UC Irvine

Nitrogen oxides (NO _x ) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO _x concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO _x concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO _y ), indicative of NO _x exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO _y concentration, the inferred equivalent noontime OH concentrations increased by at least 250% from 0.6 × 10⁶ to 1.6 × 10⁶ cm^-3. The conclusion is that, compared to background conditions of low NO _x concentrations over the Amazon forest, pollution increased NO _x concentrations and amplified OH concentrations, indicating the susceptibility of the atmospheric oxidation capacity over the forest to anthropogenic influence and reinforcing the important role of NO _x in sustaining OH concentrations.