Abstract
Abstract. The Amazon rainforest presents a unique, natural laboratory for the study of surface–atmosphere interactions. Its alternation between a near-pristine marine-influenced atmosphere during the wet season and a vulnerable system affected by periodic intrusions of anthropogenic pollution during the dry season provides an opportunity to investigate some fundamental aspects of boundary-layer chemical processes. This study presents the first simultaneous hourly measurements of concentrations, fluxes, and deposition velocities of the inorganic trace gases NH3, HCl, HONO, HNO3, and SO2 as well as their water-soluble aerosol counterparts NH4+, Cl−, NO2-, NO3- and SO42- over the Amazon. Species concentrations were measured in the dry season (from 6 October to 5 November 2017), at the Amazon Tall Tower Observatory (ATTO) in Brazil, using a two-point gradient wet-chemistry instrument (GRadient of AErosols and Gases Online Registration, GRAEGOR) sampling at 42 and 60 m. Fluxes and deposition velocities were derived from the concentration gradients using a modified form of the aerodynamic gradient method corrected for measurement within the roughness sub-layer. Findings from this campaign include observations of elevated concentrations of NH3 and SO2 partially driven by long-range transport (LRT) episodes of pollution and the substantial influence of coarse Cl− and NO3- particulate on overall aerosol mass burdens. From the flux measurements, the dry season budget of total reactive nitrogen dry deposition at the ATTO site was estimated as −2.9 kg N ha-1a-1. HNO3 and HCl were deposited continuously at a rate close to the aerodynamic limit. SO2 was deposited with an average daytime surface resistance (Rc) of 28 s m−1, whilst aerosol components showed average surface deposition velocities of 2.8 and 2.7 mm s−1 for SO42- and NH4+, respectively. Deposition rates of NO3- and Cl− were higher at 7.1 and 7.8 mm s−1, respectively, reflecting their larger average size. The exchange of NH3 and HONO was bidirectional, with NH3 showing emission episodes in the afternoon and HONO in the early morning hours. This work provides a unique dataset to test and improve dry deposition schemes for these compounds for tropical rainforest, which have typically been developed by interpolation from conditions in temperate environments. A future campaign should focus on making similar measurements in the wet season in order to provide a complete view of the annual pattern of inorganic trace gas and coarse aerosol biosphere–atmosphere exchange over tropical rainforest.
Highlights
The Amazon rainforest is one of the last remaining wildernesses on Earth, which – through a select combination of environmental and geographical factors – acts as a critical, living driver of global climate (Malhi et al, 2008)
We present here the concentrations, fluxes and deposition velocities of the trace gases NH3, hydrogen chloride (HCl), HONO, HNO3 and SO2 as well as their associated aerosol counterparts NH+4, Cl−, NO−2, NO−3 and SO24− as measured by GRadient of AErosols and Gases Online Registration (GRAEGOR) wet-chemistry two-point gradient system during a period of the 2017 dry season at the Amazon Tall Tower Observatory (ATTO) site
In the comparison of GRAEGOR and ToF-ACSM concentration measurements outlined in Sect. 3.2, we found a reasonable agreement for NH+4 and SO24−, considering the difference in measurement height and instrumentation
Summary
The Amazon rainforest is one of the last remaining wildernesses on Earth, which – through a select combination of environmental and geographical factors – acts as a critical, living driver of global climate (Malhi et al, 2008) It is a vast region of near-undisturbed verdant growth, covering almost 60 % of the total land area of Brazil and constituting almost 40 % of global tropical forest cover (Baccini et al, 2012). The strong coupling between the forest and the atmosphere (and the sensitive feedbacks between them that regulate atmospheric composition) has earned the Amazon rainforest the sobriquet of the “Green Ocean” (Martin et al, 2016; Roberts et al, 2001; Williams et al, 2002) and the “biogeochemical reactor” (Pöhlker et al, 2012; Andreae, 2001) It is a near-pristine microcosm of the pre-Anthropocene and acts as a continental “natural laboratory” to study unmodified surface– atmosphere exchange processes. Emissions of pollutants from agricultural activities, biomass burning and deforestation in the vicinity of the rainforest can perturb its surface–atmosphere exchange processes (Ganzeveld and Lelieveld, 2004) and cause changes in the local, regional and even global climate (Lenton et al, 2008)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
