Spatial-temporal variations and source contributions to forest ozone exposure in China.

As surface ozone (O3 ) concentrations have significantly increased in many regions of China, it is concerned that O3 may cause negative impacts on forests in the country. To estimate the risks of O3 exposure to forest health, several frequently used O3 exposure indices (M7, M24, N100, SUM60, W126, and AOT40f) were calculated for the entire year of 2013 and the source contributions to exposure in April and August were quantified using a source-oriented version of the Community Multiscale Air Quality (CMAQ) model. Critical loads for natural ecosystems (12 ppm-h for SUM60) and for moderately sensitive plant species (23.8 ppm-h for W126) are exceeded in 85% and 75% of the forest areas in China, respectively. About 90% of the forest areas have AOT40f higher than the critical load of 10 ppm-h. Forests in the western rim of the Sichuan Basin (WSCB), parts of the southern rim of the Qinghai-Tibetan Plateau (SQTP), Yunnan Province, and the North China Plain (NCP) have higher O3 exposure than that of other areas. In spring, transport of O3 and precursors from Southeast Asia have significant contributions to O3 exposure for forests in Yunnan, SQTP and to a less extent in WSCB. In both spring and summer, industries and transportation sectors have large contributions to O3 exposure along the WSCB and in the NCP. A higher priority in future field investigations to assess O3 impacts on forests in China should be given to these regions. The results also suggest that O3 production in China is much more due to nitrogen oxides (NOx ) than due to volatile organic compounds (VOCs). In order to protect China's forest from O3 , it would be most efficient to reduce NOx emissions from industries, transportation, and other countries in general.