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River ecosystems contribute significantly to CO2 emissions. However, estimates of global riverine CO2 emissions remain greatly uncertain owing to the absence of a comprehensive and spatially resolved CO2 emission measurement. Based on intensive field measurements using floating chambers, riverine CO2 evasion in the Wuding River catchment on the Loess Plateau was investigated. Lateral carbon derived from soil respiration and chemical weathering played a central role in controlling the variability of riverine CO2 partial pressure (pCO2). In addition, in‐stream processing of allochthonous organic carbon was an also important source of CO2 excess, modulating the influence of lateral carbon inputs. All the surveyed streams were net CO2 sources, exhibiting pronounced spatial and seasonal variabilities. The mean CO2 efflux was 172, 116, and 218 mmol m−2 d−1 in spring, summer, and autumn, respectively. Unlike the commonly observed strongest CO2 emissions in headwater streams, the increasing CO2 efflux with stream order in the Wuding River catchment reflects its unique geomorphologic landscape in controlling CO2 emissions. While in reservoirs, the pCO2 was more controlled by primary production with aquatic photosynthetic assimilation constraining it to a lower level. Both the magnitude and direction of CO2 evasion from reservoirs have been greatly altered. Contrast to streams with large CO2 effluxes, reservoirs were small carbon sources and even carbon sinks, due primarily to greatly reduced turbulence and enhanced photosynthesis. In view of the large number of reservoirs on the Loess Plateau, assessing the resulting changes to CO2 emissions and their implications for regional carbon budgets warrants further research.

CO 2 outgassing across water-air interface is an important, but poorly quantified, component of riverine carbon cycle, largely because the data needed for flux calculations are spatially and temporally sparse. Based on compiled data sets measured throughout the Yellow River watershed and chamber measurements on the main stem, this study investigates CO 2 evasion and assesses its implications for riverine carbon cycle. Fluxes of CO 2 evasion present significant spatial and seasonal variations. High effluxes are estimated in regions with intense rock weathering or severe soil erosion that mobilizes organic carbon into the river network. By integrating seasonal changes of water surface area and gas transfer velocity (k), the CO 2 efflux is estimated at 7.9 ± 1.2 Tg C yr À1 with a mean k of 42.1 ± 16.9 cm h À1. Unlike in lake and estuarine environments where wind is the main generator of turbulence, k is more correlated with flow velocity changes. CO 2 evasion in the Yellow River network constitutes an important pathway in its riverine carbon cycling. Analyzing the watershed-scale carbon budget indicates that 35% of the carbon exported into the Yellow River network from land is degassed during fluvial transport. The CO 2 efflux is comparable to the carbon burial rate, while both larger than the fluvial export to the ocean. Comparing CO 2 evasion with ecosystem productivity in the Yellow River watershed shows that its ecosystem carbon sink has previously been overestimated by >50%. Present efflux estimates are associated with uncertainty, and future work is needed to mechanistically understand CO 2 evasion from the highly turbid waters.

Abstract. Coordinated control of fine particulate matter (PM2.5) and ozone (O3) has become a new and urgent issue for China’s air pollution control. Biogenic volatile organic compounds (BVOCs) are important precursors of O3 and secondary organic aerosol (SOA) formation. China experienced a rapid increase in BVOC emissions as a result of increased vegetation biomass. We applied WRF-Chem3.8 coupling with MEGAN2.1 to conduct long-term simulations for impacts of BVOC emissions on O3 and SOA during 1981–2018, using the emission factors extrapolated by localized emission rates and annual vegetation biomass. In summer of 2018, BVOC emissions are 9.91 Tg (in June), which lead to an average increase of 8.6 ppb (16.75 % of the total) in daily maximum 8-h (MDA8) O3 concentration and 0.84 μg m−3 (73.15 % of the total) in SOA over China. The highest contribution to O3 is concentrated in the Great Khingan Mountains, Qinling Mountains, and most southern regions, while southern areas for SOA. Isoprene has the greatest contribution to O3 while monoterpene has the largest SOA production. BVOC emissions have distinguished impacts in different regions. Chengdu-Chongqing (CC) region has the highest O3 and SOA generated by BVOCs while Beijing-Tianjin-Hebei (BTH) region has the lowest. From 1981 to 2018, the interannual variation of BVOC emissions caused by increasing leaf biomass results in O3 concentration increasing by 7.38 % at an average rate of 0.11 ppb yr−1, and SOA increasing by 39.30 % at an average rate of 0.008 μg m−3 yr−1. Due to the different changing trends of leaf biomass by regions and vegetation types, O3 and SOA show different interannual variations. Fenwei Plain (FWP), Yangtze River Delta (YRD), and Pearl River Delta (PRD) regions have the most rapid O3 increment while the increasing rate of SOA in CC is the highest. BTH has the smallest enhancement in O3 and SOA concentration. This study will help to recognize the impact of historical BVOC emissions on O3 and SOA, and further provide the reliable scientific basis for the precise prevention and control of air pollution in China.