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Water-insoluble organic carbon (WIOC) constitutes a substantial portion of organic carbon (OC) and contributes significantly to light absorption by brown carbon (BrC), playing pivotal roles in climate forcing. China is a hotspot region with high levels of OC and BrC, but information regarding the sources and light-absorbing properties of WIOC on a national scale remains scarce. Here, we investigated the light-absorbing properties and sources of WIOC in 10 representative urban cities in China. On average, WIOC made up 33.4 ± 7.66 % and 40.5 ± 9.73 % of concentrations and light absorption at 365 nm (Abs 365) of extractable OC (EX-OC), which includes relatively hydrophobic OC (WIOC and humic-like substances, HULIS-C) and hydrophilic OC (non-humic-like substances, non-HULIS-C). The mass absorption efficiency of WIOC at 365 nm (MAE 365) was (1.59 ± 0.55 m2(gC)-1) comparable to that of HULIS (1.54 ± 0.57 m2(gC)-1) but significantly higher than non-HULIS (0.71 ± 0.28 m2(gC)-1), indicating that hydrophobic OC possesses a stronger light-absorbing capacity than hydrophilic OC. Biomass burning (31.0 %) and coal combustion (31.1 %) were the dominant sources of WIOC, with coal combustion sources exhibiting the strongest light-absorbing capacity. Moreover, employing the simple forcing efficiency (SFE 300–700nm) method, we observed that WIOC exhibited the highest SFE 300–700nm (6.57 ± 5.37 Wg-1) among the EX-OC fractions. The radiative forcing of EX-OC was predominantly contributed by hydrophobic OC (WIOC – 39.4 ± 15.5 % and HULIS – 39.5 ± 12.1 %). Considering the aromaticity, sources, and atmospheric processes of different carbonaceous components, we propose a light-absorbing carbonaceous continuum, revealing that components enriched with fossil sources tend to possess stronger light-absorbing capacity, higher aromatic levels, increased molecular weights, and greater recalcitrance in the atmosphere. Reducing fossil fuel emissions emerges as an effective means of mitigating both gaseous (CO2) and particulate light-absorbing carbonaceous warming components. [ABSTRACT FROM AUTHOR]

The optical properties of secondary brown carbon (BrC) aerosols are poorly understood, hampering quantitative assessments of their impact. We propose a new method for estimating secondary source of BrC using excitation‐emission matrix (EEM) fluorescence spectroscopy, combined with parallel factor analysis (PARAFAC) and partial least squares regression (PLSR). Experiments were conducted on a collection of PM2.5 samples from urban areas in five Chinese cities during winter and summer. The humic‐like component with long‐emission wavelengths (L‐HULIS) was identified as a secondary source tracer of BrC. This was confirmed by correlating PARAFAC components with secondary organic aerosol tracers and molecular oxidation indices obtained from Fourier transform ion cyclotron resonance mass spectrometry analysis. Using L‐HULIS as a secondary tracer of BrC, it was determined that the contribution of secondary sources to water‐soluble BrC (WS‐BrC) in source emission samples is significantly smaller than in PM2.5 from five Chinese cities, supporting our method. In the five cities, secondary source derived via L‐HULIS contributes a dominant potion (80% ± 3.5%) of WS‐BrC at 365 nm during the summer, which is approximately twice as high as during the winter (45% ± 4.9%). Radiocarbon isotope (14C) analysis provides additional constraints to the sources of L‐HULIS‐derived secondary WS‐BrC in urban PM2.5, suggesting that aged biomass burning is the dominant contributor to secondary WS‐BrC in winter, and biogenic emission is dominant during summer. This study is the first report on identification of secondary sources of BrC using the fluorescence technique. It demonstrates the potential of this method in characterizing non‐fossil source secondary BrC in the atmosphere. Plain Language Summary: Brown carbon (BrC) originates from primary combustion emissions and secondary formation, with large source‐dependent uncertainties of radiative forcing. Direct measurements to separate the primary and secondary BrC are challenging due to the chemical complexity. Recent online studies have shown that excitation‐emission matrix fluorescence spectroscopy coupled with parallel factor (PARAFAC) analysis identified some fluorescent components that may be linked to secondary sources. However, there is a knowledge gap on whether PARAFAC components correlate closely with atmospheric secondary chemical components, particularly biogenic and anthropogenic secondary organic aerosol, as their precursors can also form secondary BrC chromophores. We established the correlations between PARAFAC components and secondary organic aerosol tracers and compound oxidations to identify the long‐emission‐wavelength humic‐like component as a secondary source tracer of BrC. Then, we estimated non‐fossil source secondary BrC in urban aerosols during the winter and summer. Our studies provide references for quantifying secondary sources of BrC in the atmosphere. Key Points: A fluorescence‐based method was developed to investigate secondary sources of water‐soluble brown carbon in five cities in ChinaThe contribution of secondary sources to water‐soluble brown carbon in the summer is approximately twice as high as during the winterThis secondary water‐soluble brown carbon was more associated with aging biomass burning in winter and biogenic emissions in summer [ABSTRACT FROM AUTHOR]

Water-insoluble carbon (WIOC) constitutes a substantial portion of organic carbon (OC) and contributes significantly to light absorption by brown carbon (BrC), playing pivotal roles in climate forcing and human health. China as hotspots regions with high level of OC and BrC, information regarding the sources and light-absorbing properties of WIOC on national scale remains scarce. Here, we investigated the light-absorbing properties and sources of WIOC in ten representative urban cities across China. On average, WIOC accounted for 33.4 ± 7.66 % and 40.5 ± 9.73 % of the concentrations and light-absorbing efficiency at 365 nm (Abs365) of extractable OC (EX-OC, comprising relatively hydrophobic OC [WIOC and humic-like substances: HULIS-C], and hydrophilic OC [non-humic-like substances: non-HULIS-C]). The mass absorption efficiency of WIOC at 365 nm (MAE365) was (1.59 ± 0.55 m2/gC) comparable to that of HULIS (1.54 ± 0.57 m2/gC) but significantly higher than non-HULIS (0.71 ± 0.28 m2/gC), indicating that hydrophobic OC possesses a stronger light-absorbing capacity than hydrophilic OC. Biomass burning (31.0 %) and coal combustion (31.1 %) were the dominant sources of WIOC, with coal combustion sources exhibited the strongest light-absorbing capacity. Moreover, employing the simple forcing efficiency (SFE300–700nm) method, we observed that WIOC exhibited the highest SFE300–700nm (6.57 ± 5.37 W/g) among the EX-OC fractions. The radiative forcing of EX-OC was predominantly contributed by hydrophobic OC (WIOC: 39.4 ± 15.5 % and HULIS: 39.5 ± 12.1 %). Considering the aromaticity, sources, and atmospheric processes of different carbonaceous components, we propose a light-absorbing carbonaceous continuum, revealing that components enriched with fossil sources tend to possess stronger light-absorbing capacity, higher aromatic levels, increased molecular weights, and greater recalcitrance in the atmosphere. Reducing fossil fuel emissions emerges as an effective means of mitigating both gaseous (CO2) and particulate light-absorbing carbonaceous warming components. [ABSTRACT FROM AUTHOR]

Humic‐like substances (HULIS) are significant contributor to the light absorption of water‐soluble brown carbon (WSBrC), which contains certain strong light‐absorbing chemical components that are not well understood, impeding the assessment of WSBrC's climate impact. China as the hotspot regions with high loading of WSBrC characterized by high light‐absorbing capacity, here, we investigated the sources and atmospheric processes (δ13C–Δ14C), molecular composition (Fourier transform ion cyclotron resonance mass spectrometry), and light absorption properties (UV spectrophotometry) of HULIS in PM2.5 from 10 Chinese cities. HULIS‐C was major contributor to the light absorption coefficient (70.5 ± 6.6%) of WSBrC at 365 nm, which was more enriched with fossil sources (48.0 ± 9.0% vs. 30.3 ± 13.9%) but depleted in 13C (δ13C: −25.6 ± 0.9‰ vs. −22.4 ± 1.0‰) relative to non‐HULIS‐C. This suggests that the fossil components in HULIS are more recalcitrant to oxidative aging and exhibit higher light‐absorbing capacity, while the non‐fossil organic carbon is more likely to be oxidatively bleached into small, colorless, and highly polar molecules (i.e., non‐HULIS). Aromatic components are the major strong light‐absorbing fossil components in HULIS, dominantly originating from coal combustion (>77%). Non‐negative matrix factorization model showed that aromatic molecules from coal combustion have higher molecular weight and lower oxidation levels than biomass burning, potentially making them to be photo‐recalcitrant compounds. Our finding that coal combustion‐derived BrC maybe more persistent in the atmosphere and has greater long‐term impact on climate than BrC derived from biomass burning is an important consideration in climate models and mitigation policies. Plain Language Summary: Both the loading and light‐absorbing capacity of water‐soluble brown carbon (WSBrC) are high in China, but little is known about sources and atmospheric processes of strong light‐absorbing components in WSBrC. Humic‐like substances (HULIS) account for a significant fraction of light absorption by WSBrC, which may contain strong light‐absorbing components. Through dual carbon isotopic analysis, we discovered that HULIS‐C from 10 Chinese cities had relatively higher fossil sources contribution but lower δ13C values compared to non‐HULIS‐C. This indicates that the fossil components in HULIS not only possess a higher light‐absorbing capacity but are also more resistant to oxidative aging than non‐fossil components. The higher light‐absorbing capacity and resilience of fossil components can be attributed to the enrichment of aromatic compounds derived from coal combustion. Using the non‐negative matrix factorization model, we found that coal combustion tends to generate aromatic compounds with higher molecular weight and lower oxidation‐to‐carbon (O/C: 0.31 ± 0.03 vs. 0.50 ± 0.07) ratios than biomass burning. This characteristic may contribute to the recalcitrant property of coal combustion‐derived HULIS. Our findings highlight that coal combustion‐derived BrC may persists longer in the atmosphere and has a greater long‐term impact on climate than that derived from biomass burning. [ABSTRACT FROM AUTHOR]

The Bohai Sea (BS) as the most polluted area of PAHs in China has been received wide attention in recent decades. To characterize the variations of concentrations and sources of PAHs from June 2014 to May 2019, fifteen congeners of PAHs (∑15PAHs) were measured from atmospheric samples (N=228) collected at 12 sites around the BS, and health risk and direct medical costs associated with lung cancer exposed to PAHs were also estimated. The annual daily average concentration of ∑15PAHs was 56.8 ± 4.7 ng m-3, dominated by low molecular weight (LMW-PAHs, 3-ring) (58.9 ± 7.8 %). During the five-year sampling period, the atmospheric ∑15PAHs concentration reduced by 17.5 % for the whole BS, especially in the tightly controlled area of Tianjin (TJ) with a drop of 51.7 %, which was mainly due to the decrease of high molec-ular weight PAHs (HMW-PAHs, 5-6 ring) concentration. Generally, the concentration of ∑15PAHs was highest in winter and lowest in summer, mainly attributed to the change of LMW-PAHs concentration. Based on PMF model, PAHs at the BS were mainly ascribed to coal combustion and biomass burning. And the contribution of coal combustion and motor vehicle to PAHs had a different performance between the BS (coal combustion rose by 6.7 %, motor vehi-cle fell by 22.7 %) and TJ (coal combustion fell by 13.2 %, motor vehicle rose by 6.7 %). The in-cidence of lung cancer (ILCR) caused by exposing to atmospheric PAHs at the BS and TJ de-creased by 74.1 % and 91.6 % from 2014 to 2018, respectively. That was mainly due to the de-crease of the concentration of highly toxic HMW-PAHs. It was reflected on the savings of $10.7 million in direct medical costs of lung cancer exposed PAHs, which was accounted 46.1 % before air prevention and control around the BS. And there was a higher cost reduction of 54.5 % in TJ. Hence, this study proved that implementing pollution prevention and control not only effective-ly reduced the concentration of pollutants and the caused risks, but also significantly reduced the medical costs of diseases caused by corresponding expose. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen‐containing organic compounds (NOCs) are critical components of global nitrogen deposition and light‐absorption species. The sources and compositions of NOCs are complex and remain largely unknown. Here, NOCs in 55 ambient aerosol samples collected in Guangzhou, South China, were analyzed via ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometry in negative‐ion and positive‐ion electrospray ionization (ESI) modes. The molecular compositions of NOCs measured via ESI– and ESI+ exhibited considerable differences. NOCs detected in the negative mode were mainly composed of highly oxygenated organic nitrates (O/N = 6), whereas NOCs detected in the positive mode were mainly composed of reduced nitrogen‐containing compounds (e.g., amides and amino acids). CHN compounds potentially corresponding to amines and alkaloids showed low abundance in the detection modes. Non‐metric multidimensional scaling and individual compound correlation analyses showed that the molecular compositions of NOCs were mainly affected by anthropogenic activities and meteorological parameters. For example, anthropogenic activities such as biomass burning and secondary nitrogen‐chemistry processes led to the accumulation of aromatic and highly oxygenated NOCs during winter. During summer, higher OH radical concentrations and temperatures will result in more prevalent or persistent reduced aliphatic NOCs, particularly lipid‐like amines. Some variables (e.g., relative humidity) have distinct effects on the variation of different types of NOCs. More research is needed to reveal the influencing mechanisms. This study clarifies the molecular compositions of NOCs and the mechanisms by which various factors influence the molecular variations. The findings can guide the assessment of NOCs evolution and deposition. Plain Language Summary: Negative and positive electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (ESI‐FT‐ICR‐MS) was used to characterize the molecular composition of nitrogen‐containing organic compounds (NOCs) from aerosol samples in Guangzhou. It was determined that a large amount of various NOCs are present in the samples, with NOCs in highly oxidative and reduced states can be roughly identified by distinct ionization modes. By combining the ESI‐FT‐ICR‐MS results and statistical analysis, the factors that associated with the distribution of NOCs and how these factors worked were identified. Anthropogenic activities and meteorological parameters (e.g., temperature and oxidation levels) are the two important drivers that led to the variations of NOCs, resulting to inverse molecular patterns with different response to aromatic/highly unsaturated and saturated structures, respectively. This study provides a useful clue for studying NOCs in complex system, bridges the links between the filed observation and knowledge about sources and formation pathways of NOCs that derived from controlled laboratory studies. Key Points: Most of identified CHON− species had O/N ≥ 3 and showed higher O/C ratios than CHON+ species; CHN only constituted small fraction of nitrogen‐containing organic compounds (NOCs)Biomass burning and secondary inorganic‐nitrogen processes lead to the accumulation of aromatic and phenolic NOCs during winterHigher OH radical concentrations and temperatures result in more prevalent or persistent reduced aliphatic NOCs in summer [ABSTRACT FROM AUTHOR]

Polyaromatic organic matter (POM) is an important group of pollutants and light absorbers in ambient aerosols, which consists of a wide range of chemicals with fused benzene rings. POM in ambient aerosols is mainly derived from the incomplete combustion of fossil fuel and biomass. Source apportionment of POM is crucial for advising efficient mitigation of anthropogenic emissions, but that is a challenge due to the complicated composition of POM and ambient aerosols. Benzene polycarboxylic acids (BPCAs) have recently been introduced as molecular markers of atmospheric POM. Compound-specific radiocarbon analysis of BPCAs is expected to be a powerful tool for apportioning fossil sources and contemporary (i.e., biomass burning) sources of POM in ambient aerosols, yet this application is still lacking. We developed a method for radiocarbon analysis of BPCAs substituted with 3–6 carboxylic groups (B6CA, B5CA, B4CAs and B3CAs). BPCAs were isolated with preparative liquid chromatography with high recoveries (≥85%). The method is validated with reference materials with fossil and contemporary radiocarbon signatures. Successful radiocarbon analysis of BPCAs was achieved for these reference materials after correcting for the presence of average blanks (B6CA: 1.2 ± 0.2 μg, B5CA: 2.3 ± 0.6 μg, individual B4CAs: 2.7 ± 0.2 μg and individual B3CAs: 6.9 ± 0.7 μg). Source apportionment of POM based on radiocarbon analysis of BPCAs was performed for an urban dust reference material, as well as bulk aerosols and water-soluble, methanol-soluble fractions of ambient aerosols from a megacity of southern China. The results show varied contributions of fossil and contemporary sources among different POM fractions. [Display omitted] • Benzene polycarboxylic acids (BPCAs) are used to track polyaromatic organic matter (POM). • A method of compound-specific radiocarbon analysis (CSRA) of BPCAs is developed. • Successful CSRA of BPCAs is achieved for reference and ambient materials. • Sources of POM in ambient aerosols are apportioned by CSRA of BPCAs. • Fossil-contemporary source patterns vary between different POM fractions. [ABSTRACT FROM AUTHOR]

Black carbon (BC) is an important component of the brown clouds, contributing to climate and health effects of air pollution. In this work, we developed a new method for measuring BC concentrations and its radiocarbon isotope (14C) using quartz wool disk passive air samplers (Pas-QW). A field calibration study was conducted at two sampling sites (Guangzhou and Yantai), using an improved thermal/optical method to measure BC. Uptake profiles of Pas-QW were linear over the 77 days that the samplers were deployed, and the average sampling rate of BC measured was 1.1 ± 0.2 m3/day. The method was found to be reproducible with coefficients of variation as low as 17% for BC measured in ambient passive samples. Furthermore, passive sampling was carried out at six sites in the Indo-China Peninsula (ICP) from January to April and June to September of 2016. The high BC concentrations and the high fraction of modern carbon (f M) values emphasizes that ICP is still dominated by biomass burning emissions (residential cooking and agricultural burning), both in dry and wet seasons. This study provides a proof concept and methodology for the application of Pas-QW to cost-effectively expand measurements of BC at the global scale. [Display omitted] • A new method for measuring BC and its 14C using quartz wool disk passive samplers. • The method requires no sample processing and have good reproducibility for BC. • The relative contribution of biomass burning to BC can reach 67 ± 13% in ICP, with obvious spatiotemporal heterogeneity. [ABSTRACT FROM AUTHOR]

[Display omitted] Field-based sampling can provide more accurate evaluation than MODIS in regional biomass burning (BB) emissions given the limitations of MODIS on unresolved fires. Polyurethane foam-based passive air samplers (PUF-PASs) are a promising tool for collecting atmospheric monosaccharides. Here, we deployed PUF-PASs to monitor monosaccharides and other BB-related biomarkers and presented a dataset of 31 atmospheric BB-related biomarkers in the Indo-China Peninsula (ICP) and Southwest China. The peak concentrations of monosaccharides in the ICP occurred before monsoon season. The highest concentrations were in the eastern Mekong plain, while the lowest were along the eastern coast. BB-related biomarkers displayed elevated concentrations after April, particularly in the monsoon season; however, fewer active fires were recorded by MODIS. This revealed the importance of MODIS unresolved fires (e.g., indoor biofuel combustion, small-scale BB incidents, and charcoal fires) to the regional atmosphere. The PAS derived levoglucosan concentrations indicated that, with the inclusion of MODIS unresolved fires, the estimated top-down emissions of PM (4194–4974 Gg/yr), OC (1234–1719 Gg/yr) and EC (52–384 Gg/yr) would be higher than previous bottom-up estimations in the ICP. Future studies on these MODIS unresolved fires and regional monitoring data of BB are vital for improving the modeling of regional BB emissions. [ABSTRACT FROM AUTHOR]