Optical source profiles of brown carbon in size-resolved particulate matter from typical domestic biofuel burning over Guanzhong Plain, China

Emission and optical characteristics of brown carbon in size-segregated particles from three types of Chinese ships

Brown carbon (BrC) is one of the important light absorption substances that have high light absorption ability under short wavelength light. However, limit studies have focused on the BrC emission from ships. In this study, size-segregated particulate matters (PM) were collected from three different types of ships, light absorption characteristics and size distribution of methanol-soluble BrC and water-soluble BrC in PM from ship exhausts were investigated. Results showed that four-stroke low-power diesel fishing boat (4-LDF) had the highest mass concentrations of methanol-soluble organic carbon (MSOC) and water-soluble organic carbon (WSOC), followed by 2-stroke high-power heavy-fuel-oil vessel (2-HHV), and four-stroke high-power marine-diesel vessel (4-HMV). While 2-HHV had obviously higher light absorption coefficients of methanol-soluble BrC (Abs365,M) and water-soluble BrC (Abs365,W) in unit weight of PM than the other two types of ships. The tested ships presented comparable or higher absorption efficiency of BrC in water extracts (MAE365,W) compared with other BrC emission sources. Majority of BrC was concentrated in fine particles, and the particle size distributions of both Abs365,M and Abs365,W showed bimodal patterns, peaking at 0.43-0.65 µm and 4.7-5.8 µm, respectively. However, different particle size distributions were found for MAE365,M between diesel and heavy fuel oil ships. Besides, different wavelength dependence in particles with different size were also detected. Ship exhaust could be confirmed as a non-ignorable BrC emission source, and complex influencing factor could affect the light absorption characteristics of ship emissions. Particle size should also be considered when light absorption ability of BrC was evaluated.

Spatiotemporal distribution, light absorption characteristics, and source apportionments of black and brown carbon in China

Black carbon (BC) and brown carbon (BrC) are aerosols that absorb light and thereby contribute to climate change. In this study, the light absorption properties and spatiotemporal distributions of equivalent BC (eBC) and BrC aerosols were determined based on continuous measurements of aerosol light absorption from January to August 2017, using a seven-channel aethalometer at 49 sampling sites in China. The source apportionments of BC and BrC were identified using the BC/PM2.5, absorption Ångström exponent, the concentration-weighted trajectory method, and the random forest model. Based on the results, BC was the dominant light absorber, whereas BrC was responsible for a higher proportion of the light absorption in northern compared to southern China. The light absorption of BrC was highest in winter (34.3 Mm-1), followed by spring (19.0 Mm-1) and summer (3.6 Mm-1). The combustion of liquid fuels accounted for over 50 % of the light absorption coefficient of BC in most cities and the importance of carbon monoxide (CO) and nitrogen dioxide (NO2) was over 10 % for BC emitted by liquid fuel combustion, based on the random forest model. The contribution of solid fuel combustion to BC in the north was larger than that in the southern regions as coal combustion and crop residue burning are important emission sources of BC in most northern cities. The contribution of primary BrC to light absorption was high in some northern cities, whereas that of secondary BrC was prevalent in some southern cities. The diurnal variations in secondary BrC were affected by changes in odd oxygen and relative humidity, which promoted the photobleaching of the chromophores and aqueous-phase reactions of secondary BrC.

Insight into the Role of NH3/NH4+ and NOx/NO3- in the Formation of Nitrogen-Containing Brown Carbon in Chinese Megacities

Particulate brown carbon (BrC) plays a crucial role in the global radiative balance due to its ability to absorb light. However, the effect of molecular formation on the light absorption properties of BrC remains poorly understood. In this study, atmospheric BrC samples collected from six Chinese megacities in winter and summer were characterized through ultrahigh-performance liquid chromatography coupled with Orbitrap mass spectrometry (UHPLC-Orbitrap MS) and light absorption measurements. The average values of BrC light absorption coefficient at a wavelength of 365 nm (babs365) in winter were approximately 4.0 times higher than those in summer. Nitrogen-containing organic molecules (CHNO) were identified as critical components of light-absorbing substances in both seasons, underscoring the importance of N-addition in BrC. These nitrogen-containing BrC chromophores were more closely related to nitro-containing compounds originating from biomass burning and nitrogen oxides (NOx)/nitrate (NO3-) reactions in winter. In summer, they were related to reduced N-containing compounds formed in ammonia (NH3)/ammonium (NH4+) reactions. The NH3/NH4+-mediated reactions contributed more to secondary BrC in summer than winter, particularly in southern cities. Compared with winter, the higher O/Cw, lower molecule conjugation indicator (double bond equivalent, DBE), and reduced BrC babs365 in summer suggest a possible bleaching mechanism during the oxidation process. These findings strengthen the connection between molecular composition and the light-absorbing properties of BrC, providing insights into the formation mechanisms of BrC chromophores across northern and southern Chinese cities in different seasons.

Characterization of water-soluble brown carbon in atmospheric fine particles over Xi'an, China: Implication of aqueous brown carbon formation from biomass burning

Brown carbon (BrC) aerosols can affect not only the climate but also human health, however, the light absorption, chemical compositions, and formation mechanisms of BrC are still uncertain, which leads to uncertainties in the accurate estimation of its climate and health impacts. In this study, highly time - resolved brown carbon (BrC) in fine particles was investigated in Xi'an using offline aerosol mass spectrometer analysis. The light absorption coefficient (b_abs365) and mass absorption efficiency (MAE₃₆₅) at 365 nm of water-soluble organic aerosol (WSOA) generally increased with oxygen-to-carbon (O/C) ratios, indicating that oxidized OA could have more impacts on BrC light absorption. Meanwhile, the light absorption appeared to increase generally with the increases of nitrogen-to-carbon (N/C) ratios and water-soluble organic nitrogen; strong correlations (R of 0.76 for C_xH_yN_p⁺ and R of 0.78 for C_xH_yO_zN_p⁺) between b_abs365 and the N - containing organic ion families were observed, suggesting that the N - containing compounds are the effective BrC chromophores. b_abs365 correlated relatively well with BBOA (r of 0.74) and OOA (R of 0.57), but weakly correlated with CCOA (R of 0.33), indicating that BrC in Xi'an was likely to be associated with biomass burning and secondary sources. A multiple linear regression model was applied to apportion b_abs365 to contributions of different factors resolved from positive matrix factorization on water-soluble organic aerosols (OA) and obtained MAE₃₆₅ values of different OA factors. We found that biomass-burning organic aerosol (BBOA) dominated the b_abs365 (48.3 %), followed by oxidized organic aerosol (OOA, 33.6 %) and coal combustion organic aerosol (CCOA, 18.1 %). We further observed that nitrogen-containing organic matter (i.e., C_xH_yN_p⁺ and C_xH_yO_zN_p⁺) increased with the increase of OOA/WSOA and the decrease of BBOA/WSOA, especially under high ALWC conditions. Our work offered proper observation evidence that BBOA is oxidized through the aqueous formation to produce BrC in Xi'an, China.

Effect of carbonaceous components of biodiesel combustion particles on optical properties

This paper studies the influence of carbonaceous components on the optical properties of particulate matter (PM) in biodiesel combustion by conducting a bench test on an electronically controlled high-pressure common-rail diesel engine. In addition, the PM produced by the combustion of diesel oil, soybean oil methyl ester (SME), waste edible oil methyl ester (WME), and palm oil methyl ester (PME) was collected. The carbonaceous composition and optical properties of diesel and three biodiesel particulates were then analyzed. The obtained results showed that the ratio of organic carbon (OC) to total carbon (TC) in diesel PM was 0.25 and the ratio of OC/EC was 0.33. The OC to TC ratio of biodiesel PM was significantly greater than that of diesel PM, ranging between 0.59 and 0.65, with OC/EC values in the range of 1.44-1.86. The mass absorption cross-section (MAC) values of three kinds of biodiesel particles were all higher than those of diesel particles. When the incident laser wavelength increased, the difference of MAC values among four kinds of fuel particles gradually decreased. The MAC values of all the three biodiesel particles were higher than those of the diesel particles, and the difference between the MAC values of the four fuel particles gradually decreased with the increase of the incident laser wavelength. Afterwards, the "shell-core" model of particles was developed with 80 nm EC sphere as the core. At the two refractive indices, the scattering cross section, absorption cross section, and extinction cross section of the particles decrease with the increase of the incident light wavelength, and the scattering cross section, absorption cross section, and extinction cross section of the particles increase with the increase of the OC coating thickness.

Significant coal combustion contribution to water-soluble brown carbon during winter in Xingtai, China: Optical properties and sources

To understand the characteristics of atmospheric brown carbon (BrC), daily PM_2.5 samples in Xingtai, a small city in North China Plain (NCP), during the four seasons of 2018-2019, were collected and analyzed for optical properties and chemical compositions. The light absorption at 365 nm (abs_{λ=365 nm}) displayed a strong seasonal variation with the highest value in winter (29.0±14.3 M/m), which was 3.2∼5.4-fold of that in other seasons. A strong correlation of abs_{λ=365 nm} with benzo(b)fluoranthene (BbF) was only observed in winter, indicating that coal combustion was the major source for BrC in the season due to the enhanced domestic heating. The mass absorbing efficiency of BrC also exhibited a similar seasonal pattern, and was found to correlate linearly with the aerosol pH, suggesting a positive effect of aerosol acidity on the optical properties and formation of BrC in the city. Positive matrix factorization (PMF) analysis further showed that on a yearly basis the major source for BrC was biomass burning, which accounted for 34% of the total BrC, followed by secondary formation (26.7%), coal combustion (21.3%) and fugitive dust (18%). However, the contribution from coal combustion was remarkably enhanced in winter, accounting for ∼40% of the total. Our work revealed that more efforts of "shifting coal to clean energy" are necessary in rural areas and small cities in NCP in order to further mitigate PM_2.5 pollution in China.

Optical properties, molecular characterizations, and oxidative potentials of different polarity levels of water-soluble organic matters in winter PM2.5 in six China's megacities

Humic-like substances (HULIS) accounted for a great fraction of water-soluble organic matter (WSOM) in PM_2.5, which efficiently absorb ultraviolet (UV) radiation and pose climate and health impacts. In this study, the molecular structure, optical properties, and oxidative potential (OP) of acid- and neutral-HULIS (denoted as HULIS-a, and HULIS-n, respectively), and high-polarity WSOM (HP-WSOM) were investigated in winter PM_2.5 collected at six China's megacities. For both carbon levels and optical absorption coefficients (b_{abs_365}), HULIS-a/HULIS-n/HP-WSOM showed significant spatial differences. For each city, the carbon levels and b_{abs_365} follow a similar order of HULIS-n > HULIS-a > HP-WSOM. Besides, the b_{abs_365} of HULIS-n and HULIS-a showed the same order of Harbin > Beijing ≈ Wuhan > Xi'an > Guangzhou > Chengdu, while HP-WSOM exhibited an order of Wuhan > Chengdu > Xi'an > Harbin > Beijing > Guangzhou. Both HULIS-a and HULIS-n were abundant in aromatic and aliphatic compounds, whereas HP-WSOM was dominated by a carboxylic acid group. The OP (in unit of nmol H₂O₂ μg^-1C) followed the order of HP-WSOM > HULIS-a > HULIS-n in all the cities. The OPs of HULIS-a, HULIS-n, and HP-WSOM in Harbin and Beijing were much higher than those of other cities, attributing to the high contribution from biomass burning. Highly positive correlations between reactive oxygen species (ROS) of HULIS-a and MAE₃₆₅ were obtained in Chengdu, Wuhan, and Harbin, but ROS of HULIS-n had stronger correlation with MAE₃₆₅ in Harbin, Chengdu, and Xi'an.

Field measurements of PM2.5 emissions from typical solid fuel combustion in rural households in Fenhe Basin, China

Solid fuel is the most widely used energy source for cooking and heating in the rural households in developing countries. In this study, emissions from 13 fuel-stove combinations were studied in two typical rural villages in the Fenhe Basin, Shanxi Province, China. This study gathered data on the emission characteristics of particles with an aerodynamic diameter of ≤2.5 μm (PM_2.5), organic carbon (OC), elemental carbon (EC), and 21 parent and oxygenated polycyclic aromatic hydrocarbons (pPAHs and oPAHs, respectively); the mechanism of gas formation was also determined. The PM_2.5 EFs of biomass burning ranged from 4.11 ± 2.12 to 138 ± 47.2 g/kg, which was higher than that of coal combustion (1.57 ± 0.89 to 4.11 ± 0.63 g/kg). Notably, the average PM_2.5 EFs of biomass burning in a traditional stove and elevated kang were 50.9 ± 13.8 and 23.0 ± 3.99 g/kg, respectively, suggesting that the elevated kang had superior emission mitigation. Wood pellet burning in a biomass furnace yielded lower PM_2.5 EFs than firewood burning in the biomass furnace, which demonstrated wood pellet combustion's superior emission reduction effect. The relative contribution of OC4 to OC subfractions may be useable as tools for identifying the sources of coal and biomass burning. Regarding PAHs, biomass with abundant lignin pyrolysis produced numerous hydroxyl radicals that were conducive to the release of greater proportions of oPAHs. By contrast, pPAHs had greater relative contributions in coal combustion. Regarding gaseous pollutants, its formation mechanism varied with combustion phase. Emission differences between the two phases were mainly determined by the relative contributions of volatile C/N and char. Clarifying the pollutant formation mechanism can better guide the implementation of emission control from household solid fuel combustion.

Characteristics and health risks of parent, alkylated, and oxygenated PAHs and their contributions to reactive oxygen species from PM2.5 vehicular emissions in the longest tunnel in downtown Xi'an, China

A vehicular emission study was conducted in the longest inner-city tunnel in Xi'an, northwestern China in four time periods (I: 07:30-10:30, II: 11:00-14:00, III: 16:30-19:30, and IV: 20:00-23:00 LST). A sum of 40 PAHs, including parent (p-PAHs), alkylated (a-PAHs), and oxygenated (o-PAHs) in fine particulate matter (PM_2.5) were quantified. The relationships between the PAHs and the formation of reactive oxygen species (ROS) were also studied. The average total quantified PAHs concentration was 236.3 ± 48.3 ng m^-3. The p-PAHs were found to be the most dominated group, accounting for an average of 88.1% of the total quantified PAHs, followed by a-PAHs (6.1%) and o-PAHs (5.8%). On the base of the number of aromatic rings, the groups of ≤5 rings (92.5 ± 1.2%) had higher fractions than the high ones (≥6 rings, 7.5 ± 1.2%) for pPAHs. Diurnal variations of PAHs subgroups exhibited the highest levels in Period III, consistent with the largest traffic counts in evening rush hours. However, less reduction of few PAHs in the night period demonstrates that the emissions of compressed natural gas (CNG) and methanol-fueled vehicles cannot be ignored while their contribution increased. High ROS activity levels were observed in the traffic-dominated samples, implying the potential oxidative damages to humans. Additionally, diurnal variation of the ROS activity was consistent with the total quantified PAHs and toxic equivalency of benzo[a]pyrene. Good correlations (R > 0.6, p < 0.05) were seen between individual groups of PAHs (especially for 3-5 rings p-PAHs, 4 rings a-PAHs, and 2-3 rings o-PAHs) and ROS activity, supporting that the vehicular emitted PAHs possibly initiate oxidative stress. The multiple linear regression analysis further illustrated that chrysene contributed the highest (25.0%) to ROS activity. In addition to highlighting the potential hazards to the PAHs from the vehicular emission, their roles to mitigate the health effects by formations of ROS were firstly reported in northwestern China.

Spatial characteristics of VOCs and their ozone and secondary organic aerosol formation potentials in autumn and winter in the Guanzhong Plain, China

As critical precursors of tropospheric ozone (O₃) and secondary organic aerosol (SOA), volatile organic compounds (VOCs) largely influence air quality in urban environments. In this study, measurements of 102 VOCs at all five major cities in the Guanzhong Plain (GZP) were conducted during Sep.09-Oct. 13, 2017 (autumn) and Nov. 14, 2017-Jan. 19, 2018 (winter) to investigate the characteristics of VOCs and their roles in O₃ and SOA formation. The average concentrations of total VOCs (TVOCs) at Xi'an (XA), Weinan (WN), Xianyang (XY), Tongchuan (TC), and Baoji (BJ) sites were in the range of 55.2-110.2 ppbv in autumn and 42.4-74.3 ppbv in winter. TVOCs concentrations were reduced by 22.4%-43.5% from autumn to winter at XA, WN and BJ. Comparatively low concentrations of TVOCs were observed in XY and TC, ranging from 53.5 to 62.7 ppbv across the sampling period. Alkanes were the major components at all sites, accounting for 26.4%-48.9% of the TVOCs during the sampling campaign, followed by aromatics (4.2%-26.4%). The average concentration of acetylene increased by a factor of up to 4.8 from autumn to winter, indicating the fuel combustion in winter heating period significantly impacted on VOCs composition in the GZP. The OH radical loss rate and maximum incremental reactivity method were employed to determine photochemical reactivities and ozone formation potentials (OFPs) of VOCs, respectively. The VOCs in XA and WN exhibited the highest reactivities in O₃ formation, with the OFP of 168-273 ppbv and the OH loss rates of 19.3-40.8 s^-1. Alkenes and aromatics primarily related to on-road and industrial emissions contributed 57.8%-76.3% to the total OFP. The contribution of aromatics to the SOA formation at all sites reached 94.1%-98.6%. Considering the potential source-area of VOCs, regional transport of VOCs occurred within the GZP cities.

Source profiles of molecular structure and light absorption of PM2.5 brown carbon from residential coal combustion emission in Northwestern China

Residential coal combustion is a prominent source of brown carbon (BrC) aerosols, but knowledge of their molecular structures and optical absorption were limited, which have notable used in ambient BrC source identification and radiative forcing calculation. In this study, the Fourier transform-ion cyclotron resonance mass spectrometry combined with partial least squares regression analysis as well as Fourier transform infrared spectroscopy analysis were used to insight the molecular compounds and structures of BrC from anthracite and bituminous coal combustions between traditional and improved stoves. The absorption Ångström exponents (AAE) and mass absorption efficiency (MAE) values for the BrC emitted from the combinations of bituminous were both 1.2-2.5 times lower than those of anthracite, interpreting that the BrC from the anthracite emissions had greater light-absorbing capacity. In contrast, the emission factor of light absorption (EF_Abs) at 365 nm for the bituminous coal combusted in the traditional stove was the highest among all the tested scenarios, which revealed that the incomplete combustion of bituminous coal could emit more BrC. It was noted that primary BrC emitted from the coal combustion with traditional stoves contains higher aromaticity groups of C-C and C＝O and higher S containing organics, whereas more aliphatic groups were found in BrC using the improved stoves. N-containing (CHON and CHONS) compounds were dominated in the total molecular formula of BrC, whereas the sum of CHON and CHO groups had high double-bond equivalent (DBE) values contributed 53.5%-87.1% to the total BrC absorption. Moreover, for CHOS, the lowest of estimated molecular absorption, DBE, and DBE/C should attribute to the non-chromophoric or weak absorptive S-containing compounds. This study supplied an effective evaluation method to compare BrC emissions and their absorption for coal combustion on regional scale.

Variations of Personal Exposure to Particulate Nitrated Phenols from Heating Energy Renovation in China: The First Assessment on Associated Toxicological Impacts with Particle Size Distributions

The clean heating renovation has been executed for improving particulate matter (PM) pollution in northern China since 2017. This study determined particle size distributions of nitrated phenols (NPs) in personal exposure samples and their associations with biomarkers in saliva and urine from homemakers in rural households of the Fenwei Plain, China. Remarkable reductions of 28.6-66.3% and 52.2-82.4% on PMs and total quantified NPs, respectively, were found with the substitutions of raw coal chunk and biomass by advanced clean coal. 4-Nitroguaiacol (4NG) showed the largest reductions of 81.2% among individual NP. In addition, the clean coal efficiently reduced interleukin-6 (IL-6) and 8-hydrox-2'-deoxyguanosine (8-OHdG) in the urine and saliva by 12-72%. Furthermore, significant positive correlations between urinary 8-OHdG with most of NPs in all particle sizes, urinary IL-6 with 4NG for particles with D_p > 2.5 μm and D_p = 0.25-1.0 μm and salivary IL-6 with 4-nitrocatechol and 4-methyl-5-nitrocatechol for particles with D_p > 2.5 μm, D_p = 0.5-1.0 μm, and D_p < 0.25 μm were observed but not for salivary 8-OHdG or PMs. The results provide scientific support for the clean energy reformation and demonstrate the strong particle size dependence between NPs and biomarkers.

Emission factors, characteristics, and gas-particle partitioning of polycyclic aromatic hydrocarbons in PM2.5 emitted for the typical solid fuel combustions in rural Guanzhong Plain, China

Solid fuel is a the most dominant energy source for household usages in developing countries. In this study, emission characteristics on organic carbon (OC), elemental carbon (EC) and fifty-two polycyclic aromatic hydrocarbons (PAHs) in gaseous and particulate phases from seven fuel-stove combinations were studied in a typical rural village in northwest China. For the PAHs, the highest gaseous and particulate phase emission factors (EFs) were both observed for bituminous coal with one-stage stoves, ranging from 459 ± 154 to 1.09 ± 0.36 × 10³ mg kg^-1. In contrast, the PAHs EFs for the clean briquette coal with two-stage stoves were two orders of magnitude lower than those of the bituminous coals. For parent PAHs (pPAHs) and total quantified PAHs (∑PAHs), they mainly contributed in gaseous phases with compositions of 69-79% and 64-70%, respectively. The gas-to-particle partitioning was mostly governed by the absorption. Moreover, the correlation coefficient (r) between EC and ∑PAHs, OC and parent PAHs (pPAHs), OC and nitro PAHs (nPAHs) were 0.81, 0.67 and 0.85, respectively, supporting that the PAHs species were potential precursors to the EC formation during the solid fuel combustion. The correlation analyses in this study further deduced that the formations of pPAHs and nPAHs were more closely related to that of OC than alkylated PAHs (aPAHs) and oxygenated PAHs (oPAHs). Diagnostic ratios of selective PAHs were calculated and evaluated as well. Among those, the ratio of retene (RET)/[RET + chyrene (CHR)] was found to be an efficient tool to distinguish coal combustion and biomass burning. In general, it was found that the amounts of pollutant emissions from clean briquette coal combustion were definitely lower than those from bituminous coal and biomass combustions. It is thus necessary to introduce and recommend the use of cleaner briquette coal as energy source.

Water-soluble brown carbon in atmospheric aerosols along the transport pathway of Asian dust: Optical properties, chemical compositions, and potential sources

As an important type of light-absorbing aerosol, brown carbon (BrC) has the potential to affect the atmospheric photochemistry and Earth's energy budget. A comprehensive field campaign was carried out along the transport pathway of Asian dust during the spring of 2016, including a desert site (Erenhot), a rural site (Zhangbei), and an urban site (Jinan), in northern China. Optical properties, bulk chemical compositions, and potential sources of water-soluble brown carbon (WS-BrC) were investigated in atmospheric total suspended particulate (TSP) samples. Samples from Zhangbei had higher mass absorption efficiency at 365 nm (MAE₃₆₅, 1.32 ± 0.34 m² g^-1) than those from Jinan (1.00 ± 0.23 m² g^-1) and Erenhot (0.84 ± 0.30 m² g^-1). Compere to the non-dust samples, elevated water-soluble organic carbon (WSOC) concentrations and MAE₃₆₅ values of dust samples from Erenhot are related to the input of high molecular weight organic compounds and biogenic matter from the Gobi Desert, while lower values from Zhangbei and Jinan are attributed to the dilution effect caused by strong northwesterly winds. Based on fluorescence excitation-emission matrix spectra and parallel factor analysis, two humic-like (C1 and C2) and two protein-like (C3 and C4) substances were identified. Together, C1 and C2 accounted for ~64% of total fluorescence intensity at the highly polluted urban Jinan site; C3 represented ~45% at the rural Zhangbei site where local biomass burning affects; and C4 contributed ~24% in the desert region (Erenhot) due to dust-sourced biogenic substances. The relative absorptive forcing of WS-BrC compared to black carbon at 300-400 nm was about 31.3%, 13.9%, and 9.2% during non-dust periods at Erenhot, Zhangbei, and Jinan, respectively, highlighting that WS-BrC may significantly affect the radiative balance of Earth's climate system and should be included in radiative forcing models.

Chemical source profiles of particulate matter and gases emitted from solid fuels for residential cooking and heating scenarios in Qinghai-Tibetan Plateau

Incomplete combustion of solid fuels (animal dung and bituminous coal) is a common phenomenon during residential cooking and heating in the Qinghai-Tibetan Plateau (QTP), resulting in large amounts of pollutants emitted into the atmosphere. This study investigated the pollutant emissions from six burning scenarios (heating and cooking with each of the three different fuels: yak dung, sheep dung, and bitumite) in the QTP's pastoral dwellings. Target pollutants such as carbon monoxide (CO), gas-phase polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), fine particles (PM_2.5, particulate matter with an aerodynamic diameter < 2.5 μm), carbonaceous aerosols, water-soluble ions, and particle-phase PAHs were investigated. Emission factors (EFs) (mean ± standard deviation) of PM_2.5 from the six scenarios were in the range of 1.21 ± 0.47-7.03 ± 1.95 g kg^-1, of which over 60% mass fractions were carbonaceous aerosols. The ratio of organic carbon to elemental carbon ranged from 9.6 ± 2.7-33.4 ± 11.5 and 81.7 ± 30.4-91.9 ± 29.0 for dung and bitumite burning, respectively. These values were much larger than those reported in the literature, likely because of the region's high altitudes-where the oxygen level is approximately 65% of that at the sea level-thus providing a deficient air supply to stoves. However, the toxicity and carcinogenicity of PAHs emitted from solid fuel combustion in the QTP are significant, despite a slightly lower benzo(a)pyrene-equivalent carcinogenic potency (Bap_eq) in this study than in the literature. The gas-to-particle partitioning coefficient of PAHs and VOC emission profiles in the QTP differed significantly from those reported for other regions in the literature. More attention should be paid to the emissions of PAH derivatives (oxygenated PAHs and nitro-PAHs), considering their enhanced light-absorbing ability and high BaP_eq from solid fuel combustion in the QTP.

Light absorption properties and molecular profiles of HULIS in PM2.5 emitted from biomass burning in traditional "Heated Kang" in Northwest China

Humic-like substances (HULIS) in PM_2.5 emitted from biomass burning (BB), including maize cob, wheat straw, maize straw, wood branch, and wood, in a traditional "Heated Kang" were investigated. The relative abundances, optical properties, chemical functional groups, and molecular components in HULIS were characterized using total organic carbon (TOC) analyzer, ultraviolet-visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR/MS), respectively. The emission factors (EF) of HULIS-C (in term of carbon weight, EF_HULIS-C) from BB were in the range of 0.83 to 5.17 g/kg fuel, with a mean value of 1.93 ± 1.31 g/kg fuel. The HULIS-C accounted for 15.0-37.8% and 9.1-12.6% of fractions in organic carbon (OC) and PM_2.5, respectively, suggesting that BB is an important emission source of atmospheric HULIS. The FT-IR spectra showed BB HULIS mainly contain O-containing, aliphatic CH, and aromatic CC functional groups. The presences of carboxyl group and OH band demonstrated the uniqueness of maize straw and wood burning. Moreover, the higher ratio of CH₃ and -CH₂ groups could be used to distinguish the wood branches from the maize cob. CHO and CHON were much dominant in BB HULIS, which accounted for 44.6-47.6% and 50.1-54.2%, respectively, to the total molecular mass. The positive correlation between MAE₃₆₅ and AAE in term of number concentration of CHNO implied that the CHNO species could greatly influence on the light absorption properties of the BB HULIS. The CHO and S-containing compounds (i.e., CHNOS and CHOS, that is CHNOS+CHOS) showed weak light absorbances of the BB HULIS. The BB HULIS from maize straw had relatively high molecular weight in comparison to that in other BB emissions. The highest and lowest aromaticity were seen on the wood burning and maize cob, respectively.

Saccharides Emissions from Biomass and Coal Burning in Northwest China and Their Application in Source Contribution Estimation

Saccharides are important tracers in aerosol source identification but results in different areas varied significantly. In this study, six saccharides (levoglucosan, arabitol, glucose, mannitol, inositol, and sucrose) were determined for their emission factors and diagnostic ratios from domestic combustion of typical biomass and coal fuels in Northwest China. Three types of coal (i.e., anthracitic coal, bituminous coal, and briquettes) and five types of biomass (i.e., maize straw, wheat straw, corn cob, wood branches, and wood block) collected from regional rural areas were selected. Overall, the ranking of the fuel types in terms of the emission factor of particulate matter less than 2.5 μm in diameter (PM2.5) was coal < firewood fuel < straw fuel, with a range of 0.14–36.70 g/kg. Furthermore, the emission factor (e.g., organic carbon (OC) levels) of traditional stove-Heated Kang in the Guanzhong Plain differed significantly from that of wood stoves burning the same fuel, which is attributable to differences in the combustion conditions. The combined diagnostic ratios of levoglucosan (LG)/OC and arabitol/elemental carbon can be used to accurately distinguish the source contribution from coal and biomass combustion to atmospheric PM. Estimation of the biomass burning (BB) contribution to PM2.5 had an uncertainty of −2.7% to 41.0% and overestimation of 9.9–28.2% when LG was used as the sole tracer, despite its widespread use in other studies; thus, these estimation methods are inadequate and require improvement. The results also revealed that specialized emission control and clean energy strategies are required for both residential BB and non-BB sources on a regional scale.

Spatial distribution and sources of winter black carbon and brown carbon in six Chinese megacities

The light-absorbing carbonaceous aerosols, including black carbon (BC) and brown carbon (BrC), influenced heavily on aerosol environmental quality and the Earth's radiation. Here, a winter campaign to characterize BC and BrC in PM_2.5 was conducted simultaneously in six Chinese megacities (i.e., Harbin, Beijing, Xi'an, Shanghai, Wuhan, and Guangzhou) using continual aethalometers. The combinations of advanced aethalometer and generalized additive model (GAM) were used to precisely quantify the BC and BrC sources in these megacities. The averaged light-absorbing coefficients of BC (b_abs-BC) and BrC (b_abs-BrC) were 28.6 and 21.8 Mm^-1 in northern cities, they were 1.4 and 2.7 times higher than those in southern cities. The BrC dominated the total b_abs (>40%) in northern cities but low to 20% in southern cities. On the other hand, the BC fractions were high in the southern cities, with the contributions of 62.4-79.7%, whereas much lower values of 53.7-59.4% in the northern cities. Source apportionment showed that the combustion of liquid fuels (e.g., gasoline or diesel) was highly dominant to b_abs-BC (>80%) in Guangzhou and Wuhan. This was further supported by the high NO₂ loadings in the GAM model. Solid fuels (i.e., biomass or coal) contributed a substantial portion to total b_abs-BC in the other four cities where the high abundances of primary b_abs-BrC were observed. The diurnal trend showed the peaks of secondary-BrC (b_abs-BrC_S) and b_abs-BrC_S/ΔCO in the northern cities occurred at high relative humidity in nighttime, implying the secondary BrC formation was possibly related to aqueous reactions in winter. In contrast, in the southern cities of Shanghai and Guangzhou, the accumulation of vehicle emissions during the morning traffic rush hours lead the formation of secondary BrC through photochemical reactions. The results of this work can be applied for the development of more effective practices to control BC and BrC on regional scale.

Spectral absorption properties of organic carbon aerosol during a polluted winter in Beijing, China

A fraction of organic carbon (OC) is found to exhibit the capability to absorb solar radiation. However, the absorption properties of OC remain poorly characterized partly due to uncertainties in determination methods. In this study, the absorption coefficient (b_ap) of OC (b_ap,OC) in Beijing during a polluted winter was estimated on the basis of the combined measurements of black carbon (BC) size distribution and total aerosol b_ap (b_ap,meas). The bare BC b_ap (b_ap,bareBC) calculated using Mie theory on the basis of measured size distribution exhibited weak wavelength dependence, with a mean absorption Ångström exponent (AAE) of 0.56 ± 0.04 within the 470-660 nm wavelength range, which was lower than the value of 1 commonly used for freshly emitted BC. The calculated b_ap,bareBC was compared with b_ap,meas at 950 nm to derive the coating thickness of BC, from which the calculation of coated BC b_ap (b_ap,coatBC) within 370-660 nm was based using the core-shell Mie model. Given the thick coatings, the AAE of coated BC, with a mean of 0.53 ± 0.12, was slightly lower than that of bare BC. Subsequently, b_ap,OC was obtained by subtracting b_ap,coatBC from b_ap,meas, accounting for 59.57 ± 4.82% of b_ap,meas at 370 nm on average. The average mass absorption efficiency of OC was estimated to be 1.48 ± 0.36 m² g^-1 at 370 nm. b_ap,OC significantly decreased as wavelength increased, deriving an AAE of OC with a mean of 2.72 ± 0.32 within the 370-660 nm range. The level of b_ap,OC estimated on the basis of a widely used attribution method assuming a constant BC AAE of 1 was ~60% lower than the currently presented value, probably underestimating OC radiative effect by a factor of >3. More accurate estimations of b_ap,OC based on more advanced measurements and suitable theory calculations are recommended to provide more reliable assessments of OC radiative effects.

Effects of combustion condition and biomass type on the light absorption of fine organic aerosols from fresh biomass burning emissions over Korea

In this study, the light absorption properties of fine organic aerosols from the burning emissions of four biomass materials were examined using UV-spectrophotometry and Aethalometer-measurements, respectively. For wood chips and palm trees, the burning experiments were carried out with different combustion temperatures (200, 250, and 300 ^οC) in an adjustable, electrically heated combustor. The light absorptions of water and methanol extracts of aerosols, and smoke particles showed strong spectral dependence on the burning emissions of all biomass materials. However, the burning aerosols of wood chips showed stronger absorption than those of the other biomass burning (BB) emissions. For the burning aerosols of wood chips and palm trees, organic carbon/elemental carbon (OC/EC) decreased as the combustion temperature increased from 200 to 300 °C. Absorption Ångström exponent (AAE) values tended to decrease when combustion temperature increased for smoke aerosols and methanol extracts in smoke samples. The mass absorption efficiency at 365 nm (MAE₃₆₅, m² g^-1∙C^-1) of water- and methanol-extractable OC fractions was highest in wood chip burning smoke samples. MAE₃₆₅ values of methanol extracts for rice straw, pine needles, wood chips, and palm trees burning emission samples were 1.35, 0.92, 2.36-3.37, and 0.86-1.42, respectively. For wood chip and palm tree burning emissions, AAE_320-430nm values of methanol extracts were strongly correlated with OC/EC (i.e., combustion temperature) with slopes of 0.11 (p < 0.001) and 0.02 (p < 0.001), and R² values of 0.87 and 0.74, respectively. Moreover, a linear regression between MAE₃₆₅ of methanol extractable OC and OC/EC showed slopes of -0.05 (p < 0.001) and -0.004 (p < 0.001) and R² of 0.72 and 0.74, respectively. The results of this study clearly demonstrate that burning condition and biomass type influence the light absorption properties of organic aerosols from BB emissions.

Recycled moisture in an enclosed basin, Guanzhong Basin of Northern China, in the summer: Contribution to precipitation based on a stable isotope approach

Recycled moisture, mainly originated from evapotranspiration (surface evaporation and transpiration), is the main sources of precipitation. Influenced on the different regional/local environments, the contributions of recycled moisture to precipitation present as different proportions. Recycled moisture has an important impact on the hydrological cycle, further occurred a series of environmental effect for regional/local. Aimed to estimate the contribution of recycled moisture to precipitation in an enclosed basin, Guanzhong Basin of northern China, precipitation and lake/reservoir samples were collected. The isotope ratio analysis was done for the summer season, and a three-component mixing model based on the stable hydrogen and oxygen isotopes was applied. The results indicated that the averaged contribution of recycled moisture to precipitation was 17.44% in Guanzhong Basin of northern China, while the mean proportions of surface evaporation moisture and transpiration moisture were found to be 0.38% and 16.97%, respectively. Comparatively, most of the recycled moisture mainly comes from transpiration moisture rather than evaporation moisture, suggesting that transpiration moisture from cropland, vegetation, and plants instead of evaporation is dominant in moisture recycling of the Guanzhong Basin.

Chemical Composition and Light Absorption of PM2.5 Observed at Two Sites near a Busy Road during Summer and Winter

To examine the difference in the major chemical composition of fine particulate matter (PM2.5) between two roadway sites, 24 h integrated PM2.5 samples were simultaneously collected both 15 m (Buk-Ku District Office (BKO) site) and 150 m (Chonnam National University campus (CNU) site) away from busy roads during the summer and winter periods; these samples were taken to determine the concentrations of organic and elemental carbon (OC and EC), water-soluble organic carbon (WSOC), and water-soluble inorganic species. In addition, the real-time aerosol light absorption coefficients (Abs) were measured using a dual-spot seven-wavelength aethalometer at the CNU site to evaluate the influence of traffic and biomass burning (BB) emissions on the concentrations of organic aerosol particles. The hourly NO2 concentration was also observed at an air pollution monitoring network that is about 2 km away from the CNU site. During summer, 24 h PM2.5 concentrations (PM2.5 episode) which exceeded the Korean PM2.5 standard (35 μg/m3) were linked to increases in organic matter (OM) and SO42− concentrations that accounted for on average 35–41% and 26–30%, respectively, of the PM2.5 at the two sites. The increased SO42− concentration was most likely attributable to the inflow of long-range transported aerosols, rather than local production, as demonstrated by both the MODIS (Moderate Resolution Imaging Spectroradiometer) images and transport pathways of air masses reaching the sites. On the other hand, the OM, WSOC, and EC concentrations were directly attributable to traffic emissions at the sampling sites, as supported by the tight correlation between the OC and EC. A small difference between the absorption Ångström exponent (AAE) values calculated at wavelengths of 370–950 nm (AAE370–950nm) and 370–520 nm (AAE370–520nm), and the poor correlation of absorption coefficient by brown carbon (BrC) at 370 nm (AbsBrC370nm) with K+ (R2 = 0.00) also suggest a significant contribution of traffic emissions to OM. However, the wintertime PM2.5 episode was strongly related to the enhanced OM and NO3− concentrations, which contributed 26–28% and 22–23% of the PM2.5 concentration, respectively. It is interesting to note that there were two distinct OC/EC ratios in winter: a lower OC/EC (~3.0), which indicates a significant contribution of traffic emissions to the OC and EC, and a higher OC/EC (~6.5), which suggests an additional influence of BB emissions as well as traffic emissions at the sites. Strong correlations between the OC and EC (R2 = 0.72–0.83) and the enhanced AAE370–520nm values compared to the AAE370–950nm support that BB emissions were also an important contributor to the wintertime OM concentrations as well as traffic emissions at the two sites. A good correlation between the gaseous NO2 and NO3− and meteorological conditions (e.g., low wind speed and high relative humidity) suggest that the heterogeneous oxidation of NO2 on moist particles could be an important contributor to wintertime particulate NO3− formation at the sites. The OC concentrations during summer and winter were higher at the BKO site, with a higher traffic flow and a shorter distance from the roadway than at the CNU site. However, there were slight differences in the concentrations of secondary inorganic species (NO3−, SO42−, and NH4+) between the sites during summer and winter.

Aerosol-photolysis interaction reduces particulate matter during wintertime haze events

Aerosol-radiation interaction (ARI) plays a significant role in the accumulation of fine particulate matter (PM_2.5) by stabilizing the planetary boundary layer and thus deteriorating air quality during haze events. However, modification of photolysis by aerosol scattering or absorbing solar radiation (aerosol-photolysis interaction or API) alters the atmospheric oxidizing capacity, decreases the rate of secondary aerosol formation, and ultimately alleviates the ARI effect on PM_2.5 pollution. Therefore, the synergetic effect of both ARI and API can either aggravate or even mitigate PM_2.5 pollution. To test the effect, a fully coupled Weather Research and Forecasting (WRF)-Chem model has been used to simulate a heavy haze episode in North China Plain. Our results show that ARI contributes to a 7.8% increase in near-surface PM_2.5 However, API suppresses secondary aerosol formation, and the combination of ARI and API results in only 4.8% net increase of PM_2.5 Additionally, API increases the solar radiation reaching the surface and perturbs aerosol nucleation and activation to form cloud condensation nuclei, influencing aerosol-cloud interaction. The results suggest that API reduces PM_2.5 pollution during haze events, but adds uncertainties in climate prediction.

Investigation of Primary and Secondary Particulate Brown Carbon in Two Chinese Cities of Xi'an and Hong Kong in Wintertime

Brown carbon (BrC), an aerosol carbonaceous matter component, impacts atmospheric radiation and global climate because of its absorption in the near-ultraviolet-visible region. Simultaneous air sampling was conducted in two megacities of Xi'an (northern) and Hong Kong (southern) in China in winter of 2016-2017. The aim of this study is to determine and characterize the BrC compounds in collected filter samples. Characteristic absorption peaks corresponding to aromatic C-C stretching bands, organo-nitrates, and C═O functional groups were seen in spectra of Xi'an samples, suggesting that the BrC was derived from freshly smoldering biomass and coal combustion as well as aqueous formation of anthropogenic secondary organic carbon. In Hong Kong, the light absorption of secondary BrC accounted for 76% of the total absorbances of BrC. The high abundance of strong C═O groups, biogenic volatile organic compounds (BVOCs) and atmospheric oxidants suggest secondary BrC was likely formed from photochemical oxidation of BVOCs in Hong Kong. Several representative BrC molecular markers were detected using Fourier transform ion cyclotron resonance mass spectrometry and their absorption properties were simulated by quantum chemistry. The results demonstrate that light absorption capacities of secondary anthropogenic BrC with nitro-functional groups were stronger than those of biogenic secondary BrC and anthropogenic primary BrC.

Optical properties and source identification of black carbon and brown carbon: comparison of winter and summer haze episodes in Xi'an, Northwest China

Summer and winter fine particulate matter (PM2.5) samples were collected to provide insight into the seasonal variations of the optical properties and source profiles of PM2.5 black carbon (BC) and brown carbon (BrC) in Xi'an, China. The average PM2.5 mass concentration during the winter haze (WH) period was 292.5 μg m-3, which was 2.6, 5.0 and 9.2 times higher than that during winter non-haze (WNH), summer haze (SH), and summer non-haze (SNH) periods, respectively. Regarding optical properties, the PM2.5 chemical-derived light extinction coefficient was the highest during the WH period (1019.2 Mm-1) and decreased by approximately one-fourth in the SH period (237.6 Mm-1). During the WH period, the light absorption coefficient of BC (babs-BC) was considerably higher than that during the SH period; this is attributable to the thick coatings of inorganic ions on BC and intensive fossil fuel and biomass burning emissions in winter. Source apportionment also proved that fossil fuels were the major emission source of BC in SH and WH periods with high light absorption coefficient babs_FF (fossil fuel) fractions (>70%). Biomass burning contributed to 25.8% of BC in the winter haze period, but to only 5.4% of BC in the summer haze period. The mass absorption coefficient of BC (MAC-BC) was higher in summer, as it was considerably influenced by vehicle emissions, whereas it was lower in winter due to the strong influences of biomass burning. Moreover, the high light absorption coefficient of BrC (babs-BrC) in both WH and WNH indicated substantial light absorption during winter; however, this coefficient was considerably lower in summer. A remarkable difference in the diurnal pattern of haze between babs-BrC and babs-BC indicated that BC leads to a severe visibility reduction during traffic rush hours. In addition, the BrC abundance observed in Xi'an revealed different diurnal patterns in WH and SH periods, which can be attributed to different secondary formation processes. SH BrC was generally contributed by photochemical-derived secondary organic carbon (SOC) whereas the abundant WH BrC was mainly transformed from aqueous-SOC.

High time resolution observation of PM2.5 Brown carbon over Xi'an in northwestern China: Seasonal variation and source apportionment

There is growing evidence suggesting the enhancement of brown carbon (BrC) in severe haze episodes. In this study, hourly measurements of BrC in PM_2.5 were conducted in Xi'an, a typical city in northwestern China during winter and summer. The absorption coefficient for methanol exacts at 365 nm (b_{abs365, methanol}, which is typically used as a proxy of methanol-soluble BrC) in the winter sampling period was over 7 times than that in summer. The mass absorption cross-section for methanol extracts (MAC_{365, methanol}, normalized by b_{abs365, methanol} to organic carbon, OC) in winter sampling period was nearly 1.5 times of that in the summer. During the winter haze days, the average b_{abs365,methanol} peaked at midnight and the lowest values in the morning, in contrast to high levels in afternoon and low levels at night in non haze days. Unlike the diurnal patterns in winter, summer b_{abs365, methanol} diurnal variation presented high midday and low afternoon levels in haze days. However, in non haze days, the pattern showed high morning levels and night low levels. Haze and non haze variations of chemical species levels, b_{abs365, methanol}, and MAC_{365, methanol} during winter and summer sampling time showed that the effects of atmospheric aging were complex and could either enhance or reduce light absorption of BrC. Source apportionment based on positive matrix factorization receptor model and multiple linear regressions showed that primary emission was an important contributor to BrC emissions during the winter sampling period, whereas secondary formation played an important role in summer.

Brown carbon in the continental outflow to the North Indian Ocean

In this paper, we synthesize the size distribution and optical properties of the atmospheric water-soluble fraction of light-absorbing organic carbon (brown carbon; BrC) in the continental outflow from the Indo-Gangetic Plain (IGP) in South Asia to the North Indian Ocean. A comparison of the mass absorption coefficient of water-soluble BrC (babs-WSBrC-365nm) in PM2.5 with that in PM10 sampled over the Bay of Bengal reveals the dominance of BrC in fine mode. Furthermore, the babs-BrC-365nm shows a significant linear relationship with mass concentrations of airborne particulate matter, water-soluble organic carbon and non-sea-salt-K+ in the continental outflow from the IGP. This observation emphasizes the ubiquitous nature and significant contribution of water-soluble BrC from biomass burning emissions (BBEs). Comparing the absorption properties from this study with global datasets, it is discernible that BBEs dominate BrC absorption. Furthermore, the imaginary refractive index of water-soluble BrC (kWSBrC-365nm) in marine aerosols sampled over the North Indian Ocean during November is significantly higher than during December to January. Thus, significant temporal variability is associated with crop-residue burning emissions in the IGP on the composition of BrC over the North Indian Ocean. Our estimates show that the babs-WSBrC-365nm and kWSBrC-365nm from post-harvest crop-residue burning emissions in the IGP are much higher than the BBEs from the southeastern United States and Amazonian forest fires. Another major finding of this study is the lack of significant relationship between kWSBrC-365nm and the mass ratio of elemental carbon to particulate organic matter, as previously suggested by chamber experiments to model varying BrC absorption properties in ambient aerosols. Therefore, considerable spatio-temporal variability prevails among emission sources (wood burning vs. crop-residue burning), which needs to be considered when assessing the regional radiative forcing of BrC relative to major absorbing elemental carbon.