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Improving carbon efficiency and reducing carbon intensity are effective means of mitigating climate change. Carbon emissions due to urban residential energy consumption have increased significantly; however, there is a lack of research on urban residential carbon intensity. This paper examines the spatiotemporal variation of carbon intensity in the residential sector during 2001-2015, and then identifies the causes of the variation by utilizing the logarithmic mean Divisia index (LMDI) with the help of Microsoft Excel 2016 for 620 county-level cities in 30 Chinese provinces. The results show that high carbon intensity is mainly found in large cities, such as Beijing, Tianjin, and Shanghai. However, these cities showed a downward trend in carbon intensity. In terms of influencing factors, the energy consumption per capita, urban sprawl, and land demand are the three most influential factors in determining the changes in carbon intensity. The effect of energy consumption per capita mainly increases the carbon intensity, and its impact is higher in the municipal districts of provincial capital cities than in other types of cities. Similarly, the urban sprawl effect also promotes increases in carbon intensity, and a higher degree of influence appears in large cities. However, as urban expansion plateaus, the effect of urban sprawl decreases. The land-demand effect reduces the carbon intensity, and the degree of influence of the land-demand effect on carbon intensity is also clearly stronger in big cities. Our findings show that lowering the energy consumption per capita and optimizing the land-use structure are a reasonable direction of efforts, and the effects of differences in influencing factors should be paid more attention to reduce carbon intensity.

Sulfate, as a major component of aerosol particles, greatly contributes to haze formation and affects global climate change. Although formation pathways of sulfate aerosols from the conversion of SO 2 have been extensively studied, the discrepancy between field observations and model simulations suggests that there are still unknown sulfate sources. Herein, we report for the first time a photoinduced SO 2 uptake and oxidation pathway in Beijing urban PM 2.5 aerosols. In comparison with the NO 2 - and O 3 -induced SO 2 oxidation pathways, this SO 2 photo-oxidation in Beijing urban PM 2.5 could make an important contribution to the daytime sulfate formation. Reactive species, such as • OH radicals and H 2 O 2 , are the major oxidants leading to sulfate formation in PM 2.5 . The water-soluble matter (WSM) and water-insoluble organic matter (WISOM) in PM 2.5 were identified as the main photo-oxidant producers. Our work highlights an important daytime sulfate source in the atmosphere and provides new insight into the photochemical aging of ambient aerosols.

Photochemical aging represents an important transformation process of aerosol particles in the atmosphere, which greatly influences the physicochemical properties and the environmental impact of aerosols. In this work, we find that Beijing urban PM 2.5 aerosol particles release substantial HONO, a significant precursor of •OH radicals, into the gas phase during the photochemical aging process. The generation of HONO exhibits a high correlation with the amount of nitrate in PM 2.5 . The formation rate of HONO becomes gradually decreased with the irradiation time, but can be restored by introducing the acidic proton, indicative of the essential role of the acidic proton in the HONO production. Other environmental factors such as relative humidity, light intensity, and reaction temperature also possess important influences on HONO production. The normalized photolysis rate constant for HONO ( J HNO3→HONO ) is in the range of 1.22 × 10 -5 s -1 ∼ 4.84 × 10 -4 s -1 , which is 1-3 orders of magnitude higher than the reported photolysis rate constant of gaseous HNO 3 . The present study implies that the photochemical aging of Beijing PM 2.5 is an important atmospheric HONO production source.