Ozonesondes reaching the tropical tropopause layer (TTL) over
the west Pacific have occasionally measured layers of very low ozone
concentrations – less than 15 ppbv – raising the question of how ...prevalent
such layers are and how they are formed. In this paper, we examine aircraft
measurements from the Airborne Tropical
Tropopause Experiment (ATTREX), the Coordinated Airborne Studies in the Tropics (CAST) and the Convective Transport
of Active Species in the Tropics (CONTRAST) experiment campaigns based in Guam in
January–March 2014 for evidence of very low ozone concentrations and their
relation to deep convection. The study builds on results from the ozonesonde
campaign conducted from Manus Island, Papua New Guinea, as part of CAST,
where ozone concentrations as low as 12 ppbv were observed between 100 and
150 hPa downwind of a deep convective complex. TTL measurements from the Global Hawk unmanned aircraft show a marked
contrast between the hemispheres, with mean ozone concentrations in profiles
in the Southern Hemisphere between 100 and 150 hPa of between 10.7 and
15.2 ppbv. By contrast, the mean ozone concentrations in profiles in the
Northern Hemisphere were always above 15.4 ppbv and normally above 20 ppbv
at these altitudes. The CAST and CONTRAST aircraft sampled the atmosphere
between the surface and 120 hPa, finding very low ozone concentrations only
between the surface and 700 hPa; mixing ratios as low as 7 ppbv were
regularly measured in the boundary layer, whereas in the free troposphere
above 200 hPa concentrations were generally well in excess of 15 ppbv.
These results are consistent with uplift of almost-unmixed boundary-layer air
to the TTL in deep convection. An interhemispheric difference was found in
the TTL ozone concentrations, with values < 15 ppbv measured extensively
in the Southern Hemisphere but seldom in the Northern Hemisphere. This is
consistent with a similar contrast in the low-level ozone between the two
hemispheres found by previous measurement campaigns. Further evidence of a
boundary-layer origin for the uplifted air is provided by the
anticorrelation between ozone and halogenated hydrocarbons of marine origin
observed by the three aircraft.
We use the GEOS-Chem global 3-D atmospheric chemistry transport model to interpret atmospheric observations of bromoform (CHBr3) and dibromomethane (CH2Br2) collected during the CAST and CONTRAST ...aircraft measurement campaigns over the western Pacific, January–February 2014. We use a new linearized, tagged version of CHBr3 andCH2Br2, allowing us to study the influence of emissions from specific geographical regions on observed atmospheric variations. The model describes 32 %–37 % of CHBr3 and 15 %–45 % ofCH2Br2 observed variability during CAST and CONTRAST, reflecting model errors in vertical transport. The model has a mean positive bias of 30 % that is larger near the surface, reflecting errors in the poorly constrained prior emission estimates. We find using the model that observed variability of CHBr3 and CH2Br2 is driven by open ocean emissions where there is deep convection. Atmospheric variability above6 km includes a significant contribution from coastal oceans, but it is still dominated by emissions from the open ocean and by older air masses that originate upwind. In the absence of reliable ocean emission estimates, we use a new physical age-of-air simulation to determine the relative abundance of halogens delivered by CHBr3 and CH2Br2 to the tropical transition layer (TTL). We find that 76 % (92 %) of air masses that originate from the ocean reach the TTL within two (three) atmospherice-folding lifetimes of CHBr3 and almost all of them reach the TTL within one e-folding lifetime of CH2Br2. Over the duration of CAST and CONTRAST, and over our study region, oceans delivered a mean (range)CHBr3 and CH2Br2 mole fraction of 0.46 (0.13–0.72) and 0.88 (0.71–1.01) pptv, respectively, to the TTL, and a mean (range) Bry mole fraction of 3.14 (1.81–4.18) pptv from source gases to the upper troposphere.
Accurate determination of the tropopause is important for applications such as dynamical analysis and forecasting, radiative transfer calculations, and the diagnosis of chemical transport in the ...atmosphere. In this paper, we examine how well the extratropical tropopause is determined in the National Centers for Environmental Prediction high‐resolution Global Forecast System (GFS) model analysis over the continental United States using high‐resolution aircraft and radiosonde data. The GFS analyses and sounding data compare well, with RMS differences of approximately 600 m, which is comparable to the vertical resolution of the model. The GFS tropopause is a good proxy in areas without in situ observations, but near the subtropical jet the GFS analysis often mistakenly identifies the secondary rather than the primary tropopause. We also explore an alternative method to identify the tropopause by fitting a smoothed step function to the static stability profile. This new approach provides a measure of the depth of the troposphere‐stratosphere transition and facilitates the study of the dynamical behavior of the tropopause region. In particular, using the transition depth, we are able to identify the statistical behavior of temperature in profiles with deep or shallow tropopause transition layers.
The Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC super(4)RS) field mission based at Ellington Field, Texas, during August and September 2013 ...employed the most comprehensive airborne payload to date to investigate atmospheric composition over North America. The NASA ER-2, DC-8, and SPEC Inc. Learjet flew 57 science flights from the surface to 20km. The ER-2 employed seven remote sensing instruments as a satellite surrogate and eight in situ instruments. The DC-8 employed 23 in situ and five remote sensing instruments for radiation, chemistry, and microphysics. The Learjet used 11 instruments to explore cloud microphysics. SEAC super(4)RS launched numerous balloons, augmented AErosol RObotic NETwork, and collaborated with many existing ground measurement sites. Flights investigating convection included close coordination of all three aircraft. Coordinated DC-8 and ER-2 flights investigated the optical properties of aerosols, the influence of aerosols on clouds, and the performance of new instruments for satellite measurements of clouds and aerosols. ER-2 sorties sampled stratospheric injections of water vapor and other chemicals by local and distant convection. DC-8 flights studied seasonally evolving chemistry in the Southeastern U.S., atmospheric chemistry with lower emissions of NO sub(x) and SO sub(2) than in previous decades, isoprene chemistry under high and low NO sub(x) conditions at different locations, organic aerosols, air pollution near Houston and in petroleum fields, smoke from wildfires in western forests and from agricultural fires in the Mississippi Valley, and the ways in which the chemistry in the boundary layer and the upper troposphere were influenced by vertical transport in convective clouds. Key Points * The SEAC4RS field mission was based near Houston, Texas during August and September of 2013 * The paper overviews the mission to aid those interested in this data set to understand its context * The data can be accessed at http://www-air.larc.nasa.gov/cgi-bin/ArcView/seac4rs
The Asian summer monsoon（ASM） anticyclone isolates upper-tropospheric air within the interior of the anticyclone from the outside. Forward trajectory simulations in previous studies have shown that ...much of the air within the ASM anticyclone can be trapped for up to two or three weeks, not only laterally but also vertically. Here, we investigate the locations of exit points for upper-tropospheric air trapped within the ASM anticyclone, especially the preferred tropopause-crossing locations, using a 3-dimensional trajectory model. Forward trajectory calculations show two-thirds of the air crosses the tropopause at the southern part of the anticyclone via upward diabatic transport. Furthermore, some air crosses at northern and eastern parts via isentropic shedding, but air crosses rarely through the center of the anticyclone. However, calculations also show that many stratospheric parcels within the anticyclone are traceable from the upper-tropospheric anticyclone. This implies they cannot break through the tropopause directly overhead but instead enter the stratosphere via other entry points.
Ozonesondes launched from Beijing, China, over a 3 year time period (September 2002 to July 2005) are used to evaluate the performance of ozone profile retrievals in the upper troposphere and lower ...stratosphere (UTLS) from two new spaceborne instruments, the Atmospheric Infrared Sounder (AIRS) on the NASA Aqua satellite and the Microwave Limb Sounder (MLS) on the NASA Aura satellite. Since the Global Positioning System ozone sensors (GPSO3) used in Beijing ozonesondes are new, comparisons with simultaneously launched Vaisala ECC sensors, and comparisons with an ozonesonde climatology from Sapporo, Japan, are presented. The results show that although the new GPSO3 sensor has a positive bias (about 20–30%) below 200 hPa and a negative bias (about 5–10%) above 60 hPa relative to known sensors, the measured ozone variability is consistent with Vaisala ECC ozonesondes, particularly in the UTLS region. The GPSO3 ozonesonde profiles over Beijing are then used to evaluate coincident ozone profiles from AIRS version 4 retrieval and MLS version 1.5 retrieval. Qualitatively, both satellite data sets can reproduce the gradients and variability of ozone in the UTLS region. Quantitatively, the agreement between the AIRS and ozonesonde ozone profiles is largely within 10% in the UTLS region (from 400 to 70 hPa). The statistical difference between the retrieval and ozonesonde data is minimum in the vicinity of the tropopause. The MLS ozone profiles also show good quality in the UTLS region with the best performance between 147 and 46 hPa.
1. Overview
The upper troposphere-lower stratosphere （UTLS） of the Asian summer monsoon （ASM） region is characterized by a continental-scale anticyclonic circulation, which is dynami- cally active ...and coupled to monsoonal convection. The monsoon anticyclone exhibits anomalous chemical and aerosol characteristics, linked to the outflow of deep convection and the large-scale circulation, and strongly influences the global UTLS composition during boreal summer.
Convective transport from the marine boundary layer to the upper troposphere (UT) is investigated using airborne in situ measurements of chemical species over the tropical western Pacific. Using 42 ...volatile organic compounds with photochemical lifetimes ranging from shorter than a day to multiple decades, we derive a transit time spectrum G(t) and the associated modal and mean transit times for the UT air mass over the convectively dominant tropical western Pacific region. G(t) describes relative contributions of air masses transported from the marine boundary layer to the UT via all transport paths with different transit times. We further demonstrate that the volatile organic compound‐derived transit time scale is broadly comparable to that estimated from convective mass flux. The observation‐based transit time spectrum not only provides insights into convective transport pathways, but also has the potential to serve as an effective diagnostic for evaluating the representation of convective transport in global models.
Plain Language Summary
Tropical deep convection is an important mechanism whereby air mass and chemical species are transported from near the surface to the upper troposphere and lower stratosphere. This transport process is difficult to quantify by observations or represent in models because of the small spatial scales and short temporal scales involved. In this study, we present a method to characterize convective transport using the framework of transit time spectrum, which describes the relative contributions of different transport paths with different transit times. We demonstrate that convective transport transit time spectrum can be derived using airborne in situ measurements of chemical species with a wide range of lifetimes, and we illustrate the wealth of information they provide for quantifying transport processes. This analysis method has the potential to serve as a unique diagnostic for evaluating the representation of convective transport in global models.
Key Points
Airborne in situ measurements of VOCs with a wide range of lifetimes are used to quantify a convective‐transport transit time spectrum
The transit time scale derived from VOC measurements is broadly comparable to that estimated from convective mass flux
The estimated transit time spectrum has the potential to serve as an effective diagnostic for evaluating convective transport in CCMs
We report measurements of bromine monoxide (BrO) and use an observationally constrained chemical box model to infer total gas-phase inorganic bromine (Bry) over the tropical western Pacific Ocean ...(tWPO) during the CONTRAST field campaign (January–February 2014). The observed BrO and inferred Bry profiles peak in the marine boundary layer (MBL), suggesting the need for a bromine source from sea-salt aerosol (SSA), in addition to organic bromine (CBry). Both profiles are found to be C-shaped with local maxima in the upper free troposphere (FT). The median tropospheric BrO vertical column density (VCD) was measured as 1.6×1013 molec cm−2, compared to model predictions of 0.9×1013 molec cm−2 in GEOS-Chem (CBry but no SSA source), 0.4×1013 molec cm−2 in CAM-Chem (CBry and SSA), and 2.1×1013 molec cm−2 in GEOS-Chem (CBry and SSA). Neither global model fully captures the C-shape of the Bry profile. A local Bry maximum of 3.6 ppt (2.9–4.4 ppt; 95 % confidence interval, CI) is inferred between 9.5 and 13.5 km in air masses influenced by recent convective outflow. Unlike BrO, which increases from the convective tropical tropopause layer (TTL) to the aged TTL, gas-phase Bry decreases from the convective TTL to the aged TTL. Analysis of gas-phase Bry against multiple tracers (CFC-11, H2O ∕ O3 ratio, and potential temperature) reveals a Bry minimum of 2.7 ppt (2.3–3.1 ppt; 95 % CI) in the aged TTL, which agrees closely with a stratospheric injection of 2.6 ± 0.6 ppt of inorganic Bry (estimated from CFC-11 correlations), and is remarkably insensitive to assumptions about heterogeneous chemistry. Bry increases to 6.3 ppt (5.6–7.0 ppt; 95 % CI) in the stratospheric "middleworld" and 6.9 ppt (6.5–7.3 ppt; 95 % CI) in the stratospheric "overworld". The local Bry minimum in the aged TTL is qualitatively (but not quantitatively) captured by CAM-Chem, and suggests a more complex partitioning of gas-phase and aerosol Bry species than previously recognized. Our data provide corroborating evidence that inorganic bromine sources (e.g., SSA-derived gas-phase Bry) are needed to explain the gas-phase Bry budget in the upper free troposphere and TTL. They are also consistent with observations of significant bromide in Upper Troposphere–Lower Stratosphere aerosols. The total Bry budget in the TTL is currently not closed, because of the lack of concurrent quantitative measurements of gas-phase Bry species (i.e., BrO, HOBr, HBr, etc.) and aerosol bromide. Such simultaneous measurements are needed to (1) quantify SSA-derived Bry in the upper FT, (2) test Bry partitioning, and possibly explain the gas-phase Bry minimum in the aged TTL, (3) constrain heterogeneous reaction rates of bromine, and (4) account for all of the sources of Bry to the lower stratosphere.
We infer surface fluxes of bromoform (CHBr3) and
dibromoform (CH2Br2) from aircraft observations over the western
Pacific using a tagged version of the GEOS-Chem global 3-D atmospheric
chemistry ...model and a maximum a posteriori inverse model. Using GEOS-Chem (GC) as
an intermediary, we find that the distribution of a priori ocean
emissions of these gases are reasonably consistent with observed atmospheric
mole fractions of CHBr3 (r=0.62) and CH2Br2
(r=0.38). These a priori emissions result in a positive model bias
in CHBr3 peaking in the marine boundary layer, but reproduce
observed values of CH2Br2 with no significant bias by virtue of
its longer atmospheric lifetime. Using GEOS-Chem, we find that observed
variations in atmospheric CHBr3 are determined equally by sources
over the western Pacific and those outside the study region, but observed
variations in CH2Br2 are determined mainly by sources outside
the western Pacific. Numerical closed-loop experiments show that the spatial
and temporal distribution of boundary layer aircraft data have the potential
to substantially improve current knowledge of these fluxes, with improvements
related to data density. Using the aircraft data, we estimate aggregated
regional fluxes of 3.6±0.3×108 and 0.7±0.1×108 g month−1 for CHBr3 and CH2Br2 over
130–155∘E and 0–12∘ N, respectively, which represent
reductions of 20 %–40 % of the prior inventories by Ordóñez
et al. (2012) and substantial spatial deviations from different a
priori inventories. We find no evidence to support a robust linear
relationship between CHBr3 and CH2Br2 oceanic
emissions, as used by previous studies. We find that over regions with dense
observation coverage, our choice of a priori inventory does not
significantly impact our reported a posteriori flux estimates.
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