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Following wood pyrolysis, tar ball aerosols were laboratory generated from wood tar separated into polar and nonpolar phases. Chemical information of fresh tar balls was obtained from a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and single-particle laser desorption/resonance enhanced multiphoton ionization mass spectrometry (SP-LD-REMPI-MS). Their continuous refractive index (RI) between 365 and 425 nm was retrieved using a broadband cavity enhanced spectroscopy (BBCES). Dynamic changes in the optical and chemical properties for the nonpolar tar ball aerosols in NOx-dependent photochemical process were investigated in an oxidation flow reactor (OFR). Distinct differences in the chemical composition of the fresh polar and nonpolar tar aerosols were identified. Nonpolar tar aerosols contain predominantly high-molecular weight unsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs), while polar tar aerosols consist of a high number of oxidized aromatic substances (e.g., methoxy-phenols, benzenediol) with higher O : C ratios and carbon oxidation states. Fresh tar balls have light absorption characteristics similar to atmospheric brown carbon (BrC) aerosol with higher absorption efficiency towards the UV wavelengths. The average retrieved RI is 1.661+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols, respectively, with an absorption Ångström exponent (AAE) between 5.7 and 7.8 in the detected wavelength range. The RI fits a volume mixing rule for internally mixed nonpolar/polar tar balls. The RI of the tar ball aerosols decreased with increasing wavelength under photochemical oxidation. Photolysis by UV light (254 nm), without strong oxidants in the system, slightly decreased the RI and increased the oxidation state of the tar balls. Oxidation under varying OH exposure levels and in the absence of NOx diminished the absorption (bleaching) and increased the O : C ratio of the tar balls. The photobleaching via OH radical initiated oxidation is mainly attributed to decomposition of chromophoric aromatics, nitrogen-containing organics, and high-molecular weight components in the aged particles. Photolysis of nitrous oxide (N2O) was used to simulate NOx-dependent photochemical aging of tar balls in the OFR. Under high-NOx conditions with similar OH exposure, photochemical aging led to the formation of organic nitrates, and increased both oxidation degree and light absorption for the aged tar ball aerosols. These observations suggest that secondary organic nitrate formation counteracts the bleaching by OH radical photooxidation to eventually regain some absorption of the aged tar ball aerosols. The atmospheric implication and climate effects from tar balls upon various oxidation processes are briefly discussed.

Background Carbonaceous aerosols emitted from indoor and outdoor biomass burning are major risk factors contributing to the global burden of disease. Wood tar aerosols, namely, tar ball particles, compose a substantial fraction of carbonaceous emissions, especially from biomass smoldering. However, their health-related impacts and toxicity are still not well known. This study investigated the toxicity of the water-soluble fraction of pyrolyzed wood tar aerosols in exposed mice and lung epithelial cells. Results Mice exposed to water-soluble wood tar aerosols showed increased inflammatory and oxidative stress responses. Bronchial epithelial cells exposed to the same water-soluble wood tar aerosols showed increased cell death with apoptotic characteristics. Alterations in oxidative status, including changes in reactive oxygen species (ROS) levels and reductions in the expression of antioxidant genes related to the transcription factor Nrf2, were observed and were confirmed by increased levels of MDA, a lipid peroxidation adduct. Damage to mitochondria was observed as an early event responsible for the aforementioned changes. Conclusions The toxicity and health effect-related mechanisms of water-soluble wood tar were investigated for the first time in the context of biomass burning. Wood tar particles may account for major responses such as cell death, oxidative stress, supression of protection mechnaisms and mitochondrial damaged cause by expsoure to biomass burning aerosols.

Polycyclic aromatic hydrocarbons (PAHs) are toxic compounds in the atmosphere and have adverse effects on public health, especially through the inhalation of particulate matter (PM). At present, there is limited understanding of the size distribution of particulate-bound PAHs and their health risks on a continental scale. In this study, we carried out a PM campaign from October 2012 to September 2013 at 12 sampling sites simultaneously, including urban, suburban and remote sites in different regions of China. Size-segregated PAHs and typical tracers of coal combustion (picene), biomass burning (levoglucosan) and vehicle exhaust (hopanes) were measured. The annual averages of the 24 total measured PAHs (∑24PAHs) and benzo[a]pyrene (BaP) carcinogenic equivalent concentration (BaPeq) ranged from 7.56 to 205 ng/m3 with a mean of 53.5 ng/m3 and from 0.21 to 22.2 ng/m3 with a mean of 5.02 ng/m3, respectively. At all the sites, ∑24PAHs and BaPeq were dominant in the ultrafine particles with aerodynamic diameter < 1.1 µm, followed by those in the size ranges of 1.1–3.3 µm and > 3.3 µm. Compared with southern China, northern China witnessed much higher ∑24PAHs (87.36 vs. 17.56 ng/m3), BaPeq (8.48 vs. 1.34 ng/m3) and PAHs' inhalation cancer risk (7.4 × 10−4 vs. 1.2 × 10−4). Nationwide increases in both PAH levels and inhalation cancer risk occurred in winter. The unfavorable meteorological conditions and enhanced emissions of coal combustion and biomass burning together led to severe PAHs' pollution and high cancer risk in the atmosphere of northern China, especially during winter. Coal combustion is the major source of BaPeq in all size particles at most sampling sites. Our results suggested that the reduction of coal and biofuel consumption in the residential sector could be crucial and effective in lowering PAH concentrations and their inhalation cancer risk in China.

The multi-pass photoacoustic spectrometer (PAS) is an important tool for the direct measurement of light absorption by atmospheric aerosol. Accurate PAS measurements heavily rely on accurate calibration of their signal. Ozone is often used for calibrating PAS instruments by relating the photoacoustic signal to the absorption coefficient measured by an independent method such as cavity ring down spectroscopy (CRD-S), cavity-enhanced spectroscopy (CES) or an ozone monitor. We report here a calibration method that uses measured absorption coefficients of aerosolized, light-absorbing organic materials and offer an alternative approach to calibrate photoacoustic aerosol spectrometers at 404 nm. To implement this method, we first determined the complex refractive index of nigrosin, an organic dye, using spectroscopic ellipsometry and then used this well-characterized material as a standard material for PAS calibration.

Oxidized Organic Aerosol (OOA), a major component of fine atmospheric particles, impacts climate and human health. Previous experiments and atmospheric models emphasize the importance of nocturnal OOA formation from NO 3 · oxidation of biogenic VOCs. This seasonal study extends the understanding by showing that nocturnal oxidation of biomass-burning emissions can account for up to half of total OOA production in fall and winter. It is the first to distinguish nocturnal OOA characteristics from daytime OOA across all seasons using bulk aerosol measurements. Summer observations of nocturnal OOA align well with regional chemistry transport model predictions, but discrepancies in other seasons reveal a common model deficiency in representing biomass-burning emissions and their nocturnal oxidation. This study underscores the significance of near-ground nocturnal OOA production, proposes a method to differentiate it using bulk aerosol measurements, and suggests model optimization strategies. These findings enhance the understanding and prediction of nighttime OOA formation.

Long-term data on PM.sub.2.5 chemical composition provide essential information for evaluating the effectiveness of air pollution control measures and understanding the evolving mechanisms of secondary species formation in the real atmosphere. This study presented field measurements of PM.sub.2.5 and its chemical composition at a regional background site in the Pearl River Delta (PRD) from 2007 to 2020. PM.sub.2.5 concentration declined significantly from 87.1 ± 15.5 to 34.0 ± 11.3 µg m.sup.-3 (-4.0 µg m.sup.-3 yr.sup.-1). The proportion of secondary species increased from 57 % to 73 % with the improvement in air quality. Among these species, sulfate (SO42-) showed a sharp decline, while nitrate (NO3-) exhibited a moderate decrease. Consequently, the proportion of NO3- in 2020 doubled relative to 2007. In addition, we further found that SO42- reduction (-10 % yr.sup.-1) lagged behind SO.sub.2 reduction (-13 % yr.sup.-1 ), while NO3- reduction (-6 % yr.sup.-1) outpaced that of NO.sub.2 (-3 % yr.sup.-1). These contrasting trends were associated with an increase in sulfur oxidation rate (SOR) and a decrease in nitrogen oxidation rate (NOR). Changes in PM.sub.2.5 chemical composition also influenced aerosol physicochemical properties, such as aerosol pH (0.04 yr.sup.-1 ), aerosol liquid water content (ALWC, -1.1 µg m.sup.-3 yr.sup.-1 ), and the light extinction coefficient (-21.44 Mm.sup.-1 yr.sup.-1). Given important roles of aerosol acidity and ALWC in the heterogeneous reactions, these changes may further inhibit the formation of secondary species in the atmosphere, particularly secondary organic aerosols.

Long-term data on PM2.5 chemical composition provide essential information for evaluating the effectiveness of air pollution control measures and understanding the evolving mechanisms of secondary species formation in the real atmosphere. This study presented field measurements of PM2.5 and its chemical composition at a regional background site in the Pearl River Delta (PRD) from 2007 to 2020. PM2.5 concentration declined significantly from 87.1 ± 15.5 to 34.0 ± 11.3 µg m−3 (−4.0 µg m−3 yr−1). The proportion of secondary species increased from 57 % to 73 % with the improvement in air quality. Among these species, sulfate (SO42-) showed a sharp decline, while nitrate (NO3-) exhibited a moderate decrease. Consequently, the proportion of NO3- in 2020 doubled relative to 2007. In addition, we further found that SO42- reduction (−10 % yr−1) lagged behind SO2 reduction (−13 % yr−1), while NO3- reduction (−6 % yr−1) outpaced that of NO2 (−3 % yr−1). These contrasting trends were associated with an increase in sulfur oxidation rate (SOR) and a decrease in nitrogen oxidation rate (NOR). Changes in PM2.5 chemical composition also influenced aerosol physicochemical properties, such as aerosol pH (0.04 yr−1), aerosol liquid water content (ALWC, −1.1 µg m−3 yr−1), and the light extinction coefficient (−21.44 Mm−1 yr−1). Given important roles of aerosol acidity and ALWC in the heterogeneous reactions, these changes may further inhibit the formation of secondary species in the atmosphere, particularly secondary organic aerosols.

Accurate Rayleigh scattering and absorption cross sections of atmospheric gases are essential for understanding the propagation of electromagnetic radiation in planetary atmospheres. Accurate extinction cross sections are also essential for calibrating high-finesse optical cavities and differential optical absorption spectroscopy and for accurate remote sensing. In this study, we measured the scattering and absorption cross sections of carbon dioxide, nitrous oxide, sulfur hexafluoride, oxygen, and methane in the continuous wavelength range of 307–725 nm using broadband cavity-enhanced spectroscopy (BBCES). The experimentally derived Rayleigh scattering cross sections for CO2, N2O, SF6, O2, and CH4 agree with refractive index-based calculations, with a difference of (0.4 ± 1.2) %, (−0.6 ± 1.1) %, (0.9 ± 1.4) %, (2.8 ± 1.2) %, and (0.9 ± 2.2) %, respectively. The O2–O2 collision-induced absorption and absorption by methane are obtained with high precision at the 0.8 nm resolution of our BBCES instrument in the 307–725 nm wavelength range. New dispersion relations for N2O, SF6, and CH4 were derived using data in the UV–vis wavelength range. This study provides dispersion relations for refractive indices, n-based Rayleigh scattering cross sections, and absorption cross sections based on more continuous and more extended wavelength ranges than available in the current literature.

Secondary organic aerosol (SOA) is a dominant constituent of fine particulate matter, exerting significant impacts on both climate and human health. Oxalic acid (C2), a key end-product formed from the oxidation of volatile organic compounds, can provide insights into the formation mechanism of SOA. Thus, long-term measurements of C2 and related compounds help understand the changes in SOA formation with decreasing pollutant levels. In this study, C2 and its homologs, along with five primary anthropogenic source markers and three SOA markers, were measured in the Pearl River Delta (PRD) during 2007–2018. The concentrations of C2 did not exhibit significant downward trends, despite substantial reductions in anthropogenic emissions, such as biomass burning (−11 % yr−1), vehicle emissions (−17 % yr−1), and cooking emissions (−7 % yr−1). Correlation analysis revealed that aerosol liquid water content (ALWC) and Ox (O3 + NO2) were the main drivers of C2 variations. Moreover, the relative contribution of biogenic SOA increased under cleaner conditions. A machine learning model was applied to quantify the impacts of changes in anthropogenic precursor emissions, biogenic precursor emissions, aqueous-phase oxidation processes, and gas-phase oxidation processes on C2 variability. As pollution levels declined, the impacts of gas-phase oxidation increased from 37 % to 55 %, whereas that of aqueous-phase oxidation declined from 42 % to 30 %. This shift indicated a transition from aqueous-phase to gas-phase pathways in C2 and SOA formation. Our findings highlight the increasing importance of gas-phase oxidation under low-pollution conditions and underscore the need for effective ozone control strategies to further reduce SOA in the future.

Long-term data on PM2.5 chemical composition provide essential information for evaluating the effectiveness of air pollution control measures and understanding the evolving mechanisms of secondary species formation in the real atmosphere. This study presented field measurements of PM2.5 and its chemical composition at a regional background site in the Pearl River Delta (PRD) from 2007 to 2020. PM2.5 concentration declined significantly from 87.1 ± 15.5 to 34.0 ± 11.3 µg m−3 (-4.0 µg m−3 yr−1). The proportion of secondary species increased from 57 % to 73 % with the improvement in air quality. Among these species, sulfate (SO42-) showed a sharp decline, while nitrate (NO3-) exhibited a moderate decrease. Consequently, the proportion of NO3- in 2020 doubled relative to 2007. In addition, we further found that SO42- reduction (-10 % yr−1) lagged behind SO2 reduction (-13 % yr−1), while NO3- reduction (-6 % yr−1) outpaced that of NO2 (-3 % yr−1). These contrasting trends were associated with an increase in sulfur oxidation rate (SOR) and a decrease in nitrogen oxidation rate (NOR). Changes in PM2.5 chemical composition also influenced aerosol physicochemical properties, such as aerosol pH (0.04 yr−1), aerosol liquid water content (ALWC, -1.1 µg m−3 yr−1), and the light extinction coefficient (-21.44 Mm−1 yr−1). Given important roles of aerosol acidity and ALWC in the heterogeneous reactions, these changes may further inhibit the formation of secondary species in the atmosphere, particularly secondary organic aerosols.