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A wide range of estimates on global wetland methane (CH 4 ) fluxes has been reported during the recent two decades. This gives rise to urgent needs to clarify and identify the uncertainty sources, and conclude a reconciled estimate for global CH 4 fluxes from wetlands. Most estimates by using bottom-up approach rely on wetland data sets, but these data sets show largely inconsistent in terms of both wetland extent and spatiotemporal distribution. A quantitative assessment of uncertainties associated with these discrepancies among wetland data sets has not been well investigated yet. By comparing the five widely used global wetland data sets (GISS, GLWD, Kaplan, GIEMS and SWAMPS-GLWD), it this study, we found large differences in the wetland extent, ranging from 5.3 to 10.2 million km 2 , as well as their spatial and temporal distributions among the five data sets. These discrepancies in wetland data sets resulted in large bias in model-estimated global wetland CH 4 emissions as simulated by using the Dynamic Land Ecosystem Model (DLEM). The model simulations indicated that the mean global wetland CH 4 emissions during 2000–2007 were 177.2 ± 49.7 Tg CH 4 yr −1 , based on the five different data sets. The tropical regions contributed the largest portion of estimated CH 4 emissions from global wetlands, but also had the largest discrepancy. Among six continents, the largest uncertainty was found in South America. Thus, the improved estimates of wetland extent and CH 4 emissions in the tropical regions and South America would be a critical step toward an accurate estimate of global CH 4 emissions. This uncertainty analysis also reveals an important need for our scientific community to generate a global scale wetland data set with higher spatial resolution and shorter time interval, by integrating multiple sources of field and satellite data with modeling approaches, for cross-scale extrapolation. [ABSTRACT FROM AUTHOR]

Heterogeneous aging of organic aerosols can significantly alter their physicochemical properties and thus their impacts on the radiative balance of the earth system. In this study, kinetics and mechanisms of heterogeneous reactions between suspended squalane particles and hydroxyl (OH) radicals were investigated using a self-made photooxidation flow tube and a high-resolution long time-of-flight chemical ionization mass spectrometer equipped with a Filter Inlet for Gases and AEROsols (FIGAERO-HR-LToF-CIMS). In addition to the measurements of the heterogeneous reaction rate constants for squalane and its multi-generation functionalization products whose carbon backbone are retaining, identification and semi-quantification of functionalization and fragmentation products from OH radical-initiated aging of squalane were carried out, which were utilized to develop a plausible reaction scheme. Our results show that the heterogeneous oxidation mechanism for particulate squalane is similar to those for OH radical initiated-oxidation of gaseous alkanes, but intermolecular hydrogen abstraction, in addition to reactions with O 2 , isomerization, and decomposition, could be an important heterogeneous reaction pathway for alkoxy radicals, leading to the formation of functionalization products of alcohols. The relative abundances of fragmentation products suggest that OH radicals preferably attack the tertiary carbon. The evolution of products indicates that functionalization plays a leading role in the early stage of the reaction and the fragmentation products become more and more important with increased OH exposure. Our study depicts heterogeneous reaction pathways of organic aerosols with OH radicals at the molecular level, and helps to better understand the chemical evolution of complex organic aerosols in the atmosphere. • Hydroxyl radical-initiated aging of particulate squalene was investigated. • A plausible reaction scheme was developed. • Intermolecular hydrogen abstraction is important for particulate alkoxy radicals. • Functionalization plays a leading role in the early stage of the reaction. [ABSTRACT FROM AUTHOR]

It is highly uncertain on how human and natural environmental factors have altered methane (CH 4 ) emissions from terrestrial ecosystems in India. Using a process-based, Dynamic Land Ecosystem Model (DLEM) driven by climate, land cover and land use change (LCLUC), atmospheric nitrogen deposition (NDEP), atmospheric carbon dioxide (CO 2 ) concentration, and tropospheric ozone (O 3 ) pollution, we examined CH 4 flux from terrestrial in India during 1901–2010. The DLEM simulations have shown that total CH 4 flux over the country ranged from 2.9 Tg C year −1 to 6.5 Tg C year −1 with significant inter-annual variations driven by climate during 1901–2010. Contemporary CH 4 emissions have primarily occurred from rice fields (3.9 ± 0.9 Tg C year −1 ) while wetlands contributed to 2.1 ± 0.6 Tg C year −1 in the 2000s. During 1901–2010, total CH 4 emission from the terrestrial biosphere has increased by ∼2.1 Tg C year −1 . LCLUC has increased CH 4 emissions by 2.3 Tg C year −1 primarily due to increase in the rice-based cropping systems as well as irrigation expansion during the study period. Elevated CO 2 concentration stimulated plant biomass production in both rice fields and wetlands that increased CH 4 emissions by 0.7 Tg C year −1 . On the contrary, climate change decreased net CH 4 emissions by ∼1.2 Tg C year −1 due to negative effects of extreme high temperature as well as occurrences of extreme drought events on plant growth. Our study suggests that LCLUC and elevated CO 2 concentration have significantly increased CH 4 emissions from terrestrial ecosystems in India. [ABSTRACT FROM AUTHOR]