scholarly journals Hydrogen cyanide in the upper troposphere: GEM-AQ simulation and comparison with ACE-FTS observations

2009 ◽  
Vol 9 (1) ◽  
pp. 2165-2194 ◽  
Author(s):  
A. Lupu ◽  
J. W. Kaminski ◽  
L. Neary ◽  
J. C. McConnell ◽  
K. Toyota ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Atmospheric Chemistry ◽  
Hydrogen Cyanide ◽  
Weather Forecasting ◽  
Boreal Summer ◽  
Air Quality Model ◽  
Quality Model ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
The Tropics

Abstract. We investigate the spatial and temporal distribution of hydrogen cyanide (HCN) in the upper troposphere through numerical simulations and comparison with observations from a space-based instrument. To perform the simulations, we used the Global Environmental Multiscale Air Quality model (GEM-AQ), which is based on the three-dimensional global multiscale model developed by the Meteorological Service of Canada for operational weather forecasting. The model was run for the period 2004–2006 on a 1.5°×1.5° global grid with 28 hybrid vertical levels from the surface up to 10 hPa. Objective analysis data from the Canadian Meteorological Centre were used to update the meteorological fields every 24 h. Fire emission fluxes of gas species were generated by using year-specific inventories of carbon emissions with 8-day temporal resolution from the Global Fire Emission Database (GFED) version 2. The model output is compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument onboard the Canadian SCISAT-1 satellite. High values of up to a few ppbv are observed in the tropics in the Southern Hemisphere; the enhancement in HCN volume mixing ratios in the upper troposphere is most prominent in October. Low upper-tropospheric mixing ratios of less than 100 pptv are mostly recorded at middle and high latitudes in the Southern Hemisphere in May–July. Mixing ratios in Northern Hemisphere peak in the boreal summer. The amplitude of the seasonal variation is less pronounced than in the Southern Hemisphere. Our model results show that in the upper troposphere GEM-AQ performs well globally for all seasons, except at Northern high and middle latitudes in summer, where the model has a large negative bias, and in the tropics in winter and spring, where it exhibits large positive bias. This may reflect inaccurate emissions or possible inaccuracies in the emission profile. The model is able to explain most of the observed variability in the upper troposphere HCN field, including the interannual variations in the observed mixing ratio. The estimated average global emission equals 1.3 Tg N yr−1. The average atmospheric burden is 0.53 Tg N, and the corresponding lifetime is 4.9 months.

scholarly journals Hydrogen cyanide in the upper troposphere: GEM-AQ simulation and comparison with ACE-FTS observations

2009 ◽  
Vol 9 (13) ◽  
pp. 4301-4313 ◽  
Author(s):  
A. Lupu ◽  
J. W. Kaminski ◽  
L. Neary ◽  
J. C. McConnell ◽  
K. Toyota ◽  
...  
Keyword(s):  
Seasonal Variation ◽  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Atmospheric Chemistry ◽  
Hydrogen Cyanide ◽  
Boreal Summer ◽  
Air Quality Model ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
The Tropics

Abstract. We investigate the spatial and temporal distribution of hydrogen cyanide (HCN) in the upper troposphere through numerical simulations and comparison with observations from a space-based instrument. To perform the simulations, we used the Global Environmental Multiscale Air Quality model (GEM-AQ), which is based on the three-dimensional global multiscale model developed by the Meteorological Service of Canada for operational weather forecasting. The model was run for the period 2004–2006 on a 1.5°×1.5° global grid with 28 hybrid vertical levels from the surface up to 10 hPa. Objective analysis data from the Canadian Meteorological Centre were used to update the meteorological fields every 24 h. Fire emission fluxes of gas species were generated by using year-specific inventories of carbon emissions with 8-day temporal resolution from the Global Fire Emission Database (GFED) version 2. The model output is compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument onboard the Canadian SCISAT-1 satellite. High values of up to a few ppbv are observed in the tropics in the Southern Hemisphere; the enhancement in HCN volume mixing ratios in the upper troposphere is most prominent in October. Low upper-tropospheric mixing ratios of less than 100 pptv are mostly recorded at middle and high latitudes in the Southern Hemisphere in May–July. Mixing ratios in Northern Hemisphere peak in the boreal summer. The amplitude of the seasonal variation is less pronounced than in the Southern Hemisphere. The comparison with the satellite data shows that in the upper troposphere GEM-AQ performs well globally for all seasons, except at northern high and middle latitudes in summer, where the model has a large negative bias, and in the tropics in winter and spring, where it exhibits large positive bias. This may reflect inaccurate emissions or possible inaccuracies in the emission profile. The model is able to explain most of the observed variability in the upper troposphere HCN field, including the interannual variations in the observed mixing ratio. A complementary comparison with daily total columns of HCN from two middle latitude ground-based stations in Northern Japan for the same simulation period shows that the model captures the observed seasonal variation and also points to an underestimation of model emissions in the Northern Hemisphere in the summer. The estimated average global emission equals 1.3 Tg N yr−1. The average atmospheric burden is 0.53 Tg N, and the corresponding lifetime is 4.9 months.

scholarly journals Carbon and hydrogen isotopic ratios of atmospheric methane in the upper troposphere over the Western Pacific

2012 ◽  
Vol 12 (4) ◽  
pp. 9035-9077 ◽  
Author(s):  
T. Umezawa ◽  
T. Machida ◽  
K. Ishijima ◽  
H. Matsueda ◽  
Y. Sawa ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Isotopic Ratio ◽  
Lower Troposphere ◽  
Western Pacific ◽  
Boreal Summer ◽  
Additional Contribution ◽  
Mixing Ratio ◽  
Upper Troposphere ◽  
The Tropics ◽  
The Western Pacific

Abstract. We present the mixing ratio, δ13C and δD of atmospheric CH4 using commercial aircraft in the upper troposphere (UT) over the Western Pacific for the period December 2005–September 2010. The observed results were compared with those obtained using commercial container ships in the lower troposphere (LT) over the same region. In the Northern Hemisphere (NH), the UT CH4 mixing ratio shows high values in the boreal summer–autumn, when the LT CH4 mixing ratio reaches a seasonal minimum. From tagged tracer experiments made using an atmospheric chemistry transport model, we found that such high CH4 values are due to rapid transport of air masses influenced by CH4 sources in South Asia and East Asia. The observed isotopic ratio data suggest that CH4 sources in these areas have relatively low δ13C and δD signatures, implying biogenic sources. Latitudinal distributions of the annual average UT and LT CH4 mixing ratio intersect each other in the tropics; the mixing ratio value is lower in the UT than in the LT in the NH and the situation is reversed in the Southern Hemisphere (SH), due mainly to the NH air intrusion into the SH through the UT. Such intersection of the latitudinal distributions is observable in δD but not in δ13C, implying additional contribution of a reaction of CH4 with active chlorine in the marine boundary layer. δ13C and δD show low values in the NH and high values in the SH both in the UT and in the LT. We also observed an increase in the CH4 mixing ratio and decreases in δ13C and δD during 2007–2008 in the UT and LT over the Western Pacific, possibly due to enhanced biogenic emissions in the tropics and NH.

scholarly journals Impact of the new HNO<sub>3</sub>-forming channel of the HO<sub>2</sub>+NO reaction on tropospheric HNO<sub>3</sub>, NO<sub>x</sub>, HO<sub>x</sub> and ozone

2008 ◽  
Vol 8 (14) ◽  
pp. 4061-4068 ◽  
Author(s):  
D. Cariolle ◽  
M. J. Evans ◽  
M. P. Chipperfield ◽  
N. Butkovskaya ◽  
A. Kukui ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Atmospheric Chemistry ◽  
Tropospheric Ozone ◽  
Transport Model ◽  
Chemical Transport Model ◽  
Temperature Dependent ◽  
Upper Troposphere ◽  
The Mean ◽  
The Tropics ◽  
The Impact

Abstract. We have studied the impact of the recently observed reaction NO+HO2→HNO3 on atmospheric chemistry. A pressure and temperature-dependent parameterisation of this minor channel of the NO+HO2→NO2+OH reaction has been included in both a 2-D stratosphere-troposphere model and a 3-D tropospheric chemical transport model (CTM). Significant effects on the nitrogen species and hydroxyl radical concentrations are found throughout the troposphere, with the largest percentage changes occurring in the tropical upper troposphere (UT). Including the reaction leads to a reduction in NOx everywhere in the troposphere, with the largest decrease of 25% in the tropical and Southern Hemisphere UT. The tropical UT also has a corresponding large increase in HNO3 of 25%. OH decreases throughout the troposphere with the largest reduction of over 20% in the tropical UT. The mean global decrease in OH is around 13%, which is very large compared to the impact that typical photochemical revisions have on this modelled quantity. This OH decrease leads to an increase in CH4 lifetime of 5%. Due to the impact of decreased NOx on the OH:HO2 partitioning, modelled HO2 actually increases in the tropical UT on including the new reaction. The impact on tropospheric ozone is a decrease in the range 5 to 12%, with the largest impact in the tropics and Southern Hemisphere. Comparison with observations shows that in the region of largest changes, i.e. the tropical UT, the inclusion of the new reaction tends to degrade the model agreement. Elsewhere the model comparisons are not able to critically assess the impact of including this reaction. Only small changes are calculated in the minor species distributions in the stratosphere.

scholarly journals Impact of the new HNO<sub>3</sub>-forming channel of the HO<sub>2</sub>+NO reaction on tropospheric HNO<sub>3</sub>, NO<sub>x</sub>, HO<sub>x</sub> and ozone

2008 ◽  
Vol 8 (1) ◽  
pp. 2695-2713 ◽  
Author(s):  
D. Cariolle ◽  
M. J. Evans ◽  
M. P. Chipperfield ◽  
N. Butkovskaya ◽  
A. Kukui ◽  
...  
Keyword(s):  
Southern Hemisphere ◽  
Atmospheric Chemistry ◽  
Tropospheric Ozone ◽  
Transport Model ◽  
Chemical Transport Model ◽  
Temperature Dependent ◽  
Upper Troposphere ◽  
The Tropics ◽  
The Impact ◽  
Model Agreement

Abstract. We have studied the impact of the recently established reaction NO+HO2→HNO3 on atmospheric chemistry. A pressure and temperature-dependent parameterisation of this minor channel of the NO+HO2→NO2+OH reaction has been included in both a 2-D stratosphere-troposphere model and a 3-D tropospheric chemical transport model (CTM). Significant effects on the nitrogen species and hydroxyl radical concentrations are found throughout the troposphere, with the largest percentage changes occurring in the tropical upper troposphere (UT). Including the reaction leads to a reduction in NOx everywhere in the troposphere, with the largest decrease of 25% in the tropical and southern hemisphere UT. The tropical UT also has a corresponding large increase in HNO3 of 25%. OH decreases throughout the troposphere with the largest reduction of over 20% in the tropical UT. Mean global decreases in OH are around 13% which leads to a increase in CH4 lifetime of 5%. Due to the impact of decreased NOx on the OH:HO2 partitioning, modelled HO2 actually increases in the tropical UT on including the new reaction. The impact on tropospheric ozone is a decrease in the range 5 to 12%, with the largest impact in the tropics and southern hemisphere. Comparison with observations shows that in the region of largest changes, i.e. the tropical UT, the inclusion of the new reaction tends to degrade the model agreement. Elsewhere the model comparisons are not able to critically assess the impact of including this reaction. Only small changes are calculated in the minor species distributions in the stratosphere.

scholarly journals Global stratospheric chlorine inventories for 2004–2009 from Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) measurements

2013 ◽  
Vol 13 (9) ◽  
pp. 23491-23548 ◽  
Author(s):  
A. T. Brown ◽  
M. P. Chipperfield ◽  
S. Dhomse ◽  
C. Boone ◽  
P. F. Bernath
Keyword(s):  
Southern Hemisphere ◽  
Northern Hemisphere ◽  
Atmospheric Chemistry ◽  
Transport Model ◽  
Montreal Protocol ◽  
Chemical Transport Model ◽  
Mixing Ratios ◽  
Ozone Depleting Substances ◽  
The Tropics ◽  
Chemistry Experiment

Abstract. We present chlorine budgets calculated between 2004 and 2009 for four latitude bands (70° N–30° N, 30° N–0° N, 0° N–30° S, and 30° S–70° S). The budgets were calculated using ACE-FTS version 3.0 retrievals of the volume mixing ratios (VMRs) of 9 chlorine-containing species: CCl4, CFC-12 (CCl2F2), CFC-11 (CCl3F), COCl2, COClF, HCFC-22 (CHF2Cl), CH3Cl, HCl and ClONO2. These data were supplemented with calculated VMRs from the SLIMCAT 3-D chemical transport model (CFC-113, CFC-114, CFC-115, H-1211, H-1301, HCFC-141b, HCFC-142b, ClO and HOCl). The total chlorine profiles are dominated by chlorofluorocarbons (CFCs) and halons up to 24 km in the tropics and 19 km in the extra-tropics. In this altitude range CFCs and halons account for 58% of the total chlorine VMR. Above this altitude HCl increasingly dominates the total chlorine profile, reaching a maximum of 95% of total chlorine at 54 km. All total chlorine profiles exhibit a positive slope with altitude, suggesting that the total chlorine VMR is now decreasing with time. This conclusion is supported by the time series of the mean stratospheric total chlorine budgets which show mean decreases in total stratospheric chlorine of 0.38 ± 0.03% per year in the Northern Hemisphere extra-tropics, 0.35 ± 0.07% per year in the Northern Hemisphere tropical stratosphere, 0.54 ± 0.16% per year in the Southern Hemisphere tropics and 0.53 ± 0.12% per year in the Southern Hemisphere extra-tropical stratosphere for 2004–2009. Globally stratospheric chlorine is decreasing by 0.46 ± 0.02% per year. Both global warming potential-weighted chlorine and ozone depletion potential-weighted chlorine are decreasing at all latitudes. These results show that the Montreal Protocol has had a significant effect in reducing emissions of both ozone-depleting substances and greenhouse gases.

scholarly journals Impact of a new condensed toluene mechanism on air quality model predictions in the US

2010 ◽  
Vol 3 (4) ◽  
pp. 2291-2314
Author(s):  
G. Sarwar ◽  
K. W. Appel ◽  
A. G. Carlton ◽  
R. Mathur ◽  
K. Schere ◽  
...  
Keyword(s):  
Air Quality ◽  
Los Angeles ◽  
Production Efficiency ◽  
Sensitivity Study ◽  
Chemical Mechanism ◽  
Ozone Production ◽  
Daily Maximum ◽  
Air Quality Model ◽  
Quality Model ◽  
Mixing Ratios

Abstract. A new condensed toluene mechanism is incorporated into the Community Multiscale Air Quality Modeling system. Model simulations are performed using the CB05 chemical mechanism containing the existing (base) and the new toluene mechanism for the western and eastern US for a summer month. With current estimates of tropospheric emission burden, the new toluene mechanism increases monthly mean daily maximum 8-h ozone by 1.0–3.0 ppbv in Los Angeles, Portland, Seattle, Chicago, Cleveland, northeastern US, and Detroit compared to that with the base toluene chemistry. It reduces model mean bias for ozone at elevated observed ozone mixing ratios. While the new mechanism increases predicted ozone, it does not enhance ozone production efficiency. Sensitivity study suggests that it can further enhance ozone if elevated toluene emissions are present. While changes in total fine particulate mass are small, predictions of in-cloud SOA increase substantially.

Investigation of atmospheric hydrogen cyanide: a modelling perspective

2021 ◽  
Author(s):  
Antonio G. Bruno ◽  
Jeremy J. Harrison ◽  
David P. Moore ◽  
Martyn P. Chipperfield ◽  
Richard J. Pope
Keyword(s):  
Biomass Burning ◽  
Atmospheric Chemistry ◽  
Hydrogen Cyanide ◽  
Three Dimensional ◽  
Oxidation Reactions ◽  
Loss Mechanism ◽  
Mixing Ratios ◽  
The Individual ◽  
Atmospheric Loss ◽  
Physical And Chemical

&lt;p&gt;Hydrogen cyanide (HCN) is one of the most abundant cyanides present in the global atmosphere, and is a tracer of biomass burning, especially for peatland fires. The HCN lifetime is 2&amp;#8211;5 months in the troposphere but several years in the stratosphere. Understanding the physical and chemical mechanisms of HCN variability is important due to its non-negligible role in the nitrogen cycle. The main source of tropospheric HCN is biomass burning with minor contributions from industry and transport. The main loss mechanism of atmospheric HCN is the reaction with the hydroxyl radical (OH). Ocean uptake is also important, while in the stratosphere oxidation by reaction with O(&lt;sup&gt;1&lt;/sup&gt;D) needs to be considered.&lt;/p&gt;&lt;p&gt;HCN variability can be investigated using chemical model simulations, such as three-dimensional (3-D) chemical transport models (CTMs). Here we use an adapted version of the TOMCAT 3-D CTM at a 1.2&amp;#176;x1.2&amp;#176; spatial resolution from the surface to ~60 km for 12 idealised HCN tracers which quantify the main loss mechanisms of HCN, including ocean uptake, atmospheric oxidation reactions and their combinations. The TOMCAT output of the HCN distribution in the period 2004-2020 has been compared with HCN profiles measured by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) over an altitude grid from 6 to 42 km. HCN model data have also been compared with ground-based measurements of HCN columns from NDACC FTIR stations and with in-situ volume mixing ratios (VMRs) from NOAA ground-based measurement sites.&lt;/p&gt;&lt;p&gt;The model outputs for the HCN tracer with full treatment of the loss processes generally agree well with ACE-FTS measurements, as long as we use recent laboratory values for the atmospheric loss reactions. Diagnosis of the individual loss terms shows that decay of the HCN profile in the upper stratosphere is due mainly to the O(&lt;sup&gt;1&lt;/sup&gt;D) sink. In order to test the magnitude of the tropospheric OH sink and the magnitude of the ocean sink, we also show the comparisons of the model tracers with surface-based observations. The implications of our results for understanding HCN and its variability are then discussed.&lt;/p&gt;

scholarly journals Seasonal ozone vertical profiles over North America using the AQMEII group of air quality models: model inter-comparison and stratospheric intrusions

2018 ◽  
Author(s):  
Marina Astitha ◽  
Ioannis Kioutsoukis ◽  
Ghezae Araya Fisseha ◽  
Roberto Bianconi ◽  
Johannes Bieser ◽  
...  
Keyword(s):  
United States ◽  
Air Quality ◽  
North America ◽  
Vertical Profiles ◽  
Lateral Boundary ◽  
Air Quality Model ◽  
Quality Model ◽  
Mixing Ratios ◽  
Lateral Boundary Conditions ◽  
Stratospheric Intrusions

Abstract. This study evaluates simulated vertical ozone profiles produced in the framework of the third phase of the Air Quality Model Evaluation International Initiative (AQMEII3) against ozonesonde observations in North America for the year 2010. Four research groups from the United States (U.S.) and Europe have provided ozone vertical profiles to conduct this analysis. Because some of the modeling systems differ in their meteorological drivers, wind speed and temperature are also included in the analysis. In addition to the seasonal ozone profile evaluation for 2010, we also analyze chemically inert tracers designed to track the influence of lateral boundary conditions on simulated ozone profiles within the modeling domain. Finally, cases of stratospheric ozone intrusions during May–June 2010 are investigated by analyzing ozonesonde measurements and the corresponding model simulations at Intercontinental Chemical Transport Experiment Ozonesonde Network Study (IONS) experiment sites in the western United States. The evaluation of the seasonal ozone profiles reveals that at a majority of the stations, ozone mixing ratios are under-estimated in the 1–6 km range. The seasonal change noted in the errors follows the one seen in the variance of ozone mixing ratios, with the majority of the models exhibiting less variability than the observations. The analysis of chemically inert tracers highlights the importance of lateral boundary conditions up to 250 hPa for the lower tropospheric ozone mixing ratios (0–2 km). Finally, for the stratospheric intrusions, the models are generally able to reproduce the location and timing of most intrusions but underestimate the magnitude of the maximum mixing ratios in the 2–6 km range and overestimate ozone up to the first km possibly due to marine air influences that are not accurately described by the models. The choice of meteorological driver appears to be a greater predictor of model skill in this altitude range than the choice of air quality model.

scholarly journals Global stratospheric fluorine inventory for 2004–2009 from Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) measurements and SLIMCAT model simulations

2014 ◽  
Vol 14 (1) ◽  
pp. 267-282 ◽  
Author(s):  
A. T. Brown ◽  
M. P. Chipperfield ◽  
N. A. D. Richards ◽  
C. Boone ◽  
P. F. Bernath
Keyword(s):  
Atmospheric Chemistry ◽  
Radiative Forcing ◽  
Transport Model ◽  
Similar Rate ◽  
Montreal Protocol ◽  
Negative Slope ◽  
Mixing Ratios ◽  
Inorganic Species ◽  
The Tropics ◽  
The Impact

Abstract. Fluorine-containing species can be extremely effective atmospheric greenhouse gases. We present fluorine budgets using organic and inorganic species retrieved by the ACE-FTS satellite instrument supplemented with output from the SLIMCAT 3-D chemical transport model. The budgets are calculated between 2004 and 2009 for a number of latitude bands: 70–30° N, 30–00° N, 00° N–30° S, and 30–70° S. At lower altitudes total fluorine profiles are dominated by the contribution from CFC-12, up to an altitude of 20 km in the extra-tropics and 29 km in the tropics; above these altitudes the profiles are dominated by hydrogen fluoride (HF). Our data show that total fluorine profiles at all locations have a negative slope with altitude, providing evidence that overall fluorine emissions (measured by their F content) have been increasing with time. Total stratospheric fluorine is increasing at a similar rate in the tropics: 32.5 ± 4.9 ppt yr−1 (1.31 ± 0.20% per year) in the Northern Hemisphere (NH) and 29.8 ± 5.3 ppt yr−1 (1.21 ± 0.22% per year) in the Southern Hemisphere (SH). Extra-tropical total stratospheric fluorine is also increasing at a similar rate in both the NH and SH: 28.3 ± 2.7 ppt per year (1.12 ± 0.11% per year) in the NH and 24.3 ± 3.1 ppt per year (0.96 ± 0.12% per year) in the SH. The calculation of radiative efficiency-weighted total fluorine allows the changes in radiative forcing between 2004 and 2009 to be calculated. These results show an increase in radiative forcing of between 0.23 ± 0.11% per year and 0.45 ± 0.11% per year, due to the increase in fluorine-containing species during this time. The decreasing trends in the mixing ratios of halons and chlorofluorocarbons (CFCs), due to their prohibition under the Montreal Protocol, have suppressed an increase in total fluorine caused by increasing mixing ratios of hydrofluorocarbons (HFCs). This has reduced the impact of fluorine-containing species on global warming.

scholarly journals Validation of Aura MLS retrievals of temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere over the Tibetan Plateau during boreal summer

2016 ◽  
Author(s):  
X. L. Yan ◽  
J. S. Wright ◽  
X. D. Zheng ◽  
N. Livesey ◽  
H. Vömel ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Water Vapour ◽  
Boreal Summer ◽  
The Tibetan Plateau ◽  
Mixing Ratio ◽  
Upper Troposphere ◽  
Mixing Ratios ◽  
Dry Bias ◽  
Temperature Water ◽  
Middle Stratosphere

Abstract. We validate Aura Microwave Limb Sounder (MLS) version 3 (v3) and version 4 (v4) retrievals of summertime temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere (UTLS; 10–316 hPa) against balloon soundings collected during the Study of Ozone, Aerosols and Radiation over the Tibetan Plateau (SOAR-TP). Mean v3 and v4 profiles of temperature, water vapour and ozone in this region during the measurement campaigns are almost identical through most of the stratosphere (10–68 hPa), but differ in several respects in the upper troposphere and tropopause layer. Differences in v4 relative to v3 include slightly colder mean temperatures from 100–316 hPa, smaller mean water vapour mixing ratios in the upper troposphere (215–316 hPa), and a more vertically homogeneous profile of mean ozone mixing ratios below the climatological tropopause (100–316 hPa). These changes substantially improve agreement between ozonesondes and MLS ozone retrievals in the upper troposphere, but slightly worsen existing cold and dry biases in the upper troposphere. Aura MLS v3 and v4 temperature profiles contain significant cold biases relative to collocated temperature measurements in several layers of the lower–middle stratosphere (mean biases of −1.3 to −1.8 K centered at 10–12 hPa, 26–32 hPa and 68– 83 hPa) and in the upper troposphere (mean biases of approximately −2.3±0.3 K in v3 and −2.6±0.4 K in v4 between 147 and 261 hPa). MLS v3 and v4 profiles of water vapour volume mixing ratio generally compare well with collocated measurements, with a slight dry bias (v4: −8±4%) near 22–26 hPa, a slight wet bias (v4: +12±5%) near 68–83 hPa, and a more substantial dry bias (v4: −32±11%) in the upper troposphere (121–261 hPa). MLS v3 and v4 retrievals of ozone volume mixing ratio are biased high relative to collocated ozonesondes through most of the stratosphere (18–83 hPa), but are biased low at 100 hPa. The largest positive biases in ozone retrievals are located at 83 hPa (approximately +70%); this peak was not identified by earlier validations and may be regionally or seasonally specific. Ozone retrievals are substantially improved in v4 relative to v3, with smaller biases in the tropopause layer, reduced variance below 68 hPa, larger data yields, and smoother gradients in the vertical profile of ozone biases in the upper troposphere.

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