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. 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. A fluorescence-based method was developed to investigate secondary sources of water-soluble brown carbon in five cities in China The contribution of secondary sources to water-soluble brown carbon in the summer is approximately twice as high as during the winter This secondary water-soluble brown carbon was more associated with aging biomass burning in winter and biogenic emissions in summer

To elucidate the molecular composition and sources of organic aerosols in Central Asia, carbonaceous compounds, major ions, and 15 organic molecular tracers of total suspended particulates (TSP) were analyzed from September 2018 to August 2019 in Dushanbe, Tajikistan. Extremely high TSP concentrations (annual mean +/- std: 211 +/- 131 mu gm(-3)) were observed, particularly during summer (seasonal mean +/- std: 333 +/- 183 mu g m(- 3)). Organic carbon (OC: 11.9 +/- 7.0 mu gm(-3)) and elemental carbon (EC: 5.1 +/- 2.2 mu gm(-3)) exhibited distinct seasonal variations from TSP, with the highest values occurring in winter. A high concentration of Ca2+ was observed (11.9 +/- 9.2 mu g m(-3)), accounting for 50.8% of the total ions and reflecting the considerable influence of dust on aerosols. Among the measured organic molecular tracers, levoglucosan was the predominant compound (632 +/- 770 ng m(-3)), and its concentration correlated significantly with OC and EC during the study period. These findings highlight biomass burning (BB) as an important contributor to the particulate air pollution in Dushanbe. High ratios of levoglucosan to mannosan, and syringic acid to vanillic acid suggest that mixed hardwood and herbaceous plants were the main burning materials in the area, with softwood being a minor one. According to the diagnostic tracer ratio, OC derived from BB constituted a large fraction of the primary OC (POC) in ambient aerosols, accounting for an annual mean of nearly 30% and reaching 63% in winter. The annual contribution of fungal spores to POC was 10%, with a maximum of 16% in spring. Measurements of plant debris, accounting for 3% of POC, divulged that these have the same variation as fungal spores.

Runoff processes in glacier and paramo catchments in the Andean region are of interest as they are vitally important to serve the water needs of surrounding communities. Particularly in Northern Ecuador, the runoff processes are less well-known due to the high variability of precipitation, young volcanic ash soil properties, soil moisture dynamics and other local factors. Previous studies have shown that the melting of glaciers contributes to runoff generation and that the paramo ecosystem plays an important role in regulating runoff during periods of low precipitation. Data collection and experimental investigations were carried out in a catchment of 15.2 km(2) and altitude ranging between 4000 and 5700 m above sea level. Environmental tracers and hydrochemical catchment characterization were used for identifying runoff sources and their respective contributions during dry and wet conditions. Dry conditions are defined as periods where precipitation was absent for at least three consecutive days and wet conditions imply rainfall events. This study highlights the importance of the paramo on contributing to total runoff during baseflow (70% of total runoff) and the capacity of the paramo to dissipate the stream energy and buffer the peak flow during rainfall conditions. Electrical conductivity together with stable isotopes were identified as conservative tracers that characterize the end-member concentrations.

Aerosols affect the radiative forcing of the global climate and cloud properties. Organic aerosols are among the most important, yet least understood, components of the sensitive Tibetan Plateau atmosphere. Here, the concentration of and the seasonal and diurnal variations in biomass burning and biogenic aerosols, and their contribution to organic aerosols in the inland Tibetan Plateau were investigated using molecular tracers. Biomass burning tracers including levoglucosan and its isomers, and aromatic acids showed higher concentrations during winter than in summer. Molecular tracers of primary and secondary biogenic organic aerosols were more abundant during summer than those in winter. Meteorological conditions were the main factors influencing diurnal variations in most organic molecular tracers during both seasons. According to the tracer-based method, we found that biogenic secondary organic aerosols (38.5 %) and fungal spores (14.4 %) were the two dominant contributors to organic aerosols during summer, whereas biomass burning (15.4 %) was an important aerosol source during winter at remote continental background site. Results from the positive matrix factor source apportionment also demonstrate the importance of biomass burning and biogenic aerosols in the inland Tibetan Plateau. During winter, the long-range transport of biomass burning from South Asia contributes to organic aerosols. In contrast, the precursors, biogenic secondary organic aerosols, and fungal spores from local emissions/long-range transport are the major sources of organic aerosols during summer. Further investigation is required to distinguish between local emissions and the long-range transport of organic aerosols. In-depth insights into the organic aerosols in the Tibetan Plateau are expected to reduce the uncertainties when evaluating aerosol effects on the climate system in the Tibetan Plateau.

Methane emissions in the Arctic are important, and may be contributing to global warming. While methane emission rates from Arctic lakes are well documented, methods are needed to quantify the relative contribution of active layer groundwater to the overall lake methane budget. Here we report measurements of natural tracers of soil/groundwater, radon, and radium, along with methane concentration in Toolik Lake, Alaska, to evaluate the role active layer water plays as an exogenous source for lake methane. Average concentrations of methane, radium, and radon were all elevated in the active layer compared with lake water (1.6 x 10(4) nM, 61.6 dpm.m(-3), and 4.5 x 10(5) dpm.m(-3) compared with 1.3 x 10(2) nM, 5.7 dpm.m(-3), and 4.4 x 10(3) dpm.m(-3), respectively). Methane transport from the active layer to Toolik Lake based on the geochemical tracer radon (up to 2.9 g.m(-2).y(-1)) can account for a large fraction of methane emissions from this lake. Strong but spatially and temporally variable correlations between radon activity and methane concentrations (r(2) > 0.69) in lake water suggest that the parameters that control methane discharge from the active layer also vary. Warming in the Arctic may expand the active layer and increase the discharge, thereby increasing the methane flux to lakes and from lakes to the atmosphere, exacerbating global warming. More work is needed to quantify and elucidate the processes that control methane fluxes from the active layer to predict how this flux might change in the future and to evaluate the regional and global contribution of active layer water associated methane inputs.

Arctic river basins are amongst the most vulnerable to climate change. However, there is currently limited knowledge of the hydrological processes that govern flow dynamics in Arctic river basins. We address this research gap using natural hydrochemical and isotopic tracers to identify water sources that contributed to runoff in river basins spanning a gradient of glacierization (0-61%) in Svalbard during summer 2010 and 2011. Spatially distinct hydrological processes operating over diurnal, weekly and seasonal timescales were characterized by river hydrochemistry and isotopic composition. Two conceptual water sources (meltwater' and groundwater') were identified and used as a basis for end-member mixing analyses to assess seasonal and year-to-year variability in water source dynamics. In glacier-fed rivers, meltwater dominated flows at all sites (typically >80%) with the highest contributions observed at the beginning of each study period in early July when snow cover was most extensive. Rivers in non-glacierized basins were sourced initially from snowmelt but became increasingly dependent on groundwater inputs (up to 100% of total flow volume) by late summer. These hydrological changes were attributed to the depletion of snowpacks and enhanced soil water storage capacity as the active layer expanded throughout each melt season. These findings provide insight into the processes that underpin water source dynamics in Arctic river systems and potential future changes in Arctic hydrology that might be expected under a changing climate. Copyright (c) 2013 John Wiley & Sons, Ltd.