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Nitrated aromatic compounds (NACs) are a group of key chromophores for brown carbon (light-absorbing organic carbon, i.e., BrC) aerosol, which affects radiative forcing. The chemical composition and sources of NACs and their contributions to BrC absorption, however, are still not well understood. In this study, PM 2.5 -bound NACs in Xi'an, Northwest China, were investigated for 112 daily PM 2.5 filter samples from 2015 to 2016. Both the total concentrations and contributions from individual species of NACs show distinct seasonal variations. The seasonally averaged concentrations of NACs are 2.1 (spring), 1.1 (summer), 12.9 (fall), and 56 ng m -3 (winter). Thereinto, 4-nitrophenol is the major NAC component in spring (58 %). The concentrations of 5-nitrosalicylic acid and 4-nitrophenol dominate in summer (70 %), and the concentrations of 4-nitrocatechol and 4-nitrophenol dominate in fall (58 %) and winter (55 %). The NAC species show different seasonal patterns in concentrations, indicating differences in emissions and formation pathways. Source apportionment results using positive matrix factorization (PMF) further show large seasonal differences in the sources of NACs. Specifically, in summer, NACs were highly influenced by secondary formation and vehicle emissions (∼ 80 %), while in winter, biomass burning and coal combustion contributed the most (∼ 75 %). Furthermore, the light absorption contributions of NACs to BrC are wavelength-dependent and vary greatly by season, with maximum contributions at ∼ 330 nm in winter and fall and ∼ 320 nm in summer and spring. The differences in the contribution to light absorption are associated with the higher mass fractions of 4-nitrocatechol (λmax⁡= 345 nm) and 4-nitrophenol (λmax⁡= 310 nm) in fall and winter, 4-nitrophenol in spring, and 5-nitrosalicylic acid (λmax⁡= 315 nm) and 4-nitrophenol in summer. The mean contributions of NACs to BrC light absorption at a wavelength of 365 nm in different seasons are 0.14 % (spring), 0.09 % (summer), 0.36 % (fall), and 0.91 % (winter), which are about 6–9 times higher than their mass fractional contributions of carbon in total organic carbon. Our results indicate that the composition and sources of NACs have profound impacts on the BrC light absorption. [ABSTRACT FROM AUTHOR]

To mitigate haze pollution in China, a better understanding of the sources of carbonaceous aerosols is required due to the complexity in multiple emissions and atmospheric processes. Here we combined the analysis of radiocarbon and the stable isotope 13 C to investigate the sources and formation of carbonaceous aerosols collected in two Chinese megacities (Beijing and Xi'an) during severe haze events of a "red alarm" level from December 2016 to January 2017. The haze periods with daily PM 2.5 concentrations as high as ∼ 400 µ g m -3 were compared to subsequent clean periods (i.e., PM 2.5 less than median concentrations during the winter 2016/2017) with PM 2.5 concentrations below 100 µ g m -3 in Xi'an and below 20 µ g m -3 in Beijing. In Xi'an, liquid fossil fuel combustion was the dominant source of elemental carbon (EC; 44 %–57 %), followed by biomass burning (25 %–29 %) and coal combustion (17 %–29 %). In Beijing, coal combustion contributed 45 %–61 % of EC, and biomass burning (17 %–24 %) and liquid fossil fuel combustion (22 %–33 %) contributed less. Non-fossil sources contributed 51 %–56 % of organic carbon (OC) in Xi'an, and fossil sources contributed 63 %–69 % of OC in Beijing. Secondary OC (SOC) was largely contributed by non-fossil sources in Xi'an (56±6 %) and by fossil sources in Beijing (75±10 %), especially during haze periods. The fossil vs. non-fossil contributions to OC and EC did not change drastically during haze events in both Xi'an and Beijing. However, compared to clean periods, the contribution of coal combustion to EC during haze periods increased in Xi'an and decreased in Beijing. During clean periods, primary OC from biomass burning and fossil sources constituted ∼ 70 % of OC in Xi'an and ∼ 53 % of OC in Beijing. From clean to haze periods, the contribution of SOC to total OC increased in Xi'an but decreased in Beijing, suggesting that the contribution of secondary organic aerosol formation to increased OC during haze periods was more efficient in Xi'an than in Beijing. In Beijing, the high SOC fraction in total OC during clean periods was mainly due to an elevated contribution from non-fossil SOC. In Xi'an, a slight day–night difference was observed during the clean period with enhanced fossil contributions to OC and EC during the day. This day–night difference was negligible during severe haze periods, likely due to the enhanced accumulation of pollutants under stagnant weather conditions. [ABSTRACT FROM AUTHOR]

The impact of brown carbon aerosol (BrC) on the Earth's radiative forcing balance has been widely recognized but remains uncertain, mainly because the relationships among BrC sources, chromophores and optical properties of aerosol are poorly understood. In this work, the light absorption properties and chromophore composition of BrC were investigated for samples collected in Xi'an, northwestern China, from 2015 to 2016. Both absorption Ångström exponent (AAE) and mass absorption efficiency (MAE) show distinct seasonal differences, which could be attributed to the differences in sources and chromophore composition of BrC. Three groups of light-absorbing organics were found to be important BrC chromophores, including compounds that have multiple absorption peaks at wavelengths > 350 nm (12 polycyclic aromatic hydrocarbons and their derivatives) and compounds that have a single absorption peak at wavelengths < 350 nm (10 nitrophenols and nitrosalicylic acids and 3 methoxyphenols). These measured BrC chromophores show distinct seasonal differences and contribute on average about 1.1 % and 3.3 % of light absorption of methanol-soluble BrC at 365 nm in summer and winter, respectively, about 7 and 5 times higher than the corresponding carbon mass fractions in total organic carbon. The sources of BrC were resolved by positive matrix factorization (PMF) using these chromophores instead of commonly used non-light-absorbing organic markers as model inputs. Our results show that vehicular emissions and secondary formation are major sources of BrC (∼ 70 %) in spring, coal combustion and vehicular emissions are major sources (∼ 70 %) in fall, biomass burning and coal combustion become major sources (∼ 80 %) in winter, and secondary BrC dominates (∼ 60 %) in summer. [ABSTRACT FROM AUTHOR]

To investigate the sources and formation mechanisms of carbonaceous aerosols, a major contributor to severe particulate air pollution, radiocarbon (14C) measurements were conducted on aerosols sampled from November 2015 to November 2016 in Xi'an, China. Based on the 14C content in elemental carbon (EC), organic carbon (OC) and water-insoluble OC (WIOC), contributions of major sources to carbonaceous aerosols are estimated over a whole seasonal cycle: primary and secondary fossil sources, primary biomass burning, and other non-fossil carbon formed mainly from secondary processes. Primary fossil sources of EC were further sub-divided into coal and liquid fossil fuel combustion by complementing 14C data with stable carbon isotopic signatures. The dominant EC source was liquid fossil fuel combustion (i.e., vehicle emissions), accounting for 64 % (median; 45 %–74 %, interquartile range) of EC in autumn, 60 % (41 %–72 %) in summer, 53 % (33 %–69 %) in spring and 46 % (29 %–59 %) in winter. An increased contribution from biomass burning to EC was observed in winter (∼28 %) compared to other seasons (warm period; ∼15 %). In winter, coal combustion (∼25 %) and biomass burning equally contributed to EC, whereas in the warm period, coal combustion accounted for a larger fraction of EC than biomass burning. The relative contribution of fossil sources to OC was consistently lower than that to EC, with an annual average of 47±4 %. Non-fossil OC of secondary origin was an important contributor to total OC (35±4 %) and accounted for more than half of non-fossil OC (67±6 %) throughout the year. Secondary fossil OC (SOC fossil) concentrations were higher than primary fossil OC (POC fossil) concentrations in winter but lower than POC fossil in the warm period. Fossil WIOC and water-soluble OC (WSOC) have been widely used as proxies for POC fossil and SOC fossil , respectively. This assumption was evaluated by (1) comparing their mass concentrations with POC fossil and SOC fossil and (2) comparing ratios of fossil WIOC to fossil EC to typical primary OC-to-EC ratios from fossil sources including both coal combustion and vehicle emissions. The results suggest that fossil WIOC and fossil WSOC are probably a better approximation for primary and secondary fossil OC, respectively, than POC fossil and SOC fossil estimated using the EC tracer method. [ABSTRACT FROM AUTHOR]

Sources of organic carbon (OC) and elemental carbon (EC) in Xi'an, China, are investigated based on 1-year radiocarbon and stable carbon isotope measurements. The radiocarbon results demonstrate that EC is dominated by fossil sources throughout the year, with a mean contribution of 83±5% (7±2 μgm-3). The remaining 17±5% (1:5±1 μgm-3) is attributed to biomass burning, with a higher contribution in the winter (~ 24 %) compared to the summer (~ 14 %). Stable carbon isotopes of EC (δ13CEC) are enriched in winter (-23:2±0.4 %) and depleted in summer (-25:9±0:5 %), indicating the influence of coal combustion in winter and liquid fossil fuel combustion in summer. By combining radiocarbon and stable carbon signatures, relative contributions from coal combustion and liquid fossil fuel combustion are estimated to be 45% (median; 29 %-58 %, interquartile range) and 31% (18 %-46 %) in winter, respectively, whereas in other seasons more than one half of EC is from liquid fossil combustion. In contrast with EC, the contribution of non-fossil sources to OC is much larger, with an annual average of 54±8% (12±10 μgm-3). Clear seasonal variations are seen in OC concentrations both from fossil and non-fossil sources, with maxima in winter and minima in summer because of unfavorable meteorological conditions coupled with enhanced fossil and non-fossil activities in winter, mainly biomass burning and domestic coal burning. (δ13cOC exhibited similar values to δ13CEC, and showed strong correlations (r2 D 0:90) in summer and autumn, indicating similar source mixtures with EC. In spring, δ13cOC is depleted (1.1 %-2.4 %) compared to δ13CEC, indicating the importance of secondary formation of OC (e.g., from volatile organic compound precursors) in addition to primary sources. Modeled mass concentrations and source contributions of primary OC are compared to the measured mass and source contributions. There is strong evidence that both secondary formation and photochemical loss processes influence the final OC concentrations. [ABSTRACT FROM AUTHOR]

A thermal desorption aerosol gas chromatograph coupled to a high resolution -- time of flight -- aerosol mass spectrometer (TAG-AMS) was connected to an atmospheric chamber for the molecular characterization of the evolution of organic aerosol (OA) emitted by woodstove appliances for residential heating. Two log woodstoves (old and modern) and one pellet stove were operated under typical conditions. Emissions were aged during a time equivalent to 5 h of atmospheric aging. The five to seven samples were collected and analyzed with the TAG-AMS during each experiment. We detected and quantified over 70 compounds, including levoglucosan and nitrocatechols. We calculate the emission factor (EF) of these tracers in the primary emissions and highlight the influence of the combustion efficiency on these emissions. Smoldering combustion contributes to a higher EF and a more complex composition.We also demonstrate the effect of atmospheric aging on the chemical fingerprint. The tracers are sorted into three categories according to the evolution of their concentration: primary compounds, non-conventional primary compounds, and secondary compounds. For each, we provide a quantitative overview of their contribution to the OA mass at different times of the photooxidative process. [ABSTRACT FROM AUTHOR]

Wheat straw, rice straw, and corn stalks, the major agricultural crop residues in China, were collected from six major crop producing regions, and burned in a laboratory combustion chamber to determine PM 2.5 source profiles and speciated emission factors (EFs). Organic carbon (OC) and water-soluble ions (the sum of NH 4 + , Na + , K + , Mg 2+ , Ca 2+ , Cl − , NO 3 − and SO 4 2− ) are major constituents, accounting for 43.1 ± 8.3% and 27.4 ± 14.6% of PM 2.5 , respectively. Chloride (Cl − ) and water-soluble potassium (K + ) are the dominant ionic species, with an average abundance of 14.5 ± 8.2% and 6.4 ± 4.4% in PM 2.5 , respectively. The average K + /Cl − ratio is ∼0.4, lower than 2.8–5.4 for wood combustion. Similarity measures (i.e., Student's t -test, coefficient of divergence, correlations, and residual to uncertainty ratios) show the crop profiles are too similar for the species measured to be resolved from one another by receptor modeling. The largest difference was found between rice straw and corn stalk emissions, with higher OC and lower Cl − and K + abundances (50%, 8%, and 3% of PM 2.5 , respectively) for corn stalks; lower OC, and higher Cl − and K + abundances (38%, 21%, and 10% of PM 2.5 , respectively) for rice straw. Average EFs were 4.8 ± 3.1 g kg −1 for OC, 1.3 ± 0.8 g kg −1 for Cl − and 0.59 ± 0.56 g kg −1 for K + . Flaming and smoldering combustions resulted in an average modified combustion efficiency (MCE) of 0.92 ± 0.03, and low elemental carbon (EC) EFs (0.24 ± 0.12 g kg −1 ). OC/EC ratios from individual source profiles ranged from 12.9 ± 4.3 for rice straw to 24.1 ± 13.5 for wheat straw. The average K + /EC ratio was 2.4 ± 1.5, an order of magnitude higher than those from residential wood combustion (0.2–0.76). Elevated emission rates were found for OC (387 Gg yr −1 ) and Cl − (122 Gg yr −1 ), accounting for 44% and 14% of 2008 PM 2.5 emissions in China. [ABSTRACT FROM AUTHOR]

A better knowledge of the local and regional sources of the atmospheric particulate matter provides policy makers with the proper awareness when acting to improve air quality, in order to protect public health. A source apportionment study of the carbonaceous aerosol in Naples (Italy) is presented here, in order to improve this understanding in a vulnerable urban area. The aim of this study is quantifying directly fossil and non-fossil contributions to carbonaceous aerosol, by means of radiocarbon measurements. This is the first time that such an approach is implemented in this area. Fine particles with diameter ≤ 2.5 µm (PM2.5) were collected daily on top of a building in the city center, from November 2016 until January 2017. The carbonaceous aerosol was separated into organic carbon (OC) and elemental carbon (EC), by a two-step thermal desorption method. Subsequent radiocarbon analysis enabled the partitioning of the major sources of carbonaceous aerosol into fossil and non-fossil ones by applying radiocarbon isotopic mass balance. The PM2.5 concentration was on average 29 ± 3 µg⁄m3 (mean ± standard error; n = 18), with a maximum of 68.6 ± 0.7 µg⁄m3 on a day when air masses back-trajectories suggest a local origin and stagnant airflow conditions in the region. The carbonaceous component accounts for roughly half of the PM2.5 mass. Fossil fuel emissions are a minor source of OC (23%), but the dominant source of EC (66%), which is directly emitted during combustion processes. However, overall only 30% of the total carbon is of fossil origin, accounting for 14% of PM2.5 mass. Surprisingly, a comparable contribution is due to primary biomass burning carbon, which accounts in total for 15% of PM2.5 mass. Traffic pollution, the main cause of fossil fuel emissions in urban areas, is a significant, but not the predominant source of carbonaceous particle concentration. These findings support the conclusion of a predominant contribution from non-fossil sources to the carbon in airborne particulate matter, which policy makers should take into account when planning mitigation strategies to improve urban air quality. [ABSTRACT FROM AUTHOR]

The health effects of trace metal elements in atmospheric fine particulate matter (PM 2.5) are widely recognized, however, the emission factor profiles and chemical fractionation of metal elements in different sources were poorly understand. In this study, sixteen metal elements, including Cd, Pb, V, Zn, Ba, Sb, As, Fe, Sr, Cr, Rb, Co, Mn, Cu, Ni and Sn from biomass burning, bituminite and anthracite combustion, as well as dust, were quantified. The results show different emission sources were associated with distinct emission profiles, holding important implications for source apportionment of ambient particulate metals. Specifically, Fe was the dominant metal species (28-1922 mg/kg) for all samples, and was followed by different metals for different samples. For dust, Mn (39.9 mg/kg dust) had the second-highest emission factor, while for biomass burning, it was Cr and Ba (7.5 and 7.4 mg/kg biomass , respectively). For bituminous coal combustion, the emission factor of Zn and Ba was 6.2 and 6.0 mg/kg bituminous , respectively, while for anthracite combustion the corresponding emission factor was 5.6 and 4.3 mg/kg anthracite , respectively. Moreover, chemical fractionation (i.e., the exchangeable, reducible fraction, oxidizable, and residual fraction) and the bioavailability index (BI) values of the metal elements from different sources were further investigated to reveal the link between different emission sources and the potential health risk. The findings from this study hold important implications for source apportionment and source-specific particulate metal-associated health effects. [Display omitted] [ABSTRACT FROM AUTHOR]

Stable carbon isotopes provide information on aerosol sources, but no extensive long-term studies of these isotopes have been conducted in China, and they have mainly been used for qualitative rather than quantitative purposes. Here, 24 h PM 2.5 samples (n = 58) were collected from July 2008 to June 2009 at Xi'an, China. The concentrations of organic and elemental carbon (OC and EC), water-soluble OC, and the stable carbon isotope abundances of OC and EC were determined. In spring, summer, autumn and winter, the mean stable carbon isotope in OC (δ 13 C OC ) were −26.4 ± 0.6, −25.8 ± 0.7, −25.0 ± 0.6 and −24.4 ± 0.8‰, respectively, and the corresponding δ 13 C EC values were −25.5 ± 0.4, −25.5 ± 0.8, −25.2 ± 0.7 and −23.7 ± 0.6‰. Large δ 13 C EC and δ 13 C OC values in winter can be linked to the burning coal for residential heating. Less biomass is burned during spring and summer than winter or fall (manifested in the levels of levoglucosan, i.e., 178, 85, 370, 935 ng m −3 in spring, summer, autumn, and winter), and the more negative δ 13 C OC in the warmer months can be explained by the formation of secondary organic aerosols. A levoglucosan tracer method combined with an isotope mass balance analysis indicated that biomass burning accounted for 1.6–29.0% of the EC, and the mean value in winter (14.9 ± 7.5%) was 7 times higher than summer (2.1 ± 0.4%), with intermediate values of 6.1 ± 5.6 and 4.5 ± 2.4% in autumn and spring. Coal combustion accounted for 45.9 ± 23.1% of the EC overall, and the percentages were 63.0, 37.2, 36.7, and 33.7% in winter, autumn, summer and spring respectively. Motor vehicles accounted for 46.6 ± 26.5% of the annual EC, and these contributed over half (56.7–61.8%) of the EC in all seasons except winter. Correlations between motor vehicle-EC and coal combustion-EC with established source indicators (B(ghi)P and As) support the source apportionment results. This paper describes a simple and accurate method for apportioning the sources of EC, and the results may be beneficial for developing model simulations as well as controlling strategies in future. [ABSTRACT FROM AUTHOR]

The Guanzhong basin is a part of the three top priority regions in China's blue sky action as of 2019. Understanding the chemical composition, sources, and atmospheric process of aerosol in this region is therefore imperative for improving air quality. In this study, we present, for the first time, the seasonal variations of organic aerosol (OA) in Xi'an, the largest city in the Guanzhong basin. Biomass burning OA (BBOA) and oxidized OA (OOA) contributed >50% of OA in both autumn and winter. The average concentrations of BBOA in autumn (14.8 ± 5.1 μg m−3) and winter (11.6 ± 6.8 μg m−3) were similar. The fractional contribution of BBOA to total OA, however, decreased from 31.9% in autumn to 15.3% in winter, because of enhanced contributions from other sources in winter. The OOA fraction in OA increased largely from 20.9% in autumn to 34.9% in winter, likely due to enhanced emissions of precursors and stagnant meteorological conditions which facilitate the accumulation and secondary formation. A large increase in OOA concentration was observed during polluted days, by a factor of ~4 in autumn and ~6 in winter compared to clean days. In both seasons, OOA formation was most likely dominated by photochemical oxidation when aerosol liquid water content was <30 μg m−3 or by aqueous-phase processes when O x was <35 ppb. A higher concentration of BBOA was observed for air masses circulated within the Guanzhong basin (16.5–18.1 μg m−3), compared to air masses from Northwest and West (10.9–14.5 μg m−3). Furthermore, compared with OA fraction in non-refractory PM 1 in other regions of China, BBOA (17–19%) and coal combustion OA (10–20%) were major emission sources in the Guanzhong Basin and the BTH region, respectively, whereas OOA (10–34%) was an important source in all studied regions. Unlabelled Image • Biomass burning and secondary formation dominated OA in both autumn and winter in Xi'an. • OOA formation was dominated by photochemical or aqueous-phase processes under different conditions. • BBOA and CCOA were major sources in the Guanzhong Basin and the BTH region, respectively. [ABSTRACT FROM AUTHOR]

The contributions of chemical composition and emission sources to aerosol optical properties were evaluated for a coastal city in southern China. The average dry light scattering coefficient (b scat,dry) and light absorption coefficient (b abs) were 32.5 ± 15.5 Mm−1 and 8.8 ± 4.7 Mm−1, respectively. Diurnal cycles in b scat,dry and b abs were observed with peak values in the morning and at night. Both b scat,dry and b abs varied with wind speeds and directions, and thus affected by transport pathways. Chemical composition data for 12-h PM 2.5 samples were used with the revised IMPROVE algorithm and a Hybrid Environmental Receptor Model to evaluate aerosol composition and source contributions to dry light extinction (b ext,dry = b scat,dry + b abs), respectively. Ammonium sulfate and organic matter were the dominant contributors to b ext,dry , followed by elemental carbon, sea salt, fine soil, and ammonium nitrate. The six PM 2.5 sources identified were secondary sulfate source, biomass burning, marine emission, fugitive dust, traffic-related emission, and shipping emission. Marine emission and secondary sulfate source were the largest contributors of b ext,dry during the daytime and nighttime, respectively. Backward trajectory analysis further explored the impact of potential sources to b ext,dry at Sanya from surrounding regions. The results of our study would be useful for improving models of radiative effects from different sources in this area. Unlabelled Image • The lower b scat,dry and b abs were consistent with better air quality at Sanya. • Ammonium sulfate and organic matter contributed most to b ext,dry. • Source contributions to b ext,dry changed from the daytime to the nighttime. • Much higher contribution of biomass burning to b ext,dry was found from Southeast Asia. [ABSTRACT FROM AUTHOR]

For the first time, a source apportionment study of the carbonaceous aerosol in the city centre of Naples (Italy) has been conducted. Fine particles with diameter < 2.5 μm (PM2.5) were collected on 24h filters on top of a building in the city centre, from the last week of November 2016 until the first week of January 2017. For source apportionment the carbonaceous aerosol fraction has been separated in organic carbon (OC) and elemental carbon (EC) fractions, by thermal separation (Dusek et al. 2014). Subsequent radiocarbon analysis on OC and EC allowed for the first time to characterize the major carbon emitting patterns of the Naples area. During this campaign, an average PM2.5 concentration of 29 ±13 μg / m3 was recorded, with a maximum of 68.6 ±0.7 μg / m3 on a day when air masses' origin appears to be very local. The carbonaceous component contributes roughly half of PM2.5, which is high compared to other European cities and shows that reducing carbonaceous aerosol concentrations could mitigate local pollution. Fossil sources produce 30% of the total carbon, but they cause the most dominant fraction of EC (∼65%), while OC is dominated by non-fossil particles. However, contribution of biomass burning is also considerable for EC, accounting for 5% of PM2.5. On average we estimate that primary biomass burning contributes 30% to total carbon and around 15-20% to PM2.5. During the same winter the stable carbon isotope 13C was analysed in OC from ambient aerosol samples and samples taken from major sources (local biomass burning, tunnel, city buses). These data can give additional insight into sources and formation processes of the organic aerosol fraction and will be presented along with the radiocarbon source apportionment data. [ABSTRACT FROM AUTHOR]