Synthesizing evidence for the external cycling of NOx in high- to low-NOx atmospheres

External cycling regenerating nitrogen oxides (NOx ≡ NO + NO2) from their oxidative reservoir, NOz, is proposed to reshape the temporal-spatial distribution of NOx and consequently hydroxyl radical (OH), the most important oxidant in the atmosphere. Here we verify the in situ external cycling of NOx in various environments with nitrous acid (HONO) as an intermediate based on synthesized field evidence collected onboard aircraft platform at daytime. External cycling helps to reconcile stubborn underestimation on observed ratios of HONO/NO2 and NO2/NOz by current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO2 lacking noontime valleys specially observed in low-NOx atmospheres. Perturbation on the budget of HONO and NOx by external cycling is also found to increase as NOx concentration decreases. Consequently, model underestimation of OH observations by up to 41% in low NOx atmospheres is attributed to the omission of external cycling in models.

Impacts of Chemical Degradation on the Global Budget of Atmospheric Levoglucosan and Its Use As a Biomass Burning Tracer

Levoglucosan has been widely used to quantitatively assess biomass burning's contribution to ambient aerosols, but previous such assessments have not accounted for levoglucosan's degradation in the atmosphere. We develop the first global simulation of atmospheric levoglucosan, explicitly accounting for its chemical degradation, to evaluate the impacts on levoglucosan's use in quantitative aerosol source apportionment. Levoglucosan is emitted into the atmosphere from the burning of plant matter in open fires (1.7 Tg yr^-1) and as biofuels (2.1 Tg yr^-1). Sinks of atmospheric levoglucosan include aqueous-phase oxidation (2.9 Tg yr^-1), heterogeneous oxidation (0.16 Tg yr^-1), gas-phase oxidation (1.4 × 10^-4 Tg yr^-1), and dry and wet deposition (0.27 and 0.43 Tg yr ^-1). The global atmospheric burden of levoglucosan is 19 Gg with a lifetime of 1.8 days. Observations show a sharp decline in levoglucosan's concentrations and its relative abundance to organic carbon aerosol (OC) and particulate K⁺ from near-source to remote sites. We show that such features can only be reproduced when levoglucosan's chemical degradation is included in the model. Using model results, we develop statistical parametrizations to account for the atmospheric degradation in levoglucosan measurements, improving their use for quantitative aerosol source apportionment.

Regional characteristics of NO2 column densities from Pandora observations during the MAPS-Seoul campaign

Vertical column density (VCD) of nitrogen dioxide (NO2) was measured using Pandora spectrometers at six sites on the Korean Peninsula during the Megacity Air Pollution Studies-Seoul (MAPS-Seoul) campaign from May to June 2015. To estimate the tropospheric NO₂ VCD, the stratospheric NO₂ VCD from the Ozone Monitoring Instrument (OMI) was subtracted from the total NO₂ VCD from Pandora. European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis wind data was used to analyze variations in tropospheric NO₂ VCD caused by wind patterns at each site. The Yonsei/SEO site was found to have the largest tropospheric NO₂ VCD (1.49 DU on average) from a statistical analysis of hourly tropospheric NO₂ VCD measurements. At rural sites, remarkably low NO₂ VCDs were observed. However, a wind field analysis showed that trans-boundary transport and emissions from domestic sources lead to an increase in tropospheric NO₂ VCD at NIER/BYI and KMA/AMY, respectively. At urban sites, high NO₂ VCD values were observed under conditions of low wind speed, which were influenced by local urban emissions. Tropospheric NO₂ VCD at HUFS/Yongin increases under conditions of significant transport from urban area of Seoul according to a correlation analysis that considers the transport time lag. Significant diurnal variations were found at urban sites during the MAPS-Seoul campaign, but not at rural sites, indicating that it is associated with diurnal patterns of NO₂ emissions from dense traffic.

Atmospheric nitrogen deposition in the Yangtze River basin: Spatial pattern and source attribution

The Yangtze River basin is one of the world's hotspots for nitrogen (N) deposition and likely plays an important role in China's riverine N output. Here we constructed a basin-scale total dissolved inorganic N (DIN) deposition (bulk plus dry) pattern based on published data at 100 observational sites between 2000 and 2014, and assessed the relative contributions of different reactive N (N_r) emission sectors to total DIN deposition using the GEOS-Chem model. Our results show a significant spatial variation in total DIN deposition across the Yangtze River basin (33.2 kg N ha^-1 yr^-1 on average), with the highest fluxes occurring mainly in the central basin (e.g., Sichuan, Hubei and Hunan provinces, and Chongqing municipality). This indicates that controlling N deposition should build on mitigation strategies according to local conditions, namely, implementation of stricter control of N_r emissions in N deposition hotspots but moderate control in the areas with low N deposition levels. Total DIN deposition in approximately 82% of the basin area exceeded the critical load of N deposition for semi-natural ecosystems along the basin. On the basin scale, the dominant source of DIN deposition is fertilizer use (40%) relative to livestock (11%), industry (13%), power plant (9%), transportation (9%), and others (18%, which is the sum of contributions from human waste, residential activities, soil, lighting and biomass burning), suggesting that reducing NH₃ emissions from improper fertilizer (including chemical and organic fertilizer) application should be a priority in curbing N deposition. This, together with distinct spatial variations in emission sector contributions to total DIN deposition also suggest that, in addition to fertilizer, major emission sectors in different regions of the basin should be considered when developing synergistic control measures.

Novel Method for Nitrogen Isotopic Analysis of Soil-Emitted Nitric Oxide

The global inventory of NO_x (NO_x = NO + NO₂) emissions is poorly constrained, with a large portion of the uncertainty attributed to soil NO emissions that result from soil abiotic and microbial processes. While natural abundance stable N isotopes (δ¹⁵N) in various soil N-containing compounds have proven to be a robust tracer of soil N cycling, soil δ¹⁵N-NO is rarely quantified due to the measurement difficulties. Here, we present a new method that collects soil-emitted NO through NO conversion to NO₂ in excess ozone (O₃) and subsequent NO₂ collection in a 20% triethanolamine (TEA) solution as nitrite and nitrate for δ¹⁵N analysis using the denitrifier method. The precision and accuracy of the method were quantified through repeated collection of an analytical NO tank and intercalibration with a modified EPA NO_x collection method. The results show that the efficiency of NO conversion to NO₂ and subsequent NO₂ collection in the TEA solution is >98% under a variety of controlled conditions. The method precision (1σ) and accuracy across the entire analytical procedure are ±1.1‰. We report the first analyses of soil δ¹⁵N-NO (-59.8‰ to -23.4‰) from wetting-induced NO pulses at both laboratory and field scales that have important implications for understanding soil NO dynamics.

PM2.5 source attribution for Seoul in May from 2009 to 2013 using GEOS-Chem and its adjoint model

Enforcement of an air quality standard for PM_2.5 in the Seoul metropolitan area (SMA) was enacted in 2015. From May to June of 2016, an international airborne and surface measurement campaign took place to investigate air pollution mechanisms in the SMA. The total and speciated PM_2.5 concentrations since 2008 have been measured at an intensive monitoring site for the SMA operated by the National Institute of Environmental Research (NIER). To gain insight on the trends and sources of PM_2.5 in the SMA in May, we analyze PM_2.5 concentrations from 2009 to 2013 using the measurements and simulations from a 3-dimensional global chemical transport model, GEOS-Chem and its adjoint. The model is updated here with the latest regional emission inventory and diurnally varying NH₃ emissions. Monthly average PM_2.5 concentration measured by β-ray attenuation ranges from 28 (2010) to 45 (2013) μg/m³, decreased from 2009 to 2010, and then continuously increased until 2013. The model shows good agreement with the measurements for the daily average PM_2.5 concentrations (R ≥ 0.5), and reproduces 10 out of 17 measured episodes exceeding the daily air quality standard (50 μg/m³). Using the GEOS-Chem adjoint model, we find that anthropogenic emissions from the Shandong region have the largest modeled influence on PM_2.5 in Seoul in May. Average contributions to the high PM_2.5 episodes simulated by the model are 39% from the Shandong region, 16% from the Shanghai region, 14% from the Beijing region, and 15% from South Korea. Anthropogenic SO₂ emissions from South Korea are negligible with 90% of the total contribution originating from China. Findings from this study may guide interpretation of observations obtained in the KORUS-AQ measurement campaign.

Observational constraints on glyoxal production from isoprene oxidation and its contribution to organic aerosol over the Southeast United States

We use a 0-D photochemical box model and a 3-D global chemistry-climate model, combined with observations from the NOAA Southeast Nexus (SENEX) aircraft campaign, to understand the sources and sinks of glyoxal over the Southeast United States. Box model simulations suggest a large difference in glyoxal production among three isoprene oxidation mechanisms (AM3ST, AM3B, and MCM v3.3.1). These mechanisms are then implemented into a 3-D global chemistry-climate model. Comparison with field observations shows that the average vertical profile of glyoxal is best reproduced by AM3ST with an effective reactive uptake coefficient γ_glyx of 2 × 10^-3, and AM3B without heterogeneous loss of glyoxal. The two mechanisms lead to 0-0.8 μg m^-3 secondary organic aerosol (SOA) from glyoxal in the boundary layer of the Southeast U.S. in summer. We consider this to be the lower limit for the contribution of glyoxal to SOA, as other sources of glyoxal other than isoprene are not included in our model. In addition, we find that AM3B shows better agreement on both formaldehyde and the correlation between glyoxal and formaldehyde (R_GF = [GLYX]/[HCHO]), resulting from the suppression of δ-isoprene peroxy radicals (δ-ISOPO₂). We also find that MCM v3.3.1 may underestimate glyoxal production from isoprene oxidation, in part due to an underestimated yield from the reaction of IEPOX peroxy radicals (IEPOXOO) with HO₂. Our work highlights that the gas-phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.

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.

Exploring the Uncertainty Associated with Satellite-Based Estimates of Premature Mortality due to Exposure to Fine Particulate Matter

The negative impacts of fine particulate matter (PM_2.5) exposure on human health are a primary motivator for air quality research. However, estimates of the air pollution health burden vary considerably and strongly depend on the datasets and methodology. Satellite observations of aerosol optical depth (AOD) have been widely used to overcome limited coverage from surface monitoring and to assess the global population exposure to PM_2.5 and the associated premature mortality. Here we quantify the uncertainty in determining the burden of disease using this approach, discuss different methods and datasets, and explain sources of discrepancies among values in the literature. For this purpose we primarily use the MODIS satellite observations in concert with the GEOS-Chem chemical transport model. We contrast results in the United States and China for the years 2004-2011. Using the Burnett et al. (2014) integrated exposure response function, we estimate that in the United States, exposure to PM_2.5 accounts for approximately 2% of total deaths compared to 14% in China (using satellite-based exposure), which falls within the range of previous estimates. The difference in estimated mortality burden based solely on a global model vs. that derived from satellite is approximately 14% for the U.S. and 2% for China on a nationwide basis, although regionally the differences can be much greater. This difference is overshadowed by the uncertainty in the methodology for deriving PM_2.5 burden from satellite observations, which we quantify to be on the order of 20% due to uncertainties in the AOD-to-surface-PM_2.5 relationship, 10% due to the satellite observational uncertainty, and 30% or greater uncertainty associated with the application of concentration response functions to estimated exposure.

Why do Models Overestimate Surface Ozone in the Southeastern United States?

Ozone pollution in the Southeast US involves complex chemistry driven by emissions of anthropogenic nitrogen oxide radicals (NO_x ≡ NO + NO₂) and biogenic isoprene. Model estimates of surface ozone concentrations tend to be biased high in the region and this is of concern for designing effective emission control strategies to meet air quality standards. We use detailed chemical observations from the SEAC⁴RS aircraft campaign in August and September 2013, interpreted with the GEOS-Chem chemical transport model at 0.25°×0.3125° horizontal resolution, to better understand the factors controlling surface ozone in the Southeast US. We find that the National Emission Inventory (NEI) for NO_x from the US Environmental Protection Agency (EPA) is too high. This finding is based on SEAC⁴RS observations of NO_x and its oxidation products, surface network observations of nitrate wet deposition fluxes, and OMI satellite observations of tropospheric NO₂ columns. Our results indicate that NEI NO_x emissions from mobile and industrial sources must be reduced by 30-60%, dependent on the assumption of the contribution by soil NO_x emissions. Upper tropospheric NO₂ from lightning makes a large contribution to satellite observations of tropospheric NO₂ that must be accounted for when using these data to estimate surface NO_x emissions. We find that only half of isoprene oxidation proceeds by the high-NO_x pathway to produce ozone; this fraction is only moderately sensitive to changes in NO_x emissions because isoprene and NO_x emissions are spatially segregated. GEOS-Chem with reduced NO_x emissions provides an unbiased simulation of ozone observations from the aircraft, and reproduces the observed ozone production efficiency in the boundary layer as derived from a regression of ozone and NO_x oxidation products. However, the model is still biased high by 8±13 ppb relative to observed surface ozone in the Southeast US. Ozonesondes launched during midday hours show a 7 ppb ozone decrease from 1.5 km to the surface that GEOS-Chem does not capture. This bias may reflect a combination of excessive vertical mixing and net ozone production in the model boundary layer.

Quantum Chemical Study on ·Cl-Initiated Atmospheric Degradation of Monoethanolamine

Recent findings on the formation of ·Cl in continental urban areas necessitate the consideration of ·Cl initiated degradation when assessing the fate of volatile organic pollutants. Monoethanolamine (MEA) is considered as a potential atmospheric pollutant since it is a benchmark and widely utilized solvent in a leading CO2 capture technology. Especially, ·Cl may have specific interactions with the N atom of MEA, which could make the MEA + ·Cl reaction have different pathways and products from those of the MEA + ·OH reaction. Hence, ·Cl initiated reactions with MEA were investigated by a quantum chemical method [CCSD(T)/aug-cc-pVTZ//MP2/6-31+G(3df,2p)] and kinetics modeling. Results show that the overall rate constant for ·Cl initiated H-abstraction of MEA is 5 times faster than that initiated by ·OH, and the tropospheric lifetimes of MEA will be overestimated by 6-46% when assuming that [·Cl]/[·OH] = 1-10% if the role of ·Cl is ignored. The MEA + ·Cl reaction exclusively produces MEA-N that finally transforms into several products including mutagenic nitramine and carcinogenic nitrosamine via further reactions with O2/NOx, and the contribution of ·Cl to their formation is about 25-250% of that of ·OH. Thus, it is necessary to consider ·Cl initiated tropospheric degradation of MEA for its risk assessment.

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.

Partitioning, sources and variability of regional and local oxidant (OX = O3 + NO 2) in a coastal rural area in the southwest of Iberian Peninsula

The purpose of this work is to investigate the behaviour and variability of oxidant levels (OX = NO2 + O3), for the first time, in a rural coastal area in the southwest of the Iberian Peninsula, affected by several air masses types. Detailed database (built-up over the years 2008 to 2011, and containing around 500,000 data) from the Atmospheric Sounding Station "El Arenosillo" was used. The observed daily cycles of NO x and OX were influenced by air masses coming from industrial and urban area. It can be seen that the concentration of OX is made up of a NO x -independent 'regional' contribution (i.e. the O3 background), and a linearly NO x -dependent 'local' contribution from primary emissions, such as traffic. The local emission is very low in this area. Also, the regional contribution is similar to unpolluted sites and presents seasonal variation, being higher in May. However, our measurements showed that the proportion of OX in the form of NO2 increases with the increase in NO x concentration during the day. The higher proportion of NO2 observed at night must be due to the conversion of NO to NO2 by the NO + O3 reaction. With regards to the source of the local NO x -dependent contribution, it may be attributed to industrial emission, or the termolecular reaction 2NO + O2 = 2NO2, at high-NO x levels and stagnant air during several days. Finally, we estimated the photolysis rate of NO2, J NO2, an important key atmospheric reaction coupled with ozone. We also present surface plots of annual variation of the daily mean NO x and OX levels, which indicate that oxidants come from transport processes instead of local emissions associated as local photochemistry.

Tropospheric methanol observations from space: retrieval evaluation and constraints on the seasonality of biogenic emissions

Methanol retrievals from nadir-viewing space-based sensors offer powerful new information for quantifying methanol emissions on a global scale. Here we apply an ensemble of aircraft observations over North America to evaluate new methanol measurements from the Tropospheric Emission Spectrometer (TES) on the Aura satellite, and combine the TES data with observations from the Infrared Atmospheric Sounding Interferometer (IASI) on the MetOp-A satellite to investigate the seasonality of methanol emissions from northern midlatitude ecosystems. Using the GEOS-Chem chemical transport model as an intercomparison platform, we find that the TES retrieval performs well when the degrees of freedom for signal (DOFS) are above 0.5, in which case the model:TES regressions are generally consistent with the model:aircraft comparisons. Including retrievals with DOFS below 0.5 degrades the comparisons, as these are excessively influenced by the a priori. The comparisons suggest DOFS >0.5 as a minimum threshold for interpreting retrievals of trace gases with a weak tropospheric signal. We analyze one full year of satellite observations and find that GEOS-Chem, driven with MEGANv2.1 biogenic emissions, underestimates observed methanol concentrations throughout the midlatitudes in springtime, with the timing of the seasonal peak in model emissions 1-2 months too late. We attribute this discrepancy to an underestimate of emissions from new leaves in MEGAN, and apply the satellite data to better quantify the seasonal change in methanol emissions for midlatitude ecosystems. The derived parameters (relative emission factors of 11.0, 0.26, 0.12 and 3.0 for new, growing, mature, and old leaves, respectively, plus a leaf area index activity factor of 0.5 for expanding canopies with leaf area index <1.2) provide a more realistic simulation of seasonal methanol concentrations in midlatitudes on the basis of both the IASI and TES measurements.

Formaldehyde columns from the Ozone Monitoring Instrument: Urban versus background levels and evaluation using aircraft data and a global model

[1] We combine aircraft measurements (Second Texas Air Quality Study, Megacity Initiative: Local and Global Research Observations, Intercontinental Chemical Transport Experiment: Phase B) over the United States, Mexico, and the Pacific with a 3-D model (GEOS-Chem) to evaluate formaldehyde column (Ω_HCHO) retrievals from the Ozone Monitoring Instrument (OMI) and assess the information they provide on HCHO across local to regional scales and urban to background regimes. OMI Ω_HCHO correlates well with columns derived from aircraft measurements and GEOS-Chem (R = 0.80). For the full data ensemble, OMI's mean bias is -3% relative to aircraft-derived Ω_HCHO (-17% where Ω_HCHO > 5 × 10¹⁵ molecules cm^-2) and -8% relative to GEOS-Chem, within expected uncertainty for the retrieval. Some negative bias is expected for the satellite and model, given the plume sampling of many flights and averaging over the satellite and model footprints. Major axis regression for OMI versus aircraft and model columns yields slopes (95% confidence intervals) of 0.80 (0.62-1.03) and 0.98 (0.73-1.35), respectively, with no significant intercept. Aircraft measurements indicate that the normalized vertical HCHO distribution, required by the satellite retrieval, is well captured by GEOS-Chem, except near Mexico City. Using measured HCHO profiles in the retrieval algorithm does not improve satellite-aircraft agreement, suggesting that use of a global model to specify shape factors does not substantially degrade retrievals over polluted areas. While the OMI measurements show that biogenic volatile organic compounds dominate intra-annual and regional Ω_HCHO variability across the United States, smaller anthropogenic Ω_HCHO gradients are detectable at finer spatial scales (∼20-200 km) near many urban areas.

CMAQ model performance enhanced when in-cloud secondary organic aerosol is included: comparisons of organic carbon predictions with measurements

Mounting evidence suggests that low-volatility (particle-phase) organic compounds form in the atmosphere through aqueous phase reactions in clouds and aerosols. Although some models have begun including secondary organic aerosol (SOA) formation through cloud processing, validation studies that compare predictions and measurements are needed. In this work, agreement between modeled organic carbon (OC) and aircraft measurements of water soluble OC improved for all 5 of the compared ICARTT NOAA-P3 flights during August when an in-cloud SOA (SOAcld) formation mechanism was added to CMAQ (a regional-scale atmospheric model). The improvement was most dramatic for the August 14th flight, a flight designed specifically to investigate clouds. During this flight the normalized mean bias for layer-averaged OC was reduced from -64 to -15% and correlation (r) improved from 0.5 to 0.6. Underpredictions of OC aloft by atmospheric models may be explained, in part, by this formation mechanism (SOAcld). OC formation aloft contributes to long-range pollution transport and has implications to radiative forcing, regional air quality and climate.