1. Global Phosgene Observations from the Atmospheric Chemistry Experiment (ACE) Mission
Dejian Fu,
C.D. Boone, P.F. Bernath,
K.A. Walker, R. Nassar,
G.L. Manney and S.D. McLeod
June 19th, 2007
In this presentation, I will describe the results of our observations of atmospheric phosgene observed with the satellite mission, ACE.
Before we go into the details, let’s look at some chemical properties of phosgene.

2. Phosgene Chemistry O John Davy synthesized phosgene in 1812. װCl C O װ
John Davy synthesized phosgene in 1812.
CO + Cl2 + sunlight → COCl2
Phosgene is a highly toxic colorless gas. Phosgene gained infamy as a chemical weapon during World War I and was stockpiled as part of U.S. military arsenals until well after World War II in the form of aerial bombs and mortar rounds.
Phosgene plays a major role in the chemical industry, particularly in the preparation of pharmaceuticals, herbicides, insecticides, synthetic foams, resins, and polymers.
Considering the health hazards associated with phosgene, the chemical industry is trying to find substitutes to eliminate its use.
Phosgene was first generated by Davy, a British chemist, nearly two hundred years ago.
He exposed the gas mixture of carbon monoxide and chlorine to strong sun light, which contains UV radiation, to synthesize COCl2. Unfortunately, phosgene was used in the first world war since it is a toxic colorless gas. It was also prepared and storage by US army till they found other more efficient chemical weapons. Phosgene is quite useful in the chemical industry. It has been used in many chemical reactions. Recently, chemists are searching for substitutes for this gas since it is harmful to human beings.

3. Atmospheric Phosgene Sources
In the troposphere, phosgene is mainly formed by the OH-initiated oxidation of chlorinated hydrocarbons such as C2HCl3, CH3CCl3, CHCl3, and C2Cl4.
In the stratosphere, phosgene is produced from the photochemical decay of CCl4 together with oxidization of its tropospheric sources that consist of C2HCl3, CH3CCl3, CHCl3, and C2Cl4.
Sinks
In the troposphere, phosgene is removed by water droplets in clouds or by deposition onto the ocean and other water surfaces. COCl2 has a life time about 70 days.
Phosgene can be slowly oxidized through ultraviolet photolysis to form ClOx, which plays an important role in stratospheric ozone depletion. COCl2 has a life time of several years since it is a weak absorber in the near UV and does not react with OH.
In the atmosphere, it is formed by the OH-initiated oxidation and photochemical decay of chlorinated carbon compounds.
In the troposphere, that is, from surface to about 12 km, there are four main species that can provide phosgene.
In the stratosphere, that is, from the top of troposphere till about 50 km, there are five that species generate phosgene. However, phosgene is mainly formed from CCl4, which contributes about 68% of total phosgene in the stratosphere.
There are two major sinks for phosgene.
In the troposphere, phosgene quickly dissolves in water droplets and has a life time about 70 days.
In the stratosphere, phosgene changes to ClOx through UV photolysis.
But the reactions are slow. Phosgene has a lifetime of several years.

4. Previous Studies Observations
Singh et al. [1976] obtain surface concentrations of phosgene at six stations in California.
Wilson et al. [1988] measured phosgene during the flight of a Lear Jet aircraft between Germany and Spitzbergen.
Toon et al. [2001] reported twelve volume mixing ratio profiles of phosgene using the solar occultation technique from data recorded during nine MkIV spectrometer balloon flights near 34ºN and 68ºN between 1992 and 2000.
Modeling
Kindler et al. [1995] studied the tropospheric and stratospheric cycles of phosgene using a two-dimensional model.
The first atmospheric phosgene observation was done by Singh thirty years ago. He measured the surface concentration of phosgene.
Wilson et al. measured phosgene in 1980s. Phosgene was measured up to about 15 km high. These two works measured phosgene in situ. Toon measured phosgene concentration profiles using JPL MkIV FT spectrometer in balloon flights. The modeling work on the tropospheric and stratospheric cycles of phosgene was done by Kindler in A two dimensional model was used in his work. To our knowledge, there are no global observations available for this interesting species. There are even no measurements in the southern hemisphere.

5. Atmospheric Chemistry Experiment
Launched date: 13th Aug. 2003
Altitude: 650 km
Period: 97.7 minutes
Inclination: 73.9º
Mass: 250 kg
Power usage: 70 W
Instruments:
1 ACE-FTS, a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 750–4400 cm-1
2 MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation), a spectrophotometer covering from 280–550 nm and 500–1030 nm with the spectral resolution of 1–2 nm
Atmospheric Chemistry Experiment, a Canadian satellite mission, provides the opportunity to determine the global distribution of phosgene using remote sensing techniques. The ACE satellite, also known as SCISAT-1, observes the atmosphere using solar occultation viewing geometry in the low earth circular obit. There are two primary instruments onboard SCISAT-1: a high resolution FTS covering the spectral range of 750 to 4400 wavenumbers and a spectrophotometer called MAESTRO. The satellite was launched in Aug for a nominal two year mission. Now it is the fourth year after launch, and ACE is still working well on orbit and providing concentration profiles of more than 30 species.
Link to the details of the ACE Mission:
Providing vertical concentration profiles of more than 30 atmospheric trace gases which relate to the ozone chemistry, climate change, and air pollution.

6. Observations and Retrievals of COCl2
6758 occultations are available for observations of phosgene during the period February 2004 to May 2006.
The phosgene profiles are from spectra recorded by the ACE-FTS. The altitude range of the retrievals extends from 8 to 30 km.
COCl2 VMR retrievals were performed using the spectral region 831 to 864 cm-1 with spectroscopic line parameters of phosgene taken from Brown et al. [1996] and Toon et al. [2001].
A two-step approach similar to that described by Nassar et al. [2005] was used to classify each of the 6758 occultations collected during the period February 2004 to May 2006 as being inside, outside, or on the edge of the vortex.
Filtering out occultations inside or on the edge of the vortex yielded 5614 extravortex occultations.
In the two and a half years of observations that we used, there are about 68 hundred measurements available for the toxic species, phosgene. The phosgene profiles are from spectra recorded by the ACE-FTS. Those profiles typical contain phosgene concentrations from 8 to 30km. Spectroscopic parameters are from the Brown and Toon’s work. To eliminate the arctic vortex effects in the phosgene distributions, we classified the ACE occulations into three classes, and we only used extravortex occultations.

7. Locations of ACE Occultations
30 km geometric tangent points
5614 ACE-FTS extravortex occultations
Feb May 2006
Red:
Blue:
Black: 2006
This plot presents the locations of about 56 hundred ACE occulations used in this work. It clearly shows that global distributions of phosgene can be obtained from ACE. Within 5 degree latitudinal zones, the observed volume mixing ratios of COCl2 show very similar characteristics in terms of peak altitudes and VMR values at the peaks. The predicted small seasonal cycle and expected annual decrease due to the decline in atmospheric chlorine loading were not considered. Therefore, (click to show next slide)

8. VMR Profiles for 5º Latitudinal Zones
The entire data set was separated into 5 degree latitude bins and all of profiles within a given bin were averaged to generate a single profile with reduced noise. Although, there are 36 bins from the north pole to the south pole, only 35 of the bins were used because there were no profiles in the region 85 to 90 degree south. In this plot, the 35 averaged profiles were shown. We can see the highest concentration of phosgene appears in the tropics and decreases poleward.
The 35 averaged COCl2 volume mixing ratio profiles for 5 degree latitudinal zones spanning from 90ºN to 85ºS during the period February 2004 to May 2006 are presented. There were no profiles in the region 85 to 90ºS.

9. Latitudinal Distribution of Averaged COCl2 VMR profiles
In the lower stratosphere, COCl2 exhibits a layer of higher concentraion (25 to 60 pptv) with a thickness of 5 to 10 km.
Within this layer, COCl2 concentrations are highest near the equator and decline poleward.
There is a core of strongly enhanced COCl2 (VMRs 40 to 60 pptv) between 22 and 27 km in the region 20ºN to 20ºS
For all latitudes, the retrieved VMR drops rapidly to zero for altitudes above the COCl2 enhancement layer.
Then, these 35 profiles were used to generate the contour plot shown here. In the troposphere, phosgene has a generally even distribution with volume mixing ratio about 15 to 20 pptv. In the lower stratosphere, there is a layer of enhanced concentration of phosgene. The thickness of this layer is about 5 to 10 km. Concentrations are from 25 to 60 pptv. In this layer, there is a core of strongly enhanced phosgene between 22 to 27 km in the tropical region, around 20 degrees south to 20 degrees north. Above the enhanced layer, phosgene concentrations drop to zero rapidly in all latitudes.

10. Possible Reasons behind Latitudinal Distribution Pattern of COCl2
Based on the latitudinal distribution of averaged COCl2 VMR profiles, the bulk of the COCl2 appears to be created over the tropics, likely because the tropics receive more insolation than higher latitudes, due to a smaller solar zenith angle.
There is also a longer sunlight exposure time in the tropics, allowing more time for the necessary chemical reactions to occur.
Tropospheric COCl2 has a lifetime of about 70 days due to fast wet removal. Stratospheric COCl2, on the other hand, is expected to have a lifetime of several years since phosgene decomposes slowly through UV photolysis and has no reaction with OH. COCl2 created in the tropics can then be transported pole ward by the Brewer-Dobson circulation.
Interestingly Kindler et al. [1995] predicts that a substantial amount of phosgene is returned to the troposphere from the stratosphere, where it is destroyed primarily by wet deposition.
The reasons of the phosgene distribution pattern can be explained by the following factors. Stratospheric phosgene is formed mainly through photolysis of CCl4. In the tropical region, there is more insolation, that is, more photons than at higher latitudes are received by the atmosphere. Also there is a longer sunlight exposure time in the tropics. That is, more time for chemical reactions. Unlike tropospheric phosgene, stratospheric phosgene has a long life time that allows it to be transported poleward by the Brewer-Dobson circulation. Hence, the high concentration of phosgene appears in the tropics, decreases polarward and shows a symmetric pattern centered on the equator. In Kindler’s study, stratospheric phosgene can return to the troposphere and be removed by water droplets.

11. Averaged COCl2 VMR Profiles for Latitudinal Zones and Hemispheres
In the stratosphere, the peak values of COCl2 VMR decrease significantly from the tropics to the poles.
In the troposphere, COCl2 VMR increases slightly from the tropics to the poles. This may result from the fact that the troposphere at higher latitudes contains less liquid water than the tropical atmosphere, providing less opportunity for wet removal.
The averaged VMR profiles for northern (0-90N) and southern (0-85S) hemispheres are very similar, suggesting that both hemispheres have similar amounts of COCl2.
We obtained averaged phosgene profiles for 5 latitude zones. They present phosgene at the high latitudes, middle latitudes and tropical regions. Also, averaged profiles for the northern hemisphere and the southern hemisphere were shown in the upper plot.
In the stratosphere, phosgene concentrations decrease towards two poles. As we saw in the previous slide, it is due to the different insolation. In the troposphere, higher COCl2 concentrations are found in the higher latitudes since there are less liquid water than the tropical atmosphere. Also the averaged volume mixing ratio profiles for two hemispheres suggest that similar amounts of phosgene in the southern hemisphere and the northern hemisphere.
We compared our work to the previous studies as shown in the lower plot.
In addition, the results of Wilson’s observations, Nassar’s results which are the average of the measurements form MkIV balloon flights, and Kindler’s model study were also included.

12. Comparisons between ACE and Previous Studies
Above the peak in all of the averaged ACE-FTS COCl2 profiles, around 22 to 25 km depending on the latitude range, COCl2 VMR decreases rapidly with increasing altitude and becomes essentially zero above 28 km. This is consistent with the results from MkIV spectrometer [Toon et al. 2001] measured near 34ºN and 68ºN between September 1992 and March Both ACE-FTS and MkIV results are inconsistent with the model results [Kindler et al. 1995].
The averaged VMR profiles for northern (0-90ºN) and southern (0-85ºS) hemispheres are very similar, suggesting that both hemispheres have similar amounts of COCl2. This observation is at variance with the model results of Kindler et al. in 1995, which predict a significant hemispheric asymmetry with an enhancement in the troposphere of the Northern Hemisphere.
Results from aircraft observations collected by Wilson et al. [1988] between Germany and Spitzbergen (50º-78ºN) at altitudes of 5-12 km are also included in previous slide. COCl2 VMRs from ACE-FTS are smaller than the observations of both Wilson et al. [1988] and Toon et al. [2001].
Generally, ACE observations show reasonable agreement with those from MkIV spectrometer considering the altitudes of peak concentration and shapes of profiles. Especially, both of them rapidly decrease to zero at 22 to 25 km in altitude. This behavior is obviously different from what the model predicts. Only ACE observations can provide averaged profiles for both hemispheres. This symmetric pattern is at variance with the model predictions. Phosgene concentrations from ACE show smaller values than those from the previous observations in 1980s and 1990s.

13. Concentration Decease in the “Parent” Molecules of Phosgene
The “parent” molecules of phosgene: CCl4, CH3CCl3, C2Cl4, CHCl3 and C2HCl3.
Northern Hemisphere
The concentration deceases in parent species of phosgene provides the explanation of this phenomenon. There are 5 main species that can decompose to form phosgene. All of their concentrations are decreasing since about These significant decreases can be seen in the plots collected in the 2006 ozone report from World Metrological Organization. Here, we are looking at the observed time series of CCl4, CH3CCl3 and C2Cl4 from several groups. All of them show a decrease in concentration in the past decade.
Montzka et al., 1996, 1999
Blake et al., 1996
Blake et al., 2001
Tompson et al., 2004
Prinn et al., 2000, 2005
WMO, 2006
Simpson et al., 2004
WMO 2006

14. Concentration Decease in the “Parent” Molecules of Phosgene
CCl4 concentrations dropped 10% between 1988 and 2005 [Montzka et al., 1996, 1999; Blake et al., 1996; Blake et al., 2001; Tompson et al., 2004; Prinn et al., 2000, 2005; WMO, 2006].
Levels of tropospheric CH3CCl3 declined rapidly between 1991 and During this period, the CH3CCl3 mixing ratio declined 85% [Prinn et al., 2000, 2005; WMO, 2006].
Between 1989 and 2002, annual mean C2Cl4 mixing ratios for the extratropical northern hemisphere dropped from 13.9 pptv to less than half this value (6.5 pptv), and global averages declined from 6.3 pptv to 3.5 pptv [Simpson et al., 2004; WMO, 2006].
Prinn et al. [2000] reported data for CHCl3 from with a trend ranging from –0.1 to –0.4 ppt/year. The decreasing rate of C2HCl3 was reported as 0.01 ppt/year during the period July 1999 to December 2004 [Simmonds et al. 2006].
The concentration of CCl4 dropped about 10% from 1988 to The CH3CCl3, the major phosgene source in troposphere decreased by more than 80% in past 15 years. Only half amount of C2Cl4 remains in the atmosphere compared to the amount in 1980s. CHCl3 and C2HCl3 are also observed to decrease with time. Hence, the decease of phosgene concentration is reasonable due to the decrease in its parent sources.

15. Summary and Conclusion
The first study of the global distribution of atmospheric phosgene (COCl2) has been performed using data from the ACE satellite mission. A total of 5614 measurements from the period February 2004 to May 2006 were used, after filtering out occultations that were inside or near the polar vortex.
No seasonal variation was observed in the data, but there was a significant variation as a function of latitude.
A major source region for atmospheric phosgene appears in the stratosphere over the tropics (around 25 km), where the highest VMRs (40-60 pptv) are observed. There are also likely enhanced abundances of the COCl2 parent species in this region, but an in-depth study of the parent species is beyond the scope of this paper. The Brewer-Dobson circulation transports the COCl2 toward the poles. A long lifetime in the lower stratosphere leads to an enhanced layer in this region. For altitudes above the enhancement layer, VMR values are small because the molecule undergoes UV photolysis. In the troposphere, COCl2 VMR values are relatively low (17-20 pptv) as a result of the 70-day lifetime which is governed by fast wet removal.
Comparisons of COCl2 VMRs between ACE-FTS observations and measurements from previous work show reasonable agreement. ACE-FTS results indicate a decline in COCl2 concentrations since those studies, as one would expect from the decline in parent species, which has occurred due to the emission restrictions required by the Montreal Protocol and its amendments.
In conclusion, we present the first global observations of phosgene using ACE observations. The latitudinal distribution of phosgene was given from the two and a half years of observations. The reasons in the formation of phosgene distribution pattern were studied. The comparison between ACE observations and previous studies show 25% decrease of phosgene in the past decade. It gives further evidence for the success of the Montreal protocol.

16. Acknowledgements Funding: Canadian Space Agency
Natural Sciences and Engineering Research Council of Canada
(NSERC), and other sources
I thank the all of coauthors in this work. With their help, this work has been sent to GRL for publication. I am also grateful for support from CSA and NSERC. Thanks you for your attention.

17. Solar occultation viewing Geometry
A schematic diagram showing the solar occultation viewing geometry used in the ACE mission. Note the distances are not scaled. Solar radiation passed through space and arrived at the upper boundary of the Earth atmosphere. These solar radiations were attenuated by the atmospheric constituents. The blue, green and gray lines indicate the atmospheric absorption lengths at layer 1, 2 and layer n, respectively. Then these attenuated solar radiations were recorded by the instruments on the SCISAT-1 in a set of tangent height during an occultation.