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1.
Environ Sci Technol ; 2020 Jul 06.
Artigo em Inglês | MEDLINE | ID: mdl-32559089

RESUMO

Biomass burning is the largest combustion-related source of volatile organic compounds (VOCs) to the atmosphere. We describe the development of a state-of-the-science model to simulate the photochemical formation of secondary organic aerosol (SOA) from biomass-burning emissions observed in dry (RH <20%) environmental chamber experiments. The modeling is supported by (i) new oxidation chamber measurements, (ii) detailed concurrent measurements of SOA precursors in biomass-burning emissions, and (iii) development of SOA parameters for heterocyclic and oxygenated aromatic compounds based on historical chamber experiments. We find that oxygenated aromatic compounds, including phenols and methoxyphenols, account for slightly less than 60% of the SOA formed and help our model explain the variability in the organic aerosol mass (R2 = 0.68) and O/C (R2 = 0.69) enhancement ratios observed across 11 chamber experiments. Despite abundant emissions, heterocyclic compounds that included furans contribute to ∼20% of the total SOA. The use of pyrolysis-temperature-based or averaged emission profiles to represent SOA precursors, rather than those specific to each fire, provide similar results to within 20%. Our findings demonstrate the necessity of accounting for oxygenated aromatics from biomass-burning emissions and their SOA formation in chemical mechanisms.

2.
J Phys Chem Lett ; 10(14): 4003-4009, 2019 Jul 18.
Artigo em Inglês | MEDLINE | ID: mdl-31264874

RESUMO

Photolytic aging has been proposed as a major mass loss mechanism for atmospheric secondary organic aerosol (SOA). However, estimated mass loss rates vary by orders of magnitude, and their impacts on modeled SOA loadings and properties are highly uncertain. In this study, photolysis rates and composition changes of α-pinene SOA are analyzed in situ over multiple days in an environmental chamber. After an initial exponential decay (τ ∼ 22 h), the mass loss rate slows dramatically, with more than ∼70-90% of the SOA particulate mass undergoing an essentially negligible photolytic degradation. Scaled to ambient conditions, SOA undergoes rapid photolysis over only its first day in the atmosphere; beyond this, the remaining SOA is photo-recalcitrant, and photolysis ceases to be a major sink compared to wet deposition time scales. Thus, extrapolation of the initial photolysis loss rate to the entire aerosol mass may significantly overestimate the role of photolysis in the removal of atmospheric SOA.

3.
Environ Sci Technol ; 53(13): 7604-7612, 2019 07 02.
Artigo em Inglês | MEDLINE | ID: mdl-31184875

RESUMO

Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.


Assuntos
Poluentes Atmosféricos , Monoterpenos , Aerossóis , Monoterpenos Bicíclicos , Íons , Espectrometria de Massas
4.
Nat Chem ; 10(4): 462-468, 2018 04.
Artigo em Inglês | MEDLINE | ID: mdl-29483638

RESUMO

The evolution of atmospheric organic carbon as it undergoes oxidation has a controlling influence on concentrations of key atmospheric species, including particulate matter, ozone and oxidants. However, full characterization of organic carbon over hours to days of atmospheric processing has been stymied by its extreme chemical complexity. Here we study the multigenerational oxidation of α-pinene in the laboratory, characterizing products with several state-of-the-art analytical techniques. Although quantification of some early generation products remains elusive, full carbon closure is achieved (within measurement uncertainty) by the end of the experiments. These results provide new insights into the effects of oxidation on organic carbon properties (volatility, oxidation state and reactivity) and the atmospheric lifecycle of organic carbon. Following an initial period characterized by functionalization reactions and particle growth, fragmentation reactions dominate, forming smaller species. After approximately one day of atmospheric aging, most carbon is sequestered in two long-lived reservoirs-volatile oxidized gases and low-volatility particulate matter.

8.
J Phys Chem Lett ; 8(7): 1503-1511, 2017 Apr 06.
Artigo em Inglês | MEDLINE | ID: mdl-28281761

RESUMO

Secondary organic aerosol (SOA) formation proceeds by bimolecular gas-phase oxidation reactions generating species that are sufficiently low in volatility to partition into the condensed phase. Advances in instrumentation have revealed that atmospheric SOA is less volatile and more oxidized than can be explained solely by these well-studied gas-phase oxidation pathways, supporting the role of additional chemical processes. These processes-autoxidation, accretion, and organic salt formation-can lead to exceedingly low-volatility species that recently have been identified in laboratory and field studies. Despite these new insights, the identities of the condensing species at the molecular level and the relative importance of the various formation processes remain poorly constrained. The thermodynamics of autoxidation, accretion, and organic salt formation can be described by equilibrium partitioning theory; a framework for which is presented here. This framework will facilitate the inclusion of such processes in model representations of SOA formation.

9.
Environ Sci Technol ; 51(5): 2519-2528, 2017 03 07.
Artigo em Inglês | MEDLINE | ID: mdl-28169528

RESUMO

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.


Assuntos
Mudança Climática , Ozônio/química , Poluição do Ar , Atmosfera/química , Ecossistema , Humanos
10.
J Phys Chem A ; 120(9): 1386-94, 2016 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-26505270

RESUMO

In this study we investigate the hypothesis that oxidants present within atmospheric particles can promote the formation of highly oxidized organic aerosol (OA) via oxidation reactions in the condensed phase. Secondary organic aerosol (SOA) was generated from the ozonolysis of α-pinene and isoprene in an environmental chamber, with seed particles systematically varied in order to assess the effects of condensed-phase oxidant levels on SOA loading and composition. The effects of particle phase (aqueous vs dry), condensed-phase oxidant source (none vs H2O2 vs Fenton chemistry), and irradiation (none vs UV) were all examined. For experiments conducted with aqueous particles but without any added oxidants, UV irradiation resulted in a small but measurable enhancement in the oxygen-to-carbon ratio (O/C). OA formed in the presence of aqueous oxidants was substantially more oxidized, with the highest oxidant concentrations leading to OA with an O/C as high as 1.4 for α-pinene and 2.0 for isoprene, strongly suggesting the formation of oxalate. High aqueous oxidant levels also resulted in increased loss of carbon from the condensed phase. This OA was more oxidized than in any other ozonolysis experiment reported to date, indicating that, when present, aqueous oxidants can have a dramatic effect on SOA formation. However, oxidant concentrations within atmospheric aqueous particles remain poorly constrained, making it difficult to assess the impacts of aqueous-phase oxidation on the loadings and oxidation state of atmospheric OA.

11.
Environ Sci Technol ; 49(22): 13483-91, 2015 Nov 17.
Artigo em Inglês | MEDLINE | ID: mdl-26461982

RESUMO

A detailed understanding of the climate and air quality impacts of mobile-source emissions requires the characterization of intermediate-volatility organic compounds (IVOCs), relatively-low-vapor-pressure gas-phase species that may generate secondary organic aerosol with high yields. Due to challenges associated with IVOC detection and quantification, IVOC emissions remain poorly understood at present. Here, we describe measurements of the magnitude and composition of IVOC emissions from a medium-duty diesel engine. Measurements are made on an engine dynamometer and utilize a new mass-spectrometric instrument to characterize the load dependence of the emissions in near-real-time. Results from steady-state engine operation indicate that IVOC emissions are highly dependent on engine power, with highest emissions at engine idle and low-load operation (≤25% maximum rated power) with a chemical composition dominated by saturated hydrocarbon species. Results suggest that unburned fuel components are the dominant IVOCs emitted at low loads. As engine load increases, IVOC emissions decline rapidly and become increasingly characterized by unsaturated hydrocarbons and oxygenated organics, newly formed from incomplete combustion processes at elevated engine temperatures and pressures. Engine transients, including a cold-start ignition and engine acceleration, show IVOC emission profiles that are different in amount or composition compared to steady-state combustion, underscoring the utility of characterizing IVOC emissions with high time resolution across realistic engine operating conditions. We find possible evidence for IVOC losses on unheated dilution and sampling surfaces, which need to be carefully accounted for in IVOC emission studies.


Assuntos
Veículos Automotores , Emissões de Veículos/análise , Compostos Orgânicos Voláteis/análise , Aerossóis , Hidrocarbonetos/análise , Espectrometria de Massas/métodos , Compostos Orgânicos Voláteis/química
12.
J Phys Chem A ; 119(44): 10767-83, 2015 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-26381466

RESUMO

Atmospheric oxidation reactions are known to affect the chemical composition of organic aerosol (OA) particles over timescales of several days, but the details of such oxidative aging reactions are poorly understood. In this study we examine the rates and products of a key class of aging reaction, the heterogeneous oxidation of particle-phase organic species by the gas-phase hydroxyl radical (OH). We compile and reanalyze a number of previous studies from our laboratories involving the oxidation of single-component organic particles. All kinetic and product data are described on a common basis, enabling a straightforward comparison among different chemical systems and experimental conditions. Oxidation chemistry is described in terms of changes to key ensemble properties of the OA, rather than to its detailed molecular composition, focusing on two quantities in particular, the amount and the oxidation state of the particle-phase carbon. Heterogeneous oxidation increases the oxidation state of particulate carbon, with the rate of increase determined by the detailed chemical mechanism. At the same time, the amount of particle-phase carbon decreases with oxidation, due to fragmentation (C-C scission) reactions that form small, volatile products that escape to the gas phase. In contrast to the oxidation state increase, the rate of carbon loss is nearly uniform among most systems studied. Extrapolation of these results to atmospheric conditions indicates that heterogeneous oxidation can have a substantial effect on the amount and composition of atmospheric OA over timescales of several days, a prediction that is broadly in line with available measurements of OA evolution over such long timescales. In particular, 3-13% of particle-phase carbon is lost to the gas phase after one week of heterogeneous oxidation. Our results indicate that oxidative aging represents an important sink for particulate organic carbon, and more generally that fragmentation reactions play a major role in the lifecycle of atmospheric OA.


Assuntos
Aerossóis/química , Carbono/química , Compostos Orgânicos/química , Cinética , Oxirredução , Tamanho da Partícula , Transição de Fase
13.
J Phys Chem Lett ; 6(12): 2388-92, 2015 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-26266621

RESUMO

Condensed-phase alkoxy (RO) radicals can undergo unimolecular (e.g., intramolecular H atom abstraction) reactions as well as bimolecular (intermolecular H atom abstraction) reactions, though the competition between these two channels is not well constrained. Here, we examine this branching by generating RO radicals from the photolysis of a large alkyl nitrite (C20H41ONO) in hexanes and nebulizing the mixture into an aerosol mass spectrometer for analysis. Product ions associated with unimolecular (isomerization) reactions were observed to increase upon photolysis. However, no formation of the C20 alcohol (C20H41OH, the expected product from RO + RH reactions) was observed, suggesting that bimolecular reactions are at most a minor channel for this condensed-phase system (involving saturated hydrocarbons). This result, combined with previous studies of liquid-phase RO radicals carried out at higher concentrations, suggests that when 1,5-H atom abstraction reactions are facile (i.e., in which a 1,5-H atom shift from a secondary or tertiary carbon can occur), this channel will dominate over bimolecular reactions.


Assuntos
Álcoois/química , Aerossóis/química , Hidrocarbonetos/química , Espectrometria de Massas , Nitritos/química , Fotólise
14.
Environ Sci Technol ; 49(7): 4129-37, 2015 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-25734883

RESUMO

The high atmospheric concentrations of toxic gases, particulate matter, and acids in the areas immediately surrounding volcanoes can have negative impacts on human and ecological health. To better understand the atmospheric fate of volcanogenic emissions in the near field (in the first few hours after emission), we have carried out real-time measurements of key chemical components of the volcanic plume from Ki̅lauea on the Island of Hawai'i. Measurements were made at two locations, one ∼ 3 km north-northeast of the vent and the other 31 km to the southwest, with sampling at each site spanning a range of meteorological conditions and volcanic influence. Instrumentation included a sulfur dioxide monitor and an Aerosol Chemical Speciation Monitor, allowing for a measurement of the partitioning between the two major sulfur species (gas-phase SO2 and particulate sulfate) every 5 min. During trade wind conditions, which sent the plume toward the southwest site, sulfur partitioning exhibited a clear diurnal pattern, indicating photochemical oxidation of SO2 to sulfate; this enabled the quantitative determination of plume age (5 h) and instantaneous SO2 oxidation rate (2.4 × 10(-6) s(-1) at solar noon). Under stagnant conditions near the crater, the extent of SO2 oxidation was substantially higher, suggesting faster oxidation. The particles within the plume were extremely acidic, with pH values (controlled largely by ambient relative humidity) as low as -0.8 and strong acidity (controlled largely by absolute sulfate levels) up to 2200 nmol/m(3). The high variability of sulfur partitioning and particle composition underscores the chemically dynamic nature of volcanic plumes, which may have important implications for human and ecological health.


Assuntos
Dióxido de Enxofre/análise , Enxofre/análise , Erupções Vulcânicas/análise , Aerossóis/análise , Gases , Hawaii , Humanos , Concentração de Íons de Hidrogênio , Espectrometria de Massas , Oxirredução , Material Particulado/análise , Sulfatos/análise , Enxofre/química , Dióxido de Enxofre/química , Vento
15.
J Phys Chem A ; 119(7): 1154-63, 2015 Feb 19.
Artigo em Inglês | MEDLINE | ID: mdl-25654760

RESUMO

The atmospheric aging of soot particles, in which various atmospheric processes alter the particles' chemical and physical properties, is poorly understood and consequently is not well-represented in models. In this work, soot aging via heterogeneous oxidation by OH and ozone is investigated using an aerosol flow reactor coupled to a new high-resolution aerosol mass spectrometric technique that utilizes infrared vaporization and single-photon vacuum ultraviolet ionization. This analytical technique simultaneously measures the elemental and organic carbon components of soot, allowing for the composition of both fractions to be monitored. At oxidant exposures relevant to the particles' atmospheric lifetimes (the equivalent of several days of oxidation), the elemental carbon portion of the soot, which makes up the majority of the particle mass, undergoes no discernible changes in mass or composition. In contrast, the organic carbon (which in the case of methane flame soot is dominated by aliphatic species) is highly reactive, undergoing first the addition of oxygen-containing functional groups and ultimately the loss of organic carbon mass from fragmentation reactions that form volatile products. These changes occur on time scales comparable to those of other nonoxidative aging processes such as condensation, suggesting that further research into the combined effects of heterogeneous and condensational aging is needed to improve our ability to accurately predict the climate and health impacts of soot particles.


Assuntos
Fuligem/química , Atmosfera/química , Oxirredução
16.
J Phys Chem A ; 119(19): 4589-99, 2015 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-25526741

RESUMO

Black carbon is an important constituent of atmospheric aerosol particle matter (PM) with significant effects on the global radiation budget and on human health. The soot particle aerosol mass spectrometer (SP-AMS) has been developed and deployed for real-time ambient measurements of refractory carbon particles. In the SP-AMS, black carbon or metallic particles are vaporized through absorption of 1064 nm light from a CW Nd:YAG laser. This scheme allows for continuous "soft" vaporization of both core and coating materials. The main focus of this work is to characterize the extent to which this vaporization scheme provides enhanced chemical composition information about aerosol particles. This information is difficult to extract from standard SP-AMS mass spectra because they are complicated by extensive fragmentation from the harsh 70 eV EI ionization scheme that is typically used in these instruments. Thus, in this work synchotron-generated vacuum ultraviolet (VUV) light in the 8-14 eV range is used to measure VUV-SP-AMS spectra with minimal fragmentation. VUV-SP-AMS spectra of commercially available carbon black, fullerene black, and laboratory generated flame soots were obtained. Small carbon cluster cations (C(+)-C5(+)) were found to dominate the VUV-SP-AMS spectra of all the samples, indicating that the corresponding neutral clusters are key products of the SP vaporization process. Intercomparisons of carbon cluster ratios observed in VUV-SP-AMS and SP-AMS spectra are used to confirm spectral features that could be used to distinguish between different types of refractory carbon particles. VUV-SP-AMS spectra of oxidized organic species adsorbed on absorbing cores are also examined and found to display less thermally induced decomposition and fragmentation than spectra obtained with thermal vaporization at 200 °C (the minimum temperature needed to quantitatively vaporize ambient oxidized organic aerosol with a continuously heated surface). The particle cores tested in these studies include black carbon, silver, gold, and platinum nanoparticles. These results demonstrate that SP vaporization is capable of providing enhanced organic chemical composition information for a wide range of organic coating materials and IR absorbing particle cores. The potential of using this technique to study organic species of interest in seeded laboratory chamber or flow reactor studies is discussed.


Assuntos
Aerossóis/análise , Espectrometria de Massas/métodos , Fuligem/análise , Carbono/análise , Cátions/análise , Ácido Cítrico/análise , Etilenos/análise , Fulerenos/análise , Compostos de Ouro/química , Nanopartículas Metálicas/química , Compostos de Platina/química , Compostos de Prata/química , Temperatura , Raios Ultravioleta , Vácuo , Volatilização
17.
Environ Sci Technol ; 48(17): 10227-34, 2014 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-25093758

RESUMO

A large number of organic species emitted into the atmosphere contain cycloalkyl groups. While cyclic species are believed to be important secondary organic aerosol (SOA) precursors, the specific role of cyclic moieties (particularly for species with multiple or fused rings) remains uncertain. Here we examine the yields and composition of SOA formed from the reaction of OH with a series of C10 (cyclo)alkanes, with 0-3 rings, in order to better understand the role of multiple cyclic moieties on aerosol formation pathways. A chamber oxidation technique using high, sustained OH radical concentrations was used to simulate long reaction times in the atmosphere. This aging technique leads to higher yields than in previously reported chamber experiments. Yields were highest for cyclic and polycyclic precursors, though yield exhibited little dependence on number of rings. However, the oxygen-to-carbon ratio of the SOA was highest for the polycyclic precursors. These trends are consistent with aerosol formation requiring two generations of oxidation and 3-4 oxygen-containing functional groups in order to condense. Cyclic, unbranched structures are protected from fragmentation during the first oxidation step, with C-C bond scission instead leading to ring opening, efficient functionalization, and high SOA yields. Fragmentation may occur during subsequent oxidation steps, limiting yields by forming volatile products. Polycyclic structures can undergo multiple ring opening reactions, but do not have markedly higher yields, likely due to enhanced fragmentation in the second oxidation step. By contrast, C-C bond scission for the linear and branched structures leads to fragmentation prior to condensation, resulting in low SOA yields. The results highlight the key roles of multigenerational chemistry and susceptibility to fragmentation in the formation and evolution of SOA.


Assuntos
Aerossóis/síntese química , Alcanos/química , Aerossóis/química , Compostos Orgânicos/síntese química , Compostos Orgânicos/química , Sulfatos/química , Fatores de Tempo
18.
J Phys Chem A ; 118(38): 8807-16, 2014 Sep 25.
Artigo em Inglês | MEDLINE | ID: mdl-25148415

RESUMO

The study of the chemistry underlying secondary organic aerosol (SOA) formation is complicated by the large number of reaction pathways and oxidation generations available to a given precursor species. Here we simplify such complexity to that of a single alkoxy radical (RO), by forming SOA via the direct photolysis of alkyl nitrite (RONO) isomers. Chamber experiments were conducted with 11 C10 RONO isomers to determine how the position of the radical center and branching of the carbon skeleton influences SOA formation. SOA yields served as a probe of RO reactivity, with lower yields indicating that fragmentation reactions dominate and higher yields suggesting the predominance of RO isomerization. The largest yields were from straight-chain isomers, particularly those with radical centers located toward the terminus of the molecule. Trends in SOA yields can be explained in terms of two major effects: (1) the relative importance of isomerization and fragmentation reactions, which control the distribution of products, and (2) differences in volatility among the various isomeric products formed. Yields from branched isomers, which were low but variable, provide insight into the degree of fragmentation of the alkoxy radicals; in the case of the two ß-substituted alkoxy radicals, fragmentation appears to occur to a greater extent than predicted by structure-activity relationships. Our results highlight how subtle differences in alkoxy radical structure can have major impacts on product yields and SOA formation.


Assuntos
Álcoois/química , Nitritos/química , Aerossóis/síntese química , Aerossóis/química , Estrutura Molecular , Fotólise
19.
J Phys Chem A ; 118(23): 4106-19, 2014 Jun 12.
Artigo em Inglês | MEDLINE | ID: mdl-24840787

RESUMO

The kinetics and products of the heterogeneous OH-initiated oxidation of squalene (C30H50, a branched alkene with 6 C═C double bonds) particles are measured. These results are compared to previous measurements of the OH-initiated oxidation of linoleic acid (C18H32O2, a linear carboxylic acid with 2 C═C double bonds) particles to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. In a 10% mixture of O2 in N2 in the flow reactor, the effective uptake coefficients (γeff) for squalene and linoleic acid are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. γeff for squalene is 2.34 ± 0.07, which is smaller than γeff for linoleic acid (3.75 ± 0.18) despite the larger number of C═C double bonds in squalene. γeff for squalene increases with [O2] in the reactor, whereas γeff for linoleic acid decreases with increasing [O2]. This suggests that the chain cycling mechanism in these two systems is different since O2 promotes chain propagation in the OH + squalene reaction but promotes chain termination in the OH + linoleic acid reaction. Elemental analysis of squalene aerosol shows that an average of 1.06 ± 0.12 O atoms are added per reactive loss of squalene prior to the onset of particle volatilization. O2 promotes particle volatilization in the OH + squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, O2 does not influence the rate of particle volatilization in the OH + linoleic acid reaction. This indicates that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.

20.
Top Curr Chem ; 339: 97-143, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-22955503

RESUMO

Organic-aerosol phase partitioning (volatility) and oxidative aging are inextricably linked in the atmosphere because partitioning largely controls the rates and mechanisms of aging reactions as well as the actual amount of organic aerosol. Here we discuss those linkages, describing the basic theory of partitioning thermodynamics as well as the dynamics that may limit the approach to equilibrium under some conditions. We then discuss oxidative aging in three forms: homogeneous gas-phase oxidation, heterogeneous oxidation via uptake of gas-phase oxidants, and aqueous-phase oxidation. We present general scaling arguments to constrain the relative importance of these processes in the atmosphere, compared to each other and compared to the characteristic residence time of particles in the atmosphere.

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