaFortA

irpark,TN

Centreville,A

LG

roundG

roundG

round9

ma.g.l. 1

Ground

Ground

10m

a.g.l.7.5

ma.g.l.

24June–13

July10

June–20July

19June–2

July22

June–19July

29June–25

July15

June–14July

14June–14

July1

June–15July

PAN

(ppt)D

aytime

258±

157519±

455259±

177764±

662765±

400583±

3601078±

519169±

129A

llday203±

138376±

346195±

159473±

439485±

366431±

316694±

487126±

110

PPN(ppt)

Daytim

en/a

25±

24n/a

92±

10672±

4739±

45141±

835±

7A

llday19±

1854±

6650±

3933±

4093±

724±

5

MPA

N(ppt)

Daytim

en/a

n/an/a

50±

4138±

2531±

2691±

489±

10A

llday37±

3230±

2120±

2164±

455±

7

PPN/

PAN

Daytim

en/a

0.068±

0.134

n/a0.118±

0.039

0.092±

0.028

0.054±

0.041

0.127±

0.036

0.033±

0.032

Allday

0.059±

0.078

0.112±

0.042

0.094±

0.034

0.061±

0.049

0.128±

0.048

0.042±

0.044

MPA

N/

PAN

Daytim

en/a

n/an/a

0.073±

0.034

0.055±

0.031

0.061±

0.045

0.105±

0.070

0.049±

0.035

Allday

0.089±

0.047

0.064±

0.036

0.049±

0.040

0.108±

0.065

0.046±

0.038

APN

3/N

Oy

Daytim

e0.125±

0.053

0.255±

0.128

0.060±

0.028

0.171±

0.132

0.140±

0.047

0.136±

0.046

0.132±

0.062

0.155±

0.081

Allday

0.091±

0.051

0.172±

0.113

0.042±

0.029

0.080±

0.090

0.101±

0.058

0.101±

0.052

0.064±

0.061

0.110±

0.067

O3

(ppb)D

aytime

56±

1052±

1463±

6.2

51±

2170±

2354±

1960±

2334±

11A

llday39±

1936±

1851±

1531±

2151±

2744±

2037±

2726±

13

NOy

(ppb)D

aytime

2.1±

0.6

2.2±

1.1

3.1±

0.9

7.3±

4.5

7.2±

4.9

5.4±

5.6

10.2±

5.3

1.1±

0.4

Allday

2.4±

0.8

2.6±

1.9

3.5±

2.0

11.1±

8.8

8.0±

5.5

5.1±

3.9

18.6±

15.5

1.2±

0.8

NOx

(ppb)D

aytime

0.8±

0.3

0.87±

0.71

0.67±

0.29

5.2±

4.4

2.0±

2.3

2.6±

5.5

5.35±

4.6

0.34±

0.22

Allday

1.1±

0.7

1.8±

2.0

1.7±

1.9

10.6±

9.5

3.8±

3.9

2.9±

3.9

16.5±

17.6

0.63±

0.60

Isoprene(ppb)

Daytim

e6.2±

3.2

5.4±

2.4

2.9±

1.2

1.3±

0.8

2.0±

1.2

0.5±

0.3

5.2±

1.9

Allday

4.7±

4.1

3.9±

3.2

2.2±

2.0

1.0±

1.0

1.7±

1.8

0.5±

0.8

3.4±

2.4

Temperature

(◦C

)D

aytime

30.9±

2.6

31±

3.3

28.4±

2.7

29.2±

2.7

25.9±

2.8

27.5±

2.5

Allday

27.8±

4.3

28.6±

4.5

25.3±

3.3

25.7±

3.9

23.1±

3.8

24.7±

3.2

4,5,6

57,8

9,10

1111,12,13

1a.g.l.m

eansabove

groundlevel.D

aytime

is10:00–16:00

localtime. 2

The

dataon

4June

were

notincluded.PPNand

MPA

Ninclude

databelow

thedetection

limit,1.8

and1.9

pptv,respectively. 3A

PNw

erecalculated

asthe

sumofPA

N,PPN

,andM

PAN

.T

hedata

were

recalculatedbased

ontim

eand

day. 4Frostetal.(1998). 5

Buhretal.(1995). 6

Cantrelletal.(1992). 7

Roberts

etal.(1998). 8B

aumann

etal.(2000). 9Starn

etal.(1998). 10N

ouaime

etal.(1998). 11

Barketetal.(2004). 12

Roberts

etal.(2002). 13T

horntonetal.(2002).

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S. Toma et al.: APN in the southeastern US during SOAS 2013 1871

ages of air from south showed 94.6, 2.8, and 3.6 pptv. Polarplots of PAN, PPN, and MPAN as a function of surface winddirection are shown in Fig. S3 in the Supplement with windfrequency. This elevated northern distribution is also seenwith NOx and O3 reflecting the influence of anthropogenicpollution sources from Tuscaloosa, Birmingham, and At-lanta.

Plots of diurnal mean separated by surface wind direc-tion (Fig. 2) indicate a noticeable pattern in PAN, PPN,and MPAN from the north and a much weaker pattern insoutherly air. Levels of all three APN were highest (alsowith greatest variance) during the daytime on average. Thediurnal cycle for PPN was less pronounced because of thelow concentrations observed over the campaign. The PANdiurnal pattern was generally similar to those reported forNashville in 1995 and 1999 (Nouaime et al., 1998; Roberts etal., 2002) resulting from early morning breakup of a noctur-nal inversion that was commonly observed at the site. Similarbehavior is observed at SOAS in ozone, NOx , and isoprene.The diurnal behavior and midday means of APN /NOy arevery similar between the Dickson site and the SOAS site(Fig. S4a).

A calculation of PAN thermal decomposition lifetime us-ing ambient temperature and [NO]/[NO2] shows that the ef-fective lifetime changes little over the course of the after-noon, which suggests that PAN levels fluctuate during earlyafternoon mostly due to dilution by boundary layer growth.Boundary layer height increased by a factor of 2–3 from09:00 to 15:00 on average, based on lidar measurements.

3.2 Historical PAN measurements in the southeasternUS over the last 23 years

APN compounds have been measured at various rural andurban locations within the southeastern US over the last23 years. Observations from six sites, Elberton (GA) 1990;Rural Oxidants in a Southern Environment (ROSE; AL) 1990and 1992; New Hendersonville (TN) 1994; Youth Inc. (TN)1995; Dickson (TN) 1999; and Cornelia Fort Airpark (TN)1999, are compared here with SOAS 2013 data (a map ofthe locations is shown in Fig. S2 and descriptive statistics ofAPN and other trace gases are summarized in Table 1).

Binned PAN concentrations during the daytime (10:00–16:00) are plotted as a function of the concentration of NOx(grouped into deciles) in Fig. 3a. Urban areas have higherPAN and NOx concentrations than rural areas. The only sitesampled in more than 1 year was ROSE, where PAN levels in1990 were more than twice as high as in 1992. Overall, thePAN concentrations were variable between sites and years.A log-normal curve fit of the data in Fig. 3a shows an asym-metric peak in the concentration at around 3.5 ppb NOx . PANconcentration increases approximately linearly with NOx upto 2 ppb and beyond the peak it decreases slowly with furtherincreases in NOx . Similar behavior was observed in the rela-

Figure 2. Diurnal behavior of PAN, PPN, and MPAN during SOAS2013 with wind from the south and north.

tionship between O3 and NOx concentration in Fig. 3b witha maximum around 1.5 ppb NOx .

The relationship of O3 production with NOx and VOCconcentrations is typically discussed in terms of “NOx-limited” and “VOC-limited” regimes (Finlayson-Pitts andPitts, 2000; Milford et al., 1994; Chameides et al., 1992),although there has been less discussion of the sensitivity ofPAN production to these reactants. This curve is reminiscentof the modeled O3 production rate as a function of NOx andHOx (≡ OH+HO2+RO2) in Thornton et al. (2002) fromOH oxidation of VOC based on measurements from Cor-nelia Fort Airpark in 1999. At low NO concentration, O3production rate increases with NO, since OH is regeneratedvia HO2+NO and the primary chain termination reactionsare HOx+HOx . At higher NO concentrations, O3 productionrate slows as OH is consumed by NOx-removing chain ter-mination reactions (HO+NO2 and RO2+NO→ RONO2)that become faster than HOx +HOx reactions. The peak inFig. 3 is related to the crossover point between NOx-limitedand NOx-saturated. A high HOx production rate enhancesthe O3 production rate with NO for low NO and the crossoverpoint shifts to higher NO. The relationship based on fielddata in Fig. 3 is phenomenological and reflects a complexcombination of production and loss processes. Untangling

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1872 S. Toma et al.: APN in the southeastern US during SOAS 2013

Figure 3. Surface (a) PAN and (b) ozone concentrations for eachground site in the southeastern US over the last 23 years for 10:00–16:00 as a function of the concentration of NOx in deciles. The solidline indicates a fit line for all measurements.

the exact mechanisms that contribute to the observed behav-ior was not investigated, although several mechanisms couldcontribute. For instance, older air masses would reflect dif-ferent PAN/NOx ratios. Based on average APN/NOy ratios(Fig. S4a), the air masses experienced at Dickson and SOASwere of similar ages. Differential loss rates for PAN and NOxor overall shorter lifetimes could yield lower absolute valuesand could also influence this ratio. As seen in Fig. 1, andexplained in Hidy et al. (2014), ozone, NOx , and photolysisrates were all lower at this ground site than in previous years,which likely results in lower overall oxidation rates due tolower radical production rate. Lower radical production ratealso increases the isoprene lifetime and leads to larger mid-day average levels of isoprene than seen in other southeasternground sites (Table 1).

The covariance between PAN and O3 (Bottenheim et al.,1994) due to their common photochemical pathway in theatmosphere suggests that the steep increase of PAN concen-trations with NOx at low NOx in Fig. 3a could result fromNOx-limited chemistry. PAN at sites with lower NOx levelsseem to be more sensitive to NOx concentrations, as is seenfor most of the rural sites in this region. The slow decrease ofPAN concentration at higher NOx levels such as those seen atmore urban sites could result from faster radical terminationrates, and thus slower VOC oxidation rates. This empiricallyderived distinction is likely related to differences in reactionrates with peroxy radicals that could be investigated compu-tationally. PAN, O3, and NOx levels in the southeast were alllowest at SOAS 2013. As NOx levels continue to decrease

in the country (Blanchard et al., 2012; Russell et al., 2012;USEPA, 2018), PAN production rates might become morewidely sensitive to NOx . Emission inventories for anthro-pogenic VOC emissions have steadily decreased in the south-east over the last few decades. Mean isobutane and isopen-tane levels measured at the Centreville site were 70 % lowerin 2013 than in 1993 (Hagerman et al., 1997). VOC measure-ments at SEARCH sites show more consistent BVOC levelsover the same time (Hagerman et al., 1997; Hidy et al., 2014;USEPA, 2018; Eric Edgerton, unpublished data).

3.3 Anthropogenic vs. biogenic contribution to PANproduction

3.3.1 Description of MLR and its statistical meaning

A multiple linear regression (MLR) has been used to quan-tify PAN precursors (Roberts et al., 2002, 1998; Williams etal., 1997). Since the thermal decomposition rates of APN aresimilar (Roberts and Bertman, 1992), and MPAN and PPNare formed from BHC and AHC, respectively, and PAN isformed from both, [PAN] can be approximately representedas a weighted linear combination of [MPAN] and [PPN]. Thecombination of BHC and AHC chemistry is indicated byMPAN and PPN. The linear model is applied as in Eq. (2).

[PAN] = A+B1[MPAN] +B2[PPN] (2)

Here, A is the intercept (describing background PAN levels)and B1 and B2 are partial regression coefficients, estimatedusing a computer software program based on field observa-tions. The MLR statistical analysis includes estimation of A,B1 and B2, overall F test and a t test, and diagnostic proce-dures (e.g., Mendenhall et al., 2008). The F test is used toinvestigate the statistical significance of the model in Eq. (2)using an analysis of variance (ANOVA) table. The strength ofthe model is evaluated using the coefficient of determinationR2 between predicted and measured [PAN] (also providedthrough this statistical analysis). The individual t test, whichis based on the Student’s t statistic, is used to investigate thestatistical significance of the individual B1 and B2. In a MLRstatistical analysis, the magnitude of the standardized partialregression coefficients, βi , which is calculated as a product ofpartial regression coefficient and the ratio between the stan-dard deviation of the respective independent variable (MPANor PPN) and the standard deviation of the dependent variable(PAN), is frequently used to compare the relative contribu-tion of independent variables. The results of MLR statisticalanalysis are summarized in Tables S1 and S2.

Tatsuoka (1971) showed that R2 from the MLR is equalto the sum of the product of the βi and the zero-order (sim-ple bivariate) correlation, ri , which are obtained as results ofMLR (see Table S2). That is, R2

=∑βiri . Therefore, we

used the fraction of R2 based on the strength of relationshipin each [MPAN] and [PPN] to [PAN] to describe the relativeimportance of BHC and AHC. Each partial R2 is obtained as

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shown in Eqs. (3) and (4).

R2BHC = β1∗rMPANvs.PAN (3)

R2AHC = β2∗rPPNvs.PAN (4)

This approach allows us to directly treat the R2 in the MLRto assess the relative importance of BHC and AHC, includingthe strength of correlation with PAN.

Results from SOAS were compared with similar PAN datacollected from Dickson, TN, in 1999, another rural south-eastern site, which show that the MLR model and regressioncoefficients for both MPAN and PPN at both sites were sta-tistically significant for predicting PAN (see Tables S1 andS2). During SOAS 2013, 60 % of the variance in the mea-surements was explained by the MLR model (R2

= 0.601).At the Dickson site in 1999, 77 % of the variance was ex-plained by the MLR model (R2

= 0.766). The R2 of MLRin the SOAS 2013 data was lower than that in Dickson 1999,which might result from the lower absolute PAN levels dur-ing SOAS 2013. In particular, SOAS MPAN and PPN dataincluded a large number of below detection limit measure-ments, while Dickson 1999 data did not. The means of PAN,MPAN, and PPN in Dickson 1999 were 3 times higher thanthe mean for SOAS 2013, while NOx levels were 5 timeshigher (see Table 1). In Fig. 4, the relative importance ofBHC and AHC was standardized to compare SOAS 2013and Dickson 1999. Based on the PAN precursors representedby the variance explained by the MLR, standardized relativepercentiles (R2

BHC/R2 and R2

AHC/R2) estimate that 66 % of

PAN at SOAS 2013 came from biogenic precursors. In com-parison, biogenic influence (mostly isoprene) from Dicksonaccounted for only 25 % of PAN. Although both samplinglocations were located in rural areas, the results using MLRindicate that the HC precursors were different.

3.3.2 Comparison of contribution of isoprene oxidationwith computational modeling

We used two computational approaches to assess the contri-bution of isoprene oxidation to PAN formation by (1) orthog-onal distance regression (ODR) between field measurementsof MPAN and PAN and (2) simulation of the production ofperoxyacetyl (PA) radicals, precursors of PAN, using an am-bient 0-D photochemical model.

In ODR, the fraction of PAN productionfrom isoprene oxidation can be expressed as(d[PAN]isoprene/dt)/(d[PAN]all/dt). Assuming that MPANis solely derived from isoprene oxidation, the relative yieldof (d[MPAN]/dt)/(d[PAN]isoprene/dt) was obtained from anisoprene oxidation chamber experiment as 0.15±0.03 RSD.The reaction was initiated with 1.37 ppm isoprene, 268 ppbNO, and 206 ppb NO2 under 5 % RH in a 5.5 m3 Tefloncylindrical bag. The OH radical was produced by photolysisof HONO. The fraction of PAN production from isopreneoxidation to total PAN formation in the field was rearranged

Figure 4. Comparison of standardized relative contribution to PANformation from biogenic and anthropogenic hydrocarbons duringthe daytime in Dickson, TN, in 1999 and SOAS, in Centreville, AL,in 2013. The std. BHC and std. AHC mean standardized the relativeimportance of biogenic hydrocarbon and anthropogenic hydrocar-bon, respectively.

as the following equation.

d[PAN]isoprene/dtd[PAN]all/dt

= 6.7×d[MPAN]/dtd[PAN]all/dt

(5)

The (d[MPAN]/dt)/(d[PAN]all/dt) was obtained from mea-surements in SOAS 2013 as the slope of the linear regressionline of [MPAN] to [PAN].

In the 0-D photochemical model simulation, the rela-tive contribution to peroxyacetyl (PA) radicals from VOCspresent at the field site is based on the Master Chemi-cal Mechanism (MCM) v3.3. The ambient model includednot only isoprene and its oxidation products (includingCH3C(O)CHO) but also acetone, acetaldehyde, and somemonoterpenes as precursors. The detail of the parameters forthe MCM set are described in Groff (2015).

These two methods were compared with the relative im-portance of BHC, β1∗rMPANvs.PAN, from the MLR model inthis work. Then, 4 days (3 June 12:30–18:00, 14 June 11:30–18:00, 26 June 11:00–18:00, and 12 July 13:00–18:00) ofdata from SOAS 2013 were selected to run the 0-D modelbecause the production ratio using ODR can only be usedwhen PAN, MPAN and NOx concentrations were appropri-ately high. The time ranges were chosen so that the boundarylayer height would be stable and any dilution effect would beminimal. Results of the comparison are plotted in Fig. 5. Al-though the relative importance of BHC in the MLR modelwas less than 40 % on 3 June, it was statistically dominanton the other 3 days accounting for more than 68 % (PPNon 3 June did not have a significant level for the predictionPAN in MLR analysis, p = 0.600). By comparison, the es-timated contribution of isoprene oxidation using ODR on3 June had the steepest slope; however, the range of the95 % confidence interval (CI) on this day was large. On theother 3 days, the relative contributions of isoprene oxida-tion using ODR were estimated at 23 %–49 %, lower than

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1874 S. Toma et al.: APN in the southeastern US during SOAS 2013

Figure 5. Estimates of the relative contribution of isoprene oxidation to PAN formation during four specific days of SOAS 2013 using threedifferent approaches: multiple regression analysis, ODR with chamber data, and simulation of PA radicals using a 0-D model. P indicatesthe calculated probability of the t test; CI= confidence interval.

the results derived from the other two methods. This mightbe due to the differences between the chamber experimentand ambient conditions. Specifically, the ratio of secondaryto primary oxidation products varies between chamber andambient conditions, with likely relatively greater primaryproducts under chamber conditions. Since PAN is formedvia the oxidation of secondary products of isoprene, the[MPAN]/[PAN]isoprene ratio would be higher in the cham-ber experiment than in ambient environment. This ratio isexpected to derive results that are biased low, when used toestimate the ambient isoprene-derived PAN concentration. Inaddition, photolysis rates also are significantly different be-tween the chamber and the field conditions. The results ofthe 0-D model suggest that isoprene oxidation significantlycontributed to PAN formation with a mean range of 55 %–73 % over all selected days. Hence, both methods, the MLRand the 0-D model, indicate that isoprene oxidation was themain source when high levels of PAN were observed duringSOAS 2013.

4 Discussion

4.1 Comparison among MACR, IN, and MPAN

MACR is a first-generation product of isoprene photooxida-tion mechanisms, and MPAN is derived from MACR oxida-tion (Bertman and Roberts, 1991; Kjaergaard et al., 2012).With enough NOx , the OH adduct of isoprene that is the pre-

cursor to MACR in these mechanisms is also a precursor ofgas-phase isomers of isoprene hydroxynitrates (IN) (Shep-son, 2007; Grossenbacher et al., 2001, 2004; Barker et al.,2003; Paulot et al., 2009a; Lockwood et al., 2010). Xiong etal. (2015) reported IN at SOAS, which affords the opportu-nity to study this aspect of NOx sensitivity of isoprene oxi-dation. In this work, the daytime (10:00–16:00) relationshipsamong MPAN, MACR, and IN at SOAS were investigatedusing the Pearson’s correlation statistical test. Missing datawere treated as pairwise deletion (not listwise deletion). Thecorrelation coefficient between the first-generation productsof isoprene, MACR, and IN was 0.528 (p < 0.001) and indi-cated a statistically significant positive correlation, as wouldbe expected. According to the known chemical pathways, astrong relationship is expected between MACR and MPAN,while a weak relationship is expected between IN and MPANgiven that IN is a primary product, while MPAN is sec-ondary. The results show, however, that daytime data over thewhole campaign did not show a statistically significant cor-relation between MACR and MPAN (r = 0.148, p = 0.104).In contrast, IN has a statistically significant positive correla-tion with MPAN (r = 0.499, p < 0.001). Likely this relation-ship is a result of the NOx dependence of both organic nitrateproducts. Because MACR can be produced in the absence ofNOx (Paulot et al., 2009b; Liu et al., 2013; Mao et al., 2013),MPAN is more dependent on NOx than on MACR at this site,and isoprene nitrates constitute a larger fraction of gas-phaseorganic nitrates from BVOC than MPAN does, consistent

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Figure 6. The relationship of daytime IN /MPAN ratio with NOx concentration (10:00–16:00). (The dashed vertical and horizontal lines areused for reference only.)

with Romer et al. (2016). Figure 6 shows that the IN /MPANratio varies over a large range when NOx < 1 ppb, corre-sponding to the range of high P (HOx) described by Thorntonet al. (2002). At higher NOx levels, the ratio remains fairlyconstant around 2.5–3. This figure is suggestive of differentchemical regimes operating under different NOx conditions,reminiscent of the behavior of differential alkane loss whendominated either by OH or halogen chemistry in Jobson etal. (1994). The higher loss rate for MPAN expected withhigher levels of HOx may drive the IN /MPAN ratio at lowerNOx levels.

4.2 Gas-phase MPAN vs. organic aerosol mass

During SOAS 2013, Lee et al. (2016) estimated that theparticle-phase organic nitrates (pONs) accounted for 3 % oftotal organic aerosol (OA) mass, on average, during the day(12:00–16:00) and BVOC precursors strongly impacted thediel trends of pONs. Laboratory experiments suggest thatMPAN can play a key role in SOA formation under highNOx conditions, C4-hydroxynitrate-PAN or hydroxymethyl-methyl-α-lactone (HMML) (Kjaergaard et al., 2012; Nguyenet al., 2015; Wennberg et al., 2018) and methacrylic acidepoxide (MAE) (Worton et al., 2013) have been proposedas precursors for uptake into the particle-phase from MPANoxidation. Nguyen et al. (2015) estimated the SOA yield asapproximately ∼ 60 % by mole from MPAN+OH reactionin the absence of NOx . IN is also expected to contribute toSOA formation (Jacobs et al., 2014). Organic nitrate involve-ment in SOA formation suggests a relationship between gas-phase MPAN and IN with particle mass, although the ni-trogen could be unretained in the particle. Figure 7a and bshow the relationship of gas-phase MPAN and IN with day-

time particle measurements. As MPAN and IN concentra-tions increase, Fig. 7a and b show that OA mass increases,while pONs mass increases very little (measured by bothHR-ToF-AMS and TD-LIF), although the slopes of MPANand IN vs. pONs are statistically different from zero. Therelative magnitude of the response of OA and pONs to in-creases in MPAN and IN suggests that they may contributeto OA growth more than to pONs growth. Although this maysuggest that if MPAN oxidation by OH is involved in par-ticle growth, the nitrogen from MPAN is not represented inaerosol organic nitrate. Oxidation of MPAN modeled fromMACR+OH results at FIXCIT (Nguyen et al., 2014) usingmeasured total peroxynitrates and kinetics of the isoprenemechanism in MCM v3.3.1 (Jenkin et al., 2015) showed apositive relationship between MPAN oxidation and pONsformation, although with a yield < 3 % (Paul Romer, per-sonal communication, 2018). Results of direct reaction ofMPAN+OH suggest that it is unlikely that pONs formationis mainly derived from MPAN+OH reaction, even in thepresence of NOx (Tran Nguyen and Paul Wennberg, personalcommunication, 2018). This small contribution of isopreneoxidation compounds to pONs formation is consistent withreported modeling of pONs formation (Xu et al., 2015; Ayreset al., 2015; Pye et al., 2015). Ayres et al. (2015) suggest thatpONs formation at SOAS was dominated by nighttime reac-tions of NO3 radicals with BVOCs rather than daytime reac-tions, and more from monoterpene oxidation than isopreneoxidation. MPAN uptake on aerosols results in the formationof aerosol-phase organosulfates (Worton et al., 2013), whichalso contributes to the weak dependence of pONs on MPAN.

Rather, MPAN is likely a precursor of low vapor pressureproducts that undergo aerosol uptake. The correlation be-tween INs and MPAN with OA may reflect that much of the

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1876 S. Toma et al.: APN in the southeastern US during SOAS 2013

Figure 7. The relationship between mass of MPAN or IN in the gas phase and in organic aerosol during the daytime 29 June–15 July (the timeperiod when data on pONs-TD-LIF were available). (a) OA is organic aerosol (without organic nitrate) and (b) pONs is particle-phase organicnitrates. MPAN has a linear slope (R2) of 27.8 (0.455) vs. OA, 0.4 (0.437) vs. pONs-HR-ToF-AMS, and 1.1 (0.120) vs. pONs-TD-LIF. INhas a linear slope (R2) of 21.4 (0.606) vs. OA, 0.2 (0.603) vs. pONs-HR-ToF-AMS, and 0.6 (0.341) vs. pONs-TD-LIF.

OA derives from BVOC oxidation, and the conditions thatlead to large rates of BVOC emission and oxidation (high Tand radiation) simultaneously produce OA, along with INsand MPAN. That OA does not correlate well with condensedphase organic nitrate reflects the fast hydrolysis of organicnitrates in the aerosol phase at a low aerosol pH (Rindelaubet al., 2016; Jacobs et al., 2014; Guo et al., 2015). While or-ganic nitrates such as the INs may partition to the aerosolphase, they are quickly converted to inorganic nitrate ion andother products by hydrolysis.

5 Conclusions

The level of APN compounds measured at the ground siteduring SOAS 2013 is lower than measured in the southeast-ern US over the past two decades. We show here that PANconcentrations at SOAS were limited by NOx availability.Russell et al. (2012) show that NOx in the eastern US hasbeen decreasing rapidly, due to effective emission control,and lower NOx levels seem to result in lower ambient PANconcentrations. As this process continues, PAN may con-tinue to be a smaller fraction of NOy , as peroxy radicalssuch as CH3C(O)OO react with HO2 and RO2 rather thanwith NOx . PAN appears to be most sensitive up to [NOx]of approximately 3.5 ppb, above which PAN concentrationsswitch to a NOx-saturated (or VOC-limited) regime. Over-all, MPAN did not show a statistically significant correlationwith MACR, but did show a statistically positive correlationwith IN. These results indicate that both organic nitrate prod-ucts were NOx dependent (MPAN being more sensitive toNOx than to MACR precursor) at SOAS.

We estimate that biogenic precursors, particularly iso-prene, account for more than half of PAN and that gas-phase MPAN shows a correlation with OA growth consis-tent with laboratory and chamber studies, but less correlationwith nitrogen-containing pONs during the daytime. This maysuggest that the nitrogen of MPAN is removed during oxida-tion to other low vapor pressure products involved in particlegrowth, as suggested by the HMML (Kjaergaard et al., 2012)and MAE mechanisms (Worton et al., 2013).

Data availability. The data from this large, collaborative study(including the data used in this paper as well as oth-ers) are all available from https://esrl.noaa.gov/csd/groups/csd7/measurements/2013senex/ (last access: 28 January 2019).

Supplement. The supplement related to this article is availableonline at: https://doi.org/10.5194/acp-19-1867-2019-supplement.

Author contributions. ST and SB measured APN by GC-ECD; CGand PBS performed the 0-D modeling; FX and PBS measured iso-prene nitrates; PR, KD, PW, and RC measured total APN by thermalconversion and LIF; KB and EE measured APN by chemilumines-cence as well as inorganic gases; ARK, JdG, and AG measured or-ganic gases by GCMS; WH and JLJ measured particles by AMS;ST wrote the first version of the paper. The co-authors contributedto the interpretation of the results and the writing of the paper.

Competing interests. The authors declare that they have no conflictof interest.

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S. Toma et al.: APN in the southeastern US during SOAS 2013 1877

Disclaimer. This publication was developed under assistanceagreements awarded by the U.S. Environmental Protection Agency.It has not been formally reviewed by the EPA. The views expressedin this document are solely those of the authors and do not neces-sarily reflect those of the Agency. The EPA does not endorse anyproducts or commercial services mentioned in this publication.

Acknowledgements. This research was supported by EPA STARgrant 83540901-0-RD. We thank the people of the SOAS 2013campaign. We thank James Roberts, David Parrish, Eric Williams,and Martin Buhr for providing NOAA data of PAN and other tracegases from Elberton, ROSE, Henderson, and Cornelia Fort groundsites. We thank Tran Nguyen and Paul Wennberg for sharing datafrom FIXCIT. Weiwei Hu and Jose L. Jimenez were supportedby NSF AGS-1822664 and EPA STAR 83587701-0. NSF grantAGS-1352972 supported Cohen group work at UC Berkeley. VOCmeasurements at the SEARCH Jefferson Street (Atlanta) site werefunded by Southern Company Services, Inc. (Birmingham, AL,USA) and the Electric Power Research Institute (Palo Alto, CA,USA).

Edited by: Jennifer G. MurphyReviewed by: two anonymous referees

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