Abstract
Abstract. The Amazon basin is important for understanding the global climate because of its carbon cycle and as a laboratory for obtaining basic knowledge of the continental background atmosphere. Aerosol particles play an important role in the climate and weather, and knowledge of their compositions and mixing states is necessary to understand their influence on the climate. For this study, we collected aerosol particles from the Amazon basin during the Green Ocean Amazon (GoAmazon2014/5) campaign (February to March 2014) at the T3 site, which is located about 70 km from Manaus, and analyzed them using transmission electron microscopy (TEM). TEM has better spatial resolution than other instruments, which enables us to analyze the occurrences of components that attach to or are embedded within other particles. Based on the TEM results of more than 10 000 particles from several transport events, this study shows the occurrences of individual particles including compositions, size distributions, number fractions, and possible sources of materials that mix with other particles. Aerosol particles during the wet season were from both natural sources such as the Amazon forest, Saharan desert, Atlantic Ocean, and African biomass burning and anthropogenic sources such as Manaus and local emissions. These particles mix together at an individual particle scale. The number fractions of mineral dust and sea-salt particles increased almost 3-fold when long-range transport (LRT) from the African continent occurred. Nearly 20 % of mineral dust and primary biological aerosol particles had attached sea salts on their surfaces. Sulfates were also internally mixed with sea-salt and mineral dust particles. The TEM element mapping images showed that several components with sizes of hundreds of nanometers from different sources commonly occur within individual LRT aerosol particles. We conclude that many aerosol particles from natural sources change their compositions by mixing during transport. The compositions and mixing states of these particles after emission result in changes in their hygroscopic and optical properties and should be considered when assessing their effects on climate.
Highlights
The Amazon basin can exhibit clean atmospheric conditions at times during the wet season and is uniquely useful for understanding aerosol particles from natural background sources (Martin et al, 2010a, 2016)
The model simulation shows the horizontal distributions of mineral dust concentrations at the surface level during the six long-range transport (LRT) periods and the vertical distributions on 7 March when the largest LRT periods occurred during intensive observation period 1 (IOP1) (Fig. 4)
Lidar measurements observed similar LRT of dust and biomass burning from Africa in the lowermost 2 km of the atmosphere during the AMAZE-2008 campaign (Baars et al, 2012). Such mixing of mineral and sea salt in the Amazon basin is consistent with studies using particle-induced X-ray emission (PIXE) for filter bulk measurements (Artaxo et al, 1990) or an electron probe microanalyzer and a scanning electron microscope for individual particle measurements (e.g., Worobiec et al, 2007; Wu et al, 2019), and we show here that sea salt occurs on the mineral particles as grains several hundred nanometers in size (Figs. 6 and S6)
Summary
The Amazon basin can exhibit clean atmospheric conditions at times during the wet season and is uniquely useful for understanding aerosol particles from natural background sources (Martin et al, 2010a, 2016). Biological emissions are being recognized as possible sources of inorganic salt particles containing elements such as K and Na (Pöhlker et al, 2012; China et al, 2018). These particles are mixed within individual particles, and, observations of their occurrence at sizes of hundreds of nanometers or smaller are needed to understand their mixing processes in the atmosphere
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