Impact of formaldehyde on ozone formation in Central China: Important role of biogenic emission in forest region

Formaldehyde (HCHO) is an important source for driving tropospheric ozone (O3) formation. This study investigated the combined effects of anthropogenic and biogenic emission on O3 formation in the Guanzhong Basin (GZB), Central China, providing useful information into the mechanisms of O3 formation due to the interaction between anthropogenic and biogenic volatile organic compounds (VOCs). A severe O3 pollution episode in summer of 2017 was simulated using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to examine the impacts of ambient HCHO on ground-level O3. Results showed secondary HCHO dominated ambient levels, peaking in the afternoon (up to 86 %), while primary emissions contributed 14 % on average. This enhanced O3 production by 7.7 % during the morning rush hour and 24.3 % in the afternoon. In addition, HCHO concentration peaked before that of O3, suggesting it plays significant role in O3 formation. Biogenic emission oxidation contributed 3.1 μg m-3 (53.1 %) of HCHO and 5.2 pptv (40.1 %) of hydroperoxyl radicals (HO2) in average urban areas, where the downwind regions of the forests had high nitrogen oxides (NOx) levels and favorable conditions for O3 production (17.3 μg m-3, 20.5 %). In forested regions, sustained isoprene oxidation led to elevated oxidized VOCs including HCHO and acetaldehyde downwind, which practiced further photolysis of O3 formation with anthropogenic NOx in urban areas. Sensitivity experiments recommend controlling industrial and traffic NOx emissions, with regional joint prevention and regulation, which are essential to reduce O3 pollution.

The fuel of atmospheric chemistry: Toward a complete description of reactive organic carbon

The Earth's atmosphere contains a multitude of emitted (primary) and chemically formed (secondary) gases and particles that degrade air quality and modulate the climate. Reactive organic carbon (ROC) species are the fuel of the chemistry of the atmosphere, dominating short-lived emissions, reactivity, and the secondary production of key species such as ozone, particulate matter, and carbon dioxide. Despite the central importance of ROC, the diversity and complexity of this class of species has been a longstanding obstacle to developing a comprehensive understanding of how the composition of our atmosphere, and the associated environmental implications, will evolve. Here, we characterize the role of ROC in atmospheric chemistry and the challenges inherent in measuring and modeling ROC, and highlight recent progress toward achieving mass closure for the complete description of atmospheric ROC.

A perspective on the development of gas-phase chemical mechanisms for Eulerian air quality models

An essential component of a three-dimensional air quality model is its gas-phase mechanism. We present an overview of the necessary atmospheric chemistry and a discussion of the types of mechanisms with some specific examples such as the Master Chemical Mechanism, the Carbon Bond, SAPRC and the Regional Atmospheric Chemistry Mechanism (RACM). The first versions of the Carbon Bond and SAPRC mechanisms were developed through a hierarchy of chemical species approach that relied heavily on chemical environmental chamber data. Now a new approach has been proposed where the first step is to develop a highly detailed explicit mechanism such as the Master Chemical Mechanism and the second step is to test the detailed explicit mechanism against laboratory and field data. Finally, the detailed mechanism is condensed for use in a three-dimensional air quality model. Here it is argued that the development of highly detailed explicit mechanisms is very valuable for research, but we suggest that combining the hierarchy of chemical species and the detailed explicit mechanism approaches would be better than either alone.Implication: Many gas-phase mechanisms are available for urban, regional and global air quality modeling. A "hierarchy of chemical species approach," relying heavily on smog-chamber data was used for the development of the early series of mechanisms. Now the development of large, explicit master mechanisms that may be condensed is a significant, trend. However, a continuing problem with air quality mechanism development is due to the high complexity of atmospheric chemistry and the current availability of laboratory measurements. This problem requires a balance between completeness and speculation so that models maintain their utility for policymakers.

Chemical Mechanisms and Their Applications in the Goddard Earth Observing System (GEOS) Earth System Model

NASA's Goddard Earth Observing System (GEOS) Earth System Model (ESM) is a modular, general circulation model (GCM), and data assimilation system (DAS) that is used to simulate and study the coupled dynamics, physics, chemistry, and biology of our planet. GEOS is developed by the Global Modeling and Assimilation Office (GMAO) at NASA Goddard Space Flight Center. It generates near-real-time analyzed data products, reanalyses, and weather and seasonal forecasts to support research targeted to understanding interactions among Earth System processes. For chemistry, our efforts are focused on ozone and its influence on the state of the atmosphere and oceans, and on trace gas data assimilation and global forecasting at mesoscale discretization. Several chemistry and aerosol modules are coupled to the GCM, which enables GEOS to address topics pertinent to NASA's Earth Science Mission. This paper describes the atmospheric chemistry components of GEOS and provides an overview of its Earth System Modeling Framework (ESMF)-based software infrastructure, which promotes a rich spectrum of feedbacks that influence circulation and climate, and impact human and ecosystem health. We detail how GEOS allows model users to select chemical mechanisms and emission scenarios at run time, establish the extent to which the aerosol and chemical components communicate, and decide whether either or both influence the radiative transfer calculations. A variety of resolutions facilitates research on spatial and temporal scales relevant to problems ranging from hourly changes in air quality to trace gas trends in a changing climate. Samples of recent GEOS chemistry applications are provided.

Long-Lived Species Enhance Summertime Attribution of North American Ozone to Upwind Sources

Ground-level ozone (O₃), harmful to most living things, is produced from both domestic and foreign emissions of anthropogenic precursors. Previous estimates of the linkage from distant sources rely on the sensitivity approach (i.e., modeling the change of ozone concentrations that result from modifying precursor emissions) as well as the tagging approach (i.e., tracking ozone produced from specific O₃ precursors emitted from one region). Here, for the first time, we tag all O₃ precursors (i.e., nitrogen oxides (NO_x), carbon monoxide (CO), and volatile organic compounds (VOCs)) from East Asia and explicitly track their physicochemical evolution without perturbing the nonlinear O₃ chemistry. We show that, even in summer, when intercontinental influence on ozone has typically been found to be weakest, nearly 3 parts per billion by volume (ppbv) seasonal average surface O₃ over North America can be attributed to East Asian anthropogenic emissions, compared with 0.7 ppbv using the sensitivity approach and 0.5 ppbv by tagging reactive nitrogen oxides. Considering the acute effects of O₃ exposure, approximately 670 cardiovascular and 300 respiratory premature mortalities within North America could be attributed to East Asia. CO and longer-lived VOCs, largely overlooked in previous studies, extend the influence of regional ozone precursors emissions and, thus, greatly enhance O₃ attribution to source region.

Highly functionalized organic nitrates in the southeast United States: Contribution to secondary organic aerosol and reactive nitrogen budgets

Speciated particle-phase organic nitrates (pONs) were quantified using online chemical ionization MS during June and July of 2013 in rural Alabama as part of the Southern Oxidant and Aerosol Study. A large fraction of pONs is highly functionalized, possessing between six and eight oxygen atoms within each carbon number group, and is not the common first generation alkyl nitrates previously reported. Using calibrations for isoprene hydroxynitrates and the measured molecular compositions, we estimate that pONs account for 3% and 8% of total submicrometer organic aerosol mass, on average, during the day and night, respectively. Each of the isoprene- and monoterpenes-derived groups exhibited a strong diel trend consistent with the emission patterns of likely biogenic hydrocarbon precursors. An observationally constrained diel box model can replicate the observed pON assuming that pONs (i) are produced in the gas phase and rapidly establish gas-particle equilibrium and (ii) have a short particle-phase lifetime (∼2-4 h). Such dynamic behavior has significant implications for the production and phase partitioning of pONs, organic aerosol mass, and reactive nitrogen speciation in a forested environment.

Organic nitrate chemistry and its implications for nitrogen budgets in an isoprene- and monoterpene-rich atmosphere: constraints from aircraft (SEAC4RS) and ground-based (SOAS) observations in the Southeast US

Formation of organic nitrates (RONO₂) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NO_x), but the chemistry of RONO₂ formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO₂) in the GEOS-Chem global chemical transport model with ∼25 × 25 km² resolution over North America. We evaluate the model using aircraft (SEAC⁴RS) and ground-based (SOAS) observations of NO_x, BVOCs, and RONO₂ from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO₂ species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50% of observed RONO₂ in surface air, and we find that another 10% is contributed by gas-phase monoterpene nitrates. Observations in the free troposphere show an important contribution from long-lived nitrates derived from anthropogenic VOCs. During both campaigns, at least 10% of observed boundary layer RONO₂ were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO₃ accounts for 60% of simulated gas-phase RONO₂ loss in the boundary layer. Other losses are 20% by photolysis to recycle NO_x and 15% by dry deposition. RONO₂ production accounts for 20% of the net regional NO_x sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NO_x emissions. This segregation implies that RONO₂ production will remain a minor sink for NO_x in the Southeast US in the future even as NO_x emissions continue to decline.

An iodide-adduct high-resolution time-of-flight chemical-ionization mass spectrometer: application to atmospheric inorganic and organic compounds

A high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (<20 ppm) and mass-resolving power (R>5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity toward more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (>1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign.

Atmospheric peroxyacetyl nitrate (PAN): a global budget and source attribution

Peroxyacetyl nitrate (PAN) formed in the atmospheric oxidation of non-methane volatile organic compounds (NMVOCs) is the principal tropospheric reservoir for nitrogen oxide radicals (NOx = NO + NO2). PAN enables the transport and release of NOx to the remote troposphere with major implications for the global distributions of ozone and OH, the main tropospheric oxidants. Simulation of PAN is a challenge for global models because of the dependence of PAN on vertical transport as well as complex and uncertain NMVOC sources and chemistry. Here we use an improved representation of NMVOCs in a global 3-D chemical transport model (GEOS-Chem) and show that it can simulate PAN observations from aircraft campaigns worldwide. The immediate carbonyl precursors for PAN formation include acetaldehyde (44% of the global source), methylglyoxal (30 %), acetone (7 %), and a suite of other isoprene and terpene oxidation products (19 %). A diversity of NMVOC emissions is responsible for PAN formation globally including isoprene (37 %) and alkanes (14 %). Anthropogenic sources are dominant in the extratropical Northern Hemisphere outside the growing season. Open fires appear to play little role except at high northern latitudes in spring, although results are very sensitive to plume chemistry and plume rise. Lightning NOx is the dominant contributor to the observed PAN maximum in the free troposphere over the South Atlantic.

Sources and processes contributing to nitrogen deposition: an adjoint model analysis applied to biodiversity hotspots worldwide

Anthropogenic enrichment of reactive nitrogen (Nr) deposition is an ecological concern. We use the adjoint of a global 3-D chemical transport model (GEOS-Chem) to identify the sources and processes that control Nr deposition to an ensemble of biodiversity hotspots worldwide and two U.S. national parks (Cuyahoga and Rocky Mountain). We find that anthropogenic sources dominate deposition at all continental sites and are mainly regional (less than 1000 km) in origin. In Hawaii, Nr supply is controlled by oceanic emissions of ammonia (50%) and anthropogenic sources (50%), with important contributions from Asia and North America. Nr deposition is also sensitive in complicated ways to emissions of SO2, which affect Nr gas-aerosol partitioning, and of volatile organic compounds (VOCs), which affect oxidant concentrations and produce organic nitrate reservoirs. For example, VOC emissions generally inhibit deposition of locally emitted NOx but significantly increase Nr deposition downwind. However, in polluted boreal regions, anthropogenic VOC emissions can promote Nr deposition in winter. Uncertainties in chemical rate constants for OH + NO2 and NO2 hydrolysis also complicate the determination of source-receptor relationships for polluted sites in winter. Application of our adjoint sensitivities to the representative concentration pathways (RCPs) scenarios for 2010-2050 indicates that future decreases in Nr deposition due to NOx emission controls will be offset by concurrent increases in ammonia emissions from agriculture.

Isoprene emissions in Africa inferred from OMI observations of formaldehyde columns

We use 2005-2009 satellite observations of formaldehyde (HCHO) columns from the OMI instrument to infer biogenic isoprene emissions at monthly 1 × 1° resolution over the African continent. Our work includes new approaches to remove biomass burning influences using OMI absorbing aerosol optical depth data (to account for transport of fire plumes) and anthropogenic influences using AATSR satellite data for persistent small-flame fires (gas flaring). The resulting biogenic HCHO columns (Ω_HCHO) from OMI follow closely the distribution of vegetation patterns in Africa. We infer isoprene emission (E _ISOP) from the local sensitivity S = ΔΩ_HCHO / ΔE _ISOP derived with the GEOS-Chem chemical transport model using two alternate isoprene oxidation mechanisms, and verify the validity of this approach using AMMA aircraft observations over West Africa and a longitudinal transect across central Africa. Displacement error (smearing) is diagnosed by anomalously high values of S and the corresponding data are removed. We find significant sensitivity of S to NO_x under low-NO_x conditions that we fit to a linear function of tropospheric column NO₂. We estimate a 40% error in our inferred isoprene emissions under high-NO_x conditions and 40-90% under low-NO_x conditions. Our results suggest that isoprene emission from the central African rainforest is much lower than estimated by the state-of-the-science MEGAN inventory.

Aerosol effects on ozone concentrations in Beijing: a model sensitivity study

Most previous O3 simulations were based only on gaseous phase photochemistry. However, some aerosol-related processes, namely, heterogeneous reactions occurring on the aerosol surface and photolysis rate alternated by aerosol radiative influence, may affect O3 photochemistry under high aerosol loads. A three-dimensional air quality model, Models-3/Community Multi-scale Air Quality-Model of Aerosol Dynamics, Reaction, Ionization, and Dissolution, was employed to simulate the effects of the above-mentioned processes on O3 formation under typical high O3 episodes in Beijing during summer. Five heterogeneous reactions, i.e., NO2, NO3, N2O5, HO2, and O3, were individually investigated to elucidate their effects on 03 formation. The results showed that the heterogeneous reactions significantly affected O3 formation in the urban plume. NO2 heterogeneous reaction increased O3 to 90 ppb, while HO2 heterogeneous reaction decreased O3 to 33 ppb. In addition, O3 heterogeneous loss decreased O3 to 31 ppb. The effects of NO2, NO3, and N2O5 heterogeneous reactions showed opposite O3 concentration changes between the urban and extra-urban areas because of the response of the reactions to the two types of O3 formation regimes. When the aerosol radiative influence was included, the photolysis rate decreased and O3 decreased significantly to 73 ppb O3. The two aerosol-related processes should be considered in the study of O3 formation because high aerosol concentration is a ubiquitous phenomenon that affects the urban- and regional air quality in China.

Isomer-selective study of the OH-initiated oxidation of isoprene in the presence of O(2) and NO: 2. the major OH addition channel

We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O2 addition to the hydroxyalkyl radical to be 1.0(-0.5)+1.7 x 10(-12) cm3 s(-1), and we find a value of 8.1-2.3+3.4 x 10(-12) cm3 s(-1) for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the delta-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 +/- 3)%. This puts a rigorous upper limit on the branching of the (E)-delta-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity.

Isoprene and nitric oxide reduce damages in leaves exposed to oxidative stress

Isoprene and nitric oxide (NO) are two volatile molecules that are produced in leaves. Both compounds were suggested to have an important protective role against stresses. We tested, in two isoprene-emitting species, Populus nigra and Phragmites australis, whether: (1) NO emission outside leaves is measurable and is affected by oxidative stresses; and (2) isoprene and NO protect leaves against oxidative stresses, both singularly and in combination. The emission of NO was undetectable, and the compensation point was very low in control poplar leaves. Both emission and compensation point increased dramatically in stressed leaves. NO emission was inversely associated with stomatal conductance. More NO was emitted in leaves that were isoprene-inhibited, and more isoprene was emitted when NO was reduced by NO scavenger c-PTIO. Both isoprene and NO reduced oxidative damages. Isoprene-emitting leaves which were also fumigated with NO, or treated with NO donor, showed low damage to photosynthesis, a reduced accumulation of H(2)O(2) and a reduced membrane denaturation. We conclude that measurable amounts of NO are only produced and emitted by stressed leaves, that both isoprene and NO are effective antioxidant molecules and that an additional protection is achieved when both molecules are released.

Spatial reduction algorithm for atmospheric chemical transport models

Numerical modeling of global atmospheric chemical dynamics presents an enormous challenge, associated with simulating hundreds of chemical species with time scales varying from milliseconds to years. Here we present an algorithm that provides a significant reduction in computational cost. Because most of the fast reactants and their quickly decomposing reaction products are localized near emission sources, we use a series of reduced chemical models of decreasing complexity with increasing distance from the source. The algorithm diagnoses the chemical dynamics on-the-run, locally and separately for every species according to its characteristic reaction time. Unlike conventional time-scale separation methods, the spatial reduction algorithm speeds up not only the chemical solver but also advection-diffusion integration. Through several examples we demonstrate that the algorithm can reduce computational cost by at least an order of magnitude for typical atmospheric chemical kinetic mechanisms.