Oxidation products of monoterpenes and isoprene have a major influence on the global secondary organic aerosol (SOA) burden and the production of atmospheric nanoparticles and cloud condensation ...nuclei (CCN). Here, we investigate the formation of extremely low volatility organic compounds (ELVOC) from O ₃ and OH radical oxidation of several monoterpenes and isoprene in a series of laboratory experiments. We show that ELVOC from all precursors are formed within the first minute after the initial attack of an oxidant. We demonstrate that under atmospherically relevant concentrations, species with an endocyclic double bond efficiently produce ELVOC from ozonolysis, whereas the yields from OH radical-initiated reactions are smaller. If the double bond is exocyclic or the compound itself is acyclic, ozonolysis produces less ELVOC and the role of the OH radical-initiated ELVOC formation is increased. Isoprene oxidation produces marginal quantities of ELVOC regardless of the oxidant. Implementing our laboratory findings into a global modeling framework shows that biogenic SOA formation in general, and ELVOC in particular, play crucial roles in atmospheric CCN production. Monoterpene oxidation products enhance atmospheric new particle formation and growth in most continental regions, thereby increasing CCN concentrations, especially at high values of cloud supersaturation. Isoprene-derived SOA tends to suppress atmospheric new particle formation, yet it assists the growth of sub-CCN-size primary particles to CCN. Taking into account compound specific monoterpene emissions has a moderate effect on the modeled global CCN budget.
Significance Extremely low volatility organic compounds (ELVOC) are suggested to promote aerosol particle formation and cloud condensation nuclei (CCN) production in the atmosphere. We show that the capability of biogenic VOC (BVOC) to produce ELVOC depends strongly on their chemical structure and relative oxidant levels. BVOC with an endocyclic double bond, representative emissions from, e.g., boreal forests, efficiently produce ELVOC from ozonolysis. Compounds with exocyclic double bonds or acyclic compounds including isoprene, emission representative of the tropics, produce minor quantities of ELVOC, and the role of OH radical oxidation is relatively larger. Implementing these findings into a global modeling framework shows that detailed assessment of ELVOC production pathways is crucial for understanding biogenic secondary organic aerosol and atmospheric CCN formation.
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
The effect of an increase in atmospheric aerosol concentrations on the distribution and radiative properties of Earth’s clouds is the most uncertain component of the overall global radiative forcing ...from preindustrial time. General circulation models (GCMs) are the tool for predicting future climate, but the treatment of aerosols, clouds, and aerosol–cloud radiative effects carries large uncertainties that directly affect GCM predictions, such as climate sensitivity. Predictions are hampered by the large range of scales of interaction between various components that need to be captured. Observation systems (remote sensing, in situ) are increasingly being used to constrain predictions, but significant challenges exist, to some extent because of the large range of scales and the fact that the various measuring systems tend to address different scales. Fine-scale models represent clouds, aerosols, and aerosol–cloud interactions with high fidelity but do not include interactions with the larger scale and are therefore limited from a climatic point of view. We suggest strategies for improving estimates of aerosol–cloud relationships in climate models, for new remote sensing and in situ measurements, and for quantifying and reducing model uncertainty.
The Model of Emissions of Gases and Aerosols from Nature (MEGANv2.1) together with the Modern-Era Retrospective Analysis for Research and Applications (MERRA) meteorological fields were used to ...create a global emission data set of biogenic volatile organic compounds (BVOC) available on a monthly basis for the time period of 1980–2010. This data set, developed under the Monitoring Atmospheric Composition and Climate project (MACC), is called MEGAN–MACC. The model estimated mean annual total BVOC emission of 760 Tg (C) yr−1 consisting of isoprene (70%), monoterpenes (11%), methanol (6%), acetone (3%), sesquiterpenes (2.5%) and other BVOC species each contributing less than 2%. Several sensitivity model runs were performed to study the impact of different model input and model settings on isoprene estimates and resulted in differences of up to ±17% of the reference isoprene total. A greater impact was observed for a sensitivity run applying parameterization of soil moisture deficit that led to a 50% reduction of isoprene emissions on a global scale, most significantly in specific regions of Africa, South America and Australia. MEGAN–MACC estimates are comparable to results of previous studies. More detailed comparison with other isoprene inventories indicated significant spatial and temporal differences between the data sets especially for Australia, Southeast Asia and South America. MEGAN–MACC estimates of isoprene, α-pinene and group of monoterpenes showed a reasonable agreement with surface flux measurements at sites located in tropical forests in the Amazon and Malaysia. The model was able to capture the seasonal variation of isoprene emissions in the Amazon forest.
Among the biogenic volatile organic compounds (BVOCs) emitted by plant foliage, isoprene is by far the most important in terms of both global
emission and atmospheric impact. It is highly reactive in ...the air, and its degradation favours the generation of ozone (in the presence of
NOx) and secondary organic aerosols. A critical aspect of BVOC emission modelling is the representation of land use and land
cover (LULC). The current emission inventories are usually based on land cover maps that are either modelled and dynamic or satellite-based and
static. In this study, we use the state-of-the-art Model of Emissions of Gases and Aerosols from Nature (MEGAN) model coupled with the canopy model MOHYCAN (Model for Hydrocarbon emissions by the CANopy) to generate and evaluate emission inventories
relying on satellite-based LULC maps at annual time steps. To this purpose, we first intercompare the distribution and evolution (2001–2016) of
tree coverage from three global satellite-based datasets, MODerate resolution Imaging Spectroradiometer (MODIS), ESA Climate Change Initiative Land Cover (ESA CCI-LC), and the Global Forest Watch (GFW), and from
national inventories. Substantial differences are found between the datasets; e.g. the global areal coverage of trees ranges from 30 to
50×106 km2, with trends spanning from −0.26 to +0.03 % yr−1 between 2001 and 2016. At the national level, the increasing
trends in forest cover reported by some national inventories (in particular for the US) are contradicted by all remotely sensed datasets. To a great
extent, these discrepancies stem from the plurality of definitions of forest used. According to some local censuses, clear cut areas and seedling or
young trees are classified as forest, while satellite-based mappings of trees rely on a minimum height. Three inventories of isoprene emissions are
generated, differing only in their LULC datasets used as input: (i) the static distribution of the stand-alone version of MEGAN, (ii) the
time-dependent MODIS land cover dataset, and (iii) the MODIS dataset modified to match the tree cover distribution from the GFW database. The mean
annual isoprene emissions (350–520 Tg yr−1) span a wide range due to differences in tree distributions, especially in isoprene-rich
regions. The impact of LULC changes is a mitigating effect ranging from 0.04 to 0.33 % yr−1 on the positive trends
(0.94 % yr−1) mainly driven by temperature and solar radiation. This study highlights the uncertainty in spatial distributions of and
temporal variability in isoprene associated with remotely sensed LULC datasets. The interannual variability in the emissions is evaluated against
spaceborne observations of formaldehyde (HCHO), a major isoprene oxidation product, through simulations using the global chemistry transport model
(CTM) IMAGESv2. A high correlation (R > 0.8) is found between the observed and simulated interannual variability in HCHO columns in most
forested regions. The implementation of LULC change has little impact on this correlation due to the dominance of meteorology as a driver of
short-term interannual variability. Nevertheless, the simulation accounting for the large tree cover declines of the GFW database over several
regions, notably Indonesia and Mato Grosso in Brazil, provides the best agreement with the HCHO column trends observed by the Ozone Monitoring Instrument (OMI). Overall, our study
indicates that the continuous tree cover fields at fine resolution provided by the GFW database are our preferred choice for constraining LULC (in
combination with discrete LULC maps such as those of MODIS) in biogenic isoprene emission models.
Natural emissions play a key role in modulating the formation of ground-level ozone (O3), especially emissions of biogenic volatile organic compounds (BVOCs) and soil nitric oxide (SNO), and their ...individual effects on O3 formation have been previously quantified and evaluated. However, their synergistic effects remain unclear and have not yet been well assessed. By applying the Weather Research and Forecasting (WRF) model coupled with the Chemistry-Model of Emissions of Gases and Aerosols from Nature (WRF/Chem-MEGAN) model, this study reveals that in the presence of sufficient BVOC emissions, which act as a fuel, SNO emissions act as a fuel additive and promote the chemical reactions of BVOCs and the subsequent production of O3. Consequently, the synergistic effects of BVOC and SNO emissions on summertime O3 production surpassed the sum of their individual effects by as much as 10–20 μg m−3 in eastern China in 2014. In order to reduce O3 concentration to a level corresponding to no natural emissions of BVOC or SNO (i.e., the BASE scenario), the anthropogenic volatile organic compound (AVOC) emissions in the scenario considers BVOC and SNO emissions must be reduced by 1.76 times that of the BASE scenario. This study demonstrates that the synergistic effects of BVOC and SNO emissions can impede ground-level O3 regulation and can subsequently impose stricter requirements on anthropogenic precursor emission control in China. The results of this study can also inform efforts in other regions that are still combating ground-level O3 pollution.
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•Synergistic effects of BVOC and SNO emissions on O3 surpass the sum of their individual effects by 10–20 μg m−3.•Synergistic effects of BVOC and SNO emissions caused a 2.4% spatial expansion of the NOx-limited regime.•An extra 76% reduction in AVOC emissions is required to reduce the O3 to a level that does not consider natural emissions.
Dry deposition of ozone is an important sink of ozone in near‐surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing ...crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short‐lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely used models. If coordinated with short‐term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long‐term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.
Plain Language Summary
The removal of tropospheric ozone at Earth's surface (often called dry deposition) is important for our understanding of air pollution, ecosystem health, and climate. Several processes contribute to dry deposition of ozone. While we have basic knowledge of these processes, we lack the ability to robustly estimate changes in ozone dry deposition through time and from one place to another. Here we review ozone deposition processes, measurements, and modeling and propose steps necessary to close gaps in understanding. A major conclusion revealed by our review is that most deposition processes can be fairly well described from a theoretical standpoint, but the relative importance of the various processes remains uncertain. We suggest that progress can be made by establishing multiyear measurements of ozone dry deposition at a limited set of sites around the world and coordinating these measurements with laboratory and field experiments that can be integrated with theory through carefully designed modeling studies.
Key Points
Ozone dry deposition through pathways other than plant stomata is critical for describing the total terrestrial ozone sink
Process‐level knowledge of ozone deposition pathways is missing from the models used to quantify deposition impacts on the Earth system
Long‐term ozone flux and related measurements are key for establishing relative importance of individual pathways
Biogenic volatile organic compound (BVOC) emissions have long been known to play vital roles in modulating the formation of ozone and secondary organic aerosols (SOAs). While early studies have ...evaluated their impact globally or regionally, the BVOC emissions emitted from urban green spaces (denoted as U-BVOC emissions) have been largely ignored primarily due to the failure of low-resolution land cover in resolving such processes, but also because their important contribution to urban BVOCs was previously unrecognized. In this study, by utilizing a recently released high-resolution land cover dataset, we develop the first set of emission inventories of U-BVOCs in China at spatial resolutions as high as 1 km. This new dataset resolved densely distributed U-BVOCs in urban core areas. The U-BVOC emissions in megacities could account for a large fraction of total BVOC emissions, and the good agreement of the interannual variations between the U-BVOC emissions and ozone concentrations over certain regions stresses their potentially crucial role in influencing ozone variations. The newly constructed U-BVOC emission inventory is expected to provide an improved dataset to enable the research community to re-examine the modulation of BVOCs on the formation of ozone, SOA, and atmospheric chemistry in urban environments.
Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that ...must be adequately represented for accurately modeling the coupled biosphere–atmosphere–climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m⁻² h⁻¹ at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO₂ reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7–17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The meganv2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events.
Over the past fifty years, considerable efforts have been devoted to measuring the concentration and chemical speciation of volatile organic compounds (VOCs) in ambient air and emissions. Recently, ...it has become possible to directly determine the overall effect of atmospheric trace gases on the oxidant hydroxyl radicals (OH), by measuring OH reactivity (OH loss frequency). Quantifying total OH reactivity is one way to characterize the roles of VOCs in formation of ground-level ozone and secondary organic aerosols (SOA). Approaches for measuring total OH reactivity in both emissions and ambient air have been progressing and have been applied in a wide range of studies. Here we evaluate the main techniques used to measure OH reactivity, including two methods directly measuring OH decay and one comparative reactivity method (CRM), and summarize the existing experimental and modeling studies. Total OH reactivity varies significantly on spatial, diurnal, seasonal and vertical bases. Comparison with individually detected OH sinks often reveals a significant missing reactivity, ranging from 20% to over 80% in some environments. Missing reactivity has also been determined in most source emission studies. These source measurements, as well as numerical models, have indicated that both undetected primary emissions and unmeasured secondary products could contribute to missing reactivity. A quantitative understanding of total OH reactivity of various sources and ambient environments will enhance our understanding of the suite of compounds found in emissions as well as chemical processes, and will also provide an opportunity for the improvement of atmospheric chemical mechanisms.
•Three total OH reactivity measuring techniques are compared.•Missing reactivity in ambient air, source and laboratory experiments are evaluated.•Contributions of primary and secondary VOCs to missing reactivity are discussed.•The study of reactivity enclosure for air quality model improvement are suggested.
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