Diel Cycle Impacts on the Chemical and Light Absorption Properties of Organic Carbon Aerosol from Wildfires in the Western United States

Organic aerosol (OA) emissions from biomass burning have been the subject of intense research in recent years, involving a combination of field campaigns and laboratory studies. These efforts have aimed at improving our limited understanding of the diverse processes and pathways involved in the atmospheric processing and evolution of OA properties, culminating in their accurate parameterizations in climate and chemical transport models. To bring closure between laboratory and field studies, wildfire plumes in the western United States were sampled and 5 characterized for their chemical and optical properties during the ground-based segment of the 2019 Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) field campaign. Using a custom-developed multiwavelength integrated photoacoustic-nephelometer (MIPN) spectrometer in conjunction with a suite of instruments, including an oxidation flow reactor equipped to generate hydroxyl (OH∙) or nitrate (NO3∙) radicals to mimic daytime or nighttime oxidative aging processes, we investigated the effects of multiple equivalent days or 10 nights of OH∙/NO3∙ exposure on the chemical composition and mass absorption cross-sections (MAC(λ)) at 488 and 561 nm of OA emitted from wildfires in Arizona and Oregon. We found that OH∙ exposure reduced the wavelengthdependent MAC(λ) by a factor of 0.72 ± 0.08, consistent with previous laboratory studies. On the other hand, NO3∙ exposure increased it by a factor of up to 1.69 ± 0.38. The MAC enhancement following NO3∙ exposure was correlated with an enhancement in CHO1N and CHOgt1N ion families measured with an aerosol mass spectrometer. 15

properties during the Fire Influence on Regional to Global Environments and Air Quality (FIREX-AQ) campaign, an interagency mission led by NASA and NOAA, conducted during the wildfire season of 2019 (Warneke, 2018    measurements of hydrogen cyanide (HCN) as a tracer for biomass smoke plumes (Li et al., 2000). Upon identification of a suitable location, the AML parked with the sample inlet on the front of the truck facing into the wind to avoid 100 self-sampling of its own exhaust. A PM2.5 cyclone with a small mesh screen to filter out extremely large ash particles was attached to the inlet. A schematic of the OFR experimental setup is given in Fig. 4, and Section 2.2 discusses instrumentation specifically relevant to the OFR-based field measurements in more detail.   EMAC(λ) was calculated using the average of aerosol MAC(λ) during a given oxidation step divided by the average of the ambient steps immediately prior and after.

Aerosol Mass Spectrometer
The SP-AMS is a standard Aerodyne high resolution time of flight aerosol mass spectrometer (HR-ToF-AMS) with an intracavity, CW laser vaporizer (Onasch et al., 2012). The AMS was operated to provide online chemically-140 speciated mass and sizing measurements of both non-refractory and refractory particles between approximately 70 -2500 nm in aerodynamic diameter. A PM2.5 inlet lens was installed on the AMS for this study, extending the range of 100% transmission efficiency of particles through the lens up to 2.5 μm in diameter. The SP-AMS laser was operated with approximately a 50% duty cycle. When the laser was off, the system was operated as a conventional AMS.
During OHArizona, OHOregon, and NO3,Oregon, the SP-AMS was used. During NO3,Arizona, the conventional AMS was used. 145 The instrument was run with 20 second time resolution, and data points included both chemical speciation and mass loading by mass spectral analysis and particle sizing by species for each data point. In the SP-AMS, particles containing refractory materials (i.e. BC and many metals) are vaporized with a 1064 nm laser. The resulting vapor is ionized via electron impact and detected with the HR-TOF-AMS. In addition to the SP-AMS vaporization, the conventional AMS heater (a heated tungsten surface at 600 °C, (Jayne et al., 2000;Canagaratna et al., 2007) was also 150 used to measure the composition of any non-refractory particles. When the instrument was run as a conventional AMS, this was the only heater used.

Potential Aerosol Mass (PAM) OFR
The

Experiment Design
The total instrument plus makeup flowrate through the OFR was 6. valve was switched to connect the instruments to the OFR, and OFR-processed air was sampled for 5 to 10 min. After each step, the OH• or NO3• exposure was changed, and the above measurements were repeated. After each experiment, 165 the OFR was cleaned out by setting both sets of lamps to maximum output and overblowing the inlet with humidified zero air until AMS measurements of background organic mass were below 0.3 g m -3 . To account for dilution and particle wall losses in the OFR refractory black carbon (rBC) monitored with the SP2 was used as a chemically conserved tracer. During all experiments, rBC accounted for ~ 2% to 5% of total aerosol mass (Figs. S1 and S2).
In the following sections, ambient steps are denoted "ambient_X" and oxidation steps are denoted "OFR_OH_X" for 170 OH• experiments, and for NO3• experiments, the ozone-only step is denoted "NO3PAM_O3" and NO3• oxidation steps are denoted "NO3PAM_NO3_X". "X" indicates the step number. Each experiment includes a "background" step, where ambient air was sampled through the dark OFR without oxidant generation.

NO3 Experiment Design and Analysis
To generate NO3•, N2O5 was first generated in the gas phase from the reaction NO2 + O3 → NO3• + O2 followed by 175 the reaction NO3• + NO2 → N2O5 in a 152.4 cm long x 2.22 cm ID perfluoroalkoxy laminar flow reactor (LFR) coupled to the OFR (Lambe et al., 2020). Separate flows containing NO2 (1% in N2, Praxair) and O3 were added to the LFR.
In these experiments, the NO2 +N2 flow rate was set between 0 and 40 cm 3 min −1 , and O3 was generated by passing 1.8 L min -1 of O2 through an ozone chamber housing a mercury fluorescent lamp (GPH212T5VH, Light Sources, Inc.). The O3 mixing ratio that was input to the LFR was approximately 250 ppmv during NO3•-OFR experiments. 180 The NO2 + N2 and O2 flow rates were set using mass flow controllers. The N2O5 generated in the LFR thermally decomposed at room temperature inside the OFR to generate NO3•. The first oxidation step of NO3•-OFR experiments was with ozone only ("NO3PAM_O3") to assess the effect of O3 exposure on BrC composition and optical properties relative to ambient BrC. During NO3,Arizona, NO2 was stepped down from 40 to 20, 5, and 3 cm 3 min -1 to generate the various oxidation time scales (Fig. S3) Where a through f are fit coefficients tabulated in Table S2, O3 is the ozone mixing ratio measured at the exit of the OFR (molec cm -3 ), OHRext is the external OH• reactivity (s -1 ), which was calculated from the summed products of

NO3• Oxidative aging of BrC
Equivalent nighttime oxidation of between 3.28 ± 0.00 to 7.67 ± 0.17 days was performed across the NO3,Arizona and 235 NO3,Oregon OFR experiments. EMAC(λ) is shown in Fig. 5 and summarized in Table 2  A breakdown of the chemical speciation from NO3,Arizona was obtained from the AMS data from before and after the NO3PAM_NO3_1 step (refer to Fig. S3), the first oxidation step after ozonolysis. During this step, 217 ppm NO2 was flowing into the LFR. NO3,EXT before and after this step was 17.56 and 13.90 s -1 , respectively, for an estimated While knowledge of the individual m/z enhancement and depletion may inform future investigations, it is perhaps more illustrative to consider ion families writ large. Figure 6 shows the enhancement and depletion of the ion families 265 in Figs. S5 and S6 on the basis of total ion mass in that family. Enhancement is calculated by summing the relative abundances of all ions within a family and taking the ratio of oxidized to ambient, similar to how EMAC(λ) is calculated.

O3 Oxidation Effects
The first step in the NO3• experiments was an O3-only oxidation experiment in case the effects of ozonolysis on biomass burning smoke were significant. Since the NO3• oxidation steps carry a significant amount of residual O3, the contribution of O3 to changes in absorption behavior should be quantified and treated separately from NO3•. Li et al. EMAC(λ) due to O3 during NO3,Arizona was 1.06 ± 0.96 and 1.05 ± 0.94 for 488 and 561 nm, respectively, indicating that 285 O3 effects during the PAM experiments were negligible, or at least dominated by NO3•.   Table 3.

[Figure 7]
While OHEXT values were large, it is noteworthy that they were typically exceeded by NO3,EXT values, which, as noted 305 in the previous section, indicates the presence of BBVOCs that are more reactive toward NO3• than toward OH•. The exact nature of these sensitivities requires further research.
[ Table 3] The same scatterplot analysis was applied to the OFR_1 step (refer to Fig. S4) of the OHArizona experiment, however, 310 it is less illustrative than with NO3• because the primary driver of MAC(λ) diminishment is fragmentation reactions.
Before this step, OHEXT was 74.95 s -1 and after, it was 59.37 s -1 , giving an approximate equivalent age of 18.27 ± 7.54 days. The enhancement ratios of the individual ion families show a decrease in CHO1N and an increase in CHOgt1N, though to a lesser degree than under NO3• aging. The ambient CHOgt1N mass fraction was approximately 25% higher in the NO3,Arizona, as wellthe comparatively lower relative abundances of nitroaromatics during OHArizona (both pre-315 and post-OFR_1) may obfuscate any meaning in the enhancement ratios. Figure S7 shows the OHArizona scatterplot with markers, and Fig. S8 shows the individual m/z measured by the SP-AMS. Qualitatively, it can be observed that there is less spread in the scatterplot and the points are grouped closer to the 1:1 line, further suggesting that the dominant mechanism is fragmentation. Figure 8 shows the ion family enhancement ratios through the same OFR_1 step corroborating the conclusions drawn 320 from the scatterplots.

Conclusions and Future Work -Synthesizing Daytime and Nighttime Aging
The observations of EMAC(λ) and the associated chemistry from FIREX-AQ represent the first attempt to use an OFR in a mobile setting to sample biomass burning at their source, as well as the first application of the novel MIPN v1 to 325 a field study. Observations track closely with previous laboratory studies.
These results show the difficulty in naïvely applying a particular aging model to atmospheric aerosol to constrain their long-term behavior in climate models. Aerosol does not age along any single pathway for more than half of a diurnal cycle: at night, oxidative aging of BrC by NO3• increases MAC, whereas daytime oxidative aging by OH• decreases MAC. Overall, our results suggest that explicit characterization of the effect of diel aging on atmospheric aerosol 330 optical, chemical, and physical properties represents the best possible input to climate models.  (Sumlin, 2021). 335 Supplement. Includes eight figures detailing rBC fraction observed during experiments (S1 and S2), sample timelines for oxidation experiments (S3 and S4), and AMS signal enhancements for various ion families upon oxidation (S5 through S8). Also includes two tables (S1 and S2) with fit coefficients for Eqs. (1) and (2).