Marine low clouds play an important role in the climate system, and their properties are sensitive to cloud condensation nuclei concentrations. While new particle formation represents a major source ...of cloud condensation nuclei globally, the prevailing view is that new particle formation rarely occurs in remote marine boundary layer over open oceans. Here we present evidence of the regular and frequent occurrence of new particle formation in the upper part of remote marine boundary layer following cold front passages. The new particle formation is facilitated by a combination of efficient removal of existing particles by precipitation, cold air temperatures, vertical transport of reactive gases from the ocean surface, and high actinic fluxes in a broken cloud field. The newly formed particles subsequently grow and contribute substantially to cloud condensation nuclei in the remote marine boundary layer and thereby impact marine low clouds.
Anthropogenic emissions and land use changes have modified atmospheric aerosol concentrations and size distributions over time. Understanding preindustrial conditions and changes in organic aerosol ...due to anthropogenic activities is important because these features (1) influence estimates of aerosol radiative forcing and (2) can confound estimates of the historical response of climate to increases in greenhouse gases. Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, represents a major fraction of global submicron‐sized atmospheric organic aerosol. Over the past decade, significant advances in understanding SOA properties and formation mechanisms have occurred through measurements, yet current climate models typically do not comprehensively include all important processes. This review summarizes some of the important developments during the past decade in understanding SOA formation. We highlight the importance of some processes that influence the growth of SOA particles to sizes relevant for clouds and radiative forcing, including formation of extremely low volatility organics in the gas phase, acid‐catalyzed multiphase chemistry of isoprene epoxydiols, particle‐phase oligomerization, and physical properties such as volatility and viscosity. Several SOA processes highlighted in this review are complex and interdependent and have nonlinear effects on the properties, formation, and evolution of SOA. Current global models neglect this complexity and nonlinearity and thus are less likely to accurately predict the climate forcing of SOA and project future climate sensitivity to greenhouse gases. Efforts are also needed to rank the most influential processes and nonlinear process‐related interactions, so that these processes can be accurately represented in atmospheric chemistry‐climate models.
Plain Language Summary
Secondary organic aerosol (SOA), formed in the atmosphere by oxidation of organic gases, often represents a major fraction of global submicron‐sized atmospheric organic aerosol. Myriad processes affect SOA formation, several of which relate to interactions between natural biogenic emissions and predominantly anthropogenic species such as SO2, NOx, sulfate, nitrate, and ammonium. Many of these key processes are nonlinear and can be synergistic or act to compensate each other in terms of climate forcing. Current atmospheric chemistry‐climate models mostly do not treat these processes. We highlight a number of process‐level mechanisms related to the interactions between anthropogenic and biogenic SOA precursors, for which the corresponding impacts on the radiative effects of SOA need to be investigated in atmospheric chemistry‐climate models. Ultimately, climate models need to capture enough important features of the chemical and dynamic evolution of SOA, in terms of both aerosol number and aerosol mass, as a function of atmospheric variables and anthropogenic perturbations to reasonably predict the spatial and temporal distributions of SOA. A better understanding of SOA formation mechanisms and physical properties is needed to improve estimates of the extent to which anthropogenic emissions and land use changes have modified global aerosol concentrations and size distributions since preindustrial times.
Key Points
We review some important developments in secondary organic aerosol (SOA) that could impact aerosol radiative forcing and response of climate to greenhouse gases
We highlight some of the important processes that involve interactions between natural biogenic emissions and anthropogenic emissions
We discuss fundamental SOA properties volatility and viscosity and their relation to evolution of aerosol mass and number concentrations in the atmosphere
Marine low clouds strongly influence global climate, and their radiative
effects are particularly susceptible to the concentration of cloud
condensation nuclei (CCN). One major source of CCN is the ...condensational growth
of pre-CCN particles, and sulfate has long been considered the major
condensing species in the remote marine boundary layer. While some studies
have suggested that secondary organic species can contribute to particle
growth, its importance remains unclear. Here we present the first long-term
observational evidence that organics play an important role in particle
growth over remote oceans. To the contrary of traditional thinking, sulfate
dominated condensational growth for only a small (∼18 %)
fraction of the 62 observed growth events, even fewer than the
organic-dominated events (24 %). During most (58 %) growth events, the
major condensing species included both organics and sulfate. Potential
precursors of the secondary organics are volatile organic compounds from
ocean biological activities and those produced by the air–sea interfacial
oxidation. Our results indicate that the condensation of secondary organics
contributes strongly to the growth of pre-CCN particles and thereby the CCN
population over remote oceans.
This paper describes simple modifications to thermally diffusive laminar flow ultrafine condensation particle counters (UCPCs) that allow detection of ∼1 nm condensation nuclei with much higher ...efficiencies than have been previously reported. These non-destructive modifications were applied to a commercial butanol-based UCPC (TSI 3025A) and to a diethylene glycol-based UCPC (UMN DEG-UCPC). Size and charge dependent detection efficiencies using the modified UCPCs (BNL 3025A and BNL DEG-UCPC) were measured with high resolution mobility classified aerosols composed of NaCl, W, molecular ion standards of tetra-alkyl ammonium bromide, and neutralizer-generated ions. With negatively charged NaCl aerosol, the BNL 3025A and BNL DEG-UCPC achieved detection efficiencies of 37% (90× increase over TSI 3025A) at 1.68 nm mobility diameter (1.39 nm geometric diameter) and 23% (8× increase over UMN DEG-UCPC) at 1.19 nm mobility diameter (0.89 nm geometric diameter), respectively. Operating conditions for both UCPCs were identified that allowed negatively charged NaCl and W particles, but not negative ions of exactly the same mobility size, to be efficiently detected. This serendipitous material dependence, which is not fundamentally understood, suggests that vapor condensation might sometimes allow for the discrimination between air "ions" and charged "particles." As a detector in a scanning mobility particle spectrometer (SMPS), a UCPC with this strong material dependence would allow for more accurate measurements of sub-2 nm aerosol size distributions due to the reduced interference from neutralizer-generated ions and atmospheric ions, and provide increased sensitivity for the determination of nucleation rates and initial particle growth rates.
Copyright 2012 American Association for Aerosol Research
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.
Aerosol measurements during the DOE ARM Layered Atlantic Smoke Interactions with Clouds (LASIC) campaign were used to quantify the differences between clean and smoky cloud condensation nuclei (CCN) ...budgets. Accumulation‐mode particles accounted for ∼70% of CCN at supersaturations <0.3% in clean and smoky conditions. Aitken‐mode particles contributed <20% and sea‐spray‐mode particles <10% at supersaturations <0.3%, but at supersaturations >0.3% Aitken particles contributions increased to 30%–40% of clean CCN. For clean conditions, the Hoppel minimum diameter was correlated to the accumulation‐mode number concentration, indicating aerosol‐correlated cloud activation was controlling the lower diameter cutoff for which particles serve as CCN. For smoky conditions, the contributions of Aitken particles increase and the correlation of cloud activation to accumulation‐mode particles is masked by the lower‐hygroscopicity smoke. These results provide the first multi‐month in situ quantitative constraints on the role of aerosol number size distributions in controlling cloud activation in the tropical Atlantic boundary layer.
Plain Language Summary
Tiny airborne particles provide the “seeds” on which cloud droplets form, and clouds are in turn important for regulating climate around the world. The small number of measurements characterizing these particles in conditions that are not affected by man‐made emissions make it difficult to represent these cloud processes in computer models that compare current climate to pre‐industrial conditions. Aerosol measurements collected for 17 months on an isolated island in the tropical Atlantic Ocean show how the size and number of particles affect cloud characteristics. The long timescale and wide range from very clean to very smoky aerosol conditions revealed not only differences in the particles that activate in clouds but also in the mechanisms that control that droplet formation process. In clean air, the size required to form a cloud droplet is influenced by the number of particles, as well as how quickly particles take up water during growth in cloud. However, in smoky air, the larger number and size of particles mean that cloud activation processes are less affected by the number of particles that take up water.
Key Points
Clean cloud condensation nuclei (CCN) at <0.3% supersaturation were ∼70% accumulation, <10% sea spray, and <20% Aitken mode particles
Hoppel minimum diameters correlated to accumulation‐mode particles showing aerosol‐correlated activation for clean conditions (<400 cm−3)
Smoky accumulation‐mode particles were 30 nm larger and had 15%–30% more CCN, which dampened correlations to cloud activation
Climate models show that particles formed by nucleation can affect cloud cover and, therefore, the earth's radiation budget. Measurements worldwide show that nucleation rates in the atmospheric ...boundary layer are positively correlated with concentrations of sulfuric acid vapor. However, current nucleation theories do not correctly predict either the observed nucleation rates or their functional dependence on sulfuric acid concentrations. This paper develops an alternative approach for modeling nucleation rates, based on a sequence of acid-base reactions. The model uses empirical estimates of sulfuric acid evaporation rates obtained from new measurements of neutral molecular clusters. The model predicts that nucleation rates equal the sulfuric acid vapor collision rate times a prefactor that is less than unity and that depends on the concentrations of basic gaseous compounds and preexisting particles. Predicted nucleation rates and their dependence on sulfuric acid vapor concentrations are in reasonable agreement with measurements from Mexico City and Atlanta.
We report a new scanning mobility particle spectrometer (SMPS) for measuring number size distributions of particles down to ∼1 nm mobility diameter. This SMPS includes an aerosol charger, a TSI 3085 ...nano differential mobility analyzer (nanoDMA), an ultrafine condensation particle counter (UCPC) using diethylene glycol (DEG) as the working fluid, and a conventional butanol CPC (the "booster") to detect the small droplets leaving the DEG UCPC. The response of the DEG UCPC to negatively charged sodium chloride particles with mobility diameters ranging from 1-6 nm was measured. The sensitivity of the DEG UCPC to particle composition was also studied by comparing its response to positively charged 1.47 and 1.70 nm tetra-alkyl ammonium ions, sodium chloride, and silver particles. A high resolution differential mobility analyzer was used to generate the test particles. These results show that the response of this UCPC to sub-2 nm particles is sensitive to particle composition. The applicability of the new SMPS for atmospheric measurement was demonstrated during the Nucleation and Cloud Condensation Nuclei (NCCN) field campaign (Atlanta, Georgia, summer 2009). We operated the instrument at saturator and condenser temperatures that allowed the efficient detection of sodium chloride particles but not of air ions having the same mobility. We found that particles as small as 1 nm were detected during nucleation events but not at other times. Factors affecting size distribution measurements, including aerosol charging in the 1-10 nm size range, are discussed. For the charger used in this study, bipolar charging was found to be more effective for sub-2 nm particles than unipolar charging. No ion induced nucleation inside the charger was observed during the NCCN campaign.
OVERVIEW OF THE HI-SCALE FIELD CAMPAIGN Fast, Jerome D.; Berg, Larry K.; Alexander, Lizbeth ...
Bulletin of the American Meteorological Society,
05/2019, Letnik:
100, Številka:
5
Journal Article
Recenzirano
Odprti dostop
Shallow convective clouds are common, occurring over many areas of the world, and are an important component in the atmospheric radiation budget. In addition to synoptic and mesoscale meteorological ...conditions, land–atmosphere interactions and aerosol–radiation–cloud interactions can influence the formation of shallow clouds and their properties. These processes exhibit large spatial and temporal variability and occur at the subgrid scale for all current climate, operational forecast, and cloud-system-resolving models; therefore, they must be represented by parameterizations. Uncertainties in shallow cloud parameterization predictions arise from many sources, including insufficient coincident data needed to adequately represent the coupling of cloud macrophysical and microphysical properties with inhomogeneity in the surface-layer, boundary layer, and aerosol properties. Predictions of the transition of shallow to deep convection and the onset of precipitation are also affected by errors in simulated shallow clouds. Coincident data are a key factor needed to achieve a more complete understanding of the life cycle of shallow convective clouds and to develop improved model parameterizations. To address these issues, the Holistic Interactions of Shallow Clouds, Aerosols and Land Ecosystems (HI-SCALE) campaign was conducted near the Atmospheric Radiation Measurement (ARM) Southern Great Plains site in north-central Oklahoma during the spring and summer of 2016. We describe the scientific objectives of HI-SCALE as well as the experimental approach, overall weather conditions during the campaign, and preliminary findings from the measurements. Finally, we discuss scientific gaps in our understanding of shallow clouds that can be addressed by analysis and modeling studies that use HI-SCALE data.
During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) campaign, size-resolved cloud condensation nuclei (CCN) spectra were characterized at a research site (T3) 60 km ...downwind of the city of Manaus, Brazil, in central Amazonia for 1 year (12 March 2014 to 3 March 2015). Particle hygroscopicity (κCCN) and mixing state were derived from the size-resolved CCN spectra, and the hygroscopicity of the organic component of the aerosol (κorg) was then calculated from κCCN and concurrent chemical composition measurements. The annual average κCCN increased from 0.13 at 75 nm to 0.17 at 171 nm, and the increase was largely due to an increase in sulfate volume fraction. During both wet and dry seasons, κCCN, κorg, and particle composition under background conditions exhibited essentially no diel variations. The constant κorg of ∼ 0. 15 is consistent with the largely uniform and high O : C value (∼ 0. 8), indicating that the aerosols under background conditions are dominated by the aged regional aerosol particles consisting of highly oxygenated organic compounds. For air masses strongly influenced by urban pollution and/or local biomass burning, lower values of κorg and organic O : C atomic ratio were observed during night, due to accumulation of freshly emitted particles, dominated by primary organic aerosol (POA) with low hygroscopicity, within a shallow nocturnal boundary layer. The O : C, κorg, and κCCN increased from the early morning hours and peaked around noon, driven by the formation and aging of secondary organic aerosol (SOA) and dilution of POA emissions into a deeper boundary layer, while the development of the boundary layer, which leads to mixing with aged particles from the residual layer aloft, likely also contributed to the increases. The hygroscopicities associated with individual organic factors, derived from PMF (positive matrix factorization) analysis of AMS (aerosol mass spectrometry) spectra, were estimated through multivariable linear regression. For the SOA factors, the variation of the κ value with O : C agrees well with the linear relationship reported from earlier laboratory studies of SOA hygroscopicity. On the other hand, the variation in O : C of ambient aerosol organics is largely driven by the variation in the volume fractions of POA and SOA factors, which have very different O : C values. As POA factors have hygroscopicity values well below the linear relationship between SOA hygroscopicity and O : C, mixtures with different POA and SOA fractions exhibit a steeper slope for the increase in κorg with O : C, as observed during this and earlier field studies. This finding helps better understand and reconcile the differences in the relationships between κorg and O : C observed in laboratory and field studies, therefore providing a basis for improved parameterization in global models, especially in a tropical context.
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