Atmospheric brown carbon (BrC), a short-lived climate forcer, absorbs solar radiation and is a substantial contributor to the warming of the Earth ' s atmosphere. BrC composition, its absorption properties, and their evolution are poorly represented in climate models, especially during atmospheric aqueous events such as fog and clouds. These aqueous events, especially fog, are quite prevalent during wintertime in Indo-Gangetic Plain (IGP) and involve several stages (e.g., activation, formation, and dissipation, etc.), resulting in a large variation of relative humidity (RH) in the atmosphere. The huge RH variability allowed us to examine the evolution of water-soluble brown carbon (WS-BrC) diurnally and as a function of aerosol liquid water content (ALWC) and RH in this study. We explored links between the evolution of WS-BrC mass absorption efficiency at 365 nm (MAE WS- BrC-365 ) and chemical characteristics, viz., low-volatility organics and water-soluble organic nitrogen (WSON) to water-soluble organic carbon (WSOC) ratio (org-N/C), in the field (at Kanpur in central IGP) for the first time worldwide. We observed that WSON formation governed enhancement in MAE WS-BrC-365 diurnally (except during the afternoon) in the IGP. During the afternoon, the WS-BrC photochemical bleaching dwarfed the absorption enhancement caused by WSON formation. Further, both MAE WS-BrC-365 and org-N/C ratio increased with a decrease in ALWC and RH in this study, signifying that evaporation of fog droplets or bulk aerosol particles accelerated the formation of nitrogen-containing organic chromophores, resulting in the enhancement of WS-BrC absorptivity. The direct radiative forcing of WS-BrC relative to that of elemental carbon (EC) was -19 % during wintertime in Kanpur, and - 40 % of this contribution was in the UV -region. These findings highlight the importance of further examining the links between the evolution of BrC absorption behavior and chemical composition in the field and incorporating it in the BrC framework of climate models to constrain the predictions.

Intra-annual variability of tree-ring oxygen stable isotopes (delta O-18) can record seasonal climate variability and a tree's ecophysiological response to it. Variability of sub-annual tree-ring delta O-18 maxima and minima, which usually occur in different parts of the growing season, may exhibit different climatic signals and can help in understanding past seasonal moisture conditions, especially in Asian monsoon areas. We developed minimum and maximum tree-ring delta O-18 series based on sub-annual tree-ring delta O-18 measurements ofPinus massonianaat a humid site in southeastern China. We found that interannual variability in minimum tree-ring delta O-18 is primarily controlled by the July-September soil water supply and source water delta O-18, whereas the maximum latewood tree-ring delta O-18 is primarily controlled by the relative humidity (RH) in October. The maximum of variability of earlywood tree-ring delta O-18 records the RH of October of the previous year. We used minimum and maximum tree-ring delta O-18 to develop two reconstructions (1900-2014) of seasonal moisture availability. The summer soil water supply (July-September self-calibrated Palmer drought severity index) and the RH in fall show contrasting trends, which may be related to late-growing seasonal warming leading to a high vapor capacity and high atmospheric moisture. Our findings are valuable for research that aims to explore seasonal moisture changes under anthropogenic climate change and the ecological implications of such contrasting trends.

The monthly/seasonal characteristics of the concentration and aerosol optical depth (AOD) of four aerosol components (water-soluble, insoluble, black carbon (BC), and sea-salt) and their direct radiative forcing (DRF) were investigated at three environmental locations in Southeast Texas. We used fine particulate matter (PM2.5) samples measured at one urban residential (Aldine (AD)) and two suburban (Deer Park (DP) and West Liberty (WL)) sites located around Houston during 2016-2017, and performed model-based analysis using the mass concentrations of the four aerosol components to evaluate their impact on the DRF. Overall, the concentrations, AODs, and DRFs of all four aerosol components at AD were higher than those at DP and WL during the study period. In particular, the water-soluble component was the most dominant contributor, except for absorbing BC. The monthly AOD patterns of the four individual aerosol components (especially, water-soluble and BC) at the three sites were found to have strong associations with their concentrations and/or relative humidity (RH). The DRFs at the top of the atmosphere (DRFTOA) and surface level (DRFSFC) for most of the aerosol components were found to be highest in winter 2017 (AD), spring 2016 and winter 2017 (DP), and winter 2016 and fall 2017 (WL). The exceptions were sea-salt and insoluble components, which showed a peak in summer 2016 and no distinct monthly variation, respectively. Uncertainties in the DRFs of the four target aerosol components calculated using in-situ RH measurements were found to be less than 20%, with the exception of the water-soluble component at WL (24%). A sensitivity test showed that the DRFs of the aerosol components were slightly and significantly influenced by changes in AOD and single scattering albedo, respectively; additionally, sensitively changed with RH.

We estimated the current (base years) and future (2021-2100) direct radiative forcing ( DRF) of four aerosol components (water-soluble, insoluble, black carbon (BC), and sea-salt) at urban (Yeonsan (Busan) and Gwangjin (Seoul)) and background sites (Aewol and Gosan (Jeju Island)), based on a modeling approach. The analysis for base years was conducted using PM2.5 samples measured at two urban and two background sites (Yeonsan and Gwangjin: 2016, Aewol and Gosan: 2014). The future DRFs were estimated according to changes in relative humidity (RH) of RCP8.5 climate change scenario at the same sites during four different periods (PI: 2021 similar to 2040, PII: 2041 similar to 2060, PIII: 2061 similar to 2080, and PIV: 2081 similar to 2100). In addition, we compared the differences between the DRFs of future (PI similar to PIV) and base years (2016 and 2014). Overall, the water-soluble component was predominant over all other components in terms of the concentrations, optical parameters (e.g., AOD), and DRFs, regardless of sites. For the base years, the monthly patterns of total DRFs for all components and the DRFs for the water-soluble component varied with sites, and months of their highest and lowest DRFs were different depending on sites. This might be due to the combined effect of the monthly patterns of the concentrations and RHs for each site. For the differences between the DRFs of future and base years, the highest future DRFs at Yeonsan and Aewol ranged from -59 to -63 W/m(2) increasing -20 (July in PII) to -28 W/m(2) (August in PIII) compared to the base years and from -73 to -74 W/m(2) increasing -31 (July in PII) to -41 W/m(2) (September in PIV), respectively. These DRFs at Gwangjin and Gosan ranged from -79 to -84 W/m(2) increasing -29 (June in PII and PIII) to -34 W/m(2) (June in PI) and from -58 to -92 W/m(2) increasing -14 (July in PII) to -26 W/m(2) (May in PI), respectively. The high heating rates at Yeonsan (up to 4.4 K/day in November) and Aewol (up to 3.7 K/day in February) of BC component might be caused by its strong radiative absorption.

The concentration characteristics of four aerosol chemical components (water-soluble, insoluble, black carbon (BC), and sea-salt) and their direct radiative forcing (DRF) were analyzed using the daily or hourly data (PM2.5) measured at urban (Yeonsan, Gwangbok, Hakjang, and Gijang in Busan) and background sites (Aewol in Jeju Island) during haze events, based on a modeling approach. Overall, the concentrations of water-soluble component and its impact on the DRF were predominant over all other components at most of the sites (especially at Aewol, Gwangbok and/or Hakjang). The DRFs at the surface (DRFSFC), top of the atmosphere (DRFTOA), and the atmosphere (DRFATM) for most aerosol components (except for BC) at most of the sites (except for Yeonsan) were high in spring or winter and low in summer or fall. Meanwhile, the DRFs at Yeonsan were highest in summer (for DRFTOA) or fall (for DRFSFC) and lowest in spring (for both). These seasonal DRF characteristics in the study sites might be closely related to the seasonal patterns of aerosol component concentrations and/ or meteorological conditions (e.g., relative humidity). In addition, the positive DRFATM of BC in the study sites was highest among the all aerosol components due to strong radiative absorption. The differences in DRFs for water-soluble component between haze and non-haze periods were largest in the all study sites. In particular, the DRFTOA (and DRFSFC) of water-soluble at the sites of Gwangbok and Aewol during the haze periods were higher by a factor of 1.8 and 2.3 (and a factor of 1.9 and 2.4) than those during the non-haze periods.

The short wave direct Aerosol Radiative Forcing (ARF) at a semi urban coastal location near Chennai (12.81 degrees N, 80.03 degrees E, similar to 45 m amsl), a mega city on the east coast of India has been estimated for all the four seasons in the year 2013 using the SBDART (Santa Barbara Discrete ordinate Atmospheric Radiative Transfer) model. As inputs to this model, measured aerosol parameters together with modeled aerosol and atmospheric parameters are used. The ARF in the atmosphere is found to be higher in the pre-monsoon and winter seasons compared to the other seasons whereas at the surface, it is found to be higher in the south-west (SW) monsoon and winter seasons. The estimated ARF values are compared with those reported over other locations in India. The effect of Relative Humidity on ARF has been investigated for the first time in the present study. It is found that the ARF increases with increasing RH in the SW monsoon and winter seasons. An unique feature of the present study is the comparison of the net surface short wave fluxes estimated from the model (SBDART) and measured fluxes using CNR 4 net radiometer. This comparison between the estimated and measured fluxes showed good agreement, providing a 'closure' for the estimates. (C) 2015 Elsevier Ltd. All rights reserved.