River Breezes for Pollutant Dispersion in GoAmazon2014/5

Abstract. The effect of river breezes on pollutant plume dispersion or canalization in the central Amazon was evaluated. A pollution plume changes atmospheric composition downwind of Manaus, a city of 2 million people positioned at the confluence between two wide rivers. Herein, to evaluate the effects of river breezes, two cases were modeled at the mesoscale for March 2014. The first case, "with rivers" (wR), simulated the transport and chemistry of the Manaus pollution plume as the rivers were in reality. The second case, "without rivers" (woR), carried out simulations for which all rivers and floodable areas were replaced by forest. The three main conclusions are as follows: (1) Between the two cases, alterations in wind speeds were maximum at local noon, and river breezes influenced horizontal wind fields from surface up to 150 m in altitude, suggesting a capping height of 150 m on most days for the influence of river breezes on pollutant concentrations. In agreement with this modeling result, data sets collected at 500 m by aircraft flights showed no apparent influence of the underlying rivers on plume dispersion. The flights traversed the plume downwind of Manaus during the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) Experiment. (2) Between the wR and wOR cases, changes to downwind concentrations of O₃, NO_x, and CO pollutants were < 6 % as a monthly average at the supersite "T3" of GoAmazon2014/5, which was 70 km downwind of Manaus and located between the two main rivers. As single events at T3, maximum one-hour concentration differences were 39 ppb for O₃, 5 ppb for NO_x, and 26 ppb for CO. (3) For a focus on the surface layer of the rivers (0 to 150 m in height), river breezes increased the monthly average O₃, NO_x, and CO surface concentrations by 25 %, 25 %, and < 5 %, respectively. In addition, strong canalization occurred 5 % of the time based on a difference of 10 ppb in the surface concentrations of at least two of O₃, NO_x, and CO between the wR and wOR cases. In conclusion, although pollutants dispersed by river breezes could at times be a strong effect on observed pollutant concentrations in the surface river boundary layer, overall most pollution was transported at heights well above the effects of the river breezes and moved downwind along the trajectories of the dominant trade winds.