This study introduces a first glance at Amazonian aerosols in the N–Dg–σ phase space. Aerosol data, measured from May 2021 to April 2022 at the Amazon Tall Tower Observatory (ATTO), were fitted by a ...multi-modal lognormal function and separated into three modes: the sub-50 nm, the Aitken (50–100 nm), and the accumulation modes. The fit results were then evaluated in the N–Dg–σ phase space, which represents a three-dimensional space based on the three lognormal fit parameters. These parameters represent, for a given mode i, the number concentration (Ni), the median geometric diameter (Dg,i), and the geometric standard deviation (σi). Each state of a particle number size distribution (PNSD) is represented by a single dot in this space, while a collection of dots shows the delimitation of all PNSD states under given conditions. The connections in ensembles of data points show trajectories caused by pseudo-forces, such as precipitation regimes and vertical movement. We showed that all three modes have a preferential arrangement in this space, reflecting their intrinsic behaviors in the atmosphere. These arrangements were interpreted as volumetric figures, elucidating the boundaries of each mode. Time trajectories in seasonal and diurnal cycles revealed that fits with the sub-20 nm mode are associated with rainfall events that happen in the morning and in the afternoon. But in the morning they grow rapidly into the Aitken mode, and in the afternoon they remain below 50 nm. Also, certain modes demonstrated well-defined curves in the space, e.g., the seasonal trajectory of the accumulation mode follows an ellipsoid, while the diurnal cycle of the sub-50 nm mode in the dry season follows a linear trajectory. As an effect of the precipitation on the PNSDs and vice versa, N and Dg were found to increase for the sub-50 nm mode and to decrease for the Aitken and accumulation modes after the precipitation peak. Afternoons with precipitation were preceded by mornings with larger particles of the accumulation mode, whose Dg was ∼ 10 nm larger than in days without precipitation. Nevertheless, this large Dg in the morning seems to influence subsequent rainfall only in the dry season, while in the wet season both N and Dg seem to have the same weight of influence. The observed patterns of the PNSDs in the N–Dg–σ phase space showed to be a promising tool for the characterization of atmospheric aerosols, to contribute to our understanding of the main processes in aerosol–cloud interactions, and to open new perspectives on aerosol parameterizations and model validation.
Pollen grains emitted from vegetation can release subpollen particles (SPPs)
that contribute to the fine fraction of atmospheric aerosols and may act as
cloud condensation nuclei (CCN), ice nuclei ...(IN), or aeroallergens. Here, we
investigate and characterize the hygroscopic growth and CCN activation of
birch, pine, and rapeseed SPPs. A high-humidity tandem differential mobility
analyzer (HHTDMA) was used to measure particle restructuring and water
uptake over a wide range of relative humidity (RH) from 2 % to 99.5 %,
and a continuous flow CCN counter was used for size-resolved measurements of
CCN activation at supersaturations (S) in the range of 0.2 % to 1.2 %.
For both subsaturated and supersaturated conditions, effective
hygroscopicity parameters, κ, were obtained by Köhler model
calculations. Gravimetric and chemical analyses, electron microscopy, and
dynamic light scattering measurements were performed to characterize further
properties of SPPs from aqueous pollen extracts such as chemical composition
(starch, proteins, DNA, and inorganic ions) and the hydrodynamic size
distribution of water-insoluble material. All investigated SPP samples
exhibited a sharp increase of water uptake and κ above
∼95 % RH, suggesting a liquid–liquid phase separation
(LLPS). The HHTDMA measurements at RH >95 % enable closure
between the CCN activation at water vapor supersaturation and hygroscopic
growth at subsaturated conditions, which is often not achieved when hygroscopicity tandem differential mobility analyzer (HTDMA) measurements are performed at lower RH where the water uptake and effective
hygroscopicity may be limited by the effects of LLPS. Such effects may be
important not only for closure between hygroscopic growth and CCN activation
but also for the chemical reactivity, allergenic potential, and related
health effects of SPPs.
New particle formation (NPF), referring to the nucleation of molecular clusters and their subsequent growth into the cloud condensation nuclei (CCN) size range, is a globally significant and ...climate-relevant source of atmospheric aerosols. Classical NPF exhibiting continuous growth from a few nanometers to the Aitken mode around 60–70 nm is widely observed in the planetary boundary layer (PBL) around the world but not in central Amazonia. Here, classical NPF events are rarely observed within the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50 nm (CN<50) particles into the PBL and their subsequent growth. Central aspects of our understanding of these processes in the Amazon have remained enigmatic, however. Based on more than 6 years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO; February 2014 to September 2020), we analyzed the diurnal and seasonal patterns as well as meteorological conditions during 254 of such Amazonian growth events on 217 event days, which show a sudden occurrence of particles between 10 and 50 nm in the PBL, followed by their growth to CCN sizes. The occurrence of events was significantly higher during the wet season, with 88 % of all events from January to June, than during the dry season, with 12 % from July to December, probably due to differences in the condensation sink (CS), atmospheric aerosol load, and meteorological conditions. Across all events, a median growth rate (GR) of 5.2 nm h−1 and a median CS of 1.1 × 10−3 s−1 were observed. The growth events were more frequent during the daytime (74 %) and showed higher GR (5.9 nm h−1) compared to nighttime events (4.0 nm h−1), emphasizing the role of photochemistry and PBL evolution in particle growth. About 70 % of the events showed a negative anomaly of the equivalent potential temperature (Δθe′) – as a marker for downdrafts – and a low satellite brightness temperature (Tir) – as a marker for deep convective clouds – in good agreement with particle injection from the UT in the course of strong convective activity. About 30 % of the events, however, occurred in the absence of deep convection, partly under clear-sky conditions, and with a positive Δθe′ anomaly. Therefore, these events do not appear to be related to downdraft transport and suggest the existence of other currently unknown sources of sub-50 nm particles.
Interactions between atmospheric aerosols, clouds, and precipitation impact Earth's radiative balance and air quality, yet remain poorly constrained. Precipitating clouds serve as major sinks for ...particulate matter, but recent studies suggest that precipitation may also act as a particle source. The magnitude of the sources versus sinks, particularly for cloud condensation nuclei (CCN) numbers, remain unquantified. This study analyzes multi‐year in situ observations from tropical and boreal forests, as well as Arctic marine environment, showing links between recent precipitation and enhanced particle concentrations, including CCN‐sized particles. In some cases, the magnitude of precipitation‐related source equals or surpasses corresponding removal effect. Our findings highlight the importance of cloud‐processed material in determining near‐surface particle concentrations and the value of long‐term in situ observations for understanding aerosol particle life cycle. Robust patterns emerge from sufficiently long data series, allowing for quantitative assessment of the large‐scale significance of new phenomena observed in case studies.
Plain Language Summary
Atmospheric aerosols, clouds, and precipitation play a significant role in Earth's temperature regulation and air quality. However, understanding their interactions is still a challenge. While clouds and precipitation help remove particles from the atmosphere, recent research suggests rain could also introduce new particles. The extent of this particle source and its impact on climate are still unknown. In this study, we analyzed years of observational data from clean environments, including tropical and boreal forests and the Arctic marine boundary layer. We discovered that after precipitation, new particles were sometimes added to the surface atmosphere. In some cases, rain introduced as many or even more particles than it removed. Our findings highlight the importance of considering how clouds and rain recycle particles when studying air quality and climate. Long‐term, real‐world observations help us understand atmospheric particle life cycles and identify consistent patterns, ultimately improving our knowledge of the complex interactions between aerosols, clouds, and precipitation.
Key Points
Precipitation can act as a source for particles of varying sizes depending on the environment, reflecting diverse underlying mechanisms
Recycling cloud‐processed material influences near‐surface particle concentrations, emphasizing its relevance for climate model implementation
Studying the time‐dependent instead of total accumulated precipitation elucidates direct versus indirect effects on aerosol populations
This study evaluates the effect of weather events on the aerosol particle size distribution (PSD) at the Amazon Tall Tower Observatory (ATTO). This research combines in situ measurements of PSD and ...remote sensing data of lightning density, brightness temperature, cloud top height, cloud liquid water, and rain rate and vertical velocity. Measurements were obtained by scanning mobility particle sizers (SMPSs), the new generation of GOES satellites (GOES-16), the SIPAM S-band radar and the LAP 3000 radar wind profiler recently installed at the ATTO-Campina site. The combined data allow exploring changes in PSD due to different meteorological processes. The average diurnal cycle shows a higher abundance of ultrafine particles (N.sub.UFP) in the early morning, which is coupled with relatively lower concentrations in Aitken (N.sub.AIT) and accumulation (N.sub.ACC) mode particles. From the early morning to the middle of the afternoon, an inverse behavior is observed, where N.sub.UFP decreases and N.sub.AIT and N.sub.ACC increase, reflecting a typical particle growth process. Composite figures show an increase of N.sub.UFP before, during and after lightning was detected by the satellite above ATTO. These findings strongly indicate a close relationship between vertical transport and deep convective clouds. Lightning density is connected to a large increase in N.sub.UFP, beginning approximately 100 min before the maximum lightning density and reaching peak values around 200 min later. In addition, the removal of N.sub.ACC by convective transport was found. Both the increase in N.sub.UFP and the decrease in N.sub.ACC appear in parallel with the increasing intensity of lightning activity. The N.sub.UFP increases exponentially with the thunderstorm intensity. In contrast, N.sub.AIT and N.sub.ACC show a different behavior, decreasing from approximately 100 min before the maximum lightning activity and reaching a minimum at the time of maximum lightning activity. The effect of cloud top height, cloud liquid water and rain rate shows the same behavior, but with different patterns between seasons. The convective processes do not occur continually but are probably modulated by gravity waves in the range of 1 to 5 h, creating a complex mechanism of interaction with a succession of updrafts and downdrafts, clouds, and clear-sky situations.
Abstract
Smoke from vegetation fires affects air quality, atmospheric cycling, and the climate in the Amazon rain forest. A major unknown has remained the quantity of long-range transported smoke ...from Africa in relation to local and regional fire emissions. Here we quantify the abundance, seasonality, and properties of African smoke in central Amazonia. We show that it accounts for ~ 60% of the black carbon concentrations during the wet season and ~ 30% during the dry season. The African smoke influences aerosol-radiation interactions across the entire Amazon, with the strongest impact on the vulnerable eastern basin, a hot spot of climate and land use change. Our findings further suggest that the direct influence of African smoke has been historically relevant for soil fertilization, the carbon and water cycles, and, thus, the development of the Amazon forest ecosystem, even in the pre-industrial era.
We present a new approach of analyzing and interpreting vertical profiles of cloud microstructure obtained by satellite remote sensing. The method is based on a spectral bin microphysics adiabatic ...parcel model and aims to elucidate the effects of aerosols on the evolution of convective clouds and related microphysical processes, including the activation of cloud condensation nuclei (CCN), the growth of cloud droplets, and the formation of precipitation. Characteristic features in the vertical profiles of effective radius (re) and temperature (T) reveal different microphysical zones in convective clouds related to the change increase of re with decreasing T. The classification of the different microphysical zones includes the (a) condensational growth of droplets, (b) growth by coalescence, (c) rainout, (d) secondary droplet activation zone (SAZ), (e) mixed‐phase of ice particles and water droplets, and (f) glaciation of the cloud. The detection of the SAZ is introduced here for the first time. This method allows us to identify the activation of aerosol particles above cloud base and their role in the invigoration of deep convective clouds.
Plain Language Summary
Using satellite remote sensing, we can obtain the vertical profiles of cloud microphysical processes in developed clouds. In this study, we present a new way of analyzing these profiles and understand the different processes that cloud droplets undergo during the development of the cloud. These processes include the turning of aerosols into cloud droplets, and the droplets' coalescence into rain drops. The change of droplets' size with height, as obtained by the satellite, reflect the different microphysical processes inside the cloud. While expecting the droplets to grow with height, a decrease of the droplets size suggests either rainout of the larger droplets from the cloud; or creation of small new droplets at much greater heights than the cloud base, where they are usually created. Those processes can be related to the extent of decrease of cloud drop size with height. The detection of new droplet formation above the cloud base with satellite is introduced here for the first time and it allows to understand this microphysical process and its effect of the development of the clouds and precipitation.
Key Points
A new method to interpret the cloud microstructure of deep convective clouds from satellite remote sensing is presented in this study
The microphysical process of secondary activation of droplets above cloud bases is demonstrated with a model and detected in observations
The satellite detection of the secondary activation microphysical zone is introduced here for the first time
The wet-season atmosphere in the central Amazon resembles natural conditions with minimal anthropogenic influence, making it one of the rare preindustrial-like continental areas worldwide. Previous ...long-term studies have analyzed the properties and sources of the natural Amazonian background aerosol. However, the vertical profile of the planetary boundary layer (PBL) has not been assessed systematically. Since 2017, such a profile assessment has been possible with the 325 m high tower at the Amazon Tall Tower Observatory (ATTO), located in a largely untouched primary forest in the central Amazon. This study investigates the variability of submicrometer aerosol concentration, size distribution, and optical properties at 60 and 325 m in the Amazonian PBL. The results show significant differences in aerosol volumes and scattering coefficients in the vertical gradient. The aerosol population was well-mixed throughout the boundary layer during the daytime but became separated upon stratification during the nighttime. We also found a significant difference in the spectral dependence of the scattering coefficients between the two heights. The analysis of downdrafts and the related rainfall revealed changes in the aerosol populations before and after rain events, with absorption and scattering coefficients decreasing as optically active particles are removed by wet deposition. The recovery of absorption and scattering coefficients is faster at 325 m than at 60 m. Convective events were concomitant with rapid increases in the concentrations of sub-50 nm particles, which were likely associated with downdrafts. We found that the aerosol population near the canopy had a significantly higher mass scattering efficiency than at 325 m. There was also a clear spectral dependence, with values for λ=450, 525, and 635 nm of 7.74±0.12, 5.49±0.11, and 4.15±0.11 m2 g-1, respectively, at 60 m, while at 325 m the values were 5.26±0.06, 3.76±0.05, and 2.46±0.04 m2 g-1, respectively. The equivalent aerosol refractive index results, which were obtained for the first time for the wet season in the central Amazon, show slightly higher scattering (real) components at 60 m compared to 325 m of 1.33 and 1.27, respectively. In contrast, the refractive index's absorptive (imaginary) component was identical for both heights, at 0.006. This study shows that the aerosol physical properties at 60 and 325 m are different, likely due to aging processes, and strongly depend on the photochemistry, PBL dynamics, and aerosol sources. These findings provide valuable insights into the impact of aerosols on climate and radiative balance and can be used to improve the representation of aerosols in global climate models.
Chiral chemodiversity plays a crucial role in biochemical processes such as insect and plant communication. However, the vast majority of organic aerosol studies do not distinguish between ...enantiomeric compounds in the particle phase. Here we report chirally specified measurements of secondary organic aerosol (SOA) at the Amazon Tall Tower Observatory (ATTO) at different altitudes during three measurement campaigns at different seasons. Analysis of filter samples by liquid chromatography coupled to mass spectrometry (LC-MS) has shown that the chiral ratio of pinic acid (C9H14O4) varies with increasing height above the canopy. A similar trend was recently observed for the gas-phase precursor α-pinene but more pronounced. Nevertheless, the measurements indicate that neither the oxidation of (+/−)-α-pinene nor the incorporation of the products into the particulate phase proceeds with stereo preference and that the chiral information of the precursor molecule is merely transferred to the low-volatility product. The observation of the weaker height gradient of the present enantiomers in the particle phase at the observation site can be explained by the significant differences in the atmospheric lifetimes of reactant and product. Therefore, it is suggested that the chiral ratio of pinic acid is mainly determined by large-scale emission processes of the two precursors, while meteorological, chemical, or physicochemical processes do not play a particular role. Characteristic emissions of the chiral aerosol precursors from different forest ecosystems, in some cases even with contributions from forest-related fauna, could thus provide large-scale information on the different contributions to biogenic secondary aerosols via the analytics of the chiral particle-bound degradation products.
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