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

Cl ClC

Oװ

Phosgene Chemistry

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.

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.

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.

Atmospheric Chemistry Experiment

Providing vertical concentration profiles of more than 30 atmospheric trace gases which relate to the ozone chemistry, climate change, and air pollution.

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

Link to the details of the ACE Mission:http://www.ace.uwaterloo.ca/homepage.htm

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.

Locations of ACE Occultations

30 km geometric tangent points

5614 ACE-FTS extravortex occultations

Feb 2004 - May 2006

Red: 2004Blue: 2005 Black: 2006

VMR Profiles for 5º Latitudinal Zones

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.

75-80oN

20 40 60

70-75oS

VMR (pptv)20 40 60

65-70oS

0 20 40 60

10

20

30

60-65oS

0 20 40 60

55-60oS50-55oS

50-55oS

10-15oS

00-05oN05-10oN10-15oN15-20oN20-25oN10

20

30

25-30oN

Alt

itud

e (k

m)

30-35oN35-40oN40-45oN45-50oN50-55oN10

20

30

55-60oN

60-65oN65-70oN70-75oN

45-50oS

25-30oS20-25oS15-20oS05-10oS10

20

30

00-05oS

40-45oS35-40oS10

20

30

30-35oS

80-85oN10

20

30

85-90oN

20 40 60

80-85oS

20 40 60

75-80oS

80S 60S 40S 20S 0 20N 40N 60N 80N5

10

15

20

25

30

35

Latitude (degree)

Alt

itu

de

(km

)

5 10 15 20 25 30 35 40 45 50 55 pptv

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.

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.

0 10 20 30 40 500

5

10

15

20

25

30

Volume Mixing Ratio (pptv)

Alt

itud

e (k

m)

85oS-60oS

30oS-60oS

30oS-30oN

30oN-60oN

60oN-90oN

0o-85oS

0o-90oN

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 (0-90N) and southern (0-85S) hemispheres are very similar, suggesting that both hemispheres have similar amounts of COCl2.

0 10 20 30 40 50 600

5

10

15

20

25

30

Volume Mixing Ratio (pptv)

Alt

itud

e (k

m)

Wilson et al. 1988Kindler et al. 1995Nassar et al. 2006

MkIV 34oN Sep 2003

MkIV 33oN Sep 2004

MkIV 35oN Sep 2005

ACE 30o - 35oN 2004 - 2006

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 2005. 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].

Concentration Decease in the “Parent” Molecules of

PhosgeneThe “parent” molecules of phosgene: CCl4, CH3CCl3, C2Cl4, CHCl3 and C2HCl3.

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

Northern Hemisphere

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 2004. 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 1983-1998 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].

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.

Funding: Canadian Space Agency

Natural Sciences and Engineering Research Council of Canada

(NSERC), and other sources

Acknowledgements

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.