Shortwave absorption by wildfire smoke dominated by dark brown carbon

Wildfires emit large amounts of black carbon and light-absorbing organic carbon, known as brown carbon, into the atmosphere. These particles perturb Earth’s radiation budget through absorption of incoming shortwave radiation. It is generally thought that brown carbon loses its absorptivity after emission in the atmosphere due to sunlight-driven photochemical bleaching. Consequently, the atmospheric warming effect exerted by brown carbon remains highly variable and poorly represented in climate models compared with that of the relatively nonreactive black carbon. Given that wildfires are predicted to increase globally in the coming decades, it is increasingly important to quantify these radiative impacts. Here we present measurements of ensemble-scale and particle-scale shortwave absorption in smoke plumes from wildfires in the western United States. We find that a type of dark brown carbon contributes three-quarters of the short visible light absorption and half of the long visible light absorption. This strongly absorbing organic aerosol species is water insoluble, resists daytime photobleaching and increases in absorptivity with night-time atmospheric processing. Our findings suggest that parameterizations of brown carbon in climate models need to be revised to improve the estimation of smoke aerosol radiative forcing and associated warming.


Ground-based instruments
The Aerodyne Mobile Laboratory (AML), a mobile sampling platform equipped with a suite of research-grade instrumentation, was used to sample and facilitate the in situ experiments. The AML travelled throughout Idaho, Oregon, Utah, and Arizona during FIREX-AQ, sampling continuously when not conducting oxidation experiments. The general sampling strategy was to search for smoke-filled valleys and transect plumes with the AML, using Tuneable Infrared Laser Direct Absorption Spectrometer (TILDAS, Aerodyne Research, Inc.) measurements of hydrogen cyanide (HCN) as a tracer for biomass smoke plumes. 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 selfsampling of its own exhaust. For the most part, plume sampling was conducted less than 3 km from the fire management area. The entire list of gas and particle instruments are provided in the FIREX-AQ white paper (https://www.esrl.noaa.gov/csl/projects/firex-aq/whitepaper.pdf). We provide brief description of the instruments of relevance to this project.

Multiwavelength integrated photoacoustic-nephelometer (MIPN):
The MIPN is a custom-built instrument designed and constructed for ground-based operations. During FIREX_AQ, the aerosol light absorption coefficient (Mm -1 ) was measured with the prototype version of the instrument at two wavelengths (λ = 488 and 561 nm). The photoacoustic design is based on ref 1 and the nephelometer is designed based on ref 2 . The MIPN is an improvement to the single-wavelength integrated photoacoustic-nephelometer spectrometers described in 3 and the instrument calibration procedure is detailed in ref 3 and ref 4 . The nephelometer was calibrated using NaCl aerosol and the photoacoustic spectrometer was calibrated using kerosene soot. Scattering calibration factors were determined using the slope of a linear regression of scattering versus extinction coefficient for salt aerosol, and the absorption calibration factor was determined using the slope of absorption coefficient versus extinction-scattering coefficients for kerosene soot. The multi-wavelength beam in the MIPN is split into two identical cells wherein one cell measures the absorption and scattering coefficients of the sample stream and the other cell samples particle-free air to measure gaseous background and account for noise within the system. Measurements were acquired every 2 seconds and the readings from the clean cell were subtracted from the measurement cell to account for any gas-phase absorption. The MIPN was operated during the Castle/Ikes and 204 Cow fire ground sampling for several days.

Aerosol Mass Spectrometer (AMS):
The Soot Particle-Aerosol Mass Spectrometer (SP-AMS) (Aerodyne Inc.) is a high-resolution time of flight aerosol mass spectrometer equipped with an intracavity laser vaporizer operating at 1064 nm 5 . The SP-AMS was used to measure the chemical speciation of the non-refractory as well as the refractory particles with aerodynamic diameters ranging from 70 -2500 nm. The SP-AMS operated on a 50% duty cycle and would operate as a regular AMS when the laser was turned off. The SP-AMS was used during the Oregon experiments and the regular AMS was used for the Arizona experiments. The instrument was run with a 20 second time resolution and measured chemical speciation and mass loading by mass spectral analysis.

Potential Aerosol Mass (PAM) Reactor:
A Potential Aerosol Mass (PAM, Aerodyne Research, Billerica, MA) Reactor is an oxidative flow photochemical reactor (OFR) capable of mimicking atmospheric oxidation processes using a variety of oxidants. During our experiments, the PAM was operated to generate two oxidation environments abundant in either hydroxyl (OH) or nitrate (NO3) radicals which simulated daytime and nighttime oxidation, respectively. The PAM was equipped with two sets of 185 nm lamps where the primary set had a maximum irradiance of ~70 µW/cm 2 , and the second set had a maximum irradiance of ~7 µW/cm 2 . Lamp output was controlled from a software by controlling the input voltage to the lamp ballast, from 0 V (lamps off) to 10.0 V (lamps at maximum). Irradiance, relative humidity, and temperature were monitored by internal sensors. For the OH experiments, ozone (generated via the photolysis of ultra-high purity oxygen using a 185 nm lamp) was photolyzed using 185 nm lamps to produce O( 1 d) which then reacts with water vapor to produce OH radicals 6,7 . The PAM was coupled to a laminar flow reactor (LFR) during the NO3 experiments. The NO3 radicals were generated in the LFR via the reaction of NO2 with ozone to produce N2O5, which further reacted with ozone to produce NO3 8 .

Single-Particle Soot Photometer (SP2):
The SP2 is an instrument that measures, in situ, the timedependent scattering and incandescence signals produced by individual BC-containing particles as they travel through a continuous-wave laser beam. Any particle traversing the laser beam will scatter light, and the BC component of a BC-containing particle will absorb some of the laser energy until its temperature is raised to the point at which it incandesces (hereafter we adopt the standard terminology of the SP2 community and denote any substance determined by the SP2 to be BC as refractory black carbon (rBC)). The amplitude of the rBC incandescence signal is related to the amount of refractory material contained in the illuminated particle. By binning the individual incandescence signals per unit sample volume, the mass concentration [ng/m3] of rBC is derived. By binning the individual signals by volume equivalent diameter the size distribution (dN/dlogDVED) per unit time is derived. The rBC mass loading per unit time and the rBC size distribution unit time are the core data products produced by the SP2. Additionally, the scattering channel can be used to provide information on the rBC particle population-based mixing states within ambient aerosols. The SP2 onboard the AML was calibrated using fullerene soot (lot L18U002) prior to the study and with lot L20W054 during the study.

Airborne Instruments
The NASA DC-8 aircraft conducted 23 individual flights during FIREX-AQ, including 13 flights characterizing wildfires in the western United States, 8 flights targeting prescribed burning plumes, and 2 transit flights. In this study we focus on the smoke sampling from the Shady Creek and Castle/Ikes wildfires by the NASA DC-8 aircraft during flights on July 25, August 12, and August 13, 2019. This is done to provide overlap with the ground based measurements of the same wildfire source. The entire list of gas and particle instruments are provided in the FIREX-AQ white paper (https://www.esrl.noaa.gov/csl/projects/firex-aq/whitepaper.pdf). All airborne measurement datasets are archived and made open-access via https://www-air.larc.nasa.gov/missions/firex-aq/.
A three wavelength (405 nm, 532 nm, and 664 nm) photoacoustic spectrometer (PAS) was deployed on the DC-8 providing real-time measurements of dry aerosol absorption of fine particles (diameters < 2.5 µm) 9 . A cavity ringdown (CRD) aerosol extinction spectrometer composed of 8 separate ringdown cells was also deployed 10 . The CRD measures dry (<10% RH) extinction at 405, 532, and 662 nm wavelength, while two 405 nm and 532 nm cells sampled air heated to 250 0 C in a thermodenuder to measure volatilize condensed coatings. Refractory black carbon (rBC) mass concentration was measured by a single particle soot photometer (SP2), which quantified rBC mass in individual particles in the 0.090 to 0.550 µm size range (volumetric-equivalent diameter assuming 1.8 g cm-3 void-free density) based on the incandescence signal they generated when passing through a laser beam 11 . The SP2 was calibrated using fullerene soot, lot 40971, which is a very good proxy for sensitivity to ambient rBC. Aerosol number size distribution was measured by a laser aerosol spectrometer (LAS, model 3340, TSI Incorporated, Shoreview, MN). The reported size range is from ~100 nm to 4 µm. Sampling was conducted to cover each transect of smoke, with sampling times of ~1 to 3 minutes and an airflow rate of 1.0 L/minute.

NO 3 • experiment design
To generate NO3•, N2O5 was first generated in the gas phase from the reaction NO2 + O3 → NO3• + O2 followed by 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 8 . 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. 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 ("OFR_O3") to assess the effect of O3 exposure on OA composition and optical properties relative to ambient OA. 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. The integrated NO3 exposure, defined as the product of the average NO3• concentration and the mean OFR residence time (τOFR), was calculated using an estimation equation developed by Lambe et al. (2020) 8

OH• Experiment Design
OH• was generated via photolysis of ambient O2 and H2O at λ = 185 nm plus photolysis of O3 (generated from at λ = 254 nm using low-pressure mercury (Hg) lamps): A fluorescent dimming ballast was used to regulate current applied to the lamps (GPH436T5VH/4, Light Sources, Inc.). The dimming voltage applied to the ballast ranged from 1.6 V to 10 V direct current (DC). To extend the range of OH• concentrations below what is achievable with one set of lamps at 1.6 VDC, a second set of GPH436T5VH/4 lamps with added segments of opaque heat shrink tubing applied to ~86 % of the arc length 13 was used. The OH exposure was calculated using Eq. (2) from Rowe et al. (2020) 13 : where a through f are fit coefficients, 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 ambient VOC concentrations (measured with Vocus) and their OH• rate coefficients (data available in the FIREX AQ data repository), H2O is the ambient water vapor mixing ratio (%), and τ is the residence time in the OFR.   Caution was exercised for distinguishing d-BrC tar balls in an aerosol population. The d-BrC tar balls appear as "Electronically dark" under an electron microscope 15 . Weakly absorbing or purely scattering organic matter appear as 'electronically bright'. Nearly half or more of all measured particles were classified as electronically dark particles (refer to Table 1).

Fig 3.
Size-dependent number fractions of aerosol particles in wildfire plumes from flights July 25 (Shady fire), August 12 (Castle), and August 13 (Castle). Energy dispersive X-ray spectroscopy within a TEM was used to analyze the elemental composition. ("O1" indicates a single oxygen atom in a molecule containing one or more atoms of C, H, and N) by 132 % through the OFR, with a corresponding diminishment in CHOgt1 and CHO1 (where, again, "Ogt1" and "O1" indicate the quantity of oxygen atoms in molecules of one or more atoms of C and H) to 79 % and 91 %, respectively. This does not imply that all CHO1N species come from CHO1 (or CHOgt1N from CHOgt1, similarly). The addition of nitrogen-containing functional groups is likely the cause of the observed light absorption enhancement since nitrogenated aromatic hydrocarbons form during reactions with NO3•. This has relevance for formation of nighttime secondary organic aerosol and BrC 16 .

Fig. 5.
Ratio of extinction at 664 nm with and without thermal denuding measured using the airborne NOAA cavity ring down instrument. The extinction measurements are shown in the red lines and the hollow black circles. Ratio of NOT-DENUDED to DENUDED is shown in blue circles, plotted against the right axis.