Chemical Signatures of Seasonally Unique Anthropogenic Influences on Organic Aerosol Composition in the Central Amazon.

Urbanization and fires perturb the quantities and composition of fine organic aerosol in the central Amazon, with ramifications for radiative forcing and public health. These disturbances include not only direct emissions of particulates and secondary organic aerosol (SOA) precursors but also changes in the pathways through which biogenic precursors form SOA. The composition of ambient organic aerosol is complex and incompletely characterized, encompassing millions of potential structures relatively few of which have been synthesized and characterized. Through analysis of submicron aerosol samples from the Green Ocean Amazon (GoAmazon2014/5) field campaign by two-dimensional gas chromatography coupled with machine learning, ∼1300 unique compounds were traced and characterized over two seasons. Fires and urban emissions produced chemically and interseasonally distinct impacts on product signatures, with only ∼50% of compounds observed in both seasons. Seasonally unique populations point to the importance of aqueous processing in Amazonian aerosol aging, but further mechanistic insights are impeded by limited product identity knowledge. Less than 10% of compounds were identifiable at an isomer-specific level. Overall, the findings (i) provide compositional characterization of anthropogenic influence on submicron organic aerosol in the Amazon, (ii) identify key season-to-season differences in chemical signatures, and (iii) highlight high-priority knowledge gaps in current speciated knowledge.

Lightning patterns in the Pantanal: Untangling natural and anthropogenic-induced wildfires.

The identification of fire causes and characteristics is of fundamental importance to better understand fire regimes and drivers. Particularly for Brazil, there is a gap in the quantification of lightning-caused fires. Accordingly, this work is a novel probabilistic assessment of the spatial-temporal patterns of lightning-ignited wildfires in the Pantanal wetland. Here, remote sensing information such as VIIRS active fires, MODIS burned area (BA) and STARNET lightning observations from 2012 to 2017, were combined to estimate the location, number of scars and amount of BA associated with atmospheric discharges on a seasonal basis. The highest lightning activity occurs during summer (December-February), and the lowest during winter (June-August). Conversely, the highest fire activity occurred during spring (September-November) and the lowest during autumn (March-May). Our analysis revealed low evidence of an association between fires and lightning, suggesting that human-related activities are the main source of ignitions. Weak evidence of natural-caused fire occurrence is conveyed by the low spatial-temporal match of lightning and fire throughout the studied period. Natural-caused fires accounted for only 5% of the annual total scars and 83.8% of the BA was human-caused. Most of the fires with extension larger than 1000 ha were not related to lighting. Lightning-fires seem an important element of the summer fire regime given that around half of the total BA during this season may be originated by lightning. By contrast, in the rest of the year the lightning-fires represent a minor percentage of the fire activity in the region. The density of lightning-ignited fires varies considerably, being higher in the north part of the Pantanal. This work provides a basis for a better understanding of lightning-related fire outbreaks in tropical ecosystems, particularly wetlands, which is fundamental to improve region-based strategies for land management actions, ecological studies and modeling climatic and anthropogenic drivers of wildfires.

Comparison of spatial interpolation methods for annual and seasonal rainfall in two hotspots of biodiversity in South America.

The Colombian Biogeographic Choco (CBC) and the La Plata Basin (LPB) are regions with high biodiversity. However, these areas are characterized by scarce climatological information, complex orography, and rain-gauge network unevenly distributed. Interpolated data from the ground station might overcome these aspects. For this reason, is necessary to identify the best technique for the spatial interpolation of rainfall. Hence, the spatial interpolation techniques were applied to annual and seasonal rainfall in the CBC and LPB. Geostatistical results and deterministic approaches were compared by cross-validation. Cokriging with spherical (gaussian) model is the best interpolator in the CBC (LPB), as indicated by the lowest root mean square error (RMSE) and a standardized RMSE close to one. The CBC shows three rainfall cores: the northern, 9,000 mm/year; the central-southern, 10,000 mm/year; and the southern, 7,000 mm/year. The LPB shows a west-east rainfall gradient, with a minimum to the west (450 mm/year) and a maximum in the mid-west (2,000 mm/year). To the north of the LPB, rainfall reaches 1,500 mm/year, while in the south it reaches only 900 mm/year. The results in our study may be useful for scientists and decision-makers for use in environmental and hydrological models for the CBC and the LPB.

Influence of seasonality on the aerosol microbiome of the Amazon rainforest.

The Amazon rainforest is the world's largest tropical forest, and this biome may be a significant contributor to primary biological aerosol (PBA) emissions on a global scale. These aerosols also play a pivotal role in modulating ecosystem dynamics, dispersing biological material over geographic barriers and influencing climate through radiation absorption, light scattering, or acting as cloud condensation nuclei. Despite their importance, there are limited studies investigating the effect of environmental variables on the bioaerosol composition in the Amazon rainforest. Here we present a 16S rRNA gene-based amplicon sequencing approach to investigate the bacterial microbiome in aerosols of the Amazon rainforest during distinct seasons and at different heights above the ground. Our data revealed that seasonal changes in temperature, relative humidity, and precipitation are the primary drivers of compositional changes in the Amazon rainforest aerosol microbiome. Interestingly, no significant differences were observed in the bacterial community composition of aerosols collected at ground and canopy levels. The core airborne bacterial families present in Amazon aerosol were Enterobacteriaceae, Beijerinckiaceae, Polyangiaceae, Bacillaceae and Ktedonobacteraceae. By correlating the bacterial taxa identified in the aerosol with literature data, we speculate that the phyllosphere may be one possible source of airborne bacteria in the Amazon rainforest. Results of this study indicate that the aerosol microbiota of the Amazon Rainforest are fairly diverse and principally impacted by seasonal changes in temperature and humidity.

Natural and Anthropogenically Influenced Isoprene Oxidation in Southeastern United States and Central Amazon.

Anthropogenic emissions alter secondary organic aerosol (SOA) formation chemistry from naturally emitted isoprene. We use correlations of tracers and tracer ratios to provide new perspectives on sulfate, NOx, and particle acidity influencing isoprene-derived SOA in two isoprene-rich forested environments representing clean to polluted conditions-wet and dry seasons in central Amazonia and Southeastern U.S. summer. We used a semivolatile thermal desorption aerosol gas chromatograph (SV-TAG) and filter samplers to measure SOA tracers indicative of isoprene/HO2 (2-methyltetrols, C5-alkene triols, 2-methyltetrol organosulfates) and isoprene/NOx (2-methylglyceric acid, 2-methylglyceric acid organosulfate) pathways. Summed concentrations of these tracers correlated with particulate sulfate spanning three orders of magnitude, suggesting that 1 µg m-3 reduction in sulfate corresponds with at least ∼0.5 µg m-3 reduction in isoprene-derived SOA. We also find that isoprene/NOx pathway SOA mass primarily comprises organosulfates, ∼97% in the Amazon and ∼55% in Southeastern United States. We infer under natural conditions in high isoprene emission regions that preindustrial aerosol sulfate was almost exclusively isoprene-derived organosulfates, which are traditionally thought of as representative of an anthropogenic influence. We further report the first field observations showing that particle acidity correlates positively with 2-methylglyceric acid partitioning to the gas phase and negatively with the ratio of 2-methyltetrols to C5-alkene triols.

Atomic Absorption Spectrometry Methods to Access the Metal Solubility of Aerosols in Artificial Lung Fluid.

Recent studies to quantify the health risks that fine particulate matter with an aerodynamic less than 2.5 µm (PM2.5) pose use in vitro approaches. One of these approaches is to incubate PM2.5 in artificial lysosomal fluid for a given period at body temperature. These body fluids used have a high ionic strength and as such can be challenging samples to analyze with atomic spectroscopy techniques. As PM2.5 is a primary health hazard because it is tiny enough to penetrate deep into the lungs and could, in addition, dissolve in the lung fluid it is important to quantify elements of toxic and/or carcinogenic concerns, reliably and accurately. Sophisticated instrumentation and expensive pre-treatment of challenging samples are not always available, especially in developing countries. To evaluate the applicability of graphite furnace atomic absorption spectrometry (GFAAS) without Zeeman correction capability to detect trace quantities of heavy metals leached from PM2.5 on to artificial lung fluid, univariate and multivariate approaches have been used for optimization purposes. The limits of quantification, LOQ, obtained by the optimized method were: 2 µg L-1 (Cu), 3 µg L-1 (Cr), 1 µg L-1 (Mn), and 10 µg L-1 (Pb). The addition/recovery experiments had a mean accuracy of: (Cu) 99 ± 7%; 110 ± 8% (Cr); 95 ± 9% (Mn), and 96 ± 11% (Pb). The average soluble fractions of PM2.5 incubated in artificial lysosomal fluid (ALF) for 1 h were: 1.2 ± 0.01 ng m-3 Cu, 0.4 ± 0.01 ng m-3 Cr, 0.6 ± 0.01 ng m-3 Mn, and 4.8 ± 0.03 ng m-3 Pb. Using historical elemental averages of PM2.5 in Curitiba (Cu 3.3 ng m-3, Cr 2.1 ng m-3, Mn 6.1 ng m-3, Pb 21 ng m-3), the percentage bioaccessibility were determined to be Cu 38%, Cr 20%, Mn 10%, and Pb 23%. The elemental values of the atmospheric soluble fraction of Cu, Cr, and Mn were below the inhalation risk concentrations. However, for Pb, the atmospheric soluble fraction exceeded the inhalation unit risk of 0.012 ng m-3. This robust and straightforward GF AAS method is pivotal for low and middle-income countries were most air pollution adverse effects occur and established lower-cost technologies are likely unavailable.

Impact of biomass burning on a metropolitan area in the Amazon during the 2015 El Niño: The enhancement of carbon monoxide and levoglucosan concentrations.

Extreme droughts associated with changes in the climate have occurred every 5 years in the Amazon during the 21st century, with the most severe being in 2015. The increase in biomass burning (BB) events that occurred during the 2015 drought had several negative socioeconomic and environmental impacts, one of which was a decrease in the air quality. This study is an investigation into the air quality in the Manaus Metropolitan Region (MMR) (central Amazon, Brazil) during the dry (September to October) and wet (April to May) seasons of 2015 and 2016. A strong El Niño event began during the wet season of 2015 and ended during the wet season of 2016. Particulate matter samples were collected in the MMR during 2015 and 2016, and analyses of the satellite-estimated total carbon monoxide (CO) column and observed levoglucosan concentrations were carried out. Levoglucosan has been shown to be significantly correlated with regional fires and is a well-established chemical tracer for the atmospheric particulates emitted by BB, and CO can be treated as a gaseous-phase tracer for BB. The number of BB events increased significantly during the El Niño period when compared to the average number during 2003-2016. Consequently, the total CO column and levoglucosan concentration values in the MMR increased by 15% and 500%, respectively, when compared to the normal conditions. These results indicate that during the period that was analyzed, the impacts of BB were exacerbated during the strong El Niño event as compared to the non-El Niño period. In this study, we provided evidence that the air quality in the MMR will degrade in the future if droughts and BB occurrences continue to increase.

The influence that different urban development models has on PM2.5 elemental and bioaccessible profiles.

Limited studies have reported on in-vitro analysis of PM2.5 but as far as the authors are aware, bioaccessibility of PM2.5 in artificial lysosomal fluid (ALF) has not been linked to urban development models before. The Brazilian cities Manaus (Amazon) and Curitiba (South region) have different geographical locations, climates, and urban development strategies. Manaus drives its industrialization using the free trade zone policy and Curitiba adopted a services centered economy driven by sustainability. Therefore, these two cities were used to illustrate the influence that these different models have on PM2.5 in vitro profile. We compared PM2.5 mass concentrations and the average total elemental and bioaccessible profiles for Cu, Cr, Mn, and Pb. The total average elemental concentrations followed Mn > Pb > Cu > Cr in Manaus and Pb > Mn > Cu > Cr in Curitiba. Mn had the lowest solubility while Cu showed the highest bioaccessibility (100%) and was significantly higher in Curitiba than Manaus. Cr and Pb had higher bioaccessibility in Manaus than Curitiba. Despite similar mass concentrations, the public health risk in Manaus was higher than in Curitiba indicating that the free trade zone had a profound effect on the emission levels and sources of airborne PM. These findings illustrate the importance of adopting sustainable air quality strategies in urban planning.

Intermediate-scale horizontal isoprene concentrations in the near-canopy forest atmosphere and implications for emission heterogeneity.

The emissions, deposition, and chemistry of volatile organic compounds (VOCs) are thought to be influenced by underlying landscape heterogeneity at intermediate horizontal scales of several hundred meters across different forest subtypes within a tropical forest. Quantitative observations and scientific understanding at these scales, however, remain lacking, in large part due to a historical absence of canopy access and suitable observational approaches. Herein, horizontal heterogeneity in VOC concentrations in the near-canopy atmosphere was examined by sampling from an unmanned aerial vehicle (UAV) flown horizontally several hundred meters over the plateau and slope forests in central Amazonia during the morning and early afternoon periods of the wet season of 2018. Unlike terpene concentrations, the isoprene concentrations in the near-canopy atmosphere over the plateau forest were 60% greater than those over the slope forest. A gradient transport model constrained by the data suggests that isoprene emissions differed by 220 to 330% from these forest subtypes, which is in contrast to a 0% difference implemented in most present-day biosphere emissions models (i.e., homogeneous emissions). Quantifying VOC concentrations, emissions, and other processes at intermediate horizontal scales is essential for understanding the ecological and Earth system roles of VOCs and representing them in climate and air quality models.

Radical Formation by Fine Particulate Matter Associated with Highly Oxygenated Molecules.

Highly oxygenated molecules (HOMs) play an important role in the formation and evolution of secondary organic aerosols (SOA). However, the abundance of HOMs in different environments and their relation to the oxidative potential of fine particulate matter (PM) are largely unknown. Here, we investigated the relative HOM abundance and radical yield of laboratory-generated SOA and fine PM in ambient air ranging from remote forest areas to highly polluted megacities. By electron paramagnetic resonance and mass spectrometric investigations, we found that the relative abundance of HOMs, especially the dimeric and low-volatility types, in ambient fine PM was positively correlated with the formation of radicals in aqueous PM extracts. SOA from photooxidation of isoprene, ozonolysis of α- and ß-pinene, and fine PM from tropical (central Amazon) and boreal (Hyytiälä, Finland) forests exhibited a higher HOM abundance and radical yield than SOA from photooxidation of naphthalene and fine PM from urban sites (Beijing, Guangzhou, Mainz, Shanghai, and Xi'an), confirming that HOMs are important constituents of biogenic SOA to generate radicals. Our study provides new insights into the chemical relationship of HOM abundance, composition, and sources with the yield of radicals by laboratory and ambient aerosols, enabling better quantification of the component-specific contribution of source- or site-specific fine PM to its climate and health effects.

Increasing Isoprene Epoxydiol-to-Inorganic Sulfate Aerosol Ratio Results in Extensive Conversion of Inorganic Sulfate to Organosulfur Forms: Implications for Aerosol Physicochemical Properties.

Acid-driven multiphase chemistry of isoprene epoxydiols (IEPOX), key isoprene oxidation products, with inorganic sulfate aerosol yields substantial amounts of secondary organic aerosol (SOA) through the formation of organosulfur compounds. The extent and implications of inorganic-to-organic sulfate conversion, however, are unknown. In this article, we demonstrate that extensive consumption of inorganic sulfate occurs, which increases with the IEPOX-to-inorganic sulfate concentration ratio (IEPOX/Sulfinorg), as determined by laboratory measurements. Characterization of the total sulfur aerosol observed at Look Rock, Tennessee, from 2007 to 2016 shows that organosulfur mass fractions will likely continue to increase with ongoing declines in anthropogenic Sulfinorg, consistent with our laboratory findings. We further demonstrate that organosulfur compounds greatly modify critical aerosol properties, such as acidity, morphology, viscosity, and phase state. These new mechanistic insights demonstrate that changes in SO2 emissions, especially in isoprene-dominated environments, will significantly alter biogenic SOA physicochemical properties. Consequently, IEPOX/Sulfinorg will play an important role in understanding the historical climate and determining future impacts of biogenic SOA on the global climate and air quality.

Uncovering prokaryotic biodiversity within aerosols of the pristine Amazon forest.

Biological aerosols (bioaerosol) are atmospheric particles that act as a dispersion unit of living organisms across the globe thereby affecting the biogeographic distribution of organisms. Despite their importance, there is virtually no knowledge about bioaerosols emitted by pristine forests. Here we provide the very first survey of the prokaryotic community of a bioaerosol collected inside pristine Amazon forest at 2 m above ground. Total atmospheric particles were collected at the Amazon Tall Tower Observatory, subjected to metagenomic DNA extraction and the prokaryotic diversity was determined by 16S rRNA gene amplicon sequencing. A total of 271,577 reads of 250 bp of the 16S rRNA gene amplicon were obtained. Only 27% of the reads could be classified using the 16S SILVA database. Most belonged to Proteobacteria, Actinobacteria and Firmicutes which is in good agreement with other bioaerosol studies. Further inspection of the reads using Blast searches and the 18S SILVA database revealed that most of the dataset was composed of Fungi sequences. The identified microbes suggest that the atmosphere may act as an important gateway to interchange bacteria between plants, soil and water ecosystems.

Observations of particulate matter, NO2, SO2, O3, H2S and selected VOCs at a semi-urban environment in the Amazon region.

This research aims to assess air quality in a transitional location between city and forest in the Amazon region. Located downwind of the Manaus metropolitan region, this study is part of the large-scale experiment GoAmazon2014/5. Based on their pollutant potential, inhalable particulate matter (PM2.5), nitrogen dioxide (NO2), sulfur dioxide (SO2), ozone (O3), hydrogen sulfide (H2S), benzene, toluene, ethylbenzene and meta-, orto-, para-xylene (BTEX) were selected for analysis. Sampling took place during the wet season (March-April 2014) and dry season (August-October 2014). The number of forest fires in the surroundings was higher during the dry wet season. Results show significant increase during the dry season in mass concentration (wet: <0.01-10 µg m-3; dry: 9.8-69 µg m-3), NH4+ soluble content (wet: 13-125 µg m-3; dry: 86-323 µg m-3) and K+ soluble content (wet: 11-168 µg m-3; dry 60-356 µg m-3) of the PM2.5, and O3 levels (wet: 1.4-14 µg m-3; dry: 1.0-40 µg m-3), indicating influence of biomass burning emissions. BTEX concentrations were low in both periods, but also increased during the dry season. A weak correlation in the time series of the organic and inorganic gaseous pollutants indicates a combination of different sources in both seasons and NO2 results suggest a spatial heterogeneity in gaseous pollutants levels beyond initial expectations.

Development of a hydrophilic interaction liquid chromatography (HILIC) method for the chemical characterization of water-soluble isoprene epoxydiol (IEPOX)-derived secondary organic aerosol.

Acid-catalyzed multiphase chemistry of isoprene epoxydiols (IEPOX) on sulfate aerosol produces substantial amounts of water-soluble secondary organic aerosol (SOA) constituents, including 2-methyltetrols, methyltetrol sulfates, and oligomers thereof in atmospheric fine particulate matter (PM2.5). These constituents have commonly been measured by gas chromatography interfaced to electron ionization mass spectrometry (GC/EI-MS) with prior derivatization or by reverse-phase liquid chromatography interfaced to electrospray ionization high-resolution mass spectrometry (RPLC/ESI-HR-MS). However, both techniques have limitations in explicitly resolving and quantifying polar SOA constituents due either to thermal degradation or poor separation. With authentic 2-methyltetrol and methyltetrol sulfate standards synthesized in-house, we developed a hydrophilic interaction liquid chromatography (HILIC)/ESI-HR-quadrupole time-of-flight mass spectrometry (QTOFMS) protocol that can chromatographically resolve and accurately measure the major IEPOX-derived SOA constituents in both laboratory-generated SOA and atmospheric PM2.5. 2-Methyltetrols were simultaneously resolved along with 4-6 diastereomers of methyltetrol sulfate, allowing efficient quantification of both major classes of SOA constituents by a single non-thermal analytical method. The sum of 2-methyltetrols and methyltetrol sulfates accounted for approximately 92%, 62%, and 21% of the laboratory-generated ß-IEPOX aerosol mass, laboratory-generated δ-IEPOX aerosol mass, and organic aerosol mass in the southeastern U.S., respectively, where the mass concentration of methyltetrol sulfates was 171-271% the mass concentration of methyltetrol. Mass concentrations of methyltetrol sulfates were 0.39 and 2.33 µg m-3 in a PM2.5 sample collected from central Amazonia and the southeastern U.S., respectively. The improved resolution clearly reveals isomeric patterns specific to methyltetrol sulfates from acid-catalyzed multiphase chemistry of ß- and δ-IEPOX. We also demonstrate that conventional GC/EI-MS analyses overestimate 2-methyltetrols by up to 188%, resulting (in part) from the thermal degradation of methyltetrol sulfates. Lastly, C5-alkene triols and 3-methyltetrahydrofuran-3,4-diols are found to be largely GC/EI-MS artifacts formed from thermal degradation of 2-methyltetrol sulfates and 3-methyletrol sulfates, respectively, and are not detected with HILIC/ESI-HR-QTOFMS.

Isoprene photo-oxidation products quantify the effect of pollution on hydroxyl radicals over Amazonia.

Nitrogen oxides (NO x ) emitted from human activities are believed to regulate the atmospheric oxidation capacity of the troposphere. However, observational evidence is limited for the low-to-median NO x concentrations prevalent outside of polluted regions. Directly measuring oxidation capacity, represented primarily by hydroxyl radicals (OH), is challenging, and the span in NO x concentrations at a single observation site is often not wide. Concentrations of isoprene and its photo-oxidation products were used to infer the equivalent noontime OH concentrations. The fetch at an observation site in central Amazonia experienced varied contributions from background regional air, urban pollution, and biomass burning. The afternoon concentrations of reactive nitrogen oxides (NO y ), indicative of NO x exposure during the preceding few hours, spanned from 0.3 to 3.5 parts per billion. Accompanying the increase of NO y concentration, the inferred equivalent noontime OH concentrations increased by at least 250% from 0.6 × 106 to 1.6 × 106 cm-3. The conclusion is that, compared to background conditions of low NO x concentrations over the Amazon forest, pollution increased NO x concentrations and amplified OH concentrations, indicating the susceptibility of the atmospheric oxidation capacity over the forest to anthropogenic influence and reinforcing the important role of NO x in sustaining OH concentrations.

Substantial convection and precipitation enhancements by ultrafine aerosol particles.

Aerosol-cloud interactions remain the largest uncertainty in climate projections. Ultrafine aerosol particles smaller than 50 nanometers (UAP<50) can be abundant in the troposphere but are conventionally considered too small to affect cloud formation. Observational evidence and numerical simulations of deep convective clouds (DCCs) over the Amazon show that DCCs forming in a low-aerosol environment can develop very large vapor supersaturation because fast droplet coalescence reduces integrated droplet surface area and subsequent condensation. UAP<50 from pollution plumes that are ingested into such clouds can be activated to form additional cloud droplets on which excess supersaturation condenses and forms additional cloud water and latent heating, thus intensifying convective strength. This mechanism suggests a strong anthropogenic invigoration of DCCs in previously pristine regions of the world.

Amazon boundary layer aerosol concentration sustained by vertical transport during rainfall.

The nucleation of atmospheric vapours is an important source of new aerosol particles that can subsequently grow to form cloud condensation nuclei in the atmosphere. Most field studies of atmospheric aerosols over continents are influenced by atmospheric vapours of anthropogenic origin (for example, ref. 2) and, in consequence, aerosol processes in pristine, terrestrial environments remain poorly understood. The Amazon rainforest is one of the few continental regions where aerosol particles and their precursors can be studied under near-natural conditions, but the origin of small aerosol particles that grow into cloud condensation nuclei in the Amazon boundary layer remains unclear. Here we present aircraft- and ground-based measurements under clean conditions during the wet season in the central Amazon basin. We find that high concentrations of small aerosol particles (with diameters of less than 50 nanometres) in the lower free troposphere are transported from the free troposphere into the boundary layer during precipitation events by strong convective downdrafts and weaker downward motions in the trailing stratiform region. This rapid vertical transport can help to maintain the population of particles in the pristine Amazon boundary layer, and may therefore influence cloud properties and climate under natural conditions.

Isoprene photochemistry over the Amazon rainforest.

Isoprene photooxidation is a major driver of atmospheric chemistry over forested regions. Isoprene reacts with hydroxyl radicals (OH) and molecular oxygen to produce isoprene peroxy radicals (ISOPOO). These radicals can react with hydroperoxyl radicals (HO2) to dominantly produce hydroxyhydroperoxides (ISOPOOH). They can also react with nitric oxide (NO) to largely produce methyl vinyl ketone (MVK) and methacrolein (MACR). Unimolecular isomerization and bimolecular reactions with organic peroxy radicals are also possible. There is uncertainty about the relative importance of each of these pathways in the atmosphere and possible changes because of anthropogenic pollution. Herein, measurements of ISOPOOH and MVK + MACR concentrations are reported over the central region of the Amazon basin during the wet season. The research site, downwind of an urban region, intercepted both background and polluted air masses during the GoAmazon2014/5 Experiment. Under background conditions, the confidence interval for the ratio of the ISOPOOH concentration to that of MVK + MACR spanned 0.4-0.6. This result implies a ratio of the reaction rate of ISOPOO with HO2 to that with NO of approximately unity. A value of unity is significantly smaller than simulated at present by global chemical transport models for this important, nominally low-NO, forested region of Earth. Under polluted conditions, when the concentrations of reactive nitrogen compounds were high (>1 ppb), ISOPOOH concentrations dropped below the instrumental detection limit (<60 ppt). This abrupt shift in isoprene photooxidation, sparked by human activities, speaks to ongoing and possible future changes in the photochemistry active over the Amazon rainforest.