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Anthropogenic aerosols have been shown to perturb CO2 exchange between the vegetation and the atmosphere. However, the climate effects of aerosols through carbon cycle feedback still have significant uncertainties. Taking advantage of the periodic fluctuations of aerosol loading in Beijing, we intensively measured the diurnal course of leaf microclimates and photosynthesis under different aerosol conditions during the growing season in 2014 and 2015. We found that increasing aerosol loadings altered the diurnal course of microclimates and thus sun and shade leaf photosynthesis. Our mechanistic photosynthesis model experiments further showed that aerosol‐induced increase in sun leaf photosynthesis occurred around noon and afternoon, mainly by alleviating the depression of photosynthesis caused by high leaf temperature and leaf‐air vapour pressure deficit. Meanwhile, aerosols enhanced shade leaf photosynthesis throughout the day by mitigating the light limitation within the canopy, with the highest increase occurring around noon. Overall, our study suggested that aerosol's diffuse fertilization effect, cooling effect and the accompanying low leaf‐air vapour pressure deficit collectively drove the changes in the diurnal courses of sun and shade leaf photosynthesis. Our results provided an important benchmark for assessing how anthropogenic aerosols regulate ecosystem C balance under different meteorological conditions. [ABSTRACT FROM AUTHOR]

Atmospheric pollution could significantly alter tree growth independently and synergistically with meteorological conditions. North China offers a natural experiment for studying how plant growth responds to air pollution under different meteorological conditions, where rapid economic growth has led to severe air pollution and climate changes increase drought stress. Using a single aspen clone (Populus euramericana Neva.) as a 'phytometer', we conducted three experiments to monitor aspen leaf photosynthesis and stem growth during in situ exposure to atmospheric pollutants along the urban-rural gradient around Beijing. We used stepwise model selection to select the best multiple linear model, and we used binned regression to estimate the effects of air pollutants, atmospheric moisture stress and their interactions on aspen leaf photosynthesis and growth. Our results indicated that ozone (O 3) and vapor pressure deficit (VPD) inhibited leaf photosynthesis and stem growth. The interactive effect of O 3 and VPD resulted in a synergistic response: as the concentration of O 3 increased, the negative impact of VPD on leaf photosynthesis and stem growth became more severe. We also found that nitrogen (N) deposition had a positive effect on stem growth, which may have been caused by an increase in canopy N uptake, although this hypothesis needs to be confirmed by further studies. The positive impact of aerosol loading may be due to diffuse radiation fertilization effects. Given the decline in aerosols and N deposition amidst increases in O 3 concentration and drought risk, the negative effects of atmospheric pollution on tree growth may be aggravated in North China. In addition, the interaction between O 3 and VPD may lead to a further reduction in ecosystem productivity. Image 1 • Nitrogen deposition can stimulate aspen growth by increasing canopy nitrogen uptake. • Aerosols can promote aspen growth through diffuse radiation fertilization effects. • Among all pollutants, ozone is the most important factor to inhibit aspen growth. • The effect of VPD on aspen growth is negative and aggravated by ozone pollution. The response of leaf photosynthesis and stem growth to multiple atmospheric pollutants. [ABSTRACT FROM AUTHOR]