Evolution of Aerosol Optical Properties from Wood Smoke in Real Atmosphere Influenced by Burning Phase and Solar Radiation

Photochemical evolution of the molecular composition of dissolved organic carbon and dissolved brown carbon from wood smoldering

Recently, extreme wildfires occur frequently around the world and emit substantial brown carbon (BrC) into the atmosphere, whereas the molecular compositions and photochemical evolution of BrC remain poorly understood. In this work, primary smoke aerosols were generated from wood smoldering, and secondary smoke aerosols were formed by the OH radical photooxidation in an oxidation flow reactor, where both primary and secondary smoke samples were collected on filters. After solvent extraction of filter samples, the molecular composition of dissolved organic carbon (DOC) was determined by Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). The molecular composition of dissolved BrC was obtained based on the constraints of DOC formulae. The proportion of dissolved BrC fractions accounted for approximately 1/3-1/2 molecular formulae of DOC. The molecular characteristics of dissolved BrC showed higher levels of carbon oxidation state, double bond equivalents, and modified aromaticity index than those of DOC, indicating that dissolved BrC fractions were a class of organic structures with relatively higher oxidation state, unsaturated and aromatic degree in DOC fractions. The comparative analysis suggested that aliphatic and olefinic structures dominated DOC fractions (contributing to 70.1%-76.9%), while olefinic, aromatic, and condensed aromatic structures dominated dissolved BrC fractions (contributing to 97.5%-99.9%). It is worth noting that dissolved BrC fractions only contained carboxylic-rich alicyclic molecules (CRAMs)-like structures, unsaturated hydrocarbons, aromatic structures, and highly oxygenated compounds. CRAMs-like structures were the most abundant species in both DOC and dissolved BrC fractions. Nevertheless, the specific molecular characteristics for DOC and dissolved BrC fractions varied with subgroups after aging. The results highlight the similarities and differences in the molecular compositions and characteristics of DOC and dissolved BrC fractions with aging. This work will provide insights into understanding the molecular composition of DOC and dissolved BrC in smoke.

Effects of combustion temperature on the optical properties of brown carbon from biomass burning

Biomass burning has been known as one of main sources of Brown Carbon (BrC) in atmosphere. In this study, by controlling the combustion temperature at 250°C, 350°C, and 450°C, the methanol soluble organic carbon (MSOC) and methanol insoluble carbon (MISC) from pine wood burning was collected by impinger. UV-Vis, excitation emission matrix (EEM), TEM and FTIR spectra were applied to investigate the properties of BrC collected. For MSOC at 250°C and 350°C, all the spectral profiles of UV-Vis absorption and excitation emission matrix are almost the same, while the EEM of MSOC at 450°C are different from that of the other two. For MISC fluorescence was observed only in the case of 450°C. In the FTIR spectra, with the temperature increasing the peaks associated to the oxygen-contained functions was weakened, indicating the formation of the fluorophores with larger conjugated system, especially aromatic hydrocarbons. Our results show that biomass combustion at low temperature produces more oxygen-riched BrC, which possesses relatively lower light absorption, while at high temperature produces more aromatics hydrocarbons with relatively strong light absorption. The results of this work are helpful to trace the source of brown carbon and optimize biomass energy utilization.

Wildfire plume ageing in the Photochemical Large Aerosol Chamber (PHOTO-LAC)

Plumes from wildfires are transported over large distances from remote to populated areas and threaten sensitive ecosystems. Dense wildfire plumes are processed by atmospheric oxidants and complex multiphase chemistry, differing from processes at typical ambient concentrations. For studying dense biomass burning plume chemistry in the laboratory, we establish a Photochemical Large Aerosol Chamber (PHOTO-LAC) being the world's largest aerosol chamber with a volume of 1800 m3 and provide its figures of merit. While the photolysis rate of NO2 (jNO2) is comparable to that of other chambers, the PHOTO-LAC and its associated low surface-to-volume ratio lead to exceptionally low losses of particles to the walls. Photochemical ageing of toluene under high-NOx conditions induces substantial formation of secondary organic aerosols (SOAs) and brown carbon (BrC). Several individual nitrophenolic compounds could be detected by high resolution mass spectrometry, demonstrating similar photochemistry to other environmental chambers. Biomass burning aerosols are generated from pine wood and debris under flaming and smouldering combustion conditions and subsequently aged under photochemical and dark ageing conditions, thus resembling day- and night-time atmospheric chemistry. In the unprecedented long ageing with alternating photochemical and dark ageing conditions, the temporal evolution of particulate matter and its chemical composition is shown by ultra-high resolution mass spectrometry. Due to the spacious cavity, the PHOTO-LAC may be used for applications requiring large amounts of particulate matter, such as comprehensive chemical aerosol characterisation or cell exposures under submersed conditions.

Aging effects on residential biomass burning emissions under quasi-real atmospheric conditions

Emissions from biomass burning (BB) vastly contribute to the atmospheric trace gases and particles, which affect air quality and human health. After emission, the chemical evolution changes the mass and composition of organic aerosol (OA) in the diluted and aged plume. In this study, we used a quasi-real atmospheric smog chamber system to conduct aging experiments and investigated the multiphase oxidation of primary organic aerosol (POA) and the formation of secondary organic aerosols (SOA) in residential biomass burning plumes. We found that the emissions in the gas and particle phases were interlinked during the plume evolution. During photochemical aging, more oxidized OA was produced, and SOA formation increased by a factor of 2 due to functionalization reactions of gaseous precursors such as furans, phenols, and carbonyls. On the other hand, dark aging resulted in a lower OA mass enhancement by a factor of 1.2, with weaker oxidation from gaseous reactions. Dark aging experiments resulted in the generation of substantial quantities of nitrogen-containing organic compounds in both gas and particulate phases, while photochemical aging led to a notable increase in the concentration of gaseous carboxylic acids. Our observations show that the properties of SOA are influenced by exposure to sunlight radiation and oxidants such as OH or NO3 radicals. These results reflect the aging process of BB plumes in real-world atmospheric conditions and highlight the importance of considering various aging mechanisms.

Dynamic Changes of Composition of Particulate Matter Emissions during Residential Biomass Combustion

Residential biomass combustion in developing countries produces significant primary particulate matter (PM) emissions. Highly time-resolved aerosol mass spectrometry and aethalometer measurements were used to investigate the dynamic changes of emitted PM chemical composition from a typical improved stove burning with wood and crop straw in China. Combustion temperature and organic aerosol (OA) concentration increased quickly during the ignition stage. The flaming stage was characterized by high combustion temperature and high pollutant [including OA, black carbon (BC), inorganic salts, and polycyclic aromatic hydrocarbons (PAHs)] emissions, while the burnout stage is characterized by low combustion temperature and lower pollutant emissions. OA was the primary emitted species; emission factors of OA in the flaming stage were generally higher (24.5-792%) than those in the burnout stage. Mass spectral signatures of OA were obtained. The ratio of Cl-/OA for wood combustion (0.05 ± 0.01) is much lower than that from burning crop straw (0.32 ± 0.19). Hydrocarbon OA emissions dominated during the ignition and flaming stages. A high percentage of oxidized OA was emitted during the burnout stage. The relationship between PAHs and BC/OA emissions under different burning conditions was investigated, and PAHs may act as intermediate products in the conversion of OA to BC.

Direct measurement of the deposition of submicron soot particles on leaves of Platanus acerifolia tree

Submicron soot particles (<1.0 μm in aerodynamic diameter) are responsible for global warming and health burdens worldwide. However, studies on bio-monitoring of submicron soot particles and their associated sources by using tree leaves are not comprehensively illustrated. Here, we determined the seasonal trends of submicron soot particles on the leaves of the Platanus acerifolia collected from two cities (Lu'an, Anhui Province, and Nanjing, Jiangsu Province) in the Yangtze River Delta region, China. The source apportionment of submicron soot particles was performed using stable carbon isotopic analyses. Significant seasonal trends of submicron soot particles were observed in two cities with averaged levels of 0.41-1.36 mg m^-2 in cold seasons and averaged levels of 0.13-0.24 mg m^-2 in warm seasons. The levels of δ¹³C for submicron soot at the suburban site of Lu'an city were observed to be in the range of -25.6‰ to -18.2‰ with fossil fuels dominated (∼58%) in summer and -23.0‰ to -15.6‰ with biomass burning dominated in winter (∼67%). In comparison, the ranges in the levels of δ¹³C in submicron soot were found to be from -26.5‰ to -20.4‰ in winter, and -24.2‰ to -17.9‰ in summer at the urban site of Nanjing. Fossil fuels accounted for a large fraction of submicron soot with average contributions of 53% in winter and 73% in summer, respectively. These findings demonstrate that Platanus acerifolia trees could be used as an effective and low-cost bio-monitoring tool for monitoring the pollution status of submicron soot and associated source contribution.

Characterization of Aerosol Properties from the Burning Emissions of Typical Residential Fuels on the Tibetan Plateau

The Tibet Autonomous Region in China is a unique place with high altitude and special Tibetan culture. The residents have different living habits and domestic fuels from those in other parts of China, however, knowledge on the emission characteristics of local residential fuels remain poorly understood until now. In this study, nine popular residential fuels in the Tibet are burned in situ to study the aerosol chemical compositions, mass spectral signatures, and emission characteristics from their burning emissions. Overall, emissions of particulate and gaseous pollutants depend strongly on the burning conditions, in addition to the fuel constituents themselves. Burning the biofuels of yak dung, WeiSang mixture fuels, and two powdery Tibetan incenses with relatively low combustion efficiencies can emit large amounts of CO and aerosols, especially organic aerosols (>90%) with large diameters. In contrast, burning of wood, coal, ghee lamp, stick-like Tibetan incense, and diesel can release abundant CO₂ but fewer aerosols from their flaming combustion. A comprehensive database consisting of the high-resolution mass spectra of organics and emission factors of multiple chemical components are established. Distinctly different mass spectral signatures are found among the different fuels, in particularly those unique Tibetan biofuels. All these findings have significant implications for the identification of aerosol sources, compilation of pollutant emission inventories, and assessment of potential environment effects in this remote region.

Effect of photooxidation on size distribution, light absorption, and molecular compositions of smoke particles from rice straw combustion

Organic aerosol (OA) emitted from biomass burning (BB) impacts air quality and global radiation balance. However, the comprehensive characterization of OA remains poorly understood because of the complex evolutionary behavior of OA in atmospheric processes. In this work, smoke particles were generated from rice straw combustion. The effect of OH radicals photooxidation on size distribution, light absorption, and molecular compositions of smoke particles was systematically investigated. The results showed that the median diameters of smoke particles increased by a factor of approximately 1.2 after photooxidation. In the particle compositions, although both non-polar fractions (n-hexane-soluble organic carbon, HSOC) and polar fractions (water-soluble organic carbon, WSOC) underwent photobleaching after aging, the photobleaching properties of HSOC (1.87-2.19) was always higher than that of WSOC (1.52-1.33). Besides, the light-absorbing properties of HSOC were higher than that of WSOC, showing a factor of approximately 1.75 times for mass absorption efficiency at 365 nm (MAE₃₆₅). Consequently, the simple forcing efficiency (SFE) caused by absorption showed that HSOC has higher radiation effects than WSOC. After photooxidation, the concentration of 16 PAHs in HSOC fractions significantly decreased by 15.3%-72.5%. In WSOC fractions, the content of CHO, CHONS, and CHOS compounds decreased slightly, while the content of CHON compounds increased. Meantime, the variations in molecular properties supported the decrease in light absorption of WSOC fractions. These results reveal the aging behavior of smoke particles, then stress the importance of non-polar organic fractions in particles, providing new insights into understanding the atmospheric pollution caused by BB smoke particles.

Connecting the Light Absorption of Atmospheric Organic Aerosols with Oxidation State and Polarity

The light-absorbing organic aerosol (OA) constitutes an important fraction of absorbing components, counteracting major cooling effect of aerosols to climate. The mechanisms in linking the complex and changeable chemistry of OA with its absorbing properties remain to be elucidated. Here, by using solvent extraction, ambient OA from an urban environment was fractionated according to polarity, which was further nebulized and online characterized with compositions and absorbing properties. Water extracted high-polar compounds with a significantly higher oxygen to carbon ratio (O/C) than methanol extracts. A transition O/C of about 0.6 was found, below and above which the enhancement and reduction of OA absorptivity were observed with increasing O/C, occurring on the less polar and high polar compounds, respectively. In particular, the co-increase of nitrogen and oxygen elements suggests the important role of nitrogen-containing functional groups in enhancing the absorptivity of the less polar compounds (e.g., forming nitrogen-containing aromatics), while further oxidation (O/C > 0.6) on high-polar compounds likely led to fragmentation and bleaching chromophores. The results here may reconcile the previous observations about darkening or whitening chromophores of brown carbon, and the parametrization of O/C has the potential to link the changing chemistry of OA with its polarity and absorbing properties.

Evolution of light absorption properties during photochemical aging of straw open burning aerosols

Straw burning comprises more than 30% of all types of burned biomass in Asia, while the estimation of the emitted aerosols' direct radiative forcing effect suffers from large uncertainties, especially when atmospheric aging processes are considered. In this study, the light absorption properties of primary and aged straw burning aerosols in open fire were characterized at 7 wavelengths ranging from 370 nm to 950 nm in a chamber. The primary rice, corn and wheat straw burning bulk aerosols together had a mass absorption efficiency (MAE) of 2.43 ± 1.36 m² g^-1 at 520 nm and an absorption Ångström exponent (AAE) of 1.93 ± 0.71, while the primary sorghum straw burning bulk aerosols were characterized by a relatively lower MAE of 0.95 ± 0.54 m² g^-1 and a higher AAE of 4.80 ± 0.68. Both the MAE and AAE of primary aerosols can be well parameterized by the (PM-BC)/BC ratio (in wt.). The MAE of black carbon (BC) increased by 11-190% during photoreactions equivalent to 16-60 h of atmospheric aging, which was positively correlated with the (PM-BC)/(BC) ratio. The MAE of organic aerosols first slightly increased or leveled off, and then decreased. Specifically, at 370 nm, the first growth/plateau stage lasted until OH exposure reached 0.47-1.29 × 10¹¹ molecule cm^-3 s, and the following period exhibited decay rates of 1.0-2.8 × 10^-12 cm³ molecule^-1 s^-1 against the OH radical, corresponding to half-lives of 46-134 h in a typical ambient condition. During photoreactions, competition among the lensing effect, growth/bleach of organic chromophores, and particle mass and size growth complicated the evolution of the direct radiative forcing effect. It is concluded that rice and corn straw burning aerosols maintained a warming effect after aging, while the cooling effect of fresh sorghum straw burning aerosols increased with aging.

Significant light absorption of brown carbon during the 2020 California wildfires

In this study, the contribution of brown carbon (BrC) to the absorption aerosol optical depth (AAOD) during the August to October 2020 California wildfires in Fresno, Monterey, and the University of California Santa Barbara (UCSB) was investigated using Aerosol Robotic Network (AERONET) column measurements with Moderate Resolution Imaging Spectroradiometer (MODIS) fire pixel counts. There was an approximate three to five times increase in AAOD and fine-mode aerosols during intensive wildfires in August-October 2020 compared to the wildfires in the previous 18 years (2002-2019). Substantial daily variation in the contribution of BrC to AAOD was correlated with the fire pixel counts (correlation coefficients of 0.63, 0.40, and 0.57 at Fresno, Monterey, and UCSB, respectively). This variation was influenced by regional topography, atmospheric conditions, and distance from the fire. Between August and October 2020, the average contribution of BrC to AAOD at 440 nm due to wildfires was 35.3 ± 5.6, 35.1 ± 6.8, and 40.6 ± 9.5% at Fresno, Monterey, and UCSB, respectively. This was approximately twice as high as for those sites without a direct wildfire influence. The BrC contribution with direct wildfire influence over the period of January-December 2020 at Fresno, Monterey, and UCSB (32.8 ± 7.5, 31.6 ± 7.9, and 40.0 ± 3.5%, respectively) and from 2002 to 2019 (30.7 ± 8.3, 28.5 ± 4.8, and 35.7 ± 14.6%, respectively) was approximately 20% greater than other BrC sources including vehicles, fossil fuel combustion, and residential heating.