How Different are the INP Populations in the Central and Northwestern Amazon?

The Amazon basin is usually considered a pristine environment with low aerosol concentrations during the wet season. However, besides the ubiquitous biogenic aerosol particles, anthropogenic particles can be locally emitted or transported into the Amazon throughout the year. Although it is well-known that aerosol particles are key players in cloud formation and that most of the precipitation in Tropical latitudes is originated in ice-containing clouds, the ice nucleating abilities of the aerosol particles present in this unique and pristine region are poorly understood. The only study that focuses on ice nucleating particles (INPs) in the Amazon was performed in 2008 in the Central Amazon during the wet season. The authors found INP concentrations as high as 33.7 L^-1, mainly composed of carbonaceous material (biological particles) and dust.

Aiming to improve the current understating of the role that the Amazonian aerosol particles play in mixed-phase cloud formation as well as in the local/regional precipitation patterns, the Spring NOrthern aMazon Aerosol eXperiment (SNOMAX) campaign was carried out in the northwestern Amazon (El Trueno, Guaviare, Colombia) from 14 to 27 March, 2024. Aerosol particles were collected on different substrates to characterize their chemical (elemental, ionic, and organic content) and microbiological diversity composition. Additionally, on-line sensors were deployed to measure the total particle number concentration (TPNC>30 nm), particle size distribution (300 nm to 25 μm), and particle mass concentration (PM2.5 and PM10). The INP concentration, as a function of particle size (0.32 μm – 10 μm), was obtained from the particles collected with a MicroOrifice Uniform Deposit Impactor (MOUDI) using the UNAM-Droplet Freezing Technique (UNAM-DFT). Although the sampling site is a very remote place, high aerosol particle loadings were observed (e.g., TPNC and PM10 as high as 3510 cm^-3 and 90 μg m^-3, respectively). Therefore, in the present work we will discuss the ice nucleating abilities and the most likely INP sources in the northwestern Amazon, and the influence of the biomass burning emissions on the INP population. Finally, a direct comparison of the INP populations from the Central and northwestern Amazon will be presented.