Conspectus The impact of atmospheric particulate matter (i.e., aerosols) on Earth’s radiative balance has been and continues to be the leading source of uncertainty with respect to predictions of ...future temperature increases due to climate change. Mineral dust particles transported from deserts and semiarid regions across the globe are a dominant contributor to the aerosol burden. Dust has many and diverse effects on Earth’s climate: it directly scatters and/or absorbs incoming sunlight; it reacts with trace gases leading to impacts on the oxidizing capacity of the atmosphere that affect both the lifetime of the greenhouse gas methane in addition to concentrations of tropospheric ozonea greenhouse gas and criteria air pollutant; it influences the production as well as the lifetime and radiative properties of clouds; and it deposits nutrients to aquatic and terrestrial ecosystems that can stimulate primary production and facilitate the sequestration of atmospheric carbon dioxide (CO2). This Account will focus on the last three effects. The ability of dust to affect clouds and biogeochemical cycles hinges upon the chemical nature of dust particlesin particular, whether the compounds found in dust particles are water-soluble. The solubility of nutrients found in dust is particularly critical for determining the impact of atmospheric deposition on ocean productivity. The traditional viewpoint is that dust is inherently insoluble but reactive toward trace acidic gases, a process herein referred to as chemical aging. These reactions are thought to affect the oxidizing capacity of the atmosphere while effectively transforming the chemical composition of dust by increasing its solubility. Consequently, chemical aging is hypothesized to substantially increase the impact of dust on cloud droplet formation and marine biogeochemical cycles. This Account presents recent advances in our understanding of the mechanisms that determine how efficiently dust undergoes chemical aging and what the consequences of these processes are for the different effects of dust on Earth’s climate. This Account will re-examine the traditional viewpoint that dust chemical aging strongly impacts marine biogeochemical cycles as well as the ability of dust to nucleate cloud droplets. Laboratory studies on environmental samples are combined with chemical analysis of field samples collected at dust transport receptor sites to better understand chemical aging mechanisms and determine the impact of dust on tropospheric oxidants, clouds, and biogeochemical cycles. Our results highlight the important role that dust mineralogy plays in both the nucleation of clouds as well as the kinetics responsible for the chemical aging of dust. This Account will present cases where dust contains inherently soluble minerals and does not require chemical aging in order to efficiently nucleate clouds in the atmosphere. Lastly, this Account illustrates the critical role that nondust aerosols, namely, wildfire and combustion emissions, play as a supplier of soluble nutrients important for biogeochemical cycles, particularly in marine environments. This Account will discuss these findings and highlight future research directions and recommendations to better understand dust–climate interactions and the emerging role of biomass burning aerosol in marine biogeochemical cycles.
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The deposition of phosphorus (P) from African dust is believed to play an important role in bolstering primary productivity in the Amazon Basin and Tropical Atlantic Ocean (TAO), leading to ...sequestration of carbon dioxide. However, there are few measurements of African dust in South America that can robustly test this hypothesis and even fewer measurements of soluble P, which is readily available for stimulating primary production in the ocean. To test this hypothesis, we measured total and soluble P in long-range transported aerosols collected in Cayenne, French Guiana, a TAO coastal site located at the northeastern edge of the Amazon. Our measurements confirm that in boreal spring when African dust transport is greatest, dust supplies the majority of P, of which 5% is soluble. In boreal fall, when dust transport is at an annual minimum, we measured unexpectedly high concentrations of soluble P, which we show is associated with the transport of biomass burning (BB) from southern Africa. Integrating our results into a chemical transport model, we show that African BB supplies up to half of the P deposited annually to the Amazon from transported African aerosol. This observational study links P-rich BB aerosols from Africa to enhanced P deposition in the Amazon. Contrary to current thought, we also show that African BB is a more important source of soluble P than dust to the TAO and oceans in the Southern Hemisphere and may be more important for marine productivity, particularly in boreal summer and fall.
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The reactive uptake of isoprene-derived epoxydiols (IEPOX) is thought to be a significant source of atmospheric secondary organic aerosol (SOA). However, the IEPOX reaction probability (γIEPOX) and ...its dependence upon particle composition remain poorly constrained. We report measurements of γIEPOX for trans-β-IEPOX, the predominant IEPOX isomer, on submicron particles as a function of composition, acidity, and relative humidity (RH). Particle acidity had the strongest effect. γIEPOX is more than 500 times greater on ammonium bisulfate (γ ∼ 0.05) than on ammonium sulfate (γ ≤ 1 × 10–4). We could accurately predict γIEPOX using an acid-catalyzed, epoxide ring-opening mechanism and a high Henry’s law coefficient (1.7 × 108 M/atm). Suppression of γIEPOX was observed on particles containing both ammonium bisulfate and poly(ethylene glycol) (PEG-300), likely due to diffusion and solubility limitations within a PEG-300 coating, suggesting that IEPOX uptake could be self-limiting. Using the measured uptake kinetics, the predicted atmospheric lifetime of IEPOX is a few hours in the presence of highly acidic particles (pH < 0) but is greater than 25 h on less acidic particles (pH > 3). This work highlights the importance of aerosol acidity for accurately predicting the fate of IEPOX and anthropogenically influenced biogenic SOA formation.
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Soils in the Amazon Basin are deficient in phosphorus, essential to soil fertility. Previous studies suggested that African mineral dust deposited to Amazonian soils served as an important source of ...phosphorus that enhances soil fertility. These studies lacked the quantitative measurements essential to validate estimates. Here we present daily measurements of mineral dust and PM10 carried in the trade winds at Cayenne, French Guiana, during 2002–2017. MERRA‐2 model dust concentrations showed excellent agreement with measurements over this period. Consequently, we used MERRA‐2 to estimate temporal and spatial deposition rates to Amazonia. Our annual deposition rate, 8–10 Tg dust, is substantially lower than previous estimates. Deposition rates are greatest over the northern and northeastern regions of South America. In contrast, rates are low in central Amazonia. Our results raise questions about the impact of African dust on soil fertility in Amazonia. However, African aerosol transport carries other aerosol species that could play a role in soil fertility, including biomass‐burning products known to contain substantial concentrations of phosphorus. Our study highlights the need for more measurements of aerosol and precipitation chemistry over wider expanses of South America in order to better characterize aerosol chemical and physical properties, to identify aerosol sources, and to constrain model estimates. We show that over 2002–2017 dust transport to South America was negatively correlated to rainfall over the Sahel in North Africa. Long‐term monitoring is needed to capture changes in transport from Africa that might occur as a result of climate change.
Plain Language Summary
The Amazon Basin plays a major role in climate by removing huge quantities of carbon dioxide (CO2) from the atmosphere and storing the carbon in vegetation. This removal partially offsets the impact of CO2 emitted by humans and the consequent rate of global warming. There is a concern about decreasing soil fertility in the Amazon Basin because of the loss of phosphorus, an essential plant nutrient, due to intense tropical weathering and biomass burning. Previous work suggested that large quantities of dust are transported from Africa to South America every year and deposited to the Amazon. Dust contains phosphorus and other nutrients that could mitigate soil losses and increase Amazonian soil fertility. However, estimates of the amount of dust carried to the Amazon are uncertain because of the lack of aerosol measurements. Our multiyear measurements of African dust in the trade winds in French Guiana, coupled with a chemical transport model, suggest that significant quantities of dust reach the heart of the Amazon Basin although the amounts are substantially less than had been previously estimated. This raises questions about the long‐term status of soil fertility in the Amazon and the future impact on the carbon cycle. We also find that the quantities of dust transported to South America are inversely linked to rainfall in North Africa. Consequently, climate change will affect dust transport to South America.
Key Points
We use 15 years of aerosol measurements in trade winds at Cayenne, French Guiana, to quantify the transport of African dust to South America in boreal spring
Dust transport may be supplying significant amounts of nutrients, including phosphorus, that are important for maintaining soil fertility in the Amazon Basin
The MERRA‐2 model yields deposition rates to the Amazon that are substantially smaller than rates from previous studies
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Low bulk diffusivity inside viscous semisolid atmospheric secondary organic aerosol (SOA) can prolong equilibration time scale, but its broader impacts on aerosol growth and size distribution ...dynamics are poorly understood. Here, we present quantitative insights into the effects of bulk diffusivity on the growth and evaporation kinetics of SOA formed under dry conditions from photooxidation of isoprene in the presence of a bimodal aerosol consisting of Aitken (ammonium sulfate) and accumulation (isoprene or α-pinene SOA) mode particles. Aerosol composition measurements and evaporation kinetics indicate that isoprene SOA is composed of several semivolatile organic compounds (SVOCs), with some reversibly reacting to form oligomers. Model analysis shows that liquid-like bulk diffusivities can be used to fit the observed evaporation kinetics of accumulation mode particles but fail to explain the growth kinetics of bimodal aerosol by significantly under-predicting the evolution of the Aitken mode. In contrast, the semisolid scenario successfully reproduces both evaporation and growth kinetics, with the interpretation that hindered partitioning of SVOCs into large viscous particles effectively promotes the growth of smaller particles that have shorter diffusion time scales. This effect has important implications for the growth of atmospheric ultrafine particles to climatically active sizes.
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Intense wildfires burning >360 000 acres in San Diego during October, 2007 provided a unique opportunity to study the impact of wildfires on local air quality and biomass burning aerosol (BBA) aging. ...The size-resolved mixing state of individual particles was measured in real-time with an aerosol time-of-flight mass spectrometer (ATOFMS) for 10 days after the fires commenced. Particle concentrations were high county-wide due to the wildfires; 84% of 120–400 nm particles by number were identified as BBA, with particles <400 nm contributing to mass concentrations dangerous to public health, up to 148 μg/m3. Evidence of potassium salts heterogeneously reacting with inorganic acids was observed with continuous high temporal resolution for the first time. Ten distinct chemical types shown as BBA factors were identified through positive matrix factorization coupled to single particle analysis, including particles comprised of potassium chloride and organic nitrogen during the beginning of the wildfires, ammonium nitrate and amines after an increase of relative humidity, and sulfate dominated when the air mass back trajectories passed through the Los Angeles port region. Understanding BBA aging processes and quantifying the size-resolved mass and number concentrations are important in determining the overall impact of wildfires on air quality, health, and climate.
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Abstract
The climate effects of atmospheric aerosol particles serving as cloud condensation nuclei (CCN) depend on chemical composition and hygroscopicity, which are highly variable on spatial and ...temporal scales. Here we present global CCN measurements, covering diverse environments from pristine to highly polluted conditions. We show that the effective aerosol hygroscopicity,
κ
, can be derived accurately from the fine aerosol mass fractions of organic particulate matter (
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org
) and inorganic ions (
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) through a linear combination,
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=
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κ
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+
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. In spite of the chemical complexity of organic matter, its hygroscopicity is well captured and represented by a global average value of
κ
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= 0.12 ± 0.02 with
κ
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= 0.63 ± 0.01 as the corresponding value for inorganic ions. By showing that the sensitivity of global climate forcing to changes in
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and
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is small, we constrain a critically important aspect of global climate modelling.
North African dust reaches the southeastern United States every summer. Size-resolved dust mass measurements taken in Miami, Florida, indicate that more than one-half of the surface dust mass ...concentrations reside in particles with geometric diameters less than 2.1 µm, while vertical profiles of micropulse lidar depolarization ratios show dust reaching above 4 km during pronounced events. These observations are compared to the representation of dust in the Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) aerosol reanalysis and closely related Goddard Earth Observing System model version 5 (GEOS-5) Forward Processing (FP) aerosol product, both of which assimilate satellite-derived aerosol optical depths using a similar protocol and inputs. These capture the day-to-day variability in aerosol optical depth well, in a comparison to an independent sun-photometer-derived aerosol optical depth dataset. Most of the modeled dust mass resides in diameters between 2 and 6 µm, in contrast to the measurements. Model-specified mass extinction efficiencies equate light extinction with approximately 3 times as much aerosol mass, in this size range, compared to the measured dust sizes. GEOS-5 FP surface-layer sea salt mass concentrations greatly exceed observed values, despite realistic winds and relative humidities. In combination, these observations help explain why, despite realistic total aerosol optical depths, (1) free-tropospheric model volume extinction coefficients are lower than those retrieved from the micro-pulse lidar, suggesting too-low model dust loadings in the free troposphere, and (2) model dust mass concentrations near the surface can be higher than those measured. The modeled vertical distribution of dust, when captured, is reasonable. Large, aspherical particles exceeding the modeled dust sizes are also occasionally present, but dust particles with diameters exceeding 10 µm contribute little to the measured total dust mass concentrations after such long-range transport. Remaining uncertainties warrant a further integrated assessment to confirm this study's interpretations.
Heterogeneous reactions of dinitrogen pentoxide (N2O5) on aerosol particles impact air quality and climate, yet aspects of the relevant physical chemistry remain unresolved. One important ...consideration is the competing effects of diffusion and the rate of chemical reaction within the particle, which determines the length that N2O5 travels within a particle before reacting, referred to as the reacto-diffusive length (l). Large values of l imply a dependence of the reactive uptake efficiency of N2O5, i.e., γ(N2O5), on particle size. We present measurements of the size dependence of γ(N2O5) on aqueous sodium chloride, ammonium sulfate, and ammonium bisulfate particles. γ(N2O5) on ammonium sulfate and ammonium bisulfate particles ranged from 0.016 ± 0.005 to 0.036 ± 0.001 as the surface-area-weighted particle radius increased from 39 to 127 nm, resulting in an estimated l of 32 ± 6 nm. In contrast, γ(N2O5) on sodium chloride particles was independent of particle size, suggesting a near-surface reaction dominated the uptake of N2O5. Differences in the reactivity of the N2O5 intermediate, NO2 +, with water and chloride can explain the observed dependencies. These results allow for parameterizations in atmospheric models to determine a more robust population mean value of γ(N2O5) that accounts for the distribution of particle sizes.
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