Interactive comment on “ Chemical characterization of biogenic SOA generated from plant emissions under baseline and stressed conditions : inter-and intra-species variability for six coniferous species ”

The largest global source of secondary organic aerosol in the atmosphere is derived from the oxidation of biogenic emissions. Plant stressors associated with a changing environment can alter both the quantity and composition of the compounds that are emitted. Alterations to the biogenic VOC profile could impact the characteristics of the SOA formed from those emissions. This study investigated the impacts of one global change stressor, increased herbivory, on the composition of SOA derived from real plant emissions. Herbivory was simulated via application of methyl jasmonate, a proxy compound. Experiments were repeated under pre- and post-treatment conditions for six different coniferous plant types. VOCs emitted from the plants were oxidized to form SOA via dark ozone-initiated chemistry. The SOA particle size distribution and chemical composition were measured using a scanning mobility particle sizer (SMPS) and Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS), respectively. The aerosol mass spectra of pre-treatment biogenic SOA from all plant types tended to be similar with correlations usually greater than or equal to 0.90. The presence of a stressor produced characteristic differences in the SOA mass spectra. Specifically, the following m/z were identified as a possible biogenic stress AMS marker with the corresponding HR ion(s) shownmore » in parentheses: m/z 31 (CH3O+), m/z 58 (C2H2O2+, C3H6O+) m/z 29 (C2H5+), m/z 57 (C3H5O+), m/z 59 (C2H3O2+, C3H7O+), m/z 71 (C3H3O2+, C4H7O+), and m/z 83 (C5H7O+). The first aerosol mass spectrum of SOA generated from the oxidation of the plant stress hormone, methyl jasmonate, is also presented. Elemental analysis results demonstrated an O:C range of baseline biogenic SOA between 0.3–0.47. The O:C of standard methyl jasmonate SOA was 0.52. Results presented here could be used to help identify a biogenic plant stress marker in ambient datasets collected in forest environments.« less


Figures Back Close
Full

Abstract
The largest global source of secondary organic aerosol in the atmosphere is derived from the oxidation of biogenic emissions.Plant stressors associated with a changing environment can alter both the quantity and composition of the compounds that are emitted.Alterations to the biogenic VOC profile could impact the characteristics of the SOA formed from those emissions.This study investigated the impacts of one global change stressor, increased herbivory, on the composition of SOA derived from real plant emissions.Herbivory was simulated via application of methyl jasmonate, a proxy compound.Experiments were repeated under pre-and post-treatment conditions for six different coniferous plant types.VOCs emitted from the plants were oxidized to form SOA via dark ozone-initiated chemistry.The SOA particle size distribution and chemical composition were measured using a scanning mobility particle sizer (SMPS) and Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS), respectively.The aerosol mass spectra of pre-treatment biogenic SOA from all plant types tended to be similar with correlations usually greater than or equal to 0.90.The presence of a stressor produced characteristic differences in the SOA mass spectra.Specifically, the following m/z were identified as a possible biogenic stress AMS marker with the corresponding HR ion (s)  ).The first aerosol mass spectrum of SOA generated from the oxidation of the plant stress hormone, methyl jasmonate, is also presented.Elemental analysis results demonstrated an O : C range of baseline biogenic SOA between 0.3-0.47.The O : C of standard methyl jasmonate SOA was 0.52.
Results presented here could be used to help identify a biogenic plant stress marker in ambient datasets collected in forest environments.

Introduction
Organic material comprises 20-90 % of the mass in atmospheric particles smaller than one micrometer (Jimenez et al., 2009;Zhang et al., 2007).Most of this small organic particulate material is secondary organic aerosol (SOA), and the major fraction of SOA globally is formed from the oxidation of biogenic volatile organic compounds (BVOC) released by vegetation (Hallquist et al., 2009).BVOCs are emitted by plants primarily for defensive purposes (Dudareva et al., 2006;Kesselmeier and Staudt, 1999).BVOC emission rates and emission profiles (i.e., the types of compounds emitted) can change significantly when plants are exposed to biotic and abiotic stressors (Holopainen, 2004;Peñuelas and Staudt, 2010;Pinto et al., 2010).It follows then that plant stress exposure associated with climate change could have significant impacts on SOA formation, and thus could lead to a climate feedback because atmospheric aerosols play an important role in the global radiation budget.Potential climate change feedbacks resulting from the processes linking naturally produced aerosols and the rest of the Earth system have been summarized by Carslaw et al. (2010).These processes include the production of secondary sulfate aerosol from phytoplankton emissions, physical processes that contribute to dust entrainment, and the formation of biogenic SOA from terrestrial plant emissions.Their simulations estimate that the radiative forcing resulting from these feedbacks could produce positive radiative perturbations up to 1 W m −2 by the end of the 21st century, amplifying the expected effects of climate change (Carslaw et al., 2010).Another review focused on feedbacks between the terrestrial biosphere and climate, and also included a discussion of the biogenic SOA formation process (Arneth et al., 2010).They estimated that climate feedbacks with the terrestrial biosphere could result in positive radiative perturbations of up to 1.5 W m −2 K −1 by the end of the 21st century.Both reviews make clear that more work is required to fully understand these feedbacks, stating that the current level of scientific understanding for them is "poor" (Carslaw et al., 2010) and "very low" (Arneth et al., 2010).Despite the uncertainty in these projections, the assessments of Figures both papers are in stark contrast to the previously held assumption that the overall contribution of vegetation to the changing climate system is to act as a sink for increasing CO 2 (Magnani et al., 2007).Carslaw et al. (2010) listed several research topics that need to be addressed in order to reduce the uncertainty in these predictions; resolving BVOC responses to climate change stressors and investigating the subsequent impact on biogenic SOA formation was included as a high priority for future research projects.
Most studies of how BVOC emissions respond to stressors have focused solely on the BVOC emissions themselves.Using these results to infer overall impacts on climate requires highly uncertain assumptions about how different mixtures of BVOCs could impact SOA yields and chemical composition.A few studies have examined SOA formation from real plant emissions more directly.Joutsensaari et al. (2005) were the first to report SOA formation in a laboratory chamber from the oxidation of real plant emissions.They used a methyl jasmonate treatment to induce emissions in order to investigate the role of inducible plant volatiles in particle nucleation and growth.Other studies have focused on SOA production and chemical composition from BVOC emissions under baseline conditions, rather than looking at potential feedbacks between stressors and climate (Hao et al., 2011;Kiendler-Scharr et al., 2009;Mentel et al., 2009;VanReken et al., 2006).Our own recent work showed that SOA can also form from BVOCs emitted from leaf litter, and that this aerosol is chemically very similar to SOA produced from live tree emissions (Faiola et al., 2014b).BVOC emissions from leaf litter were also found to respond to external environmental drivers, raising the possibility of additional pathways for climate feedbacks to occur.
Recently, there have been two studies that compared biogenic SOA yields for baseline emission vs. stressed conditions.Lang-Yona et al. (2010) examined the effect of increased temperature on holm oak (Quercus ilex) emissions and subsequent SOA formation, finding that increased temperature led to heightened BVOC emissions and increased SOA production.The BVOC profile was slightly altered with increasing temperature, but this did not impact the resultant SOA mass yields.In another study, Mentel et al. (2013)  biogenic SOA yields.They found that the measured impact on SOA formation was different for different stressors.For example, infestation by the aphid Cinara pilicornis resulted in emissions of large organic compounds that had higher SOA yields than the baseline emissions (33 % stress yield vs. 4-6 % baseline yield).However, if the plants were experiencing both herbivory and drought stress concurrently, emissions of small six-carbon green leaf volatiles increased, which reduced biogenic SOA yields.These results suggest that climate change could have significant impacts on biogenic SOA formation, and furthermore, that multiple stressors can interact to change the SOA formation potential of BVOC emissions in a different way than a single stressor in isolation.These previous plant stress SOA formation studies provide valuable insight into the potential impacts of climate change stressors on biogenic SOA yields.However, to date there have been no in-depth analyses to investigate how plant stress may affect biogenic SOA composition, which would have implications for aerosol radiative properties and cloud forming potential.The research described in this paper addresses these gaps in our current understanding of the variability in biogenic SOA composition -including a discussion of inter-and intra-plant species variability as well as a first look at some impacts of herbivore stress on biogenic SOA composition.

Description of dual chamber system and operation
The experiments presented here were performed using the Biogenic Aerosol Formation Facility at Washington State University.This dual chamber facility uses emissions from living vegetation as the precursor VOC source for SOA formation.This is in contrast to other systems that have historically used commercially obtained pure compounds as a proxy for biogenic emissions.The facility includes a dynamic plant enclosure and an aerosol growth chamber.The plant enclosure is a rectangular 0. plant enclosure and the on-line analytical gas chromatography (GC) system used to measure BVOC emissions is provided in a separate paper that focuses on the impacts of herbivory stress on plant emissions (Faiola et al., 2014a).The current paper focuses specifically on the composition of biogenic SOA formed from the oxidation of the plant emissions.
The aerosol growth chamber operation and SOA generation methods are similar to those described by Faiola et al. (2014b).Chamber dimensions were 1.6 m × 2.2 m × 2.2 m.All aerosol growth experiments were conducted with the chamber using a batch mode approach.Oxidation of SOA precursors was initiated with ozone that was generated with an Enaly model HG-1500 ozone generator.The chemistry in this chamber is best described as dark "ozone-initiated" chemistry because the chamber was not equipped with UV lights and no OH scavenger was used.Most experiments were unseeded.Experiments where 50 nm ammonium sulfate seed particles were used are marked with an asterisk in the experiment summary table (Table 1).When used, seed particles were produced from a TSI constant output atomizer (model 3076) and then size-selected with a differential mobility analyzer (DMA, TSI, Inc.).Temperature and relative humidity in the aerosol growth chamber were not controlled, but were monitored using a Vaisala HMP110 humidity and temperature probe.Nitrogen oxides were not measured, but the aerosol chamber likely contained some NO x due to soil emissions from the plant pots (Davidson and Kingerlee, 1997).

Tree description and experimental design
Six different plant species were used as emissions sources to generate biogenic SOA in this study: ponderosa pine (Pinus ponderosa), bristlecone pine (Pinus aristata), blue spruce (Picea pungens), western redcedar (Thuja plicata), grand fir (Abies grandis), and Douglas-fir (Pseudotsugas mensiezii).Saplings were 1-3 years old at the time of the experiments.All specimens were obtained from the University of Idaho forest nursery and were stored outdoors at the Washington State University greenhouse facility Introduction

Conclusions References
Tables Figures

Back Close
Full when they were not being used for experiments.Greenhouse staff cared for the specimens, providing regular watering and fertilization.Plants were transported to the laboratory at least 36 h before the first aerosol growth chamber experiment to allow time for acclimation to laboratory conditions.Three to nine saplings of the same species were placed in the plant enclosure (the number depended on the size of the plants).The only exceptions to this were four experiments performed using a combination of Abies grandis and Pseudotsugas menziesii specimens rather than just a single plant species (referred to as "mix" experiments).One day before an aerosol growth experiment, the aerosol growth chamber was cleaned with 1 ppm ozone and flushed with zero air for at least 18 h until ozone concentrations were less than 20 ppb (Model 1008-PC ozone monitor, Dasibi) and particle number concentrations were less than 10 cm −3 (Model 3771 condensation particle counter, TSI, Inc.).Zero air was generated with a pure air generator (Aadco model 737).Chamber flushing was stopped on the morning of the experiment, at which point the chamber was operated in batch mode.Biogenic VOC emissions were pumped from the plant enclosure to the aerosol growth chamber for three hours (flow = 9.5 LPM) using a chemically resistant vacuum pump (KNF Laboport model UN810 FTP) through PFA lines heated to 80 • C. Lines were heated to minimize losses of lower-volatility compounds.During chamber loading, a fan inside the chamber was used to facilitate mixing.
When VOC loading was complete, the oxidation chemistry was initiated by rapidly introducing 130 ppb ozone to the aerosol growth chamber.The mixing fan was turned off immediately following oxidant addition to reduce particle wall loss.Particle growth and composition were then monitored for the next 6-8 h.This process was repeated with the same batch of trees twice in one week -once before treatment was applied and again after the treatment.The treatment was either a stress application or a negative control.Both treatments are described in detail in the next section.The time required to observe maximum plant response to treatment can vary (Copolovici et al., 2011).Consequently, some of the post-treatment aerosol growth experiments were performed the day after treatment and some were performed on the same day as the treatment.Introduction

Conclusions References
Tables Figures

Back Close
Full A list of all experiments with the Experiment ID, date, and treatment approach is provided in Table 1.The naming convention for the Experiment ID is "plant species type" + "experiment number" + "experiment type".For example, "PA-1-Pre" stands for Pinus aristata, first experiment, pre-treatment and "PA-1-Post" stands for Pinus aristata, first experiment, post-treatment.One pre-treatment aerosol growth experiment performed with Picea pungens specimens on 14 May 2013 did not produce enough particle mass for AMS analysis, so it has been removed from this table and will not be considered further.There was also one SOA growth experiment performed that used a single-component standard, methyl jasmonate, as the precursor compound to generate SOA, rather than using real plant emissions.For this experiment, a 95 % methyl jasmonate standard solution (Sigma-Aldrich part #392707-5ML) was introduced into the aerosol growth chamber using a dynamic dilution system (Faiola et al., 2012).

Stress treatment
Herbivory stress was simulated by exposing plants to methyl jasmonate (MeJA).Methyl jasmonate is a plant stress hormone used in plant-plant communication for defensive purposes (Cheong and Choi, 2003).Plants emit MeJA into the gas-phase, where it induces the jasmonic acid defense pathway in neighboring plants (Farmer and Ryan, 1990) -a biochemical pathway that leads to changes in the VOC compounds produced and emitted from those plants.Consequently, exposing plants to MeJA alters BVOC emission rates, their chemical profile, and their concentrations in storage pools (Martin et al., 2003;Rodriguez-Saona et al., 2001).For the 2012 experiments, MeJA was introduced using an exogenous treatment where 20 µL of a 9 : 1 diluted ethanol : MeJA solution was applied to a cotton swab and placed in the biogenic emissions enclosure with the plants, following the methods of Rodriguez-Saona et al. (2001).The 2013 experiments used a foliar application of 10 mM MeJA in nanopure water, following the approach of Martin et al. (2003).The plant foliage was sprayed with 200 mL of this solution.The negative control treatment was a foliar application of 200 mL of nanopure water rather than the MeJA solution.Introduction

Conclusions References
Tables Figures

Back Close
Full The revised MeJA treatment employed in 2013 was intended to promote a maximal herbivory stress response.The goal was to allow us to investigate an upper limit of the potential impacts of herbivory on biogenic SOA composition, something that has not been reported previously.The foliar MeJA stress treatment elevates BVOC emissions and typically leads to much larger mass loadings relative to the pre-treatment experiments.Importantly, the purpose of these experiments was not to quantify changes to the amount of SOA formed under stressed conditions.Rather, this research seeks to fill in current gaps in knowledge by investigating changes to biogenic SOA composition due to stress.
A number of the post-treatment aerosol growth experiments were performed the same day as the foliar MeJA application.In these cases, methyl jasmonate solution remained present on the plants in the plant chamber while the aerosol chamber was being loaded.The vapor pressure of methyl jasmonate at 23 • C is 1.28 × 10 −4 mmHg (Acevedo et al., 2003), which corresponds to an effective saturation concentration (C*) for methyl jasmonate of 1500 µg m −3 .This puts methyl jasmonate at the lower end of the intermediate volatility range (C* range of 1000-100 000 µg m −3 ) approaching the semi-volatile range (C* range of 0.1-1000 µg m −3 ) (Robinson et al., 2007).To compare, the vapor pressure of alpha-pinene, a typical monoterpene, is four orders of magnitude greater, nearly 3 mmHg at 20 • C.Even with MeJA's low vapor pressure, some of the compound sprayed on the trees would volatilize and be subsequently pumped into the aerosol growth chamber.This MeJA could act as an SOA precursor in addition to the VOC emissions from the plant.Consequently, there are two types of post-treatment SOA in these experiments: pure plant emission post-treatment SOA and plant emission + MeJA post-treatment SOA.This latter SOA could still be considered a type of stress SOA because plants do emit significant quantities of plant hormones in forests when exposed to stressed conditions (Karl et al., 2008).oxidation of MeJA had an SOA mass yield of 68 %, suggesting that this is a compound that warrants further investigation.

Analytical instrumentation
SOA microphysical properties were measured with a scanning mobility particle sizer (SMPS, custom built with major components from TSI, Inc.) described previously by Faiola et al. (2014b) and Mwaniki et al. (2014).Aerosol mass spectra were continuously measured using a high resolution time-of-flight aerosol mass spectrometer (HR-AMS, Aerodyne Research, Inc.) described in detail elsewhere (Canagaratna et al., 2007;De-Carlo et al., 2006).Briefly, the HR-AMS collimates sub-micron particles into a narrow beam with an aerodynamic lens.The particle beam is directed onto a vaporizer plate held at 600 • C that volatilizes all non-refractory components.The volatilized fragments are then ionized with a tungsten filament with 70 eV electron impact ionization.These mass fragments are introduced to a Tofwerk high-resolution time-of-flight mass spectrometer where they are separated by size and quantified.The HR-AMS was operated with 1 to 4.5 min sample averaging, alternating between general mass spectrometer (MS) mode and particle time-of-flight (p-ToF) mode.Only v-mode data were used in this study because pre-treatment experiments often did not have sufficient signal for w-mode data to be used.Ionization efficiency calibrations were performed using the brute force single particle technique with monodisperse ammonium nitrate particles generated with a constant output atomizer (TSI Model 3076).

AMS data analysis
The goal of this research was to compare the aerosol mass spectra between SOA formed from the oxidation of emissions from different types of trees and between SOA formed under pre-treatment vs. post-treatment conditions.In the past, unit mass resolution (UMR) data from the Aerodyne HR-AMS has been normalized to the sum of the organic mass to compare spectra between different experiments with different mass Introduction

Conclusions References
Tables Figures

Back Close
Full  et al., 2008).One way these UMR spectra can be quantitatively compared is to calculate the square of the Pearson correlation coefficient (r 2 ), called the coefficient of determination, between the two spectra (Kiendler-Scharr et al., 2009).Using this approach, Kiendler-Scharr and colleagues observed clear differences between biogenic SOA and other types of organic aerosol including biomass burning organic aerosol (r 2 = 0.44-0.51),diesel exhaust organic aerosol (r 2 = 0.44-0.51),and ambient hydrocarbon-like organic aerosol in Pittsburgh (r 2 = 0.16-0.41).For the comparisons presented here, only those m/z that contributed to 90 % of the HR-AMS UMR organic signal in any of the experiments was used to calculate the correlations.
The composition of organic aerosol can also be described through the use of elemental analysis (Aiken et al., 2008).Results of such analyses are presented on a Van Krevelen diagram with axes of hydrogen to carbon (H : C) and oxygen to carbon (O : C) ratios.In general, laboratory SOA generation studies produce aerosol that is less oxidized than those found in the ambient atmosphere (Kroll and Seinfeld, 2008).However, laboratory chamber studies have also shown a wide variability in elemental ratios that are dependent on the precursor compounds used to generate the aerosol (Chhabra et al., 2010;Ng et al., 2010).Consequently, differences in the precursor compounds from different sources of BVOCs (e.g., different trees, or pre-treatment vs. post-treatment emissions, or the presence of near semi-volatile plant hormones) could produce differences in biogenic SOA composition that would occupy different locations in Van Krevelen space.
To perform HR-AMS data analysis, it is necessary to carefully correct the particle signals that have significant air interferences.To account for some of these interferences, Introduction

Conclusions References
Tables Figures

Back Close
Full HEPA particle filters were placed in the sampling line at the beginning and end of each experiment for a minimum of ten runs.Using these filter runs, adjustments were made to the UMR fragmentation table for m/z 15, 16, 29, and 44 and to the high resolution (HR) fragmentation table for ions 15N + , O + , and CO + 2 .When performing elemental analysis, extra care is required to identify and remove additional interferences for a few HR ions (Aiken et al., 2008).For example, the HR ions at O + , HO + , and H 2 O + are produced from organic material in particles, but also have significant interferences with particulate water.Particulate water was reduced by placing a gas sample dryer (Perma Pure, model MD-110) along the inlet, but it is very difficult to eliminate all particulate water.Consequently, the organic particle contribution to these signals was constrained using the organic particle CO + 2 signal as suggested by Aiken et al. (2008).Another important ion, the CO + ion (exact mass 27.9949), is often overwhelmed by the very large neighboring N + 2 ion (exact mass 28.0061).The N + 2 air ion is still very large even though the vacuum in the HR-AMS dilutes gas phase molecules by a factor of 10 7 relative to aerosol species (Allan et al., 2004).Evaluations of the organic particle contribution to CO + have generally found that the ratio of organic particulate CO + to organic particulate CO + 2 is approximately one (Aiken et al., 2008).We investigated this interference using p-ToF data to separate the CO + ion from the air N + 2 signal at m/z 28; fourteen of the thirty-four total experiments had high quality p-ToF signal that allowed us to directly calculate the organic CO + /CO + 2 ratio.The tail of the air N + 2 peak was subtracted from the CO + signal before integrating the peaks and calculating the ratio.The organic CO + /CO + 2 ratio ranged from 0.79-3.04 with an average value of 1.6 ± 40 %.The pre-treatment and negative control experiments (N = 8) had ratios close to the literature value of ∼ 1, with an average of 1.1 ± 0.3 (mean ± one standard deviation).The stress experiments (N = 6) had a higher ratio, with an average of 2.2 ± 0.6.Sample p-ToF spectra for both pre-treatment and post-treatment conditions are shown in Supplement Fig. S1.These results were used to inform corrections for experiments that did not have useful p-ToF data.For pre-treatment and negative control experiments where the ratio could not be directly calculated (N = 14), a default value Introduction

Conclusions References
Tables Figures

Back Close
Full of 1 was used.For stress experiments where the ratio could not be directly calculated (N = 6), elemental analysis was performed twice, once using the average measured stress value of 2.2 and once using the default literature value of 1.This was done to ensure the data treatment did not skew overall conclusions.Final results are shown using a ratio of 2.2.
Nitrogen-containing ions were detected in the aerosol mass spectra for all experiments.While not measured, we presume this was due to the presence of a small amount of NO x in the aerosol growth chamber emitted from the soil holding the plant specimens.Whatever the source, the contribution of nitrogen-containing peaks to total organic signal was low -N : C ratios ranged from 0.004 to 0.011.The nitrogencontaining signals were ignored for the analysis presented in this paper.
Some of the baseline aerosol growth experiments had low HR-AMS signal (<10 µg m −3 of organic aerosol).Consequently, the high-resolution data was screened to ensure adequate signal-to-noise S/N for further HR analysis.All elemental ratios presented from the HR analysis had a relative standard deviation less than 10 %.For the experiments with low S/N, elemental ratios of O : C and H : C were parameterized with unit-mass resolution (UMR) data, using the fractions of m/z 44 (f 44 ) and of m/z 43 (f 43 ) to the total organic signal as described by Aiken et al. (2008) and Ng et al. (2011), respectively.The approach used to calculate elemental ratios (UMR vs. HR) for each experiment is summarized in Table 2 along with other important experimental conditions.

Results and discussion
Our analysis of the SOA composition in these experiments show definite inter-and intra-species variability, but the differences are generally subtle.In this section, we first present the paired pre-treatment and negative control experiments to demonstrate the reproducibility of the chamber system and provide context for the variability that was observed in other experiments.Next, a summary of all experiments is presented with Introduction

Conclusions References
Tables Figures

Back Close
Full a discussion of the inter-and intra-species variation, followed by a discussion of the post-treatment aerosol spectra.In this section, we present the first aerosol mass spectra generated from SOA produced via the gas-phase oxidation of the plant hormone, methyl jasmonate.Finally, we present results of the SOA elemental analysis using a Van Krevelen plot and discuss the inter-species variability along with implications for stress effects on SOA composition.

Negative control summary
Three sets of paired pre-treatment/negative control experiments were performed for which AMS measurements are available-one with grand fir (Abies grandis), one with western redcedar (Thuja plicata), and one with a mix of grand fir (Abies grandis) and Douglas-fir (Pseudotsugas menziesii).Scatter plots comparing the normalized UMR organic spectra between the pre-treatment SOA and the corresponding paired negative control SOA are shown in Fig. 1.The signal at m/z 28 was removed to avoid air interferences in the UMR spectra comparisons.The coefficient of determination (r 2 ) for each comparison is shown, calculated from the square of the Pearson product moment correlation coefficient.All paired negative control experiments were very similar, with r 2 greater than or equal to 0.990.The reproducibility of the high correlations between these paired experiments suggest that any correlations less than 0.99 that were observed in other experiments do truly reflect differences in SOA mass spectra.Based on these results, we considered any correlations lower than 0.90 to indicate potentially noteworthy differences between SOA mass spectra.

Summary of UMR comparisons
Correlations (r 2 ) comparing SOA organic UMR spectra from all biogenic aerosol growth experiments are summarized in Fig. 2. Correlations ranged from 0.503 to 0.999.In general, the pre-treatment aerosol mass spectra from all tree types had higher correlation values with respect to each other than they did with respect to post-treatment aerosol Introduction

Conclusions References
Tables Figures

Back Close
Full mass spectra.One pre-treatment experiment, AG-1-Pre (#9), stands out clearly with lower correlation values when compared to all other spectra.During this experiment, plants may have been exposed to an unidentified stress before transport to the laboratory (Faiola et al., 2014a).This pre-treatment experiment will be referred to as the "UNID stress" experiment and will be discussed in detail in a later section.Other than the AG-1-Pre spectra, all other pre-treatment SOA spectra had correlations ranging from 0.806-0.997when compared to each other across all tree types.Most of the weakest correlations (excluding the AG-1-Pre spectra) were found between comparisons that included the 2013 post-treatment experiments (#21-30 in Fig. 2).Specifically, the following experiments had the lowest correlations when compared to other SOA spectra: AG-2-Post (#23), TP-1-Post (#24), TP-3-Post (#25), PM-1-Post (#28), PM-2-Post (#29), and Mix-1-Post (#30).This list includes all the experiments where the MeJA treatment and the aerosol growth experiment occurred on the same day (Table 1).In contrast, when these six aerosol spectra were compared to one another, each comparison had r 2 greater than or equal to 0.95.This suggests that the MeJA and its oxidation products may have contributed substantially to SOA formation.This hypothesis and its environmental implications are explored in detail in a later section on methyl jasmonate SOA.
To further investigate trends in the SOA spectra correlations, all comparisons were classified by the type of comparison and binned into six different ranges of correlation values: < 0.6000, 0.6000-0.6999,0.7000-0.7999,0.8000-0.8999,0.9000-0.9499,and 0.9500-0.9999.The results of this analysis are presented in Fig. 3.The top bar in the figure shows the results from all types of biogenic SOA comparisons using real plant emission as the VOC precursor for SOA formation (N = 561 total comparisons).This classification did not include any comparisons with the methyl jasmonate standard SOA spectrum.Nearly 50 % of all comparisons with biogenic SOA had an r 2 greater than or equal to 0.90.The rest of the comparison types were organized reading top to bottom from highest to lowest number of correlations that fell within the Introduction

Conclusions References
Tables Figures

Back Close
Full All three comparisons of the paired pre-treatment/negative control SOA spectra were in the highest correlation bin with r 2 greater than or equal to 0.95.The fifteen comparisons between the post-treatment SOA spectra where the methyl jasmonate treatment occurred the same day as the SOA growth experiment (SD, PostT) were all greater than or equal to 0.90.The pre-treatment SOA comparisons were more heavily weighted toward the higher correlation values than the "all comparisons" category, with nearly 80 % of the r 2 values greater than or equal to 0.90.Additionally, the pre-treatment SOA spectra were more similar to one another than the post-treatment spectra were to one another.This suggests there was more variability in VOC emissions post-treatment than there was pre-treatment between the different tree types.The negative control spectra tended to be more similar to the pre-treatment SOA than the post-treatment SOA with nearly 80 % of comparisons with r 2 greater than or equal to 0.90 for the former and only ∼ 30 % of comparisons with r 2 greater than or equal to 0.90 for the latter.SOA spectra from the same tree type were more heavily weighted toward the higher correlation bins than SOA spectra generated from different tree types.Additionally, SOA spectra generated the same year were more similar to one another than SOA generated in different years.We hypothesize that some of difference between years could be attributed to differences in tree types and tree maturity from 2012 to 2013.In the 2012 experiments, only Pinus ponderosa and Pinus aristata were used and they were 2-3 years of age at the time of the experiments.In contrast, in 2013 all tree types except for Pinus ponderosa were used and most of the experiments were performed with trees that were 1 year of age.
Thirteen paired pre-treatment/post-treatment experiments were performed.Six of these had GC-MS-FID data available to investigate whether or not a plant response to the stress treatment had occurred.For several of the intended pre-treatment/posttreatment comparisons, there were no differences in the BVOC profile between the pre-and post-treatment experiment.These paired experiments were excluded from Introduction

Conclusions References
Tables Figures

Back Close
Full our comparisons after also verifying that the stress treatment had not produced any significant differences in the SOA mass spectra (PPo-2, r 2 = 0.92; PPu-2, r 2 = 0.98; PA-3, r 2 = 0.97).All other pre-treatment/post-treatment comparisons were included in the analysis even if the BVOC profile only changed minimally after treatment (PPo-1) or if there were confounding winter dormancy effects on emissions (PA-1).All of the comparisons without GC data to confirm plant stress response were included in the analysis.Eight of the ten remaining pre-treatment/post-treatment comparisons had r 2 between 0.7-0.8999,substantially lower than the negative control spectra comparisons.This suggests there were small, but possibly significant, differences between the SOA generated under the baseline emissions scenario and the SOA generated under the herbivore-stress emissions scenario.A potential plant stress AMS "marker" in the post-treatment SOA is discussed further in Sect.3.4.2.
The weakest correlations between biogenic SOA spectra (excluding the MeJA singlecomponent standard spectra comparisons) were observed for comparisons with the UNID stress SOA from experiment AG-1-Pre (#9).All SOA spectra comparisons with the UNID stress spectrum had correlations less than 0.90 and nearly 80 % of the comparisons had r 2 less than 0.70.Due to the dissimilar nature of this SOA spectrum relative to others, we have included a detailed description of the spectral characteristics of this SOA in the following section (Sect.3.3).

A closer look at Abies grandis (grand fir) SOA
Three paired sets of aerosol growth experiments were performed with Abies grandis emissions: two pre-treatment/foliar MeJA treatment experiments (AG-1 and AG-2) and one pre-treatment/negative control experiment (AG-3).The negative control results were presented in Sect.3.1.BVOC measurements were collected during aerosol growth chamber loading for AG-1, but not for the other two sets of experiments due to a GC instrument malfunction.In the companion paper, we hypothesized that the Abies grandis saplings used in experiment AG-1 had been exposed to an unidentified external stress before being transported to the laboratory chamber (Faiola et al., 2014a).

Conclusions References
Tables Figures

Back Close
Full Consequently, this is one of the only experiments where emissions actually decreased after MeJA treatment relative to the pre-treatment value.Despite this apparent confounding stress, the MeJA treatment still induced emissions of 1,8-cineol and terpinolene.The BVOC profiles during aerosol chamber loading for experiment AG-1 are shown in Supplement (Fig. S2).
The correlation between the AG-2-Pre SOA mass spectrum and the AG-3-Pre spectrum was very strong, with an r 2 of 0.97.The AG-1-Pre SOA spectrum was less similar to the other two Abies grandis pre-treatment SOA spectra with r 2 values of 0.66 (vs. AG-2-Pre) and 0.80 (vs.AG-3-Pre).The aerosol mass spectra for AG-1-Pre and AG-2-Pre are shown in Fig. 4 to highlight some of the m/z contributing to the differences between the SOA spectra.The AG-1-Pre SOA spectrum has a significant cluster of peaks present around m/z 200 that were not observed in any other aerosol mass spectra including the other SOA spectra produced from Abies grandis emissions.This evidence further supports the hypothesis that the AG-1-Pre spectra was not representative of a "typical Abies grandis SOA baseline" and that these plants had been exposed to an unidentified stressor.To investigate differences in the relative m/z enhancements and reductions generated under the unidentified stress condition and the MeJA stress condition, AG-2-Pre was selected to use as a "typical Abies grandis baseline spectrum" for comparison.A stress response plot was generated for both the unidentified stress effect and methyl jasmonate stress effect (Fig. 5).The unidentified stress response was calculated by subtracting the normalized spectrum of the AG-2-Pre experiment (baseline Abies grandis SOA) from the normalized spectrum of the AG-1-Pre experiment (unidentified stress SOA).The MeJA stress response was calculated by subtracting the normalized spectra of the same AG-2-Pre experiment (baseline Abies grandis SOA) from its paired post-treatment MeJA stress experiment, AG-2-Post (MeJA post-treatment SOA).The changes to the m/z profile were substantially different between the two stress scenarios.The MeJA SOA stress response spectrum demonstrated the most enhanced m/z values at 15, 26, 27, 29, 31, 57, 58, 59, 71, and  43 was reduced.Recall that these spectra have been normalized to the sum of total organics so a negative value in the stress response spectra does not necessarily mean that the fragment was inhibited.Rather, it demonstrates only that the relative contribution to the total has been reduced.The fragment at m/z 43 is frequently the highest organic fragment in chamber SOA (Chhabra et al., 2010) .This list contains many more oxidized fragments than the enhanced HR ions in the unidentified stress response spectrum.
The weaker presence of oxidized HR ions in the unidentified stress SOA spectra could be the result of two possibilities or, possibly more likely, a combination of the two explanations.One explanation is that the unidentified stress induced emissions of large hydrocarbons, which produced a higher proportion of larger, less oxidized fragments in the spectra.This cause is suggested by the cluster of peaks greater than m/z 200, particularly at m/z 202, in the AG-1-Pre spectrum that were not observed in any of the other spectra.The HR ion identified here was C 16 H + 10 , a large un-oxidized fragment that could have originated from a large stress-induced hydrocarbon BVOC emission.The compounds that contributed to these large m/z fragments were not detected by the GC system so they cannot be positively identified here.However, large 16-carbon and Introduction

Conclusions References
Tables Figures

Back Close
Full 18-carbon compounds have been identified following herbivory stress in other studies (De Boer et al., 2004;Mentel et al., 2013).Another possibility is that the amount of ozone added to the chamber was not sufficient to fully oxidize these particles to the same extent as other experiments because the plant VOC emissions were so high.114 ppb of ozone was added at the start of the experiment and it had fallen to 9 ppb by the end (Table 2).With the high organic particle loadings generated in this experiment (> 500 µg m −3 ) it is possible that some of these larger emissions and their oxidation products were able to partition to the particle phase in a less oxidized state than would normally occur under lower mass loadings (Kroll and Seinfeld, 2008).Thus, the higher emissions generated a large amount of overall organic particle mass, and the combination of the presence of larger, less volatile emissions (and their oxidation products) and an oxidant-limited system promoted the partitioning of less oxidized components to the particle phase.
The correlations between the two paired pre-treatment/post-treatment Abies grandis SOA spectra were 0.86 and 0.77 for AG-1 and AG-2, respectively (Fig. 2).Thus, despite the presence of an unidentified stressor under pre-treatment conditions, the stress treatment still produced some small differences between the pre-treatment and post-treatment SOA spectra in the AG-1 experiment.This is consistent with the BVOC emission profile where emissions of 1,8-cineol were induced after treatment and the relative contribution of beta-myrcene, limonene, and terpenolene increased (Supplement Fig. S2).Five of the top ten most enhanced fragments between the AG-1-Post and AG-1-Pre spectra were also observed in the top 10 most enhanced fragments between the AG-2-Post and AG-2-Pre spectra: m/z 15, 26, 27, 31, and 58.The dominant HR ions corresponding to m/z 15, 26, and 27 were CH markers for biogenic stress SOA.This will be discussed further in the following sections while looking at more examples of the post-treatment SOA spectra in detail.

Post-treatment aerosol mass spectra
Thirteen post-treatment SOA experiments were completed for this study.Of those, three were performed in 2012 with the exogenous methyl jasmonate treatment.The PPo-1 experiment exhibited small, likely insignificant, differences between the pre-and post-treatment SOA that could have been due to natural variation in plant emissions.
The BVOC profile indicated that any stress response was weak if it existed at all.The other two experiments performed in 2012 may have had a confounding stress effect due to pulling the plants out of dormancy.For these reasons, the 2012 post-treatment experiments will not be the focus of this discussion of post-treatment SOA here.Of the ten post-treatment SOA experiments performed in 2013 with the foliar methyl jasmonate application, four were performed the day after treatment and six were performed the same day as treatment.
The four experiments that were performed the day after MeJA treatment were PPu-1-Post, PPu-2-Post, AG-1-Post, and PA-3-Post.Based on the BVOC profiles, none of the post-treatment experiments where the treatment was performed on a different day than the aerosol growth experiment would work as good candidates for identifying a biogenic stress SOA marker in the AMS spectra.The AG-1-Post SOA spectrum was discussed in detail in the previous section, showing confounding effects on the SOA spectra from an apparent unidentified stress exposure prior to transporting the plants to the laboratory.The PPu-1-Post and PPu-2-Post both appear to be representative of a stress condition for Picea pungens based on the BVOC profiles presented in Faiola et al. (2014a).The stress response for the PPu-1 experiment in particular was very high.However, the PPu-1-Pre experiment did not produce enough SOA mass to perform AMS analysis, so there is no baseline Picea pungens SOA spectra for comparison.The BVOC results from the PPu-2-Pre experiment suggest these plants may also have been stressed before being brought into the laboratory; their BVOC profile closely 25187 Introduction

Conclusions References
Tables Figures

Back Close
Full resembled the post-treatment Picea pungens BVOC profile from the previous experiment.So, no baseline Picea pungens SOA spectra were acquired for comparison with the post-treatment SOA spectra for these two experiments.Finally, no stress response was observed during the PA-3 experiment based on the BVOC profile.Consequently, this post-treatment SOA spectrum could not be used to identify a stress biogenic SOA marker either.
The remaining six post-treatment SOA spectra were AG-2-Post, PM-1-Post, PM-2-Post, TP-1-Post, TP-3-Post, and Mix-1-Post.Where BVOC data was available (PM-2 and TP-3), it suggested there was an identifiable plant stress response due to the foliar MeJA stress treatment (Faiola et al., 2014a).These six spectra also stand out distinctly on the correlation summary figure because they had lower correlations with other spectra than observed for most of the other SOA spectra comparisons (Fig. 2).However, the influence of the methyl jasmonate and its oxidation products needs to be accounted for when interpreting these spectra.A discussion of these results is provided in the next section.

Methyl jasmonate SOA
The aerosol mass spectrum of SOA generated from the oxidation of the singlecomponent methyl jasmonate standard is shown in Fig. 6.To the authors' knowledge, this is the first description of SOA generated from the plant hormone, methyl jasmonate, from ozone-initiated gas-phase oxidation.The dominant fragments in the normalized mass spectrum were m/z 28, 29, and 44.The standard MeJA SOA had more of the highly oxidized m/z 44 and less m/z 43 than observed in typical biogenic SOA generated from chamber experiments.Additionally, there were small, but observable, peaks at m/z 131 and m/z 157 that were not typical of the other biogenic SOA spectra generated in the work presented here.The lowest correlations between all SOA spectra acquired throughout these experiments were observed between biogenic SOA generated from real plant emissions and SOA derived from the oxidation of the methyl jasmonate single-component standard.This was shown in the bottom three horizontal 25188 Introduction

Conclusions References
Tables Figures

Back Close
Full bars on Fig. 3.The most similar spectra to the methyl jasmonate standard were those from the post-treatment SOA where treatment was applied the same day as the SOA growth experiment (SD, PostT).However, even these correlations were all less than or equal to 0.8999.All other comparisons between biogenic SOA spectra and singlecomponent MeJA standard spectra had r 2 less than 0.80.
The possible influence of methyl jasmonate and its oxidation products on SOA composition could have significant atmospheric implications because plant hormones can be emitted from forests at rates as high as monoterpenoids when plants experience stressed conditions in the natural environment (Karl et al., 2008).For the experiments where the MeJA foliar application occurred on the same day as the aerosol growth experiments (referred to herein as "Same-Day" experiments), the estimated amount of MeJA vapor transported to the aerosol growth chamber was between 30-70 % of the total monoterpenoid concentrations.This value was estimated based on the saturation vapor pressure of MeJA, with the range reflecting variations in monoterpenoid emission rates from experiment to experiment.

Corrected 2013 "Same-Day" post-treatment SOA
The relative contribution of methyl jasmonate to the six "Same-Day" post-treatment SOA spectra was estimated by generating a series of linear combinations of different relative amounts of the normalized pre-treatment SOA spectra and the normalized methyl jasmonate standard SOA spectra.For each series, an optimized linear combination was determined based on identifying the combination spectra that had the strongest correlation with the paired post-treatment SOA spectra.The results of this analysis for all six "Same-Day" experiments are presented in Fig. 7.In each of the six experiments, the optimized linear addition spectrum occurred when the contribution of the pre-treatment spectrum was between 40 and 60 % of the combination spectrum.Thus, methyl jasmonate and its oxidation products were estimated to contribute between 60 and 40 % of the SOA mass in the "Same-Day" post-treatment spectra.The optimized combination spectrum was then subtracted from the normalized 25189 Introduction

Conclusions References
Tables Figures

Back Close
Full post-treatment spectra to define a "residual spectrum" for each experiment.This residual should be more representative of the influence of stress-induced emissions on posttreatment spectra, having removed the presumed direct effect of the MeJA present.All six of these residual spectra are shown in Fig. 8.Only the positive values are shown to focus on the m/z fragments that were remaining after subtracting off the optimized linear addition of the paired pre-treatment and methyl jasmonate standard spectra.The residual spectra were generally very similar to one another with r 2 > 0.90 for most comparisons.The residual TP-3-Post was an exception to this with correlations ranging from 0.32-0.70 with the other residual spectra.The strongest contributions across the residual spectra were at m/z 26, 27, 29, 31, 57, 58, 59, 71, and 83.Many of these are consistent with the most enhanced fragments described earlier from the stress response spectra comparing the paired AG-1-Pre and AG-1-Post spectra (m/z 26, 27, 31, and 58).The AG-1-Post experiment was conducted the day following foliar methyl jasmonate treatment rather than on the same day as methyl jasmonate, so the contribution of methyl jasmonate and its oxidation products to SOA mass should have been minimal.This further supports the hypothesis that enhanced m/z 31 and 58 are associated with a biogenic stress response.It is also worth noting that m/z 29 was the largest fragment in each of the six residual spectra, and specifically that the HR ion C 2 H + 5 at m/z 29 was increased more significantly than the other major HR ion at m/z 29, CHO + .Other larger HR ions found prominently in the residual spectra were and C 4 H 3 O + 2 , C 5 H 7 O + , and C 6 H + 11 (m/z 83).At m/z 83, the C 5 H 7 O + was the most enhanced HR ion.The potential enhancement of these ions due to biogenic stress response merits further targeted investigation.

Elemental analysis summary
A summary of the elemental analysis results for all pre-treatment SOA and negative control SOA is shown in Fig. 9a.This figure illustrates the inter-plant variation in biogenic SOA composition.One clear outlier was the SOA generated in experiment 25190 Introduction

Conclusions References
Tables Figures

Back Close
Full AG-1-Pre -the unidentified stress (UNID Stress) experiment that was discussed previously.All H : C ratios were similar (∼ 1.5) throughout the pre-treatment experiments -this is consistent with expected oxidation products of monoterpenes during ozonolysis reactions.In contrast, the O : C ratios varied between different tree types.In fact, the elemental analysis results demonstrated a higher level of variability between pretreatment SOA than was expected from the UMR correlation coefficient analysis.This could partially be caused by the exclusion of m/z 28 in the UMR analysis.The CO + ion was accounted for in the elemental analysis but not in the UMR analysis.The contribution from organics at m/z 28 was a substantial fraction of the total signal and is commonly estimated to be around the same magnitude as m/z 44 -a significant peak for all of these spectra contributing between 4-10 % of total organic signal.Most pre-treatment SOA had an O : C within the range of 0.3-0.38.However, there were some exceptions.Specifically, the Pinus aristata SOA had a higher O : C on average than other pre-treatment biogenic SOA generated from emissions of other tree types with O : C ranging from 0.39-0.47.Similarly, one pre-treatment Picea pungens experiment and one pre-treatment Mix experiment generated biogenic SOA with higher O : C values than the average.The pre-treatment SOA from the Picea pungens emissions could have been more representative of a stress condition based on the BVOC emission profile -stress emissions of 1,8-cineol and beta-ocimene were measured (Faiola et al., 2014a).A second pre-treatment Mix experiment was performed and produced SOA with a much lower O : C than the first, so the high O : C results from the pre-treatment Mix emissions were not reproducible.Two of the three negative control SOA had some of the lowest O : C ratios that were measured (excluding the UNID stress experiment).The Thuja plicata negative control had substantially higher O : C than the others, but it was very similar to the other pre-treatment Thuja plicata experiments.
A summary of the elemental analysis results from all paired pre-and post-treatment experiments where a plant stress response was observed is presented in Fig. 9b.The pre-treatment SOA that had a paired post-treatment experiment where a stress Introduction

Conclusions References
Tables Figures

Back Close
Full response was observed had O : C that ranged from 0.32-0.41(excluding the unidentified stress experiment) or 0.32-0.37 if the possible Mix SOA outlier is excluded as well.
The paired post-treatment SOA had O : C that ranged from 0.42-0.46.For all experiments, the MeJA SOA shifted the O : C ratio to higher values relative to the paired pretreatment SOA.Each of these post-treatment experiments were performed the same day as treatment except for the Abies grandis unidentified + MeJA stress experiment (AG-1-Post).The unidentified stress post-treatment experiment resulted in an increase of O : C from 0.19 in the pre-treatment SOA to 0.29 in the post-treatment SOA.This effect could have been due to the stress treatment or it could have been due to the unidentified stress waning after the trees were transported to the laboratory -the posttreatment O : C was still not as high as most pre-treatment SOA.For all the "Same-Day" post-treatment experiments, the increased O : C could be due to the oxidation products of the plant hormone, methyl jasmonate.The elemental ratios from the SOA generated from the oxidation of the single-component methyl jasmonate standard are also shown in Fig. 9b in black.Expected elemental ratios calculated from the optimized linear addition of the baseline spectra and the MeJA standard spectra yielded elemental ratios that were within 10 % of those measured for the paired posttreatment experiment (for same day treatment/growth experiment only).This suggests that most of the increase in the O : C may have been due to the methyl jasmonate and its oxidation products rather than the influence of specific stress compounds in the SOA spectra.However, the pre-treatment Picea pungens experiment where the plants appeared to be in a stressed condition also had higher O : C in approximately the same Van Krevelen space as these other post-treatment SOA.This suggests there are compounds other than MeJA and its oxidation products that could also produce SOA in this region of the Van Krevelen plot.

Results summary
The number of experiments and types of tree species examined in this study has provided a rich, but complex data set.When experiments are grouped into categories by 25192 Introduction

Conclusions References
Tables Figures

Back Close
Full common characteristics, clear patterns emerge in the data.First, we find that the SOA generation methods used in this study were highly reproducible as evidenced by results from the three paired pre-treatment/negative control experiments where all SOA spectra comparisons produced correlations greater than 0.990.These results put all other comparisons in context and suggest that any correlations less than 0.90 do truly represent a difference between SOA mass spectra.Most of the pre-treatment SOA generated from emissions of all tree species had very similar UMR SOA spectra with nearly 80 % of all pre-treatment SOA comparisons having an r 2 greater than 0.90.This result, when combined with the diversity in pre-treatment monoterpenoid emission profiles from these trees presented in Faiola et al. (2014a), suggests that aerosol mass spectra of biogenic SOA formed under baseline conditions all look very similar even with a different mix of monoterpenes used to generate the SOA.These results are consistent with findings presented by Kiendler-Scharr et al. (2009) who found similar AMS characteristics between biogenic SOA generated from the emissions of different types of plant species.In contrast, results from HR data analysis showed a higher degree of variability between pre-treatment biogenic SOA with O : C values ranging from 0.30-0.47(excluding the UNID stress experiment).
The presence of stress led to significant differences in the UMR SOA spectra.For example, the SOA spectra that was least similar to all other SOA spectra was generated from the emissions of Abies grandis after the saplings had apparently been exposed to an unidentified stressor before being transported to the lab.Consequently, these results were not reproducible but did serve as an opportunity to investigate a plant's response to a natural stressor.The presence of substantial, discernible peaks in the UMR spectrum around m/z 200 indicated the presence of higher molecular weight emissions that were not identified with the GC system.Large 18-carbon compounds have been observed as a plant's response to certain types of herbivores and, when observed previously, resulted in substantially increased SOA yields (Mentel et al., 2013).The AG-1-Pre results may have been due to a similar phenomenon.The amount of SOA produced from these emissions was substantial (> 500 µg m −3 ) and had a significantly Introduction

Conclusions References
Tables Figures

Back Close
Full lower O : C than any other SOA reported here or reported elsewhere (Chhabra et al., 2010) The other SOA spectra that had the lowest correlation coefficients when compared to pre-treatment SOA were the 2013 post-treatment SOA.We attempted to remove the influence of MeJA, and its oxidation products, on the "Same-Day" post-treatment SOA spectra.The resulting residual spectra highlighted differences to the SOA spectra that were due to the plants response to the stress treatment.+ enhancement).The enhancement of these ions in ambient datasets should be investigated to search for this possible biogenic stress marker in aerosol spectra collected in a natural forest environment.
Additionally, our results demonstrate that plant hormones, such as methyl jasmonate, can contribute to SOA formation and produce distinctive SOA mass spectra with peaks at m/z 131 and m/z 157.The standard MeJA SOA was substantially more oxidized than other biogenic SOA as was evidenced by its high relative proportion of m/z 44 to the total organic mass and its high O : C ratio of 0.52.Plant emissions of stress hormones can equal emissions of monoterpenes under stressed conditions, and others have even suggested using ambient measurements of plant hormones to monitor for plant stress at an ecosystem scale (Karl et al., 2008).It is possible that the mass spectral markers associated with either the plants response to the stress treatment or the markers associated with the methyl jasmonate plant hormone directly could also be used to monitor for stress at an ecosystem scale.Introduction

Conclusions References
Tables Figures

Back Close
Full

Conclusions
The baseline aerosol mass spectra of biogenic SOA produced from real plant emissions were similar across six different plant species when comparing UMR results.However, the presence of stress appeared to change the composition of the SOA to the extent that the UMR aerosol mass spectra looked significantly different.This mass spectral biogenic stress marker could be indicative of an herbivory stress aerosol signature in the natural forest environment when stressed conditions produce stress-induced emissions including, but not limited to, plant hormones such as methyl jasmonate.
Future work on this topic should investigate SOA mass spectral fingerprints for other stressors that could induce emissions of non-terpenoid compounds.For example, any stressor that damages plant membranes produces bursts in OVOC products from the lipoxygenase pathway.This could be investigated in the laboratory using real herbivores or pathogens that would damage plant tissues.Additionally, tissue damage can occur under severe heat stress.Future work should also generate SOA from the plant hormone methyl salicylate, which is emitted at higher rates than methyl jasmonate and still has low enough volatility to potentially contribute to SOA formation.To our knowledge, SOA has not been generated from this major plant hormone that has been measured at significant levels in a forest environment.Other future studies should focus on analyzing ambient AMS datasets collected in forest environments to investigate whether or not the biogenic stress marker that was identified here can be observed in field measurements.This could serve as a monitoring tool to identify ecosystem-level plant stress.
The Supplement related to this article is available online at doi:10.5194/acpd-14-25167-2014-supplement.Introduction

Conclusions References
Tables Figures

Conclusions References
Tables Figures

Back Close
Full  Full  Full  1).The color scale denotes the strength of correlation between the two spectra where warmer colors are lower correlations and cooler colors are higher correlations.Due to air interferences, m/z 28 was removed from the spectra for all comparisons.The figure was organized by year followed by experiment type (pre-treatment, post-treatment, negative control) followed by tree type.NC = negative control.Introduction

Conclusions References
Tables Figures

Back Close
Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | investigated the impact of herbivory, drought, and heat stress on Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 3 m×0.3m×0.3 m FEP Teflon film dynamic enclosure where sapling trees are stored.A full description of the Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | The role of plant hormones in SOA formation has typically been ignored is plant SOA experiments.Recently, Richards-Henderson et al. (2014) demonstrated that aqueous phase Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | loadings (Sage Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 0.9500-0.9999correlation bin.The bottom three bars show results from comparisons with the standard methyl jasmonate spectra.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 97.The relative contribution of m/z Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ions are not very specific and could be generated from many organic compounds, so it is unlikely that they alone will provide help in identification of an AMS mass spectral biotic stress SOA "marker".The dominant HR ions at m/z 31 and 58 were CH 3 O + , C 2 H 2 O + 2 , C 3 H 6 O + .These ions could provide a little more insight into precursors contributing to their presence in the SOA spectra, and could possibly be the start to identifying AMS Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Hallquist, M., Wenger, J. C., Baltensperger, U., Rudich, Y., Simpson, D., Claeys, M., Dommen, J., Donahue, N. M., George, C., Goldstein, A. H., Hamilton, J. F., Herrmann, H., Hoffmann, T., Iinuma, Y., Jang, M., Jenkin, M. E., Jimenez, J. L., Kiendler-Scharr, A., Maenhaut, W., McFiggans, G., Mentel, Th.F., Monod, A., Prévôt, A. S. H., Seinfeld, J. H., Surratt, J. D., Szmigielski, R., and Wildt, J.: The formation, properties and impact of sec-Discussion Paper | Discussion Paper | Discussion Paper | Carlo, P. F., Salcedo, D.
Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper |

Figure 1 .Figure 2 .
Figure 1.Scatter plots comparing the normalized spectra of all three paired pretreatment/negative control experiments.The markers denote the m/z.Only the 89 UMR m/z signals used in the correlation analyses are plotted.The x axis is the percent contribution to total organic mass for the pre-treatment experiment and the y axis is the percent contribution to total organic mass for the paired negative control experiments.The dashed gray 1:1 lines are shown for reference.AG-3 = Abies grandis experiment.TP-2 = Thuja plicata experiment, Mix-2 = mix of Abies grandis and Pseudotsugas menziesii experiment.Correlations (r 2 ) between the negative control spectra and the pre-treatment spectra are shown in the boxes on each plot.

Figure 5 .
Figure 5. Stress response spectra comparing the effects of two different types of stress -an unidentified stress (red) and a MeJA treatment (blue).The x axis shows the m/z values and the y axis denotes the difference between the normalized stress spectrum and the normalized baseline spectrum.

Figure 7 .Figure 8 .
Figure 7. Results from the linear addition optimization for all six experiments where the posttreatment aerosol growth experiment was performed the same day as the foliar methyl jasmonate treatment was applied.The x axis denotes the fraction of the pre-treatment experiment that was included in the linear addition of the pre-treatment and MeJA standard SOA spectra.The y axis is the correlation of the linear addition spectra with the paired post-treatment SOA spectra.The fraction of pre-treatment SOA included in the linear addition spectra that produced the highest correlation with the paired post-treatment is shown in the box on each graph.
. Other enhanced m/z in the UNID stress spectra were m/z 31 and m/z 58 corresponding to HR ions CH 3 O + , C 2 H 2 O + .
The same m/z that were enhanced in the UNID spectra, m/z 31 and m/z 58 (HR ions CH 3 O + , C 2 H 2 O