Measurement report: Number size distribution of sub-40 nm particles in the Amazon rainforest

Abstract. The Amazon rainforest is a unique environment to investigate aerosol properties with limited impact from human activities, further providing a new perspective to look at the aerosol characteristics in regions heavily affected by anthropogenic emissions. Obtaining the size distributions of nucleation mode particles in the atmosphere is key to understanding aerosol formation, evolution and their impacts. Although routine and long-term aerosol measurements have been conducted in the Amazon region, information regarding sub-10 nm particles is still limited. In this study, we performed aerosol measurements from December 2022 to January 2023 on a 54-meterhigh platform of the Amazon Tall Tower Observatory (ATTO). Three advanced instruments namely the Nano Condensation Nuclei Counter (nCNC), the Neutral Cluster and Air Ion Spectrometer (NAIS), and the NanoScanning Mobility Particle Sizer (SMPS), were employed to measure the number size distributions of aerosol particles and naturally charged ions smaller than 40 nm. The results reveal that the median total number concentration of the measured particles with diameters ranging from 1.5 nm to 1000 nm was 969 cm^-3. We found concentration. There was a significant increase in the number concentration of sub-3 nm particles in January 2023 (median, 573 cm^-3) a large number of particles smaller than 3.5 nm, which accounted for up to 59 % of the measured total number compared to December 2022 (371 cm^-3). The median number concentration of particles above 3.5 nm in December and January were 481 and 335 cm^-3, respectively. No typical regional new particle formation events were observed throughout the measurement period. However, clear diurnal variations were observed for the sub-3 nm particles under pristine conditions, with the maximum concentration around noontime. Similar diurnal patterns were also observed for natural cluster ions (0.8–2 nm), with their concentration in January slightly higher than in December. Quantifying the properties of the aerosol particles in the Amazon rainforest helps to understand the processes governing the aerosol budget in the pristine atmosphere, and is essential for determining the impact of anthropogenic aerosols on climate.