Isoprene is the dominant non-methane organic compound emitted to the atmosphere
. It drives ozone and aerosol production, modulates atmospheric oxidation and interacts with the global nitrogen cycle
.... Isoprene emissions are highly uncertain
, as is the nonlinear chemistry coupling isoprene and the hydroxyl radical, OH-its primary sink
. Here we present global isoprene measurements taken from space using the Cross-track Infrared Sounder. Together with observations of formaldehyde, an isoprene oxidation product, these measurements provide constraints on isoprene emissions and atmospheric oxidation. We find that the isoprene-formaldehyde relationships measured from space are broadly consistent with the current understanding of isoprene-OH chemistry, with no indication of missing OH recycling at low nitrogen oxide concentrations. We analyse these datasets over four global isoprene hotspots in relation to model predictions, and present a quantification of isoprene emissions based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural nitrogen oxide emissions bias modelled OH and hence isoprene. Over southern Africa, we find that a prominent isoprene hotspot is missing from bottom-up predictions. A multi-year analysis sheds light on interannual isoprene variability, and suggests the influence of the El Niño/Southern Oscillation.
Urban air pollution is among the top 15 causes of death and disease worldwide, and a problem of growing importance with a majority of the global population living in cities. A important question for ...sustainable development is to what extent urban design can improve or degrade the environment and public health. We investigate relationships between satellite-derived estimates of nitrogen dioxide concentration (NO2, a key component of urban air pollution) and urban form for 83 cities globally. We find a parsimonious yet powerful relationship (model R 2 = 0.63), using as predictors population, income, urban contiguity, and meteorology. Cities with highly contiguous built-up areas have, on average, lower urban NO2 concentrations (a one standard deviation increase in contiguity is associated with a 24% decrease in average NO2 concentration). More-populous cities tend to have worse air quality, but the increase in NO2 associated with a population increase of 10% may be offset by a moderate increase (4%) in urban contiguity. Urban circularity (“compactness”) is not a statistically significant predictor of NO2 concentration. Although many factors contribute to urban air pollution, our findings suggest that antileapfrogging policies may improve air quality. We find that urban NO2 levels vary nonlinearly with income (Gross Domestic Product), following an “environmental Kuznets curve”; we estimate that if high-income countries followed urban pollution-per-income trends observed for low-income countries, NO2 concentrations in high-income cities would be ∼10× larger than observed levels.
We present 1 year of in situ proton transfer reaction mass spectrometer (PTR‐MS) measurements of isoprene and its oxidation products methyl vinyl ketone (MVK) and methacrolein (MACR) from a 244 m ...tall tower in the U.S. Upper Midwest, located at an ecological transition between isoprene‐emitting deciduous forest and predominantly non‐isoprene‐emitting agricultural landscapes. We find that anthropogenic interferences (or anthropogenic isoprene) contribute on average 22% of the PTR‐MS m/z 69 signal during summer daytime, whereas MVK + MACR interferences (m/z 71) are minor (7%). After removing these interferences, the observed isoprene and MVK + MACR abundances show pronounced seasonal cycles, reaching summertime maxima of >2500 pptv (1 h mean). The tall tower is impacted both by nearby and more distant regional isoprene sources, with daytime enhancements of isoprene (but little MVK + MACR) under southwest winds and enhancements of MVK + MACR (but little isoprene) at other times. We find that the GEOS‐Chem atmospheric model with the MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic inventory can reproduce the isoprene observations to within model uncertainty given improved land cover and temperature estimates. However, a 60% low model bias in MVK + MACR cannot be resolved, even across diverse model assumptions for NOx emissions, chemistry, atmospheric mixing, dry deposition, land cover, and potential measurement interferences. This implies that, while isoprene emissions in the immediate vicinity of the tall tower are adequately captured, they are underestimated across the broader region. We show that this region experiences a strong seasonal shift between VOC‐limited chemistry during the spring and fall and NOx‐limited or transitional chemistry during the summer, driven by the spatiotemporal distribution of isoprene emissions. Isoprene's role in causing these chemical shifts is likely underestimated due to the underprediction of its regional emissions.
Key Points
Interferences contribute on average 22% of PTR‐MS m/z 69 signal
Isoprene emissions are underestimated in the US Upper Midwest
Spatiotemporal distribution of isoprene emissions drives NOx‐VOC chemical shifts
Space‐borne formaldehyde (HCHO) column measurements from the Ozone Monitoring Instrument (OMI), with 13 × 24 km2 nadir footprint and daily global coverage, provide new constraints on the spatial ...distribution of biogenic isoprene emission from North America. OMI HCHO columns for June‐August 2006 are consistent with measurements from the earlier GOME satellite sensor (1996–2001) but OMI is 2–14% lower. The spatial distribution of OMI HCHO columns follows that of isoprene emission; anthropogenic hydrocarbon emissions are undetectable except in Houston. We develop updated relationships between HCHO columns and isoprene emission from a chemical transport model (GEOS‐Chem), and use these to infer top‐down constraints on isoprene emissions from the OMI data. We compare the OMI‐derived emissions to a state‐of‐science bottom‐up isoprene emission inventory (MEGAN) driven by two land cover databases, and use the results to optimize the MEGAN emission factors (EFs) for broadleaf trees (the main isoprene source). The OMI‐derived isoprene emissions in North America (June–August 2006) with 1° × 1° resolution are spatially consistent with MEGAN (R2 = 0.48–0.68) but are lower (by 4–25% on average). MEGAN overestimates emissions in the Ozarks and the Upper South. A better fit to OMI (R2 = 0.73) is obtained in MEGAN by using a uniform isoprene EF from broadleaf trees rather than variable EFs. Thus MEGAN may overestimate emissions in areas where it specifies particularly high EFs. Within‐canopy isoprene oxidation may also lead to significant differences between the effective isoprene emission to the atmosphere seen by OMI and the actual isoprene emission determined by MEGAN.
Indirect nitrous oxide (N
O) emissions from streams and rivers are a poorly constrained term in the global N
O budget. Current models of riverine N
O emissions place a strong focus on denitrification ...in groundwater and riverine environments as a dominant source of riverine N
O, but do not explicitly consider direct N
O input from terrestrial ecosystems. Here, we combine N
O isotope measurements and spatial stream network modeling to show that terrestrial-aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N
O in a mesoscale river network within the U.S. Corn Belt. We find that N
O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N
O across the entire river network. The delivery of soil-produced N
O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N
O input implies an important climate-N
O feedback mechanism that may enhance riverine N
O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N
O emissions may result in significant underestimations.
Indirect nitrous oxide (N2O) emissions from streams and rivers are a poorly constrained term in the global N2O budget. Current models of riverine N2O emissions place a strong focus on denitrification ...in groundwater and riverine environments as a dominant source of riverine N2O, but do not explicitly consider direct N2O input from terrestrial ecosystems. Here, we combine N2O isotope measurements and spatial stream network modeling to show that terrestrial–aquatic interactions, driven by changing hydrologic connectivity, control the sources and dynamics of riverine N2O in a mesoscale river network within the U.S. Corn Belt. We find that N2O produced from nitrification constituted a substantial fraction (i.e., >30%) of riverine N2O across the entire river network. The delivery of soil-produced N2O to streams was identified as a key mechanism for the high nitrification contribution and potentially accounted for more than 40% of the total riverine emission. This revealed large terrestrial N2O input implies an important climate–N2O feedback mechanism that may enhance riverine N2O emissions under a wetter and warmer climate. Inadequate representation of hydrologic connectivity in observations and modeling of riverine N2O emissions may result in significant underestimations.
The Salt Lake Valley experiences severe fine particulate matter pollution episodes in winter during persistent cold-air pools (PCAPs). We employ measurements throughout an entire winter from ...different elevations to examine the chemical and dynamical processes driving these episodes. Whereas primary pollutants such as NO x and CO were enhanced twofold during PCAPs, O3 concentrations were approximately threefold lower. Atmospheric composition varies strongly with altitude within a PCAP at night with lower NO x and higher oxidants (O3) and oxidized reactive nitrogen (N2O5) aloft. We present observations of N2O5 during PCAPs that provide evidence for its role in cold-pool nitrate formation. Our observations suggest that nighttime and early morning chemistry in the upper levels of a PCAP plays an important role in aerosol nitrate formation. Subsequent daytime mixing enhances surface PM2.5 by dispersing the aerosol throughout the PCAP. As pollutants accumulate and deplete oxidants, nitrate chemistry becomes less active during the later stages of the pollution episodes. This leads to distinct stages of PM2.5 pollution episodes, starting with a period of PM2.5 buildup and followed by a period with plateauing concentrations. We discuss the implications of these findings for mitigation strategies.
The Arctic is a climatically sensitive region that has experienced warming at almost 3 times the global average rate in recent decades, leading to an increase in Arctic greenness and a greater ...abundance of plants that emit biogenic volatile organic compounds (BVOCs). These changes in atmospheric emissions are expected to significantly modify the overall oxidative chemistry of the region and lead to changes in VOC composition and abundance, with implications for atmospheric processes. Nonetheless, observations needed to constrain our current understanding of these issues in this critical environment are sparse. This work presents novel atmospheric in situ proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS) measurements of VOCs at Toolik Field Station (TFS; 68°38' N, 149°36' W), in the Alaskan Arctic tundra during May-June 2019. We employ a custom nested grid version of the GEOS-Chem chemical transport model (CTM), driven with MEGANv2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) biogenic emissions for Alaska at 0.25° × 0.3125° resolution, to interpret the observations in terms of their constraints on BVOC emissions, total reactive organic carbon (ROC) composition, and calculated OH reactivity (OHr) in this environment. We find total ambient mole fraction of 78 identified VOCs to be 6.3 ± 0.4 ppbv (10.8 ± 0.5 ppbC), with overwhelming (> 80 %) contributions are from short-chain oxygenated VOCs (OVOCs) including methanol, acetone and formaldehyde. Isoprene was the most abundant terpene identified. GEOS-Chem captures the observed isoprene (and its oxidation products), acetone and acetaldehyde abundances within the combined model and observation uncertainties (±25 %), but underestimates other OVOCs including methanol, formaldehyde, formic acid and acetic acid by a factor of 3 to 12. The negative model bias for methanol is attributed to underestimated biogenic methanol emissions for the Alaskan tundra in MEGANv2.1. Observed formaldehyde mole fractions increase exponentially with air temperature, likely reflecting its biogenic precursors and pointing to a systematic model underprediction of its secondary production. The median campaign-calculated OHr from VOCs measured at TFS was 0.7 s
, roughly 5 % of the values typically reported in lower-latitude forested ecosystems. Ten species account for over 80 % of the calculated VOC OHr, with formaldehyde, isoprene and acetaldehyde together accounting for nearly half of the total. Simulated OHr based on median-modeled VOCs included in GEOS-Chem averages 0.5 s
and is dominated by isoprene (30 %) and monoterpenes (17 %). The data presented here serve as a critical evaluation of our knowledge of BVOCs and ROC budgets in high-latitude environments and represent a foundation for investigating and interpreting future warming-driven changes in VOC emissions in the Alaskan Arctic tundra.
We perform observing system simulation experiments
(OSSEs) with the GEOS-Chem adjoint model to test how well methane emissions
over North America can be resolved using measurements from the ...TROPOspheric
Monitoring Instrument (TROPOMI) and similar high-resolution satellite
sensors. We focus analysis on the impacts of (i) spatial errors in the prior
emissions and (ii) model transport errors. Along with a standard
scale factor (SF) optimization we conduct a set of inversions using
alternative formalisms that aim to overcome limitations in the SF-based
approach that arise for missing sources. We show that 4D-Var analysis of the
TROPOMI data can improve monthly emission estimates at 25 km even with a
spatially biased prior or model transport errors (42 %–93 % domain-wide
bias reduction; R increases from 0.51 up to 0.73). However, when both errors
are present, no single inversion framework can successfully improve both the
overall bias and spatial distribution of fluxes relative to the prior on the
25 km model grid. In that case, the ensemble-mean optimized fluxes have a
domain-wide bias of 77 Gg d−1 (comparable to that in the prior), with
spurious source adjustments compensating for the transport errors.
Increasing observational coverage through longer-timeframe inversions does
not significantly change this picture. An inversion formalism that optimizes
emission enhancements rather than scale factors exhibits the best
performance for identifying missing sources, while an approach combining a
uniform background emission with the prior inventory yields the best
performance in terms of overall spatial fidelity – even in the presence of
model transport errors. However, the standard SF optimization outperforms
both of these for the magnitude of the domain-wide flux. For the common
scenario in which prior errors are non-random, approximate posterior error
reduction calculations (derived via gradient-based randomization) for the
inversions reflect the sensitivity to observations but have no spatial
correlation with the actual emission improvements. This demonstrates that
such information content analysis can be used for general observing system
characterization but does not describe the spatial accuracy of the posterior
emissions or of the actual emission improvements. Findings here highlight
the need for careful evaluation of potential missing sources in prior
emission datasets and for robust accounting of model transport errors in
inverse analyses of the methane budget.
Molecular hydrogen, H2, is one of the most abundant trace gases in the atmosphere. The main known chemical source of H2 in the atmosphere is the photolysis of formaldehyde and glyoxal. Recent ...laboratory measurements and ground-state photochemistry calculations have shown other aldehydes photodissociate to yield H2 as well. This aldehyde photochemistry has not been previously accounted for in atmospheric H2 models. Here, we used two atmospheric models to test the implications of the previously unexplored aldehyde photochemistry on the H2 tropospheric budget. We used the AtChem box model implementing the nearly chemically explicit Master Chemical Mechanism at three sites selected to represent variable atmospheric environments: London, Cabo Verde and Borneo. We conducted five box model simulations per site using varying quantum yields for the photolysis of 16 aldehydes and compared the results against a baseline. The box model simulations showed that the photolysis of acetaldehyde, propanal, methylglyoxal, glycolaldehyde and methacrolein yields the highest chemical production of H2. We also used the GEOS-Chem 3-D atmospheric chemical transport model to test the impacts of the new photolytic H2 source on the global scale. A new H2 simulation capability was developed in GEOS-Chem and evaluated for 2015 and 2016. We then performed a sensitivity simulation in which the photolysis reactions of six aldehyde species were modified to include a 1 % yield of H2. We found an increase in the chemical production of H2 over tropical regions where high abundance of isoprene results in the secondary generation of methylglyoxal, glycolaldehyde and methacrolein, ultimately yielding H2. We calculated a final increase of 0.4 Tg yr−1 in the global chemical production budget, compared to a baseline production of ∼41 Tg yr−1. Ultimately, both models showed that H2 production from the newly discovered photolysis of aldehydes leads to only minor changes in the atmospheric mixing ratios of H2, at least for the aldehydes tested here when assuming a 1 % quantum yield across all wavelengths. Our results imply that the previously missing photochemical source is a less significant source of model uncertainty than other components of the H2 budget, including emissions and soil uptake.
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