The hydroxyl radical (OH) fuels tropospheric ozone production and governs the lifetime of methane and many other gases. Existing methods to quantify global OH are limited to annual and ...global-to-hemispheric averages. Finer resolution is essential for isolating model deficiencies and building process-level understanding. In situ observations from the Atmospheric Tomography (ATom) mission demonstrate that remote tropospheric OH is tightly coupled to the production and loss of formaldehyde (HCHO), a major hydrocarbon oxidation product. Synthesis of this relationship with satellite-based HCHO retrievals and model-derived HCHO loss frequencies yields a map of total-column OH abundance throughout the remote troposphere (up to 70% of tropospheric mass) over the first two ATom missions (August 2016 and February 2017). This dataset offers unique insights on near-global oxidizing capacity. OH exhibits significant seasonality within individual hemispheres, but the domain mean concentration is nearly identical for both seasons (1.03 ± 0.25 × 106 cm−3), and the biseasonal average North/South Hemisphere ratio is 0.89 ± 0.06, consistent with a balance of OH sources and sinks across the remote troposphere. Regional phenomena are also highlighted, such as a 10-fold OH depression in the Tropical West Pacific and enhancements in the East Pacific and South Atlantic. This method is complementary to budget-based global OH constraints and can help elucidate the spatial and temporal variability of OH production and methane loss.
Glyoxal (CHOCHO) is produced in the atmosphere by the oxidation of volatile organic compounds(VOCs). Like formaldehyde (HCHO), another VOC oxidation product, it is measurable from space by solar ...backscatter. Isoprene emitted by vegetation is the dominant source of CHOCHO and HCHO in most of the world. We use aircraft observations of CHOCHO and HCHO from the Southeast Nexus (SENEX) campaign over the southeast US in summer 2013 to better understand the CHOCHO time-dependent yield from isoprene oxidation, its dependence on nitrogen oxides (NO (sub x) triple bonded to NO plus NO2), the behavior of the CHOCHO-HCHO relationship, the quality of Ozone Monitoring Instrument (OMI) CHOCHO satellite observations, and the implications for using CHOCHO observations from space as constraints on isoprene emissions. We simulate the SENEX and OMI observations with the Goddard Earth Observing System chemical transport model (GEOSChem) featuring a new chemical mechanism for CHOCHO formation from isoprene. The mechanism includes prompt CHOCHO formation under low-NO (sub x) conditions following the isomerization of the isoprene peroxy radical (ISOPO2).The SENEX observations provide support for this prompt CHOCHO formation pathway, and are generally consistent with the GEOS-Chem mechanism. Boundary layer CHOCHO and HCHO are strongly correlated in the observations and the model, with some departure under low-NO (sub x) conditions due to prompt CHOCHO formation. SENEX vertical profiles indicate a free-tropospheric CHOCHO background that is absent from the model. The OMI CHOCHO data provide some support for this free-tropospheric background and show southeast US enhancements consistent with the isoprene source but a factor of 2 too low. Part of this OMI bias is due to excessive surface reflectivities assumed in the retrieval. The OMI CHOCHO and HCHO seasonal data over the southeast US are tightly correlated and provide redundant proxies of isoprene emissions. Higher temporal resolution in future geostationary satellite observations may enable detection of the prompt CHOCHO production under low-NO (sub x) conditions apparent in the SENEX data.
Recent studies suggest overestimates in current U.S. emission inventories of nitrogen oxides (NO x = NO + NO2). Here, we expand a previously developed fuel-based inventory of motor-vehicle emissions ...(FIVE) to the continental U.S. for the year 2013, and evaluate our estimates of mobile source emissions with the U.S. Environmental Protection Agency’s National Emissions Inventory (NEI) interpolated to 2013. We find that mobile source emissions of NO x and carbon monoxide (CO) in the NEI are higher than FIVE by 28% and 90%, respectively. Using a chemical transport model, we model mobile source emissions from FIVE, and find consistent levels of urban NO x and CO as measured during the Southeast Nexus (SENEX) Study in 2013. Lastly, we assess the sensitivity of ozone (O3) over the Eastern U.S. to uncertainties in mobile source NO x emissions and biogenic volatile organic compound (VOC) emissions. The ground-level O3 is sensitive to reductions in mobile source NO x emissions, most notably in the Southeastern U.S. and during O3 exceedance events, under the revised standard proposed in 2015 (>70 ppb, 8 h maximum). This suggests that decreasing mobile source NO x emissions could help in meeting more stringent O3 standards in the future.
Organic aerosol (OA) is one of the main components of the global particulate burden and intimately links natural and anthropogenic emissions with air quality and climate. It is challenging to ...accurately represent OA in global models. Direct quantification of global OA abundance is not possible with current remote sensing technology; however, it may be possible to exploit correlations of OA with remotely observable quantities to infer OA spatiotemporal distributions. In particular, formaldehyde (HCHO) and OA share common sources via both primary emissions and secondary production from oxidation of volatile organic compounds (VOCs). Here, we examine OA–HCHO correlations using data from summertime airborne campaigns investigating biogenic (NASA SEAC4RS and DC3), biomass burning (NASA SEAC4RS), and anthropogenic conditions (NOAA CalNex and NASA KORUS-AQ). In situ OA correlates well with HCHO (r=0.59–0.97), and the slope and intercept of this relationship depend on the chemical regime. For biogenic and anthropogenic regions, the OA–HCHO slopes are higher in low NOx conditions, because HCHO yields are lower and aerosol yields are likely higher. The OA–HCHO slope of wildfires is over 9 times higher than that for biogenic and anthropogenic sources. The OA–HCHO slope is higher for highly polluted anthropogenic sources (e.g., KORUS-AQ) than less polluted (e.g., CalNex) anthropogenic sources. Near-surface OAs over the continental US are estimated by combining the observed in situ relationships with HCHO column retrievals from NASA's Ozone Monitoring Instrument (OMI). HCHO vertical profiles used in OA estimates are from climatology a priori profiles in the OMI HCHO retrieval or output of specific period from a newer version of GEOS-Chem. Our OA estimates compare well with US EPA IMPROVE data obtained over summer months (e.g., slope =0.60–0.62, r=0.56 for August 2013), with correlation performance comparable to intensively validated GEOS-Chem (e.g., slope =0.57, r=0.56) with IMPROVE OA and superior to the satellite-derived total aerosol extinction (r=0.41) with IMPROVE OA. This indicates that OA estimates are not very sensitive to these HCHO vertical profiles and that a priori profiles from OMI HCHO retrieval have a similar performance to that of the newer model version in estimating OA. Improving the detection limit of satellite HCHO and expanding in situ airborne HCHO and OA coverage in future missions will improve the quality and spatiotemporal coverage of our OA estimates, potentially enabling constraints on global OA distribution.
Formaldehyde (HCHO) has been measured from space for more
than 2 decades. Owing to its short atmospheric lifetime, satellite HCHO
data are used widely as a proxy of volatile organic compounds (VOCs; ...please
refer to Appendix A for abbreviations and acronyms), providing constraints
on underlying emissions and chemistry. However, satellite HCHO products from
different satellite sensors using different algorithms have received little
validation so far. The accuracy and consistency of HCHO retrievals remain
largely unclear. Here we develop a validation platform for satellite HCHO
retrievals using in situ observations from 12 aircraft campaigns with a chemical
transport model (GEOS-Chem) as the intercomparison method. Application to
the NASA operational OMI HCHO product indicates negative biases (−44.5 %
to −21.7 %) under high-HCHO conditions, while it indicates high biases (+66.1 % to
+112.1 %) under low-HCHO conditions. Under both conditions, HCHO a priori
vertical profiles are likely not the main driver of the biases. By providing
quick assessment of systematic biases in satellite products over large
domains, the platform facilitates, in an iterative process, optimization of
retrieval settings and the minimization of retrieval biases. It is also
complementary to localized validation efforts based on ground observations
and aircraft spirals.
Hydroxyl radical (OH) plays critical roles within the troposphere, such as determining the lifetime of methane (CH4), yet is challenging to model due to its fast cycling and dependence on a multitude ...of sources and sinks. As a result, the reasons for variations in OH and the resulting CH4 lifetime (TCH4), both between models and in time, are difficult to diagnose. We apply a neural network (NN) approach to address this issue within a group of models that participated in the Chemistry-Climate Model Initiative (CCMI). Analysis of the historical specified dynamics simulations performed for CCMI indicates that the primary drivers of TCH4 differences among ten models are the flux of UV light to the troposphere (indicated by the photolysis frequency JO1D) due mostly to clouds, mixing ratio of tropospheric ozone (O3), the abundance of nitrogen oxides (NOx≡NO+NO2), and details of the various chemical mechanisms that drive OH. Water vapor, carbon monoxide (CO), the ratio of NO:NOx, and formaldehyde (HCHO) explain moderate differences in TCH4, while isoprene, CH4, the photolysis frequency of NO2 by visible light (JNO2), overhead O3 column, and temperature account for little-to-no model variation in CH4. We also apply the NNs to analysis of temporal trends in OH from 1980 to 2015. All models that participated in the specified dynamics historical simulation for CCMI demonstrate a decline in CH4 during the analysed timeframe. The significant contributors to this trend, in order of importance, are tropospheric O3, JO1D, NOx, and H2O, with CO also causing substantial interannual variability in OH burden. Finally, the identified trends in TCH4 are compared to calculated trends in the tropospheric mean OH concentration from previous work, based on analysis of observations. The comparison reveals a robust result for the effect of rising water vapor on OH and CH4, imparting an increasing and decreasing trend of about 0.5% decade(exp -1), respectively. The responses due to NOx, O3 column, and temperature are also in reasonably good agreement between the two studies, though a discrepancy in the CH4 response highlights a need for further examination of the CH4 feedback on the abundance of OH.
Wildfires emit significant amounts of pollutants that degrade air quality. Plumes from three wildfires in the western U.S. were measured from aircraft during the Studies of Emissions and Atmospheric ...Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) and the Biomass Burning Observation Project (BBOP), both in summer 2013. This study reports an extensive set of emission factors (EFs) for over 80 gases and 5 components of submicron particulate matter (PM1) from these temperate wildfires. These include rarely, or never before, measured oxygenated volatile organic compounds and multifunctional organic nitrates. The observed EFs are compared with previous measurements of temperate wildfires, boreal forest fires, and temperate prescribed fires. The wildfires emitted high amounts of PM1 (with organic aerosol (OA) dominating the mass) with an average EF that is more than 2 times the EFs for prescribed fires. The measured EFs were used to estimate the annual wildfire emissions of carbon monoxide, nitrogen oxides, total non methane organic compounds, and PM1 from 11 western U.S. states. The estimated gas emissions are generally comparable with the 2011 National Emissions Inventory (NEI). However, our PM1 emission estimate (1530 +/- 570 Gg/yr) is over 3 times that of the NEI PM2.5 estimate and is also higher than the PM2.5 emitted from all other sources in these states in the NEI. This study indicates that the source of OA from biomass burning in the western states is significantly underestimated. In addition, our results indicate that prescribed burning may be an effective method to reduce fine particle emissions.
Water vapor's contribution to Earth's radiative forcing is most sensitive to changes in its lower stratosphere concentration. One recognized pathway for rapid increases in stratospheric water vapor ...is tropopause‐overshooting convection. Since this pathway has been rarely sampled, the NASA Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) field project focused on obtaining in situ observations of stratospheric air recently affected by convection over the United States. This study reports on the extreme altitudes to which convective hydration was observed. The data show that the overworld stratosphere is routinely hydrated by convection and that past documented records of stratospheric heights of convective hydration were exceeded during several DCOTSS flights. The most extreme event sampled is highlighted, for which stratospheric water vapor was increased by up to 26% at an altitude of 19.25 km, a potential temperature of 463 K, and an ozone mixing ratio >1500 ppbv.
Plain Language Summary
When thunderstorms reach into the second layer of the atmosphere above Earth's surface, the stratosphere, they may impact the concentration and distribution of trace gases that are important to chemistry and climate. One gas that is routinely affected during these events is water vapor, which is typically scarce in the stratosphere. This study presents new aircraft observations of extreme heights in the stratosphere moistened by these thunderstorms. Since increases in stratospheric water vapor positively contribute to warming of Earth's climate and can activate chemistry that destroys ozone, better understanding of this phenomenon helps refine our understanding of its role in the climate system. The new aircraft observations provide clear evidence that water vapor is enhanced by thunderstorms at higher levels in the stratosphere than previously recognized.
Key Points
Tropopause‐overshooting convection in the United States hydrates the stratosphere to exceptional heights
Observations of the height of stratospheric convective hydration during the Dynamics and Chemistry of the Summer Stratosphere field campaign exceed all prior global records
The overworld stratosphere is routinely hydrated by midlatitude overshooting convection
Formation of organic nitrates (RONO
) during oxidation of biogenic volatile organic compounds (BVOCs: isoprene, monoterpenes) is a significant loss pathway for atmospheric nitrogen oxide radicals (NO
...), but the chemistry of RONO
formation and degradation remains uncertain. Here we implement a new BVOC oxidation mechanism (including updated isoprene chemistry, new monoterpene chemistry, and particle uptake of RONO
) in the GEOS-Chem global chemical transport model with ∼25 × 25 km
resolution over North America. We evaluate the model using aircraft (SEAC
RS) and ground-based (SOAS) observations of NO
, BVOCs, and RONO
from the Southeast US in summer 2013. The updated simulation successfully reproduces the concentrations of individual gas- and particle-phase RONO
species measured during the campaigns. Gas-phase isoprene nitrates account for 25-50% of observed RONO
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 RONO
were in the particle phase. We find that aerosol uptake followed by hydrolysis to HNO
accounts for 60% of simulated gas-phase RONO
loss in the boundary layer. Other losses are 20% by photolysis to recycle NO
and 15% by dry deposition. RONO
production accounts for 20% of the net regional NO
sink in the Southeast US in summer, limited by the spatial segregation between BVOC and NO
emissions. This segregation implies that RONO
production will remain a minor sink for NO
in the Southeast US in the future even as NO
emissions continue to decline.
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 Master Chemical Mechanism (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 γglyx 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 (RGF = GLYX/HCHO), resulting from the suppression of δ‐isoprene peroxy radicals. 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 isoprene epoxydiol (IEPOX) peroxy radicals with HO2. Our work highlights that the gas‐phase production of glyoxal represents a large uncertainty in quantifying its contribution to SOA.
Key Points
Box model‐simulated glyoxal production from three isoprene oxidation mechanisms differ greatly
Aerosol uptake of glyoxal was constrained using airborne in situ measurements and a global model
Model results show that glyoxal contributes 0–14% of SOA in the Southeast U.S. during summer
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