Seasonal changes in water-soluble brown carbon (BrC) at Nanling background station in South China

Brown carbon (BrC) is an important light-absorbing component of organic carbon (OC), causing large uncertainty in aerosol radiative forcing evaluation and being related to health issues as well. Knowledge of BrC in an atmospheric background station is beneficial to understand its role in a changing climate. A year-long sampling campaign was conducted at Nanling background station to get a comprehensive knowledge of WS-BrC, a total of seventy-two PM2.5 samples throughout a year were used. Light absorption and fluorescence spectra of WSOC were analyzed synchronously using a fluorescence spectrophotometer. The low levels of PM2.5, OC, and elemental carbon (EC) conferred a background site. The optical properties of WS-BrC were characterized using excitation-emission matrix (EEM) fluorescence spectroscopy. The WS-BrC made a significant contribution (365 nm, 18% ± 10%) to total carbonaceous aerosol absorption. The mass absorption efficiency (MAE) of WS-BrC is 0.81 ± 0.34 m2 gC–1, and varies among seasons due to the different sources or atmospheric processing. Three EEM fluorescent components were identified by parallel factor (PAFAFAC) analysis, including two humic-like substances (HULIS, C1, C2), and one phenolic-like component. The HULIS components accounted for approximately 70% of the total fluorescence intensities. Primary combustion emissions showed enhanced activity during the winter and spring seasons, but there were no significant influences on WS-BrC in spring. Secondary sources contributed significantly to WS-BrC during winter, summer, and autumn (all exceeding 50%), except for spring. Photooxidation is a significant process in the formation of secondary WS-BrC in winter and autumn, but there may be another formation pathway in summer, i.e., the ammonia pathway. This study contributes to our understanding of BrC in the background atmosphere

Dual carbon isotopes (C-14 and C-13) and optical properties of WSOC and HULIS-C during winter in Guangzhou, China

Water-soluble brown carbon (ws-BrC) exerts an important influence on climate change, but its emission sources and optical properties remain poorly understood. In this study, we isolated two ws-BrC proxies, water-soluble organic carbon (WSOC) and humic-like substance carbon (HULIS-C), from particulate matter collected in Guangzhou, China, during December 2012 for the measurement of dual carbon isotopes (C-14 and C-13) and light absorption. The mass absorption efficiencies of WSOC and HULIS-C at 365 nm were 0.81 +/- 0.16 and 1.33 +/- 0.21 m(2) g(-1) C, respectively. The C-14 results showed that two-thirds of WSOC and HULIS-C were derived from non-fossil sources (e.g., biomass burning and biogenic emission), and the remaining third was derived from fossil sources. The delta C-13 values of WSOC and HULIS-C were -23.7 +/- 1.2 parts per thousand and -24.2 +/- 0.9 parts per thousand, respectively, underlining the limited influences of C4 plants and natural gas on ws-BrC. Fitting the data to a multiple linear regression, we further concluded that approximately 80% and 10% of the light absorption at 365 nm was due to non-fossil and fossil carbon, respectively. Non-fossil sources of ws-BrC, such as the burning of agricultural residue, were responsible for the light absorption recorded in Guangzhou. (c) 2018 Elsevier B.V. All rights reserved

The influence of solvent and pH on determination of the light absorption properties of water-soluble brown carbon

Brown carbon (BrC) is a class of unidentified organic compounds that efficiently absorb solar radiation in the ultraviolet (UV) wavelengths, and its effects on climate are poorly understood. Measurement of the light absorption properties of BrC in liquid extracts is a commonly used BrC analytical method, but the optical characteristics of water-soluble BrC may be affected by pH and solvent. In this work, we investigated the effects of concentration, pH, and solvent on water-soluble BrC from ambient aerosols, biomass burning, diesel exhaust, and a humic substance standard. The results showed that pH can affect the light absorption properties of water-soluble BrC, whereas concentration had little effect, except low concentrations dissolved in methanol. Therefore, the pH of humic-like substances (HULLS) should be adjusted to the same value as water-soluble carbon (WSOC) for calculating the light-absorption contribution of HULLS to WSOC. The-light absorptivity of water-soluble BrC dissolved in methanol was higher than that in water. Considering the pH and concentration effects, extraction of WSOC with a particle: water ratio of 0.25 mg/mL is proposed as well as to get a reference pH for light absorption analysis. (C) 2017 Elsevier Ltd. All rights reserved

Molecular marker study of aerosols in the northern South China Sea: Impact of atmospheric outflow from the Indo-China Peninsula and South China

Continental outflow influences concentration and chemical composition of marine aerosols, which has an important impact on regional biogeochemistry and climate. Aerosols sampling for molecular marker study was conducted from June 2015 to May 2016 at Xieyang Island in the northern South China Sea (SCS), to learn the impact of outflow from Indo-China Peninsular (ICP) and South China (SC). Xieyang Island was under the influence of air masses from ICP and SC nearly 70% of the year. Levels of anhydrosugars were higher during October to early March. They were lower in April and May, although biomass burning events in ICP were most intensive and half of air masses back trajectories passed through ICP in this period. Significant correlations between levels of anhydrosugars, polycyclic aromatic hydrocarbons, n-alkanes, high molecular weight n-fatty acids and terephthalic acid were observed, suggesting co-emissions of these compounds. Source types of aerosol primary organic matter (POM) tracked by these markers, namely open burning of municipal wastes, fossil fuel combustion, higher plant emissions and biomass burning, largely contributed in fall and winter with SC to be an important source region. However, levels of sugar alcohols (a group of biogenic aerosol tracers) were higher in warm seasons with ICP and SCS to be the main source regions, and were poorly correlated with levels of other molecular markers. Carbon preference index of n-alkanes also increased in warm seasons, indicating enhanced higher plant wax emissions. It suggested that there was a close link between biogenic emissions of sugar alcohols (or n-alkanes) and the growing activities of related organisms. Besides continental outflow, there were marine sources of steranes, hopanes (fossil sources tracers) and mannitol (a sugar alcohol), since their levels were higher for aerosol samples basically influenced by air masses originated from SCS

Light absorption and emissions inventory of humic-like substances from simulated rainforest biomass burning in Southeast Asia

Humic-like substances (HULIS) are complex mixtures that are highly associated with brown carbon (BrC) and are important components of biomass burning (BB) emissions. In this study, we investigated the light absorption, emission factors (EFs), and amounts of HULIS emitted from the simulated burning of 27 types of regionally important rainforest biomass in Southeast Asia. We observed that HULIS had a high mass absorption efficiency at 365 nm (MAE(365)), with an average value of 2.6 +/- 0.83 m(2) g(-1) C. HULIS emitted from BB accounted for 65% +/- 13% of the amount of water-soluble organic carbon (WSOC) and 85% +/- 10% of the light absorption of WSOC at 365 nm. The EFs of HULIS from BB averaged 2.3 +/- 2.1 g kg(-1) fuel, and the burning of the four vegetation subtypes (herbaceous plants, shrubs, evergreen trees, and deciduous trees) exhibited different characteristics. The differences in EFs among the subtypes were likely due to differences in lignin content in the vegetation, the burning conditions, or other factors. The light absorption characteristics of HULIS were strongly associated with the EFs. The annual emissions (minimum-maximum) of HULIS from BB in this region in 2016 were 200-371 Gg. Furthermore, the emissions from January to April accounted for 99% of the total annual emissions of HULIS, which is likely the result of the burning activities during this season. The most significant emission regions were Cambodia, Burma, Thailand, and Laos. This study, which evaluated emissions of HULIS by simulating open BB, contributes to a better understanding of the light-absorbing properties and regional budgets of BrC in this region. (C) 2020 Elsevier Ltd. All rights reserved

The application of land use regression model to investigate spatiotemporal variations of PM2.5 in Guangzhou, China:Implications for the public health benefits of PM2.5 reduction

Understanding the intra-city variation of PM2.5 is important for air quality management and exposure assessment. In this study, to investigate the spatiotemporal variation of PM2.5 in Guangzhou, we developed land use regression (LUR) models using data from 49 routine air quality monitoring stations. The R2, adjust R2 and 10-fold cross validation R2 for the annual PM2.5 LUR model were 0.78, 0.72 and 0.66, respectively, indicating the robustness of the model. In all the LUR models, traffic variables (e.g., length of main road and the distance to nearest ancillary) were the most common variables in the LUR models, suggesting vehicle emission was the most important contributor to PM2.5 and controlling vehicle emissions would be an effective way to reduce PM2.5. The predicted PM2.5 exhibited significant variations with different land uses, with the highest value for impervious surfaces, followed by green land, cropland, forest and water areas. Guangzhou as the third largest city that PM2.5 concentration has achieved CAAQS Grade II guideline in China, it represents a useful case study city to examine the health and economic benefits of further reduction of PM2.5 to the lower concentration ranges. So, the health and economic benefits of reducing PM2.5 in Guangzhou was further estimated using the BenMAP model, based on the annual PM2.5 concentration predicted by the LUR model. The results showed that the avoided all cause mortalities were 992 cases (95% CI: 221–2140) and the corresponding economic benefits were 1478 million CNY (95% CI: 257–2524) (willingness to pay approach) if the annual PM2.5 concentration can be reduced to the annual CAAQS Grade I guideline value of 15 μg/m3. Our results are expected to provide valuable information for further air pollution control strategies in China

Biomass burning organic aerosols significantly influence the light absorption properties of polarity-dependent organic compounds in the Pearl River Delta Region, China

Atmospheric brown carbon (BrC) is an important constituent of light-absorbing organic aerosols with many unclear issues. Here, the light-absorption properties of BrC with different polarity characteristics at a regional site of Pearl River Delta Region during 2016-2017, influenced by sources and molecular compositions, were revealed using radiocarbon analysis and Fourier transform ion cyclotron resonance mass spectrometry. Humic-like substance (HULIS), middle polar (MP), and low polar (LP) carbon fractions constitute 46 +/- 17%, 30 +/- 7%, and 7 +/- 3% of total absorption coefficient from bulk extracts, respectively. Our results show that the absorption proportions of HULIS and MP to the total BrC absorption are higher than their mass proportions to organic carbon mass, indicating that HULIS and MP are the main light-absorbing components in water-soluble and water-insoluble organic carbon fractions, respectively. With decreases in non-fossil HULIS, MP, and LP carbon fractions (66 +/- 2%, 52 +/- 2%, and 36 +/- 3%, respectively), the abundances of unsaturated compounds and mass absorption efficiency at 365 nm of three fractions decreased synchronously. Increases in both nonfossil carbon and levoglucosan in winter imply that the enhanced light-absorption could be attributed to elevated levels of biomass burning organic aerosols (BBOA), which increases the number of light-absorbing nitrogencontaining compounds. Moreover, the major type of potential BrC in HULIS and MP carbon fractions are oxidized BBOA, but the potential BrC chromophores in LP are mainly associated with primary BBOA. This study reveals that biomass burning has adverse effects on radiative forcing and air quality, and probably indicates the significant influences of atmospheric oxidation reactions on the forms of chromophores

Molecular Signatures and Sources of Fluorescent Components in Atmospheric Organic Matter in South China

Excitation emission matrix (EEM) spectra coupled with parallel factor analysis (PARAFAC) have been used to characterize brown carbon (BrC). The molecular composition of PARAFAC components is not well understood in atmospheric science, which has impeded the accurate interpretation of the chemical changes and source apportionment of BrC using EEM-PARAFAC methods. We assigned numerous formulas identified by Fourier transform ion cyclotron resonance mass spectrometry (both negative and positive electrospray ionization modes) to each PARAFAC-derived fluorescent component (FC) in atmospheric PM2.5 samples. Obvious differences in molecular characteristics and sources were observed between atmospheric and hydrospheric FCs, indicating the need for caution in explaining the composition and source of atmospheric FC based on hydrospheric FC. Previously assigned protein-like components (C4) and less-oxidized humic-like FC (C1) were associated mainly with highly saturated photoresistant species and less-polar anthropogenic emissions. Highly oxidized humic-like FCs were associated with aromatic and highly unsaturated/phenolic compounds (high oxygen), which were potentially influenced by biomass burning and related secondary processes. The seasonal variations in light absorption were similar to those of fluorescence intensity and the degree of humification, which were influenced by highly unsaturated/phenolic compounds (high oxygen). These linkages indicated the potential of EEM-PARAFAC for investigating the changes of light absorption, molecular composition, and source of BrC

Molecular characteristics, sources, and formation pathways of organosulfur compounds in ambient aerosol in Guangzhou, South China

Organosulfur compounds (OrgSs), especially organosulfates, have been widely reported to be present in large quantities in particulate organic matter found in various atmospheric environments. Despite hundreds of organosulfates and their formation mechanisms being previously identified, a large fraction of OrgSs remain unexplained at the molecular level, and a better understanding of their formation pathways and critical environmental parameters is required to explain the variations in their concentrations. In this study, the abundance and molecular composition of OrgSs in fine particulate samples collected in Guangzhou were reported. The results revealed that the ratio of the annual average mass of organic sulfur to total particulate sulfur was 33 +/- 12 %, and organic sulfur had positive correlations with SO2 (r = 0.37, p = 1, indicating that they were largely in the form of oxidized organosulfates or nitrooxy organosulfates. Many OrgSs that were previously tentatively identified as having biogenic or anthropogenic origins were also present in freshly emitted aerosols derived from combustion sources. The results indicated that the formation of OrgSs through an epoxide intermediate pathway could account for up to 46 % of OrgSs from an upper bound estimation, and the oxidant levels could explain 20 % of the variation in the mass of organic sulfur. The analysis of our large dataset of FT-ICR MS results suggested that relative humidity, oxidation of biogenic volatile organic compounds via ozonolysis, and NOx-related nitrooxy organosulfate formation were the major reasons for the molecular variation of OrgSs, possibly highlighting the importance of the acid-catalyzed ring-opening of epoxides, oxidation processes, and heterogeneous reactions involving either the uptake of SO2 or the heterogeneous oxidation of particulate organosulfates into additional unrecognized OrgSs