Opposite OH reactivity and ozone cycles in the Amazon rainforest and megacity Beijing: Subversion of biospheric oxidant control by anthropogenic emissions

The Amazon rainforest in Brazil and the megacity of Beijing in China are two of the most strongly contrasting habitats on Earth. In both locations, volatile chemicals are emitted into the atmosphere affecting the local atmospheric chemistry, air quality and ecosystem health. In this study, the total reactivity in air available for reaction with the atmosphere's primary oxidant the OH radical, has been measured directly in both locations along with individual volatile organic compounds(VOC), nitrogen oxides(NOx), ozone(O3) and carbon dioxide(CO2). Peak daily OH-reactivity in the Amazon 72 s−1, (min. 27 s−1) was approximately three times higher than Beijing 26 s−1 (min. 15 s−1). However, diel ozone variation in Amazonia was small (∼5 ppb) whereas in Beijing ∼70 ppb harmful photochemical ozone was produced by early afternoon. Amazon OH-reactivity peaked by day, was strongly impacted by isoprene, and anticorrelated to CO2, whereas in Beijing OH-reactivity was higher at night rising to a rush hour peak, was dominated by NO2 and correlated with CO2. These converse diel cycles between urban and natural ecosystems demonstrate how biosphere control of the atmospheric environment is subverted by anthropogenic emissions. •Atmospheric OH reactivity was measured in a rainforest and a megacity.•Amazon reactivity peaked by day, was isoprene dominated, and anticorrelated to CO2.•Beijing reactivity peak in rush hour, was dominated by NO2 and correlated with CO2.•With high daytime reactivity in the rainforest, ozone remains low and constant.•Despite lower reactivity in megacity air, ozone increased strongly by day.