This study reports day-night and seasonal variations of aqueous brown carbon (BrCaq) and constituent humic-like substances (HULIS) (neutral and acidic HULIS: HULIS-n and HULIS-a) from the eastern Indo-Gangetic Plain (IGP) of India during 2019-2020. This is followed by the application of the receptor model positive matrix factorization (PMF) for optical source apportionment of BrCaq and the use of stable isotopic ratios (813C and 815N) to understand atmospheric processing. Nighttime BrCaq absorption and mass absorption efficiencies (MAE) were enhanced by 40-150 % and 50-190 %, respectively, compared to the daytime across seasons, possibly as a combined effect from daytime photobleaching, dark-phase secondary formation, and increased nighttime emissions. MAE250 nm/MAE365 nm (i.e., E2/E3) ratios and Angstrom Exponents revealed that BrCaq and HULIS-n were relatively more aromatic and conjugated during the biomass burning-dominated periods while BrCaq and HULIS-a were comprised mostly of nonconjugated aliphatic structures from secondary processes during the photochemistry-dominated summer. The relative radiative forcing of BrCaq with respect to elemental carbon (EC) was 10-12 % in the post-monsoon and winter in the 300-400 nm range. Optical source apportionment using PMF revealed that BrCaq absorption at 300, 365 and 420 nm wavelengths in the eastern IGP is mostly from biomass burning (60-75 %), followed by combined marine and fossil fuel-derived sources (24-31 %), and secondary processes (up to 10 %). Source-specific MAEs at 365 nm were estimated to be the highest for the combined marine and fossil fuel source (1.34 m2 g-1) followed by biomass burning (0.78 m2 g-1) and secondary processing (0.13 m2 g-1). Finally, 813C and 815N isotopic analysis confirmed the importance of summertime photochemistry and wintertime NO3--dominated chemistry in constraining BrC characteristics. Overall, the quantitative apportionment of BrCaq sources and processing reported here can be expected to lead to targeted source-specific measurements and a better understanding of BrC climate forcing in the future.

Carbonaceous particles play an important role in climate change, and an increase in their emission and deposition causes glacier melting in the Himalayas and the Tibetan Plateau (HTP). This implies that studying their basic characteristics is crucial for a better understanding of the climate forcing observed in this area. Thus, we investigated characteristics of carbonaceous particles at a typical remote site of southeastern HTP. Organic carbon and elemental carbon concentrations at this study site were 1.86 +/- 0.84 and 0.18 +/- 0.09 mu g m(-3), respectively, which are much lower than those reported for other frequently monitored stations in the same region. Thus, these values reflect the background characteristics of the study site. Additionally, the absorption coefficient per mass (alpha/rho) of water-soluble organic carbon (WSOC) at 365 nm was 0.60 +/- 0.19 m(2) g(-1), with the highest and lowest values corresponding to the winter and monsoon seasons, respectively. Multi-dimensional fluorescence analysis showed that the WSOC consisted of approximately 37% and 63% protein and humic-like components, respectively, and the latter was identified as the component that primarily determined the light absorption ability of the WSOC, which also showed a significant relationship with some major ions, including SO42-, K+, and Ca2+, indicating that combustion activities as well as mineral dust were two important contributors to WSOC at the study site. (C) 2020 Elsevier Ltd. All rights reserved.

The variability of soil carbon and nitrogen and the lack of information regarding the properties of deep soils in alpine permafrost regions hinder our understanding of ecosystem responses to climate change. The objective of this study was to examine the effects of pedogenesis and soil physicochemical parameters on the distributions of soil carbon and nitrogen and their characteristics of alpine meadows in permafrost regions. The results showed that pedogenesis was an important factor in the distribution of soil organic carbon (SOC) and total nitrogen (TN) in both the active layers and deep soils. The average water-soluble organic carbon (WSOC) content in the permafrost layer was higher than that of the active layer, which implied that the carbon pool in the permafrost layer was easily decomposable. Soil pH was an important factor that influenced soil inorganic carbon (SIC), which was closely associated with SOC in deep soils. The significant negative relationships between the SIC, pH and C/N ratios in permafrost regions implied that SIC can play an important role in the turnover of SOM and TN. (C) 2016 Elsevier B.V. All rights reserved.

The first field measurements of light absorbing water-soluble organic carbon (WSOC), referred as brown carbon (BrC), have been made in the marine atmospheric boundary layer (MABL) during the continental outflow to the Bay of Bengal (BoB) and the Arabian Sea (ARS). The absorption signal measured at 365 nm in aqueous extracts of aerosols shows a systematic linear increase with WSOC concentration, suggesting a significant contribution from BrC to the absorption properties of organic aerosols. The mass absorption coefficient (b(abs)) of BrC shows an inverse hyperbolic relation with wavelength (from 300 to 700 nm), providing an estimate of the Angstrom exponent (alpha P, range: 3-19; Av: 9 +/- 3). The mass absorption efficiency of brown carbon (sigma(abs) BrC) in the MABL varies from 0.17 to 0.72 m(2) g(-1) (Av: 0.45 +/- 0.14 m(2) g(-1)). The alpha P and sigma(abs) BrC over the BoB are quite similar to that studied from a sampling site in the Indo-Gangetic Plain (IGP), suggesting the dominant impact of organic aerosols associated with the continental outflow. A comparison of the mass absorption efficiency of BrC and elemental carbon (EC) brings to focus the significant role of light absorbing organic aerosols (from biomass burning emissions) in atmospheric radiative forcing over oceanic regions located downwind of the pollution sources.