Characteristics of fresh and aged volatile organic compounds from open burning of crop residues

Laboratory study on the characteristics of fresh and aged PM1 emitted from typical forest vegetation combustion in Southwest China

The frequency and intensity of forest fires are amplified by climate change. Substantial quantities of PM1 emitted from forest fires can undergo gradual atmospheric dispersion and long-range transport, thus impacting air quality far from the source. However, the chemical composition and physical properties of PM emitted from forest fires and its changes during atmospheric transport remain uncertain. In this study, the evolution of organic carbon (OC), elemental carbon (EC), water-soluble ions, and water-soluble metals in the particulate phase of smoke emitted from the typical forest vegetation combustion in Southwest China before and after photo-oxidation was investigated in the laboratory. Two aging periods of 5 and 9 days were selected. The OC and TC mass concentrations tended to decrease after 9-days aged compared to fresh emissions. OP, OC2, and OC3 in PM1 are expected to be potential indicators of fresh smoke, while OC3 and OC4 may serve as suitable markers for identifying aged carbon sources from the typical forest vegetation combustion in Southwest China. K+ exhibited the highest abundant water-soluble ion in fresh PM1, whereas NO3- became the most abundant water-soluble ion in aged PM1. NH4NO3 emerged as the primary secondary inorganic aerosol emitted from typical forest vegetation combustion in Southwest China. Notably, a 5-day aging period proved insufficient for the complete formation of the secondary inorganic aerosols NH4NO3 and (NH4)2SO4. After aging, the mass concentration of the water-soluble metal Ni in PM1 from typical forest vegetation combustion in Southwest China decreased, while the mean mass concentrations of all other water-soluble metals increased in varying degrees. These findings provide valuable data support and theoretical guidance for studying the atmospheric evolution of forest fire aerosols, as well as contribute to policy formulation and management of atmospheric environment safety and human health.

Effects of photochemical aging on the molecular composition of organic aerosols derived from agricultural biomass burning in whole combustion process

Biomass burning organic aerosols (BBOA) are key components of atmospheric particulate matter, yet the effects of aging process on their chemical composition and related properties remain poorly understood. In this study, fresh smoke emissions from the combustion of three types of agricultural biomass residues (rice, maize, and wheat straws) were photochemically aged in an oxidation flow reactor. The changes in BBOA composition were characterized by offline analysis using ultrahigh performance liquid chromatography coupled with Orbitrap mass spectrometry. The BBOA molecular composition varied dramatically with biomass type and aging process. Fresh and aged BBOA were predominated by CHO and nitrogen-containing CHON, CHN, and CHONS species, while with very few CHOS and other non‑oxygen species. The signal peak area variations revealed that individual molecular species underwent dynamic changes, with 77-81 % of fresh species decreased or even disappeared and 33-46 % of aged species being newly formed. A notable increase was observed in the number and peak area of CxHyO≥6 compounds in aged BBOA, suggesting that photochemical process served as an important source of highly oxygenated species. Heterocyclic CxHyN2 compounds mostly dominated in fresh CHN species, whereas CxHyN1 were more abundant in aged ones. Fragmentation and homologs oxidation by addition of oxygen-containing functional groups were important pathways for the BBOA aging. The changes in BBOA composition with aging would have large impacts on particle optical properties and toxicity. This study highlights the significance of photochemical aging process in altering chemical composition and related properties of BBOA.

Insights into the formation of secondary organic aerosols from agricultural residue burning emissions: A review of chamber-based studies

Organic aerosols (OA) are a significant component of fine particulate matter in the ambient air and are formed through primary and secondary processes. Primary organic aerosols (POA) are directly released from sources, while secondary organic aerosols (SOA) are formed through the oligomerization and/or oxidation of volatile organic compounds (VOCs) in the atmosphere. Recently, there has an increasing attention on the SOA budgets, their formation pathways, and photochemical evolution due to their impacts on climate and human health. Biomass burning (BB) is a significant source of OA, contributing around 5-30 % to the SOA burden globally. Agricultural residue burning (ARB) is a type of BB that contributes ∼10 % of total atmospheric OA mass worldwide, whereas it contributes higher in Asian regions like China and India. ARB emits a significant amount of air pollutants, including VOCs, into the atmosphere. However, there is inadequate information on the transformation of ARB emissions to SOA due to limited laboratory studies. The present review focuses on the formation mechanism of SOA from ARB emissions, summarizing the current state of the art about ARB precursors and their oxidation products from chamber-based studies, including measurement methods and analytical instrumentation. The review also discusses the role of different types of oxidants in OA mass enhancement, factors affecting the overall SOA yield, and the uncertainties involved in the process.

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.

Examination of long-time aging process on volatile organic compounds emitted from solid fuel combustion in a rural area of China

Volatile organic compounds (VOCs) emitted from solid fuels combustion (e.g., biomass and coal) are still the dominant precursors for the formation of tropospheric ozone (O₃) and secondary organic aerosols (SOAs). Limited research focused on the evolution, as known as atmospheric aging, of VOCs emitted during long-timescale observations. Here, freshly emitted and aged VOCs from common residual solid fuel combustions were collected onto absorption tubes before and after passing through an oxidation flow reactor (OFR) system, respectively. The emission factor (EF) of freshly emitted total VOCs is in descending order of corn cob ≥ corn straw > firewood ≥ wheat straw > coals. Aromatic and oxygenated VOCs (OVOCs) are the two most abundant groups, accounting for >80% of the EF of total quantified VOCs (EF_TVOCs). Briquette technology shows an effective reduction of the VOC emission, demonstrating a maximum 90.7% lower EF_TVOCs in comparison to that of biomass fuels. In contrast, each VOC shows significantly different degradation in comparison to EF of freshly emitted and after 6- and 12-equivalent day aging (actual atmospheric aging days calculated from aging simulation). The largest degradations after 6-equivalent days of aging are observed on alkenes in the biomass group (60.9% on average) and aromatics in the coal group (50.6% on average), consistent with their relatively high reactivities toward oxidation with O₃ and hydroxyl radical. The largest degraded compound is seen for acetone, followed by acrolein, benzene, and toluene. Furthermore, the results show that the distinction of VOC species based on long-timescale (12-equivalent day aging) observation is essential to further explore the effect of regional transport. The alkanes which have relatively lower reactivities but high EFs could be accumulated through long-distance transport. These results provide detailed data on fresh and aged VOCs emitted from residential fuels which could be used to explore the atmospheric reaction mechanism.

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.

Effects of atmospheric aging processes on in vitro induced oxidative stress and chemical composition of biomass burning aerosols

Biomass burning (BB) has an important impact on local/regional air quality and human health in China, but most previous studies overlooked the influence of atmospheric aging processes on cytotoxicity and chemical composition of BB aerosols. In this study, we combined a combustion chamber and an oxidation flow reactor to generate fresh and aged BB PM_2.5. Human bronchial epithelial BEAS-2B cells were exposed to PM_2.5 preparation for 24 h, and then determined for particle-induced reactive oxygen species (ROS) in vitro. The particle-induced ROS production increased by 11 %-64 % after two days of aging, suggesting an enhancement of in vitro-induced oxidative stress (OS) of aged BB particles. Chemical analysis showed that organic matter (OM) was the dominant component with no changes in relative abundance for the fresh and aged BB particles. Organic polycyclic aromatic compounds and some metals showed strong correlations with ROS in fresh particles, indicating the important effects of these harmful components on the OS of fresh BB aerosols. However, such correlations were not found for the aged particles, which is possibly related to the loss of non- or low-toxic semivolatile compounds and the formation of secondary harmful OM (such as some N-containing organic compounds) during the atmospheric aging processes.

Molecular characteristics of organic compositions in fresh and aged biomass burning aerosols

Biomass burning (BB) is the most important source of primary organic aerosols (OA) in the atmosphere that has significant impact on local/regional air quality and human health. However, few studies paid attention to the evolution of molecular characteristics of BB OA in the atmospheric aging processes. In this study, both fresh and aged PM_2.5 aerosols from burning of rice, maize, and wheat straws were collected from a combined system of combustion chamber and oxidation flow reactor, and were analyzed for >100 organic species. The emission factors (EFs) of anhydrosugars and some fatty acids showed slight variations between fresh and aged samples, indicating that these compounds are relatively stable. However, the EFs of n-alkanes, fatty alcohols, and parent-PAHs decreased 8-57% from fresh to aged samples, suggesting that they can undergo further oxidation to form other organic materials in the atmosphere. Phthalic acids, nitrophenols and isoprene-derived products were mainly secondarily formed by aging processes. Thus their EFs increased by 2-23 times from fresh to aged samples. Levoglucosan was the most abundant individual organic tracer, and its EF varied slightly between fresh and aged samples, proving its indicative role on BB emission. Moreover, the ratio of vanillic acid to levoglucosan and p-hydroxybenzoic acid to levoglucosan increased 2-13 times from fresh to aged samples. Therefore they can be used to investigate the impact of aging processes on BB aerosols in the atmosphere. RO₂ + HO₂ pathway derived 2-methyltetrols (2-MTs) predominated the EFs of isoprene-derived products (SOA_i) in the fresh samples. However, RO₂ + NO pathway derived 2-methylglyceric acid (2-MGA) increased by >30 times and became comparable with 2-MTs in aged particles. The ratio of 2-MGA/2-MTs increased from 0.06-0.27 in fresh samples to 0.94-1.18 in aged samples, because the high loading of NO_x in BB smoke enhanced the formation of SOA_i through RO₂ + NO reactions.