Aerosol characteristics at the Southern Great Plains site during the HI-SCALE campaign

5 Large uncertainties exist in global climate model predictions of radiative forcing due to insufficient understanding and simplified numerical representation of cloud formation and cloud-aerosol interactions. The Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) campaign was conducted near the DOE’s Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site in north-central Oklahoma to provide a better understanding of land-atmosphere interactions, aerosol and 10 cloud properties, and the influence of aerosol and land-atmosphere interactions on cloud formation. The HI-SCALE campaign consisted of two Intensive Observational Periods (IOPs) (April-May, and August-September, 2016), during which coincident measurements were conducted both on the G-1 aircraft platform and at the SGP ground site. In this study we focus on the observations at the SGP ground site. An Aerodyne HR-ToF Aerosol Mass Spectrometer (AMS) and an Ionicon Proton-Transfer-Reaction Mass 15 Spectrometer (PTR-MS) were deployed, characterizing chemistry of non-refractory aerosol and trace gases, respectively. Contributions from various aerosol sources, including biogenic and biomass burning emissions, were retrieved using factor analysis of the AMS data. In general, the organic aerosols at the SGP site was highly oxidized, with OOA identified as the dominant factor for both the spring and summer IOP though more aged in spring. Cases of IEPOX SOA and biomass burning events were further investigated to understand additional sources of organic aerosol. Unlike other regions largely impacted by IEPOX chemistry, the IEPOX SOA at SGP was more highly oxygenated, likely due to the relatively weak local emissions of isoprene. Biogenic emissions appear to largely control the formation of OA during HI-SCALE campaign. Potential HOM (highly-oxygenated molecule) chemistry likely contributes to the highly-oxygenated feature of aerosols at the SGP site, with impacts on new particle formation and global climate. addition, several case studies are discussed in detail to examine the impacts of seasonal variations in biogenic and anthropogenic sources, long-range transport and meteorology on aerosol properties. particle formation (NPF), they will also impact subsequent aerosol growth, 475 CCN populations, and the influence of aerosols on global climate. Actually during HI-SCALE campaign, NPF events were more frequently observed in spring than in summer (Fast et al., 2019), in agreement with the more-oxygenated feature of OA observed during the spring IOP in this study. Considering the potential climate impacts, the highly-oxygenated nature of aerosols at the SGP site is an interesting topic which should be investigated further. The mixture of anthropogenic, biogenic, and biomass burning sources of 480 OA at the SGP site provides an opportunity to rigorously evaluate explicit and parameterized treatments of a range of SOA pathway mechanisms.


SGP site description
The central facility at the SGP site is located in north-central Oklahoma, at 36. 60°N and 97.48°W, as shown in Figure S1. It was designed to measure cloud, radiation, and aerosol properties in a region that experiences a wide variety of meteorological conditions. As the first field measurement site established by 70 the Atmospheric Radiation Measurement (ARM) user facility, the SGP site is known as a "hotspot" of land-atmosphere interactions that influences the lifecycle of shallow convection (e.g., Dirmeyer et al., 2006;Koster et al., 2004;Koster et al., 2006). The central facility is located in a rural environment, immediately surrounded by cropland and pasture with a small portion of forest also in close proximity to the facility (Sisterson et al., 2016). Several urban areas are located within 200 kilometers of the site,

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including Wichita (~110 km to the north), Oklahoma City (~135 km to the south) and Tulsa (~150 km to the southeast). Several smaller towns such as Enid, Stillwater and Ponca City are located within 100 km of the site. In addition, a refinery is located approximately 45 km ENE of the site and a 1138 MW coal-fired power plant is located 50 km to the ESE. Therefore, the air masses arriving at the SGP site are diverse, originating from anthropogenic, biogenic, and biomass burning sources. During the two IOPs of the HI-80 SCALE campaign, a suite of supplemental online instruments were deployed at the SGP central facility to characterize both the gas-and particle-phase composition. Most instruments were located in the guest user facility, which is a separate trailer 300 meters from the main building of the central facility (Sisterson et al, 2016). Due to the proximity of the guest instrument trailer to the permanent aerosol equipment located at the central facility, the instruments are expected to sample the same air mass.

Instrumentation
An Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (abbreviated as AMS hereafter) was deployed at the SGP site to provide the mass concentration and chemical composition of submicron, non-refractory aerosols (Jayne et al., 2000;DeCarlo et al., 2006). The AMS was operated in the standard "V" mass spectrometer (MS) mode, with a 5-min data averaging interval. Filter blanks were performed 90 every day by diverting AMS-sampled air through a HEPA filter, and these filter periods were used to account for gas-phase interferences with isobaric particulate signals. Based on the standard deviation of these blank measurements (3) as described in the literature (DeCarlo et al., 2006), the detection limits of the AMS at the 5-min sampling interval were 0.07, 0.015, 0.006, 0.009, and 0.005 g/m 3 for organics, sulfate, nitrate, ammonium, and chloride, respectively. The AMS was operated continuously during the 95 entire spring IOP and first half of the summer IOP (August 28-September 9, 2016); the AMS suffered an ion optics failure during the second half of the summer IOP. During the campaign, the AMS was routinely calibrated using monodisperse NH 4 NO 3 particles quantified with a TSI condensation particle counter (CPC), whereas (NH 4 ) 2 SO 4 particles were applied for AMS calibration before and after campaign. Data were analyzed in Igor Pro (v6.37) using the high-resolution analysis package (Squirrel v1.57,PIKA v1.16) and 100 https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License. techniques described in the literature Kroll et al., 2011;Aiken et al., 2008;Jimenez et al., 2003). The values of atomic oxygen-to-carbon (O:C) and hydrogen-to-carbon (H:C) ratios were calculated using the updated fragmentation tables in Canagaratna et al. (2015). Positive matrix factorization (PMF) analysis was performed using the high-resolution data and the PMF Evaluation Tool (v3.05A).

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The AMS sampled air drawn from an inlet located 10-m above the ground. Sample air was drawn through a PM 2.5 cyclone, passed through a Nafion dryer, brought into the guest facility with ½" stainless steel tubing, and shared by three aerosol sampling instruments including an HR-ToF-AMS, a Single Particle Laser Ablation Time-of-Flight mass spectrometer (SPLAT II), and an SMPS. The SMPS system consisted of a TSI Model 3081 long column DMA with a recirculating sheath flow of 3 Lpm and a TSI Model 3775 CPC 110 operated in the low flow mode (0.3 Lpm), and was set to measure the particle size distribution from14 nm to 710 nm (mobility diameter) at a sampling frequency of one scan every 4 minutes. Data from the SMPS were also used in evaluating the AMS collection efficiency.
An Ionicon quadrupole high-sensitivity Proton-Transfer-Reaction Mass Spectrometer (PTR-MS) was used to measure the mixing ratios of gas-phase VOCs (Lindinger and Jordan, 1998). Similar to the aerosol 115 sampling instrument, the inlet of PTR-MS was also positioned at 10-m above the ground with an inlet filter at the end to remove particles, then connected to the instrument through Teflon tubing. The PTR-MS was run in the mass-scan mode, in which a mass spectrum from m/z 21 to m/z 250 was recorded with 1-s dwell time on each unit m/z. The time resolution for each cycle is ~ 4-min. Drift tube pressure and temperature were set at 2.2 mbar and 60 °C with a 600 V potential across the drift tube. Signal intensity of selected 120 species, including m/z 42, 45, 59, 69, 71, 79, 93, 107, 121 and 137, was then converted to ppbv using a multi-point calibration with air from a calibration cylinder (Apel Riemer Environmental Inc.) containing known concentrations of acetonitrile, acetaldehyde, acetone, isoprene, methacrolein, benzene, toluene, mxylene, trimethylbenzene (TMB), and alpha-pinene. It is assumed in our analysis that the signals at the aforementioned m/z values are entirely from the indicated species, which could be a source of uncertainty.

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The calibration was performed periodically before, during, and after the campaign. The PTR-MS background was assessed twice per day by diverting air through a stainless-steel tube filled with a Shimadzu platinum catalyst heated to 600 °C, which removes VOCs from the airstream without perturbing RH. The catalyst efficiency was tested by comparing signal from air containing VOCs passed through the catalyst with signal from VOC-free air.

Back trajectory analyses
To investigate the potential sources and transport pathways of aerosols and aerosol precursors observed at the SGP site, back trajectories were performed by utilizing the National Oceanic and Atmospheric Administration (NOAA) Air Resources Laboratory Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT; http://ready.arl.noaa.gov/HYSPLIT_traj.php) (Draxler and Rolph, 2012). 72-hour backward 135 https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License. trajectories initialized from the SGP site were computed every 3 hours at multiple altitudes. The back trajectory analyses help identify sources of aerosols for specific events during the campaign. Figure 1 shows the HYSPLIT back trajectory paths for both spring-and summer-IOPs. In the spring 140 campaign, the back trajectories suggested that the air masses arriving at the SGP site mainly originated from the north during first half of the IOP, and gradually transitioned to originating from the south. At the end of the spring IOP there was a several-day period when the dominant winds became easterly, which would bring air masses from the biogenic-rich eastern region (Parworth et al., 2015). Approximately 45% of the time during the spring IOP, air arriving at the SGP facility originated from the northern plains. This 145 set of trajectories passed primarily over grassland and cropland (Trishchenko et al., 2004), and would therefore be influenced primarily by weak biogenic emissions. Air masses from the south are also a major source impacting the SGP site, contributing 36% of the back trajectories. These trajectories passed by cities such as Houston and Oklahoma city, which are largely influenced by anthropogenic emissions. For the remaining ~20% of the trajectories, air masses traveled from the east and likely brought emissions from 150 deciduous and mixed forests in northern Arkansas, Missouri and southern Illinois. It is possible that air masses originating from the southeastern U.S. can be transported to the SGP site, but the transport period would be longer than three days.

Analysis of HYSPLIT trajectories
During the summer IOP, air masses arriving at SGP site originated from two main directions ( Figure 1) according to HYSPLIT analysis. Southerly winds dominate during the summer, accounting for ~63% of the 155 trajectories, which suggests a larger contribution of aerosols and their precursors transported from Oklahoma and eastern Texas with urban characteristics. Compared to spring IOP, a smaller fraction of the air masses originated from the north, only accounting for ~37% of the trajectories. The back trajectories during summer IOP indicate shorter transport distances than spring due to lower wind speeds.

spring-and summer-IOPs
The AMS results for non-refractory submicron aerosols (NR-PM1) observed during both the spring-and summer-IOPs are summarized in Figure 2 and Table 1. Organic aerosol (OA) contributed the largest fraction to the total NR-PM1 mass concentration during both the spring-and summer-IOPs, accounting for >60% on average. There are, however, periods where inorganics were greater than 50% of the total mass.

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Average OA loading was 2.5 g/m 3 in the spring and 3.8 g/m 3 in the summer. Similar to organics, sulfate was also more abundant in absolute mass in the summer than in the spring IOP (average concentration 0.79 g/m 3 in spring versus 1.29 g/m 3 in summer; details in Table 1), but the mass fraction is similar (20.1% during spring IOP versus 22.4% during summer IOP). In contrast, the level of nitrate is much lower in the https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License.
summer IOP (0.085 g/m 3 ) than in the spring IOP (0.244 g/m 3 ; details in Table 1). This may be due to its 170 semi-volatile nature with warmer temperatures pushing the equilibrium back to the gas phase, decreasing nitrate concentrations. Due to incomplete datasets of gas-phase NH 3 , HNO 3 and SO 2 , we were unable to directly determine if there were seasonal variations in inorganic precursor trace gas emissions, however, AMS measurements of aerosol acidity may be used to infer these potential changes. During both IOPs, anions and cations show good correlation ( Figure S2), but in summer ammonium is insufficient for full 175 neutralization of the anions, suggesting the aerosols in summer were more acidic. In most circumstances, ammonium nitrate will not partition into the condensed phase until particulate sulfate is fully neutralized (Guo et al., 2017). Thus the more acidic aerosol might be another explanation for the lower nitrate concentration in summer. In the spring IOP, ammonium is 13% higher than that required to fully balance AMS-measured anions. This may be due to the presence of seasalt particles being transported to the SGP 180 site in the spring; the AMS is not optimized for detection of seasalt and anionic species would be relatively more easily detected than Na and Ca in the particles. Alternatively, this difference may be due to slight measurement and calibration errors.
The concentrations of biogenic VOCs are influenced by ambient temperature and sunlight, with higher temperatures and more abundant sunlight, among other factors, producing higher emissions (Guenther et 185 al., 2012). The average daily temperature at SGP was 24.0 C during the summer IOP, which is significantly higher than during the spring IOP (15.9 C). Days are also longer in the summer than in the spring. Concentrations of isoprene (m/z 69), a well-known biogenic VOC precursor of SOA, are about 2 times higher during the summer IOP compared to the spring IOP ( Figure 3). As discussed in the introduction, the design of two IOPs took into consideration the potential impacts of different stages and 190 distribution of 'greenness' for cultivated crops, pasture, herbaceous, and forest vegetation types. Our isoprene observations suggest a complex relationship between emissions and vegetation. Isoprene concentrations scaled with temperature and sunlight, despite the fact that summer was significantly drier and that back trajectories suggest a smaller impact from biogenic-rich regions during summer IOP.
Monoterpenes (m/z 137), another category of biogenic VOCs emitted into the atmosphere, also show a 195 similar pattern with higher concentrations observed in summer IOP. Back trajectories suggest more prevalent transport from the south in summer, which suggests higher anthropogenic impact. However, several representative anthropogenic VOCs observed by PTR-MS, including benzene, toluene and TMB, did not show significant enhancements during the summer season (Table 2). High concentrations of benzene and toluene were observed during the first several days of summer IOP, but they were also 200 accompanied by high levels of biogenic precursors (i.e., isoprene, monoterpenes) ( Figure 3). During these time periods, back trajectories were from the southeast and the paths over the three-day period were generally short ( Figure S3). Thus, the high concentrations were probably locally accumulated due to lower wind speeds. Acetonitrile, a key tracer for biomass burning, did not show significant changes during the two IOPs. Therefore, the higher OA concentrations observed in the summer IOP relative to the spring are 205 https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License. likely related to more-intense biogenic emissions, rather than enhanced transport from urban areas or from biomass burning.
Recently, there has been intense research into the formation of highly-oxygenated organic compounds from biogenic precursors. This new group of highly-oxygenated molecule (HOMs) has been proposed to be the source of a major fraction of tropospheric submicron SOA (Bianchi et al., 2019;Ehn et al., 2017;  values observed in the spring and summer season. The first possible explanation is that the aerosol in the spring is more aged due to a longer residence time in the atmosphere, potentially different oxidant concentrations, or a combination of both. Since photochemical aging leads to an increase in f 44 de Gouw et al., 2005;Aiken et al., 2008;Kleinman et al., 2008), the level of f 44 can be considered 220 as an indicator of atmospheric aging. Shown in the triangle plot ( Figure 5A), the f 44 values in spring are generally higher than those in summer IOP, suggesting more aged aerosols arriving at the site in spring. A second possibility is that the VOCs contributing to SOA formation in summer are different than in spring.
For example, we previously discuss that higher concentrations of isoprene and monoterpenes are observed in the summer, likely due to higher emissions, whereas concentrations of anthropogenics were more 225 constant. Finally, it is possible that the more abundant biogenic VOCs in summer IOP were not transformed into a higher-oxygenated form in the aerosol phase, either due to differences in RO 2 radical chemistry, oxidants, or their residence time in the atmosphere (e.g., D'Ambro et al., 2017;Liu et al., 2016;Pye et al., 2019). Further identification of OA sources will be discussed in the next section via PMF analyses.

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In order to better understand the sources of organic aerosol, we performed PMF analysis on the high resolution mass spectra data separately for each IOP. For the spring IOP, we chose a five factor solution ( Figure S4). Shown in Figure S4, a first factor is characterized by an enhanced signal of C 5 H 6 O + (m/z 82), which is recognized as a tracer for IEPOX SOA (iSOA; Budisulistiorini et al., 2013). According to previous studies, a fractional C 5 H 6 O + signal (f C5H6O ) of 1.7 ‰ is roughly the background level (Hu et al., 235 2015), while the resolved first factor has an f C5H6O value of 4.5‰. This factor correlates with SO 4 , with r = 0.55 ( Figure 6). Based on the correlation of this factor with SO 4 , and comparison of the mass spectrum to literature, we assign this factor as IEPOX-derived SOA. During the spring IOP, IEPOX-derived SOA factor contributed 27.9% of the total OA mass on average. https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License.
The second factor features a prominent marker at m/z 60 (primarily C 2 H 4 O 2 + ) with a f 60 value of 0.9%.

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Based on previous studies (e.g., Cubison et al., 2011) this f 60 fraction is representative of air masses impacted by aged biomass burning. The trend of this factor ( Figure 6B) tracks the time evolution of C 2 H 4 O 2 + and C 3 H 5 O 2 + well, with r values of 0.96 and 0.92, respectively. The signals of these two ions are thought to represent tracers for levoglucosan, which are also tracers for biomass burning organic aerosol (BBOA) (Cubison et al., 2011;Jolleys et al., 2015). In addition, the time series of the second factor tracks a 245 biomass burning event on April 29 well, with average f 60 value of 1.0% (details will be discussed in section 3.6). Thus we identify this second factor as BBOA. We also note that this BBOA factor has a relatively high f 44 value of 0.16, which further suggested the BBOA observed at SGP was aged. The BBOA factor accounts for 10.0% of total OA mass during the spring IOP on average, but at times can rise to over 50% ( Figure 6F).

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We identify the third factor as HOA by comparison of the mass spectrum with literature spectra including the prominent signal at m/z 55 and 57 ( Figure S4). This factor exhibits similar trend in time with toluene, a typical VOC tracer for primary emissions. Interestingly, the evolution of HOA doesn't correlate strongly with CO, a well known anthropogenic tracer. Shown in Figure S5, CO appeares to be associated with both HOA and BBOA, which likely impacts the correlation between CO and a single PMF factor. Our retrieval 255 of an HOA factor from the PMF analysis contrasts with the results of a previous ACSM-based study at SGP. Based on 1.5-year of observational data, Parworth et al. (2015) suggested no HOA factor is extractable due to the rural characteristics of the SGP site. In this study, we retrieved an HOA factor, with an average contribution of 9.6% of total OA mass ( Figure 6F). It is possible that the higher S/N and time resolution of the HR-ToF-AMS used in our study relative to the quadrupole ACSM used in the Parworth 260 study allows us to extract the HOA factor.
The fourth and fifth factors are two OOA (oxygenated organic aerosol) factors that are typically representative of SOA. The average mass spectra of the two OOA factors ( Figure S4) show that the f 44 value is higher for OOA-2 (0.25) than OOA-1 (0.18), where the ion signal at m/z 44 commonly comes from the thermal decomposition of carboxylic acids and other highly oxygenated compounds on the vaporizer.

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Therefore OOA-2 is identified as more-oxidized OOA (MO-OOA) and OOA-1 is identified as less- , and C 7 H 7 + has been inferred to be a thermal decomposition product of dimers and oligomers in ISOPOOH-derived SOA (Riva et al., 2016). Shown in Figure 6D approximately 50% of the total OA mass, indicating that the majority of the OA arriving at the site is relatively aged.
During the summer IOP, we chose a four-factor PMF solution, consisting of an IEPOX SOA factor, an HOA factor and two OOA factors (Figure 7). A BBOA factor was not identified during the summer season, which is consistent with the low concentrations of BBOA observed in summer at SGP in a previous study 280 (Parworth et al., 2015). The major contribution to the total OA mass is from the two OOA factors, the sum of which contributed >60% throughout the summer IOP. Similar to the spring IOP, OOA-1 is associated with enhanced signal at m/z 91 ( Figure S6), and OOA-2 correlates best with acetone ( Figure 7). However contribution of the HOA factor during the summer IOP is 13.0%, higher than the fraction in the spring IOP.
The higher summer HOA contribution is consistent with higher fraction of air masses originating from the urbanized southern regions based on HYSPLIT analyses.

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The fourth factor, IEPOX SOA, contributed 25.3% of the total OA mass, similar to spring IOP. Another difference between the studies relates to the "triangle plots" for OA. Shown in Figure  In summary, the Parworth study suggested the organic aerosols at the SGP site were in a more oxygenated state throughout the year, whereas the OA observed in our study demonstrated more seasonal variation.
There are also significant differences in the factor analysis results between the two studies. In the Parworth 325 study, only three factors were isolated, including a BBOA factor and two OOA factors while we isolate an additional IEPOX factor in both seasons and an HOA factor in the spring. In the Parworth study, the OOA factor with a smaller f 44 value (OOA-2) was a larger fraction of the total mass in summer relative to spring, which is consistent with our observations that the summer OA appeared to be more fresh. It should be noted that the three factors retrieved in the Parworth study were based on data throughout the 19 months, 330 and the contribution by BBOA was only evident in winter and spring seasons, which is again consistent with our results. One factor the Parworth study did not extract is the HOA factor, which they attributed to the rural setting of the SGP site. In our study, although the HOA only contributed ~10% of the total OA mass on average, the occasional spikes were accompanied with high concentrations of anthrogeponic tracers and is likely associated with transition of air mass origins. With respect to the IEPOX SOA factor, 335 the Parworth study did take spatial distribution of isoprene emissions into consideration and suggested biogenic emissions likely contribute to SOA mass at SGP primarily during the summer, but this contribution was not directly attributed to IEPOX chemistry. However, Parsworth were not able to isolate an IEPOX SOA factor in their analysis. In our analyses, although the retrieved IEPOX SOA had similar fractions in spring and summer, the absolute mass concentration was indeed higher in summer, which 340 agrees with the more intense biogenic emissions and photochemistry associated with higher solar insolation and temperatures as suggested in Parworth et al. (2015).

Case study 1: IEPOX SOA events
An IEPOX SOA factor was resolved during the spring and summer IOPs and it was a substantial contribution to the total OA mass. (noted as spring iSOA event), and the period from September 4 6:00 to September 5 21:00 (UTC) for the summer IOP (noted as summer iSOA event).
The C 5 H 6 O + ion has been recognized as a unique marker for IEPOX SOA (Robinson et al., 2011;Lin et al., 2012Lin et al., , 2013Hu et al., 2015;Shilling et al., 2018). During both iSOA events, the time series of C 5 H 6 O + ion 350 track that of IEPOX SOA and SO 4 ( Figure 8A). Researchers have also identified additional ions that are representative of IEPOX SOA such as the ion of C 3 H 7 O 2 + (Budisulistiorini et al., 2016), and we observe good correlation between C 3 H 7 O 2 + (m/z 75) and C 5 H 6 O + (m/z 82) for both spring IOP and summer IOP.
We also investigated the relationship between iSOA and its gas-phase precursors ( Figure 8B). We attribute the PTR-MS signal at m/z 71 as the sum of methyl vinyl ketone (MVK), methacrolein (MACR), and 355 isoprene hydroxyhydroperoxide (ISOPOOH), all first-generation oxidation products of isoprene, and use the sum of m/z 69 (isoprene) and m/z 71 as an indicator of gas-phase iSOA precursors. During both events, generally speaking, the sum of isoprene and its first-generation oxidation products did not track the evolution of IEPOX SOA well, but at times an enhancement in IEPOX SOA followed after a peak in the sum of m/z 69 and 71. Given the good correlation between SO 4 and IEPOX SOA, we speculate that the 360 variation of IEPOX SOA at the SGP site was mostly driven by variations in the concentration and the acidity of particles, as iSOA precursors appear to be abundant during most of the campaign (Lin et al., 2012). Back trajectories provide some insights into the transport pathways associated with the iSOA events.
In both the spring and summer iSOA events, air masses passed over urban areas before reaching the SGP site, though there was some variation in the detailed trajectories. As seen in Figure  suggested higher isoprene emissions originate from regions east of the SGP, which is supported by our observations of higher m/z 69+71 in the summer iSOA event compared to the spring event ( Figure 8B).

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Higher concentrations of isoprene and isoprene oxidation products in summer were accompanied by higher abundance of the C 5 H 6 O + ion, with the summer average f C5H6O =6.51‰, much higher than that in spring (f C5H6O =3.44‰). To roughly estimate the relative age and oxidation level of the aerosol during iSOA events, we present "triangle plot" similar to those described in Hu et al. (2015) in Figure S8. The iSOA events group into distinct regions of these plots with the spring iSOA more similar to the OH-aged aerosols 380 observed in southeast US (Hu et al., 2015) and the summer iSOA more similar to fresher aerosol. Note that the f C5H6O and f CO2 values shown in Figure S8 are for the total OA, not just the IEPOX SOA factor. For https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License.
iSOA, the mean carbon oxidation state, OSc, was estimated to be -0.05 for summer event, and 0.75 for the spring event, respectively. Considering the higher levels of isoprene and its first-generation oxidation products observed during the summer iSOA events, the higher f C5H6O value might suggest that the IEPOX 385 pathway was favored over isoprene oxidation pathways producing higher generation, more oxidized products, such as ISOP(OOH) 2 (D' Ambro et al., 2017;Liu et al., 2016). In contrast, lower biogenic VOC concentrations in spring might result in a relatively higher ratio of oxidant-to-VOC, leading to the formation of more highly oxygenated organic aerosols.
Compared to other sites where IEPOX SOA were extensively studied, the IEPOX chemistry at SGP site 390 appeared to be unique, especially for the iSOA event observed during spring IOP. Hu et al. (2015) summarized the characteristics of IEPOX SOA factors retrieved from a set of ambient observations covering urban, downwind urban, and pristine regions, and suggested a range of 12-40 ‰ of f C5H6O values for ambient IEPOX SOA. In this study, the summer IEPOX SOA had an f C5H6O value of 12.1‰, at the lower end of previous results, whereas the f C5H6O value of spring IEPOX SOA was only 4.55‰. During the 395 specific period of spring iSOA event, the f C5H6O values of 3.44‰ for bulk OA was also significantly lower than the average value of 6.5‰ suggested for ambient OA strongly influenced by isoprene emission (Hu et al., 2015). In addition, during the spring iSOA event, f CO2 values of bulk OA were significantly higher than previously-reported results of ambient OA strongly impacted by isoprene emission. Recent work has suggested some major IEPOX SOA components, specifically methyltetrol sulfates, may undergo further 400 OH oxidation accompanied by the formation of HSO 4 ion (Chen et al., 2020;Lam et al., 2019). Thus, one possibility is that this mechanism produces more oxygenated IEPOX SOA in the spring, contributing to the higher f CO2 . Considering the relatively weak local emission of isoprene in spring at SGP, the higher f CO2 but lower f C5H6O values might suggest aging processes of the IEPOX SOA during long-range transport. Since most IEPOX SOA studies were previously conducted in the summer season with intense isoprene 405 emissions, the distinct observations at SGP site, especially those during spring IOP, provided a unique point of view on SOA chemistry in rural environment with weak biogenic emissions.

Case study 2: Biomass burning event on April 29
In our PMF analysis, a BBOA factor was extracted during the spring IOP but not during the summer IOP, in broad agreement with a previous observation of more substantial contribution from fire events during 410 spring (Parworth et al., 2015). In this study we selected a specific biomass burning event to examine the evolution of typical tracers, sources and characteristics.
On April 29, 2016, two spikes of organic aerosol concentration were observed, as shown in Figure 1. From 00:00-9:00 (UTC) on April 29, OA is by far the dominant component of the aerosol phase, with an average fraction of 79.13%. Particulate ammonium and nitrate also increased during these events likely due to 415 increased emissions of their precursors, such as NH 3 and NO x (Paulot et al., 2017;Souri et al., 2017). No increase in sulfate concentrations was observed.  (Cubison et al., 2011). Indeed, the PMF-resolved BBOA fraction also showed an 425 increase from the spring IOP average of 10% to as high as 77% on April 29.
Active fire data shows relatively intense fire hotspots north of the SGP site, with two concentrated areas in Kansas and Canada. Back trajectory analysis suggests that these emissions need to travel at least 6 hours before arriving at the SGP site ( Figure S9). According to previous studies, levoglucosan has an atmospheric lifetime of approximately 2 days (Hennigan et al., 2010), suggesting that a significant fraction of the 430 levoglucosan may have decayed before reaching SGP, particularly for BBOA originating in Canada. The van Krevelen analysis ( Figure S10) shows that the bulk organic aerosols are highly oxygenated, with an average O:C ratio of 0.839, much higher than typical characteristics of primary BBOA , again consistent with the relatively long-range transport that was needed to bring BBOA to the SGP site. Although the SGP site was strongly influenced by biomass burning, the oxidation state of OA during 435 the April 29 event did not show significant differences relative to other periods in spring IOP, with OSc value of 0.281 (April 29 event) versus 0.289 (spring IOP average).
Interestingly, the elevated levels of C 2 H 4 O 2 + and BBOA were not accompanied by significantly elevated levels of acetonitrile (m/z 42 measured by PTR-MS), which has been traditionally been used as a gas-phase marker of biomass burning emissions. The average acetonitrile concentration for the spring IOP was 0.11 440 ppbv while during the biomass burning period it was 0.09 ppbv (Table 1). Even during the spikes in BBOA concentrations, acetonitrile concentrations were observed to only increase moderately, up to 0.2 ppbv ( Figure S11). The specific reason we do not see a concomitant increase in acetonitrile with the clear BBOA plume remains ambiguous. We have ruled out the possibility that dilution and processing of acetonitrile reduced its concentration below the PTR-MS detection limit; BBOA concentrations remain high despite 445 dilution, acetonitrile has an atmospheric lifetime of 1.4 years, and the acetonitrile detection limit was 0.06 ppbv during the spring IOP. A second possibility is that the biomass burning did not emit significant amounts of acetonitrile, though we are unsure why this would be the case.

Conclusions and Discussions
Observations of gas-phase VOCs and particle-phase chemical composition were taken at the SGP site 450 during the HI-SCALE campaign in 2016, with two intensive operation periods covering the spring and summer season. Aerosol and trace gases were characterized using the AMS and PTR-MS as well as a suite https://doi.org/10.5194/acp-2020-1100 Preprint. The SGP site is located in a rural setting, and biogenic emissions appear to largely control the concentrations of OA during the HI-SCALE campaign. In recent years, a number of studies have focused on HOMs produced from biogenic precursors, and these components are expected to play a key role in new 470 particle formation (Ehn et al., 2014;Jokinen et al., 2015;Qi et al., 2018). The high oxidation state of the OA observed at the SGP site during HI-SCALE suggests that these molecules could indeed be important SOA components at SGP. Nevertheless, due to lack of molecule-level information, we were not able to quantitively evaluate the contribution of HOM chemistry to the oxidation state of OA at SGP site. Since HOMs likely contribute to new particle formation (NPF), they will also impact subsequent aerosol growth,

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CCN populations, and the influence of aerosols on global climate. Actually during HI-SCALE campaign, NPF events were more frequently observed in spring than in summer (Fast et al., 2019), in agreement with the more-oxygenated feature of OA observed during the spring IOP in this study. Considering the potential climate impacts, the highly-oxygenated nature of aerosols at the SGP site is an interesting topic which should be investigated further. The mixture of anthropogenic, biogenic, and biomass burning sources of 480 OA at the SGP site provides an opportunity to rigorously evaluate explicit and parameterized treatments of a range of SOA pathway mechanisms.
Author contribution: J. Fast, L. Alexander and J. Shilling designed the experiments. J. Liu, L. Alexander and R. Lindenmaier carried out the measurements. J. Liu analysed the data and prepared the manuscript with contributions from all co-authors.

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The authors declare that they have no conflict of interest.     https://doi.org/10.5194/acp-2020-1100 Preprint. Discussion started: 29 October 2020 c Author(s) 2020. CC BY 4.0 License. Figure 9. Scatter plot of the C 2 H 4 O 2 + ion (a biomass burning marker) against total OA during the spring 775 IOP. The April 29 period is clearly differentiated from much of the data with a slope of 1.0%. The slope for the remaining IOP data is 0.3%.