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To understand diurnal variations in PM2.5 composition and aerosol extract absorption, PM2.5 samples were collected at intervals of 2 hr from 8:00 to 20:00 and 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019–2020 and analyzed for bulk components, organic tracers, and light absorption of water and methanol extracts—a proxy measure of brown carbon (BrC). Diurnal patterns of measured species reflected the influences of primary emissions and atmospheric processes. Light absorption coefficients of water (Abs365,w) and methanol extracts (Abs365,m) at 365 nm shared a similar diurnal profile peaking at 18:00–20:00, generally following changes in biomass burning tracers. However, Abs365,w, Abs365,m, and their normalizations to organic aerosols increased at 14:00–16:00, earlier than that of levoglucosan in the late afternoon, which was attributed to secondarily formed BrC. The methanol extracts showed a less drastic decrease in light absorption at night than the water extracts and elevated absorption efficiency during 2:00–8:00. This is due to the fact that the water‐insoluble OC has a longer lifetime and stronger light absorption than the water‐soluble OC. According to the source apportionment results solved by positive matrix factorization (PMF), biomass burning and secondary formation were the major BrC sources in northern Nanjing, with an average total relative contribution of about 90%. Compared to previous studies, diurnal source cycles were added to the PMF simulations in this work by using time‐resolved speciation data, which avoided misclassification of BrC sources. Plain Language Summary: Light‐absorbing organic carbon (OC), also known as "brown carbon", plays an important role in influencing the Earth's radiative balance. Filter samples of particulate matter with an aerodynamic diameter of less than 2.5 μm (PM2.5) were collected at a resolution of 2 hr from 8:00 to 20:00 and a resolution of 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019 and 2020. To understand the diurnal variations of BrC and its sources, each sample was analyzed for bulk components (OC, elemental carbon, and water‐soluble ions), organic molecular markers, and light absorption by BrC, and a receptor model was used to identify potential BrC sources based on PM2.5 composition data. We found that diurnal variations of BrC generally followed changes in concentrations of levoglucosan‐a tracer of biomass burning; the increase in BrC absorption in the early afternoon was caused by secondary formation of BrC chromophores; biomass burning and secondary formation were the main sources of BrC. Time‐resolved measurements of PM2.5 components were rarely used to investigate the diurnal variations of BrC and its sources, which is very important for modeling the climate impact of BrC. Key Points: Diurnal variations in PM2.5 bulk components and organic tracers reflect diurnal cycles of sourcesLight absorption of water and methanol extracts of PM2.5 exhibited a similar diurnal profile that followed changes in levoglucosanShort‐term and high time‐resolved data of organic tracers should be used for source apportionment to avoid misclassification [ABSTRACT FROM AUTHOR]

The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH / dichloromethane (MeOH / DCM, 1:1 , v/v), MeOH / DCM (1:2 , v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ∼95 %), followed by MeOH and MeOH / DCM mixtures (<90 %), and the DMF extracts had significantly (p<0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (>280 ∘) are less soluble in MeOH (∼80 %) than in DMF (∼90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed consistent temporal variations in winter when biomass burning dominated BrC absorption, while the average light absorption of DMF extracts was more than 2 times greater than the MeOH extracts in late spring and summer. The average light absorption coefficient at 365 nm of DMF extracts was 30.7 % higher (p<0.01) than that of MeOH extracts. Source apportionment results indicated that the MeOH solubility of BrC associated with biomass burning, lubricating oil combustion, and coal combustion is similar to their DMF solubility. The BrC linked with unburned fossil fuels and polymerization processes of aerosol organics was less soluble in MeOH than in DMF, which was likely the main reason for the large difference in time series between MeOH and DMF extract absorption. These results highlight the importance of testing different solvents to investigate the structures and light absorption of BrC, particularly for the low-volatility fraction potentially originating from non-combustion sources. [ABSTRACT FROM AUTHOR]

In this study, collocated filter samples of particulate matter with aerodynamic diameter less than 2.5 μm (PM2.5) from northern Nanjing were extracted using water and methanol, followed by analysis of light absorption. A backup quartz filter was used to correct sampling artifacts caused by adsorption of gaseous organics. The collocated precision of light‐absorbing properties of water‐soluble organic carbon (WSOC) and methanol‐extractable organic carbon (MEOC) were parameterized using correlation coefficient (r), coefficient of divergence (COD), and average relative percent difference (ARPD, %). In general, the light absorption of WSOC and MEOC showed good agreement (r > 0.80, COD < 0.20) between collocated samples. Performing artifact correction is necessary and will increase the heterogeneity between collocated measurements. The duplicate‐derived ARPD values of MEOC absorption were more than 60% higher than those of WSOC absorption. Then, it would be inappropriate to assume a uniform uncertainty fraction (e.g., ∼10%) for WSOC and MEOC absorption in future studies on their climate effects and source apportionment. To apportion artifact‐corrected absorption of aerosol extracts to specific emission sources or formation pathways, positive matrix factorization was performed by using concentration data of selected bulk species and organic molecular markers. Among the nine identified factor/sources, the biomass burning factor had the highest average contributions to the absorption of both WSOC (31.6%) and MEOC (48.0%), followed by dust resuspension and coal combustion factors. Unlike combustion‐related primary emissions, the factors containing influences from atmospheric processing (e.g., secondary nitrate) contributed more fractions of WSOC absorption than MEOC absorption. Key Points: Collocated measurements provide an approach to estimate uncertainties in light‐absorbing properties of solvent‐extractable organic carbonIgnoring the adsorption of gaseous organics onto filter medium will lead to significant overestimation of brown carbon absorptionAerosol sources influenced by aging processes contributed more fractions of aqueous extracts absorption than methanol extracts absorption [ABSTRACT FROM AUTHOR]

N-containing aromatic compounds (NACs) are an important group of light-absorbing molecules in the atmosphere. They are often observed in combustion emissions, but their chemical formulas and structural characteristics remain uncertain. In this study, red oakwood and charcoal fuels were burned in cookstoves using the standard water-boiling test (WBT) procedure. Submicron aerosol particles in the cookstove emissions were collected using quartz (Qf) and polytetrafluoroethylene (PTFE) filter membranes positioned in parallel. A backup quartz filter (Qb) was also installed downstream of the PTFE filter to evaluate the effect of sampling artifacts on NAC measurements. Liquid chromatography–mass spectroscopy (LC–MS) techniques identified 17 NAC chemical formulas in the cookstove emissions. The average concentrations of total NACs in Qb samples (0.37±0.31 – 1.79±0.77 µgm-3) were greater than 50 % of those observed in the Qf samples (0.51±0.43 – 3.91±2.06 µgm-3), and the Qb -to- Qf mass ratios of individual NACs had a range of 0.02–2.71, indicating that the identified NACs might have substantial fractions remaining in the gas phase. In comparison to other sources, cookstove emissions from red oak or charcoal fuels did not exhibit unique NAC structural features but had distinct NAC composition. However, before identifying NAC sources by combining their structural and compositional information, the gas-particle partitioning behaviors of NACs should be further investigated. The average contributions of total NACs to the light absorption of organic matter at λ=365 nm (1.10 %–2.57 %) in Qf and Qb samples (10.7 %–21.0 %) are up to 10 times larger than their mass contributions (Qf : 0.31 %–1.01 %; Qb : 1.08 %–3.31 %), so the identified NACs are mostly strong light absorbers. To explain more sample extract absorption, future research is needed to understand the chemical and optical properties of high-molecular-weight (e.g., molecular weight, MW>500 Da) entities in particulate matter. [ABSTRACT FROM AUTHOR]

This study seeks to understand the compositional details of N-containing aromatic compounds (NACs) emitted during biomass burning (BB) and their contribution to light-absorbing organic carbon (OC), also termed brown carbon (BrC). Three laboratory BB experiments were conducted with two United States pine forest understory fuels typical of those consumed during prescribed fires. During the experiments, submicron aerosol particles were collected on filter media and subsequently extracted with methanol and examined for their optical and chemical properties. Significant correlations (p<0.05) were observed between BrC absorption and elemental carbon (EC)/OC ratios for individual burns data. However, the pooled experimental data indicated that EC/OC alone cannot explain the BB BrC absorption. Fourteen NAC formulas were identified in the BB samples, most of which were also observed in simulated secondary organic aerosol (SOA) from photooxidation of aromatic volatile organic compounds (VOCs) with NOx. However, the molecular structures associated with the identical NAC formula from BB and SOA are different. In this work, the identified NACs from BB are featured by methoxy and cyanate groups and are predominately generated during the flaming phase. The mass concentrations of identified NACs were quantified using authentic and surrogate standards, and their contributions to bulk light absorption of solvent-extractable OC were also calculated. The contributions of identified NACs to organic matter (OM) and BrC absorption were significantly higher in flaming-phase samples than those in smoldering-phase samples, and they correlated with the EC/OC ratio (p<0.05) for both individual burns and pooled experimental data, indicating that the formation of NACs from BB largely depends on burn conditions. The average contributions of identified NACs to overall BrC absorption at 365 nm ranged from 0.087±0.024 % to 1.22±0.54 %, which is 3–10 times higher than their mass contributions to OM (0.023±0.0089 % to 0.18±0.067 %), so the NACs with light absorption identified in this work from BB are likely strong BrC chromophores. Further studies are warranted to identify more light-absorbing compounds to explain the unknown fraction (>98 %) of BB BrC absorption. [ABSTRACT FROM AUTHOR]

Abstract Light-absorbing organic carbon (OC), also referred to as "brown carbon" (BrC), has been intensively investigated in atmospheres impacted by biomass burning. However, other BrC sources (e.g., secondary formation in the atmosphere) are rarely studied in ambient aerosols. In the current work, forty-five PM 2.5 filter samples were collected in Research Triangle Park (RTP), NC, USA from June 1st to July 15th, 2013. The bulk carbonaceous components, including OC, elemental carbon (EC), water soluble OC (WSOC), and an array of organic molecular markers were measured; an ultraviolet/visible spectrometer was used to measure the light absorption of methanol extractable OC and WSOC. The average light absorption per OC and WSOC mass of PM 2.5 samples in summer RTP are 0.36 ± 0.16 m2 gC−1 and 0.29 ± 0.13 m2 gC−1, respectively, lower than the ambient aerosol samples impacted by biomass burning and/or fossil fuel combustion (0.7–1.6 m2 gC−1) from other places. Less than 1% of the aqueous extracts absorption is attributed to the light-absorbing chromophores (nitroaromatic compounds) identified in this work. To identify the major sources of BrC absorption in RTP in the summer, Positive Matrix Factorization (PMF) was applied to a dataset containing optical properties and chemical compositions of carbonaceous components in PM 2.5. The results suggest that the formation of biogenic secondary organic aerosol (SOA) containing organosulfates is an important BrC source, contributing up to half of the BrC absorption in RTP during the summertime. Graphical abstract Image 1 Highlights • BrC absorption in summer at RTP, NC is similar to other Southeastern US locations. • Less than 1% of BrC absorption is attributed to identified nitroaromatic compounds. • Biomass burning is not the dominant source for BrC absorption in summer at RTP, NC. • Biogenic secondary organic aerosol formation is a major BrC source in this work. This study identified a linkage between atmospheric BrC and biogenic SOA formation when biomass burning is not significant in Research Triangle Park, NC during the summer. [ABSTRACT FROM AUTHOR]

Owing to the potential influence of light-absorbing organic carbon (OC), also termed "brown carbon" (BrC), on the planetary radiation budget, many studies have focused on its absorption in single-sized ranges of particulate matter (PM). However, the size distribution and organic tracer-based source apportionment of BrC absorption have not been extensively investigated. In this study, size-resolved PM samples were collected using multi-stage impactors from eastern Nanjing during each season in 2017. The light absorption of methanol-extractable OC at 365 nm (Abs 365 , Mm−1) was determined using spectrophotometry, and a series of organic molecular markers (OMMs) was measured using a gas chromatography-mass spectrometer. Fine PM with an aerodynamic diameter <2.1 μm (PM 2.1) dominated Abs 365 (79.8 ± 10.4%) of the total size ranges with maxima and minima in winter and summer, respectively. The distributions of Abs 365 shifted to larger PM sizes from winter to spring and summer due to lower primary emissions and increased BrC chromophores in dust. Except for low-volatility (p o,* L < 10−10 atm) polycyclic aromatic hydrocarbons (PAHs), the non-polar OMMs, including n- alkanes, PAHs, oxygenated PAHs, and steranes, showed a bimodal distribution pattern. Secondary products of biogenic precursors and biomass burning tracers presented a unimodal distribution peaking at 0.4–0.7 μm, while sugar alcohols and saccharides were enriched in coarse PM. Their seasonal variations in average concentrations reflected intense photochemical reactions in summer, more biomass burning emissions in winter, and stronger microbial activity in spring and summer. Positive matrix factorization was used for the source apportionment of Abs 365 in fine and coarse PM samples. Biomass burning contributed an average of 53.9% to the Abs 365 of PM 2.1 extracts. The Abs 365 of coarse PM extracts was associated with various dust-related sources where the aging processes of aerosol organics could occur. [Display omitted] • Fine particles dominated the light absorption of methanol extracts of total PM. • Low-volatile n- alkanes and steranes in coarse PM increased during warm periods. • Biomass burning contributed more than 50% to BrC absorption in fine PM. • Aging processes of organics in coarse particles could lead to BrC formation. [ABSTRACT FROM AUTHOR]

Household cookstove emissions are an important source of carbonaceous aerosols globally. The light-absorbing organic carbon (OC), also termed brown carbon (BrC), from cookstove emissions can impact the Earth's radiative balance, but is rarely investigated. In this work, PM 2.5 filter samples were collected during combustion experiments with red oak wood, charcoal, and kerosene in a variety of cookstoves mainly at two water boiling test phases (cold start CS, hot start HS). Samples were extracted in methanol and extracts were examined using spectrophotometry. The mass absorption coefficients (MAC λ , m 2 g −1 ) at five wavelengths (365, 400, 450, 500, and 550 nm) were mostly inter-correlated and were used as a measurement proxy for BrC. The MAC 365 for red oak combustion during the CS phase correlated strongly to the elemental carbon (EC)/OC mass ratio, indicating a dependency of BrC absorption on burn conditions. The emissions from cookstoves burning red oak have an average MAC λ 2–6 times greater than those burning charcoal and kerosene, and around 3–4 times greater than that from biomass burning measured in previous studies. These results suggest that residential cookstove emissions could contribute largely to ambient BrC, and the simulation of BrC radiative forcing in climate models for biofuel combustion in cookstoves should be treated specifically and separated from open biomass burning. [ABSTRACT FROM AUTHOR]

Recently, the occurrence of fog and haze over China has increased. The retrieval of trace gases from the multi-axis differential optical absorption spectroscopy (MAX-DOAS) is challenging under these conditions. In this study, various reported retrieval settings for formaldehyde (HCHO) and sulfur dioxide (SO2) are compared to evaluate the performance of these settings under different meteorological conditions (clear day, haze, and fog). The dataset from 1st December 2019 to 31st March 2020 over Nanjing, China, is used in this study. The results indicated that for HCHO, the optimal settings were in the 324.5–359 nm wavelength window with a polynomial order of five. At these settings, the fitting and root mean squared (RMS) errors for column density were considerably improved for haze and fog conditions, and the differential slant column densities (DSCDs) showed more accurate values compared to the DSCDs between 336.5 and 359 nm. For SO2, the optimal settings for retrieval were found to be at 307–328 nm with a polynomial order of five. Here, root mean square (RMS) and fitting errors were significantly lower under all conditions. The observed HCHO and SO2 vertical column densities were significantly lower on fog days compared to clear days, reflecting a decreased chemical production of HCHO and aqueous phase oxidation of SO2 in fog droplets. [ABSTRACT FROM AUTHOR]