Across Beijing, millions of weeping willows and poplar trees turn the megacity into a green metropolis. But compounds released by these trees are becoming a major contributor to air pollution1, finds a study. Beijing is not alone. Many trees planted in urban areas ' species that are often chosen for their quick growth and climate resilience ' are worsening ozone emissions, suggests the study published in Science Advances today. Rising temperatures from global warming could exacerbate the problem. Ozone forms when volatile organic compounds (VOCs) released by some vegetation, vehicles and chemical industries react with nitrogen oxides (NOx), which are emitted mostly by traffic and industry. A series of reactions between these compounds in the presence of sunlight causes ozone to form. Breathing in ozone can inflame or damage the airways, causing breathing difficulties, particularly in people with asthma or lung disease. Co-author Bin Yuan, who studies atmospheric chemistry at Jinan...
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