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This study is the first attempt to understand the synergistic impact of anthropogenic and biogenic emissions on summertime ozone (O3) formation in the Guanzhong (GZ) Basin where Xi'an, the oldest and the most populous city (with a population of 9 million) in northwestern China, is located. Month-long (August 2011) WRF-Chem simulations with different sensitivity experiments were conducted and compared with near-surface measurements. Biogenic volatile organic compounds (VOCs) concentrations was characterized from six surface sites among the Qinling Mountains, and urban air composition was measured in Xi'an city at a tower 100ma.s. The WRF-Chem control experiment reasonably reproduced the magnitudes and variations of observed O3, VOCs, NOx, PM2.5, and meteorological parameters, with normalized mean biases for each parameter within -21 %. Subsequent analysis employed the factor separation approach (FSA) to quantitatively disentangle the pure and synergistic impacts of anthropogenic and/or biogenic sources on summertime O3 formation. The impact of anthropogenic sources alone was found to be dominant for O3 formation. Although anthropogenic particles reduced NO2 photolysis by up to 60 %, the anthropogenic sources contributed 19.1 ppb O3 formation on average for urban Xi'an. The abundant biogenic VOCs from the nearby forests promoted O3 formation in urban areas by interacting with the anthropogenic NOx . The calculated synergistic contribution (from both biogenic and anthropogenic sources) was up to 14.4 ppb in urban Xi'an, peaking in the afternoon. Our study reveals that the synergistic impact of individual source contributions to O3 formation should be considered in the formation of air pollution control strategies, especially for big cities in the vicinity of forests. [ABSTRACT FROM AUTHOR]

At present, there is limited research on the effect of climate change on the growth of trees in the Qinling Mountains, and the effect of altitude has rarely been considered. In order to accurately grasp the impact of climatic factors on the natural forest in this region, the effect of water and temperature conditions on the radial growth of Pinus armandii was studied according to dendroclimatology principles. Response function and multivariate regression methods were applied to study the response of the tree-rings, including earlywood width and latewood width, of P. armandii at three elevations (1600-1800, 1800-2000, 2000-2200 m) to climate change in this region. The results showed that the radial growth of P. armandii at the three elevations contained rich climatic information. The tree-ring widths at the three elevations were significantly negatively correlated with September temperature of the previous year and May temperature of the current year (P < 0.05) and were also negatively correlated with August precipitation of the previous year and May precipitation of the current year. The late-wood width of P. armandii at 1600-1800 m and 1800-2000 m was less sensitive to temperature and precipitation than that of the earlywood. The latewood width of P. armandii at 2000-2200 m was significantly negatively correlated with December temperature of the previous year and May temperature of the current year (P < 0.01), and was significantly and positively correlated with August precipitation of the previous year (P < 0.05). Characteristic analysis of the special years showed that large quantities of precipitation in September and October of the previous year were more likely to result in the formation of a very wide tree-ring. High spring and summer temperatures and less precipitation from April to June more likely resulted in the formation of extremely narrow tree-rings. Therefore, high precipitation in autumn of the previous year was conducive to the growth of P. armandii, whereas high temperature and drought in spring and summer inhibited the growth of P. armandii. [ABSTRACT FROM AUTHOR]