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Urban forests play an important role in mitigating climate change in urban ecosystems. In order to obtain greater carbon sequestration benefits, it is crucial to advance the understanding of soil CO 2 efflux from urban forests. This study was undertaken to identify biotic and abiotic parameters that explained spatial variation in soil respiration, especially the effect of forest stand structure. To this aim, we measured soil respiration at 44 points in a 30 m × 50 m plot in an urban forest in Beijing over a 10 month period. The spatial variations of soil respiration were investigated, relating to soil climate, soil nutrients, root biomass and forest structural traits. The highest spatial variation in soil respiration was observed during the summer dry period, and similar degrees of variability were shown both in the growing and dormant season. Vegetation area index, soil moisture and nitrate nitrogen were the determinable parameters that influenced the spatial variation in soil respiration in the growing season, while nitrate nitrogen and soil temperature in the dormant season. Soil temperature, sulfur and roots biomass also affected the soil respiration during the growing season. Moreover, mean DBH and tree number within a 6-m radius had a significant influence on the spatial pattern of soil respiration at sub-plot level. Our results suggest that decreasing the soil CO 2 efflux might be an optional way to increase carbon sequestration potential for urban forest. This can be achieved by regulating forest stand structure and applying appropriate maintenance practices. [ABSTRACT FROM AUTHOR]

Abstract: Currently, China faces both the challenge of mitigating the CO2 emissions in coping with global climate change and abating air pollutants emissions in dealing with domestic environmental stresses. Although there being on-going discussions, it is still not clear for China that how to integrate greenhouse gas abatement into air pollution control policy framework now. In this study, we try to estimate CO2 reduction by major greening Olympic measures for 2008 Beijing Olympic Games, aiming to provide evidence on curbing CO2 emissions as a co-benefit for improving local air quality by mega-events. CO2 emission reduction by three types of measures has been estimated including: energy related measures, transport management, and temporary air pollution control measures. As estimated, from the perspective of long-term effect, the measure of clean energy substitution and improving energy efficiency and application of advance energy saving technologies would be cost -effectiveness measures for China addressing both air pollution control and green-house gases abatement. We suggest China government integrate CO2 reduction to local air quality improvement policy framework, and take mega-events as triggers for more sustainable urbandevelopment. [Copyright &y& Elsevier]

The mass absorption efficiency (MAE) and mass scattering efficiency (MSE) of aerosols are critical to their light extinction capacity. In winter in northern China, episodes of clear air and heavy haze alternately occur due to the high primary emission and secondary formation of aerosols, coupled with changing weather conditions. These occurrences facilitate detailed investigation of how significant changes in the pollution level impact the MAE and MSE, they also enable the determination of whether current air quality improvement measures are also beneficial to the mitigation of climate change, to avoid climatic side effects of attempts to clean the air. In the present study, a one-month observation campaign was conducted in Beijing, China, during the 2016–2017 winter season. A photoacoustic extinctiometer, which is an in situ measurement instrument, was used to investigate the aerosol absorption coefficient (σ a) and scattering coefficient (σ s) at an infra-red wavelength of 870 nm. The MAE and MSE were found to vary differently with increasing pollution level. A positive correlation between the particulate matter concentration (PM 2.5) and the MAE was observed for PM 2.5 < 100 μg m−3, with the increase in the MAE declining for PM 2.5 = 100–200 μg m−3, and the MAE subsequently stabilizing. In contrast, the MSE continuously increased with increasing PM 2.5. The single scattering albedo (SSA), which governs the climatic effect of aerosols, was observed to be a function of both σ a and σ s , increasing from 0.827 ± 0.018 (average ± standard deviation) for a bottom particulate matter concentration group of (0, 25] μg m−3 to 0.924 ± 0.007 for a top particulate matter concentration group of (300, 400] μg m−3, indicating a positive correlation between the SSA and PM 2.5. These observations suggest a risk of current clean air measures weakening or eliminating the cooling effect of ambient aerosols in northern China. It seems that no previous study has involved the simultaneous acquisition of σ a and σ s at the same wavelength with the purpose of determining how increasing air pollution differently impacts the MAE and MSE. Practical measures that can be used to decrease the black carbon-to-PM 2.5 ratio include phasing out or reducing black carbon-rich sources and promoting technologies that decrease the black carbon-to-organic carbon ratio. Image 1 • MAE and MSE were found to show different responses to escalating pollution levels. • A risk of weakened cooling effect of ambient aerosol with the clean air process. • Practical solution was proposed to against the risks in China's clean air process. [ABSTRACT FROM AUTHOR]