The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere. The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH ...concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane. It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4, 7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 ± 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns. Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.
Definitions of the extratropical tropopause are examined from the perspective of chemical composition. Fine‐scale measurements of temperature, ozone, carbon monoxide, and water vapor from ...approximately 70 aircraft flights, with ascending and descending tropopause crossings near 40°N and 65°N, are used in this analysis. Using the relationship of the stratospheric tracer O3 and the tropospheric tracer CO, we address the issues of tropopause sharpness and where the transitions from troposphere to stratosphere occur in terms of the chemical composition. Tracer relationships indicate that mixing of stratospheric and tropospheric air masses occurs in the vicinity of the tropopause to form a transition layer. Statistically, this transition layer is centered on the thermal tropopause. Furthermore, we show that the transition is much sharper near 65°N (a region away from the subtropical jet) but spans a larger altitude range near 40°N (in the vicinity of the subtropical jet). This latter feature is consistent with enhanced stratosphere‐troposphere exchange and mixing activity near the tropopause break.
The global oxidation capacity, defined as the tropospheric mean concentration of the hydroxyl radical (OH), controls the lifetime of reactive trace gases in the atmosphere such as methane and carbon ...monoxide (CO). Models tend to underestimate the methane lifetime and CO concentrations throughout the troposphere, which is consistent with excessive OH. Approximately half the oxidation of methane and non-methane volatile organic compounds (VOCs) is thought to occur over the oceans where oxidant chemistry has received little validation due to a lack of observational constraints. We use observations from the first two deployments of the NASA ATom aircraft campaign during July–August 2016 and January–February 2017 to evaluate the oxidation capacity over the remote oceans and its representation in the GEOS-Chem chemical transport model. The model successfully simulates the magnitude and vertical profile of remote OH within the measurement uncertainties. Comparisons against the drivers of OH production (water vapor, ozone, and NOy concentrations, ozone photolysis frequencies) also show minimal bias with the exception of wintertime NOy, for which a model overestimate may indicate insufficient wet scavenging and/or missing loss on seasalt aerosol but large uncertainties remain that require further studies of NOy partitioning and removal in the troposphere. During the ATom-1 deployment, OH reactivity (OHR) below 3 km is significantly enhanced, and this is not captured by the sum of its measured components (cOHRobs) or by the model (cOHRmod). This enhancement could suggest missing reactive VOCs but cannot be explained by new estimates of ocean VOC sources and additional modeled reactivity in this region would be difficult to reconcile with the full suite of ATom measurement constraints. The model generally reproduces the magnitude and seasonality of cOHRobs but underestimates the contribution of oxygenated VOC, mainly acetaldehyde, which is severely underestimated throughout the troposphere despite its calculated lifetime of less than a day. Missing model acetaldehyde in previous studies was attributed to measurement uncertainties that have been largely resolved. Observations of peroxyacetic acid (PAA) provide new support for remote levels of acetaldehyde. The underestimate in modeled acetaldehyde and PAA is present throughout the year in both hemispheres and peaks during Northern Hemisphere summer. The addition of ocean VOC sources in the model increases annual surface cOHRmod by 10 % and improves model-measurement agreement for acetaldehyde particularly in winter but cannot resolve the model summertime bias. Doing so would require a 100 Tg yr−1 source of a long-lived unknown precursor throughout the year with significant additional emissions in the Northern Hemisphere summer. Improving the model bias for remote acetaldehyde and PAA is unlikely to fully resolve previously reported model global biases in OH and methane lifetime, suggesting that future work should examine the sources and sinks of OH over land.
This study, conducted in December 2004, is the first to present observations of DMS in a snow pack covering the multi‐year sea ice of the western Weddell Sea. The snow layer is important because it ...is the interface through which DMS needs to be transported in order to be emitted directly from the ice to the overlying atmosphere. High concentrations of DMS, up to 6000 nmol m−3, were found during the first weeks of December but concentrations sharply decline as late spring‐early summer progresses. This implies that DMS contained in sea ice is efficiently vented through the snow into the atmosphere. Indeed, field measurements by relaxed eddy accumulation indicate an average release of 11 μmol DMS m−2 d−1 from the ice and snow throughout December.
The NASA Atmospheric Tomography (ATom) mission built a
photochemical climatology of air parcels based on in situ measurements with
the NASA DC-8 aircraft along objectively planned profiling transects ...through
the middle of the Pacific and Atlantic oceans. In this paper we present and
analyze a data set of 10 s (2 km) merged and gap-filled observations of the
key reactive species driving the chemical budgets of O3 and CH4
(O3, CH4, CO, H2O, HCHO, H2O2, CH3OOH,
C2H6, higher alkanes, alkenes, aromatics, NOx, HNO3,
HNO4, peroxyacetyl nitrate, and other organic nitrates), consisting of
146 494 distinct air parcels from ATom deployments 1 through 4. Six models
calculated the O3 and CH4 photochemical tendencies from this
modeling data stream for ATom 1. We find that 80 %–90 % of the
total reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top 10 %, supporting previous model-only studies that
tropospheric chemistry is driven by a fraction of all the air. Surprisingly,
the probability densities of species and reactivities averaged on a model
scale (100 km) differ only slightly from the 2 km ATom 10 s data, indicating
that much of the heterogeneity in tropospheric chemistry can be captured
with current global chemistry models. Comparing the ATom reactivities over
the tropical oceans with climatological statistics from six global chemistry
models, we find generally good agreement with the reactivity rates for
O3 and CH4. Models distinctly underestimate O3 production
below 2 km relative to the mid-troposphere, and this can be traced to lower
NOx levels than observed. Attaching photochemical reactivities to
measurements of chemical species allows for a richer, yet more
constrained-to-what-matters, set of metrics for model evaluation. This paper
presents a corrected version of the paper published under the same authors
and title (sans “corrected”) as https://doi.org/10.5194/acp-21-13729-2021.
To evaluate the utility of trajectory analysis in the tropical upper troposphere/lower stratosphere, Lagrangian predictions of ozone mixing ratio are compared to observations from the Airborne ...Tropical TRopopause EXperiment. Model predictions are based on backward trajectories that are initiated along flight tracks. Ozone mixing ratios from analysis data interpolated onto “source locations” (at trajectory termini) provide initial conditions for chemical production models that are integrated forward in time along parcel trajectories. Model sensitivities are derived from ensembles of predictions using two sets of dynamical forcing fields, four sets of source ozone mixing ratios, three trajectory formulations (adiabatic, diabatic, and kinematic), and two chemical production models. Direct comparisons of analysis ozone mixing ratios to observations have large random errors that are reduced by averaging over 75 min (~800 km) long flight tracks. These averaged values have systematic errors that motivate a similarly systematic adjustment to source ozone mixing ratios. Sensitivity experiments reveal a prediction error minimum in parameter space and, thus, a consistent diagnostic picture: The best predictions utilize the source ozone adjustment and a chemical production model derived from Whole Atmosphere Community Climate Model (a chemistry‐climate model) chemistry. There seems to be slight advantages to using ERA‐Interim winds compared to Modern‐Era Retrospective Analysis for Research and Applications and to using kinematic trajectories compared to diabatic; however, both diabatic and kinematic formulations are clearly preferable to adiabatic trajectories. For these predictions, correlations with observations typically decrease as model error is reduced and, thus, fail as a model comparison metric.
Plain Language Summary
To evaluate the utility of trajectory analysis in the tropical upper troposphere/lower stratosphere, predictions of ozone mixing ratio are compared to observations from the Airborne Tropical TRopopause EXperiment. Model predictions are based on backward trajectories that are initiated along flight tracks. Ozone mixing ratios from analysis data interpolated onto “source locations” (at trajectory termini) provide initial conditions for chemical production models that are integrated forward in time along parcel trajectories. Model sensitivities are derived from ensembles of predictions using two sets of dynamical forcing fields, four sets of source ozone mixing ratios, three trajectory formulations, and two chemical production models. Direct comparisons of analysis ozone mixing ratios to observations have large random errors that are reduced by averaging over 75 min (~800 km) long flight tracks. These averaged values have systematic errors that motivate a similarly systematic adjustment to source ozone mixing ratios. Sensitivity experiments reveal a prediction error minimum in parameter space and, thus, a consistent diagnostic picture: The best predictions utilize the source ozone adjustment and chemical production derived from National Center for Atmospheric Researchs Whole Atmosphere Community Climate Model. For these predictions, correlations with observations typically decrease as model error is reduced and, thus, fail as a model comparison metric.
Key Points
Lagrangian predictions of ozone mixing ratios are viable diagnostic tools provided that ensemble averaging and sensitivity testing are employed
Ozone mixing ratios from global models and reanalysis data have removable systematic errors
Sensitivity calculations identify the most realistic region of model parameter space and reveal subtle dynamical relationships
Observations of SF6 are used to quantify the mean time since air was in (“mean age” from) the Northern Hemisphere (NH) midlatitude surface layer. The mean age is a fundamental property of ...tropospheric transport that can be used in theoretical studies and used to evaluate transport in comprehensive models. Comparisons of simulated SF6 and an idealized clock tracer confirm that the time lag between the SF6 mixing ratio at a given location and the NH midlatitude surface provides an accurate estimate of the mean age. The ages calculated from surface SF6 measurements show large meridional gradients in the tropics but weak gradients in the extratropics, with near‐zero ages at the surface north of 30°N and ages around 1.4 years south of 30°S. Aircraft measurements show weak vertical age gradients in the lower and middle troposphere, with only slight increases of age with height in the NH and slight decreases with height in the Southern Hemisphere. There are large seasonal variations in the age at tropical stations (annual amplitudes around 0.5–1.0 year), with younger ages during northern winter, but only weak seasonal variations at higher latitudes. The seasonality and interannual variations in the tropics and Southern Hemisphere are related to changes in locations of tropical convection. There is qualitative agreement, in both spatial and temporal variations, between the simulated ages and observations. The model ages tend to be older than observed, with differences of ~0.2 year in the Northern Hemisphere upper troposphere and throughout the Southern Hemisphere troposphere.
Key Points
SF6 quantifies mean time since air was in NH mid‐latitude surface layer
Surface SF6 age varies from near zero north of 30°N to 1.4 years south of 30°S
SF6 age is useful for evaluating tropospheric transport in models
The NASA Atmospheric Tomography (ATom) mission built a
photochemical climatology of air parcels based on in situ measurements with
the NASA DC-8 aircraft along objectively planned profiling transects ...through
the middle of the Pacific and Atlantic oceans. In this paper we present and
analyze a data set of 10 s (2 km) merged and gap-filled observations of the
key reactive species driving the chemical budgets of O3 and CH4
(O3, CH4, CO, H2O, HCHO, H2O2, CH3OOH,
C2H6, higher alkanes, alkenes, aromatics, NOx, HNO3,
HNO4, peroxyacetyl nitrate, other organic nitrates), consisting of
146 494 distinct air parcels from ATom deployments 1 through 4. Six models
calculated the O3 and CH4 photochemical tendencies from this
modeling data stream for ATom 1. We find that 80 %–90 % of the total
reactivity lies in the top 50 % of the parcels and 25 %–35 % in the top
10 %, supporting previous model-only studies that tropospheric chemistry
is driven by a fraction of all the air. In other words, accurate simulation
of the least reactive 50 % of the troposphere is unimportant for global
budgets. Surprisingly, the probability densities of species and reactivities
averaged on a model scale (100 km) differ only slightly from the 2 km ATom
data, indicating that much of the heterogeneity in tropospheric chemistry
can be captured with current global chemistry models. Comparing the ATom
reactivities over the tropical oceans with climatological statistics from
six global chemistry models, we find excellent agreement with the loss of
O3 and CH4 but sharp disagreement with production of O3. The
models sharply underestimate O3 production below 4 km in both Pacific
and Atlantic basins, and this can be traced to lower NOx levels than
observed. Attaching photochemical reactivities to measurements of chemical
species allows for a richer, yet more constrained-to-what-matters, set of
metrics for model evaluation.
We measured the global distribution of tropospheric N2O
mixing ratios during the NASA airborne Atmospheric Tomography (ATom)
mission. ATom measured concentrations of ∼ 300 gas species and
aerosol ...properties in 647 vertical profiles spanning the Pacific, Atlantic,
Arctic, and much of the Southern Ocean basins, nearly from pole to pole,
over four seasons (2016–2018). We measured N2O concentrations at 1 Hz
using a quantum cascade laser spectrometer (QCLS). We introduced a new spectral
retrieval method to account for the pressure and temperature sensitivity of
the instrument when deployed on aircraft. This retrieval strategy improved
the precision of our ATom QCLS N2O measurements by a factor of three (based
on the standard deviation of calibration measurements). Our measurements show that most
of the variance of N2O mixing ratios in the troposphere is driven by
the influence of N2O-depleted stratospheric air, especially at mid- and
high latitudes. We observe the downward propagation of lower N2O mixing
ratios (compared to surface stations) that tracks the influence of
stratosphere–troposphere exchange through the tropospheric column down to
the surface. The highest N2O mixing ratios occur close to the Equator,
extending through the boundary layer and free troposphere. We observed
influences from a complex and diverse mixture of N2O sources, with
emission source types identified using the rich suite of chemical species
measured on ATom and the geographical origin calculated using an
atmospheric transport model. Although ATom flights were mostly over the
oceans, the most prominent N2O enhancements were associated with
anthropogenic emissions, including from industry (e.g., oil and gas), urban sources, and biomass
burning, especially in the tropical Atlantic outflow from Africa. Enhanced
N2O mixing ratios are mostly associated with pollution-related tracers
arriving from the coastal area of Nigeria. Peaks of N2O are often
associated with indicators of photochemical processing, suggesting possible
unexpected source processes. In most cases, the results show how
difficult it is to separate the mixture of different sources in the atmosphere,
which may contribute to uncertainties in the N2O global budget. The
extensive data set from ATom will help improve the understanding of N2O
emission processes and their representation in global models.
We present a climatology of O3, CO, and H2O for the upper troposphere and lower stratosphere (UTLS), based on a large collection of high‐resolution research aircraft data taken between 1995 and 2008. ...To group aircraft observations with sparse horizontal coverage, the UTLS is divided into three regimes: the tropics, subtropics, and the polar region. These regimes are defined using a set of simple criteria based on tropopause height and multiple tropopause conditions. Tropopause‐referenced tracer profiles and tracer‐tracer correlations show distinct characteristics for each regime, which reflect the underlying transport processes. The UTLS climatology derived here shows many features of earlier climatologies. In addition, mixed air masses in the subtropics, identified by O3‐CO correlations, show two characteristic modes in the tracer‐tracer space that are a result of mixed air masses in layers above and below the tropopause (TP). A thin layer of mixed air (1–2 km around the tropopause) is identified for all regions and seasons, where tracer gradients across the TP are largest. The most pronounced influence of mixing between the tropical transition layer and the subtropics was found in spring and summer in the region above 380 K potential temperature. The vertical extent of mixed air masses between UT and LS reaches up to 5 km above the TP. The tracer correlations and distributions in the UTLS derived here can serve as a reference for model and satellite data evaluation.
PDF
