Particles formed in the atmosphere via nucleation provide about half the number of atmospheric cloud condensation nuclei, but in many locations, this process is limited by the growth of the newly ...formed particles. That growth is often via condensation of organic vapors. Identification of these vapors and their sources is thus fundamental for simulating changes to aerosol-cloud interactions, which are one of the most uncertain aspects of anthropogenic climate forcing. Here we present direct molecular-level observations of a distribution of organic vapors in a forested environment that can explain simultaneously observed atmospheric nanoparticle growth from 3 to 50 nm. Furthermore, the volatility distribution of these vapors is sufficient to explain nanoparticle growth without invoking particle-phase processes. The agreement between observed mass growth, and the growth predicted from the observed mass of condensing vapors in a forested environment thus represents an important step forward in the characterization of atmospheric particle growth.
Abstract
Chlorine atoms (Cl) are highly reactive and can strongly influence the abundances of climate and air quality-relevant trace gases. Despite extensive research on molecular chlorine (Cl
2
), a ...Cl precursor, in the polar atmosphere, its sources in other regions are still poorly understood. Here we report the daytime Cl
2
concentrations of up to 1 ppbv observed in a coastal area of Hong Kong, revealing a large daytime source of Cl
2
(2.7 pptv s
−1
at noon). Field and laboratory experiments indicate that photodissociation of particulate nitrate by sunlight under acidic conditions (pH < 3.0) can activate chloride and account for the observed daytime Cl
2
production. The high Cl
2
concentrations significantly increased atmospheric oxidation. Given the ubiquitous existence of chloride, nitrate, and acidic aerosols, we propose that nitrate photolysis is a significant daytime chlorine source globally. This so far unaccounted for source of chlorine can have substantial impacts on atmospheric chemistry.
The authors examines the saturation vapor pressures and transition enthalpies of low-volatility organic molecules of atmospheric relevance from dicarboxylic acids to complex mixtures.
Organosulfates (OSs) with ambiguous formation mechanisms are a potential source of missing secondary organic aerosol (SOA) in current atmospheric models. In this study, we chemically characterized ...OSs and nitrooxy-OSs (NOSs) formed under the influence of biogenic emissions and anthropogenic pollutants (e.g., NOx, SO42−) in summer in Beijing. An ultrahigh-resolution mass spectrometer equipped with an electrospray ionization source was applied to examine the overall molecular composition of S-containing organics. The number and intensities of S-containing organics, the majority of which could be assigned as OSs and NOSs, increased significantly during pollution episodes, which indicated their importance for SOA accumulation. To further investigate the distribution and formation of OSs and NOSs, high-performance liquid chromatography coupled with mass spectrometry was employed to quantify 10 OSs and 3 NOS species. The total concentrations of quantified OSs and NOSs were 41.4 and 13.8 ng m−3, respectively. Glycolic acid sulfate was the most abundant species among all the quantified species, followed by monoterpene NOSs (C10H16NO7S−). The total concentration of three isoprene OSs was 14.8 ng m−3 and the isoprene OSs formed via the HO2 channel were higher than those formed via the NO ∕ NO2 channel. The OS concentration coincided with the increase in acidic sulfate aerosols, aerosol acidity, and liquid water content (LWC), indicating the acid-catalyzed aqueous-phase formation of OSs in the presence of acidic sulfate aerosols. When sulfate dominated the accumulation of secondary inorganic aerosols (SIAs; sulfate, nitrate, and ammonium; SO42− ∕ SIA > 0.5), OS formation would obviously be promoted as the increasing of acidic sulfate aerosols, aerosol LWC, and acidity (pH < 2.8). Otherwise, acid-catalyzed OS formation would be limited by lower aerosol acidity when nitrate dominated the SIA accumulation. The nighttime enhancement of monoterpene NOSs suggested their formation via the nighttime NO3-initiated oxidation of monoterpene under high-NOx conditions. However, isoprene NOSs are presumed to form via acid-catalyzed chemistry or reactive uptake of oxidation products of isoprene. This study provides direct observational evidence and highlights the secondary formation of OSs and NOSs via the interaction between biogenic precursors and anthropogenic pollutants (NOx, SO2, and SO42−). The results imply that future reduction in anthropogenic emissions can help to reduce the biogenic SOA burden in Beijing or other areas impacted by both biogenic emissions and anthropogenic pollutants.
To improve air quality, the Beijing government has employed several air
pollution control measures since the 2008 Olympics. In order to investigate
organic aerosol sources after the implementation of ...these measures, ambient
fine particulate matter was collected at a regional site in Changping (CP) and
an urban site at the Peking University Atmosphere Environment Monitoring Station (PKUERS)
during the “Photochemical Smog in China” field campaign in summer
2016. Chemical mass balance (CMB) modeling and the tracer yield method
were used to apportion primary and secondary organic sources. Our results
showed that the particle concentration decreased significantly during the
last few years. The apportioned primary and secondary sources explained
62.8 ± 18.3 and 80.9 ± 27.2 % of the measured OC at CP and
PKUERS, respectively. Vehicular emissions served as the dominant source.
Except for gasoline engine emissions, the contributions of all the other primary
sources decreased. In addition, the anthropogenic SOC, i.e., toluene SOC, also
decreased, implying that deducting primary emissions can reduce anthropogenic
SOA. In contrast to the SOA from other regions in the world where biogenic
SOA was dominant, anthropogenic SOA was the major contributor to SOA,
implying that deducting anthropogenic VOC emissions is an efficient way to
reduce SOA in Beijing. Back-trajectory cluster analysis results showed that
high mass concentrations of OC were observed when the air mass was from
the south. However, the contributions of different primary organic sources were
similar, suggesting regional particle pollution. The ozone concentration
and temperature correlated well with the SOA concentration. Different
correlations between day and night samples suggested different SOA
formation pathways. Significant enhancement of SOA with increasing particle
water content and acidity was observed in our study, suggesting that aqueous-phase acid-catalyzed reactions may be the important SOA formation mechanism
in summer in Beijing.
Trees can significantly impact the urban air chemistry by the uptake and emission of reactive biogenic volatile organic compounds (BVOCs), which are involved in ozone and particle formation. Here we ...present the emission potentials of "constitutive" (cBVOCs) and "stress-induced" BVOCs (sBVOCs) from the dominant broadleaf woody plant species in the megacity of Beijing. Based on the municipal tree census and cuvette BVOC measurements on leaf level, we built an inventory of BVOC emissions, and assessed the potential impact of BVOCs on secondary organic aerosol (SOA) formation in 2005 and 2010, i.e., before and after realizing the large tree-planting program for the 2008 Olympic Games. We found that sBVOCs, such as fatty acid derivatives, benzenoids, and sesquiterpenes, constituted a significant fraction ( ∼ 40 %) of the total annual BVOC emissions, and we estimated that the overall annual BVOC budget may have doubled from ∼ 4.8 × 109 g C year−1 in 2005 to ∼ 10.3 × 109 g C year−1 in 2010 due to the increase in urban greening, while at the same time the emission of anthropogenic VOCs (AVOCs) decreased by 24 %. Based on the BVOC emission assessment, we estimated the biological impact on SOA mass formation potential in Beijing. Constitutive and stress-induced BVOCs might produce similar amounts of secondary aerosol in Beijing. However, the main contributors of SOA-mass formations originated from anthropogenic sources (> 90 %). This study demonstrates the general importance to include sBVOCs when studying BVOC emissions. Although the main problems regarding air quality in Beijing still originate from anthropogenic activities, the present survey suggests that in urban plantation programs, the selection of low-emitting plant species has some potential beneficial effects on urban air quality.
The behaviour of secondary organic aerosols (SOA) in the atmosphere is highly dependent on their thermal properties. Here we investigate the volatility of SOA formed from α-pinene, β-pinene and ...limonene upon ozone- and OH-induced oxidation, and the effect of OH-induced ageing on the initially produced SOA. For all three terpenes, the ozone-induced SOA was less volatile than the OH-induced SOA. The thermal properties of the SOA were described using three parameters extracted from the volatility measurements: the temperature at which 50% of the volume has evaporated (TVFR0.5), which is used as a general volatility indicator; a slope factor (SVFR), which describes the volatility distribution; and TVFR0.1, which measures the volatility of the least volatile particle fraction. Limonene-derived SOA generally had higher TVFR0.5 values and shallower slopes than SOA derived from α- and β-pinene. This was especially true for the ozone-induced SOA, partially because the ozonolysis of limonene has a strong tendency to cause SOA formation and to produce extremely low volatility VOCs (ELVOCs). Ageing by OH exposure did not reduce TVFR0.5 for any of the studied terpenes but did increase the breadth of the volatility distribution by increasing the aerosols’ heterogeneity and contents of substances with different vapour pressures, also leading to increases in TVFR0.1. This stands in contrast to previously reported results from smog chamber experiments, in which TVFR0.5 always increased with ageing. These results demonstrate that there are two opposing processes that influence the evolution of SOAs’ thermal properties as they age, and that results from both flow reactors and static chambers are needed to fully understand the temporal evolution of atmospheric SOA’ thermal properties.
•Thermal properties of SOA from VOCs vary in their responses to oxidation processes.•Limonene-derived SOAs’ thermal properties differ sharply from those for α- and β-pinene.•OH-induced oxidation yielded more volatile SOA than ozonolysis.•SOA formed by ozonolysis evaporated over a narrower temperature interval.•The observed responses to OH-induced ageing explain some unresolved observations.
The Multiple Chamber Aerosol Chemical Aging Study (MUCHACHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary ...organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.
The morphological transformation of soot particles via condensation of
low-volatility materials constitutes a dominant atmospheric process with
serious implications for the optical and hygroscopic ...properties, as well as
atmospheric lifetime of the soot. We consider the morphological
transformation of soot aggregates under the influence of condensation of
vapors of sulfuric acid, and/or limonene ozonolysis products. This influence
was systematically investigated using a Differential Mobility Analyzer
coupled with an Aerosol Particle Mass
Analyzer (DMA–APM) and the Tandem DMA techniques integrated with a laminar
flow-tube system. We hypothesize that the morphology transformation of soot
results (in general) from a two-step process, i.e., (i) filling of void space
within the aggregate and (ii) growth of the particle diameter. Initially, the
transformation was dominated by the filling process followed by growth, which
led to the accumulation of sufficient material that exerted surface forces,
which eventually facilitated further filling. The filling of void space was
constrained by the initial morphology of the fresh soot as well as the nature
and the amount of condensed material. This process continued in several
sequential steps until all void space within the soot aggregate was filled.
And then “growth” of a spherical particle continued as long as vapors
condensed on it. We developed a framework for quantifying the microphysical
transformation of soot upon the condensation of various materials. This
framework used experimental data and the hypothesis of “ideal sphere
growth” and void filling to quantify the distribution of condensed materials
in the complementary filling and growth processes. Using this framework, we
quantified the percentage of material consumed by these processes at each
step of the transformation. For the largest coating experiments, 6, 10, 24,
and 58 % of condensed material went to filling process, while 94, 90, 76,
and 42 % of condensed material went to growth process for 75, 100, 150,
and 200 nm soot particles, respectively. We also used the framework to
estimate the fraction of internal voids and open voids. This information was
then used to estimate the volume-equivalent diameter of the soot aggregate
containing internal voids and to calculate the dynamic shape factor,
accounting for internal voids. The dynamic shape factor estimated based on
the traditional assumption (of no internal voids) differed significantly from
the value obtained in this study. Internal voids are accounted for in the
experimentally derived dynamic shape factor determined in the present study.
In fact, the dynamic shape factor adjusted for internal voids was close to 1
for the fresh soot particles considered in this study, indicating the
particles were largely spherical. The effective density was strongly
correlated with the morphological transformation responses to the condensed
material on the soot particle, and the resultant effective density was
determined by the (i) nature of the condensed material and (ii) morphology
and size of the fresh soot. In this work we quantitatively tracked in situ
microphysical changes in soot morphology, providing details of both fresh and
coated soot particles at each step of the transformation. This framework can
be applied to model development with significant implications for quantifying
the morphological transformation (from the viewpoint of hygroscopic and
optical properties) of soot in the atmosphere.
<正>INTRODUCTION Severe air-pollution events in many parts of China pose a major threat to health and ecosystems1.China’s air pollution is concentrated to economically developed areas,such as ...Beijing–Tianjin–Hebei(BTH)and Pearl–River–Delta(PRD)2,3.The situation has received considerable attention in international
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