River Breezes in the Central Amazon: Cluster Analysis of Meteorological and Chemical Data Sets Collected by an Unmanned Aerial Vehicle

Local atmospheric circulations induced by wide rivers in Amazonia strongly influence the transport of urban, fire, and forest emissions. Herein, a copter-type unmanned aerial vehicle (UAV) operated from a boat was used to collect vertical profiles of meteorological variables and chemical concentrations. The measurements were conducted from surface elevation to 500 m in altitude over the Rio Negro in the central Amazon from 10:00 to 17:00 during Sep-Oct 2019 of the dry season. Wind direction and speed were recorded onboard the UAV. A sensor package mounted on the UAV measured concentrations of carbon monoxide (CO) and total oxidants (Ox, defined as O3 + NO2). Multivariate statistical analysis identified four clusters for the 56 vertical profiles of meteorological variables. The clusters were closely connected to local patterns of atmospheric circulation. The findings showed that river breeze circulations were fully developed at times when the synoptic scale wind speeds were slower than 4 m s 1 in the absence of high-altitude clouds. River breeze circulations were found to significantly alter CO and Ox profiles depending on the riverbank emission conditions. Numerical experiments through large-eddy simulation supported that under some conditions a river breeze can enhance pollutant mixing perpendicular to the river orientation even when the synoptic scale winds are parallel. The results of this study advance an urgent need to quantify the occurrence and the properties of river breeze circulations in respect to microscale chemical dispersion, air quality, and human health because nearly all human settlements throughout the Amazon region are along rivers.