Influences of nitrogen oxides and isoprene on ozone-temperature relationships in the Amazon rain forest

As human encroachment increases in the Amazon rain forest, it is important to determine how anthropogenic emissions of reactive gases affect regional atmospheric chemistry. In the present study, we investigate the extent to which urban air plumes modify the levels of ozone (O₃) and nitrogen oxides (NO_X) in the downwind rain forest. The median mixing ratios of the background O₃, NO_X, and NO_X oxidation products (NO_Z) were 20 (15 parts per billion on a volume basis, ppbv), 0.6 ppbv (0.6 ppbv), and 1.0 ppbv (0.5 ppbv) during the dry (wet) season at the study site. Compared to the background environment, air plumes from the city of Manaus had enhanced median mixing ratios for O₃ and NO_Z by 30–50% and 40–90%, respectively. However, the enhancements of NO_X in the air plumes were less than 20%, indicating that the majority of NO_X was chemically converted to O₃ and NO_Z during transport. Results from a photochemical model showed that an injection of 8 ppbv of NO_X into the rain forest can cause up to 260% and 150% increases in O₃ and hydroxyl radical (OH) levels compared to the background conditions, indicating the likely extent that NO_X can modify the air quality and oxidative capacity in the Amazon rain forest. Slopes of the O₃-temperature linear relationships increased with NO_X levels from 3.7 to 6.5 ppbv per degree Kelvin during the dry season and 1.7–5.5 ppbv per degree Kelvin during the wet season. Average rates of change of the slope with respect to NO_Z were approximately 1.8 and 2.3 times higher than those with respect to NO_X for the dry and wet seasons. One key conclusion of this study is that NO_Z substantially contributed to the O₃ formation in response to temperature under enhanced NO_X conditions in the forested environment.