Thank you for the revised information that you sent regarding your estimates of the GWPs of PMM,

PTPA, and PTAA. The revisions significantly increa sed the transparency of the data and methods you used to estimate the GWPs for these compounds. As we reviewed both this revised information and your originally submitted information on the other five compounds, we noticed some inconsistencies that we wanted to ask you about. If possible, we w ould like to resolve these issues by March 5, befor e we make the GWPs and supporting information availab le to the public later in the March.

1. In your revised submission for PTAA, you state that you used two QSAR relationships to estimate that forcing. In the first QSAR, the estimated for cing of PTAA was estimated from the radiative forcing of PTPA plus the forcing of 6 perfluoroprop ane C-F bonds to yield a radiative forcing of 0.913. In the second QSAR, the radiative forcing o f PTAA was estimated from the radiative forcing of PTBA plus the forcing of 6 perfluoroprop ane C-F bonds to yield a radiative forcing of 1.25. The average of these two estimates was 1.08.

One issue that we noticed is that there are 12, not 6, more C-F bonds in PTPA than in PTAA. If one multiplies the radiative forcing you have estim ated for each additional C-F bond by 12 and adds the result to the estimated radiative forcing of PTPA, the radiative forcing of PTAA becomes 1.11 rather than 0.913. Once this change i s made, the average of the two radiative forcing estimates become 1.18. Is there something we are overlooking here?

3M Response: In evaluating the PFC radiative forci ng data that is used for the QSARs we saw it originated from 1995 and included data for five PFCs. 3M decided this would be a good opportunity to update our PFC QSAR and include data from the AR4 report as well as new data from Ivy et al 2012. The new QSAR includes da ta for 10 PFCs. The new QSAR has a slope
of 0.030. In addition we determined that the radia tive forcing for PTPA should have been 0.71
rather than 0.718 based on new data for PTPA. We h ave revised the GWP and write-up accordingly.

2. For PTBA and PMM, you state that you based radiativ e forcing estimates on temperature changes per ppbV introduction that had been calcula ted using an internal model (AER 1-D RC, Ko and Chang, 1992). Our understanding from your s ubmissions is that these temperature changes were expressed relative to the temperature change calculated by the model for CFC-11, and that to translate the temperature change into r adiative forcings, you multiplied the temperature change relative to CFC-11 by the radiat ive efficiency of CFC-11 published in the Fourth Assessment Report.

For PTBA, we can easily duplicate your result (1.05 W/m2/ppbV) using this approach. However, for PMM, we cannot. When we multiply the temperatu re change relative to CFC-11 that you report for PMM (1.8) by the radiative efficiency of CFC-11 from AR4 (0.25), we get 0.45 W/m2/ppbV rather than the 0.396 W/m2/ppbV that you report. Can you explain this?3M Response: This was a typo/carryover from an old er Excel spreadhseet. In a previous

spreadsheet the value for PMM was calculated by 3M prior to 2007 and 3M used the radiative efficiency from the SAR which is 0.22 for CFC-11 pr oducing 1.8 × 0.22 = 0.396 W/m2/ppbV.
3M now uses the most current value for CFC-11 i.e. 0.25, so 1.8 × 0.25 = 0.45 W/m2/ppbV. The GWP was updated accordingly.

3. We noticed that in your write-up of PTAA, you cited a radiative forcing value of 0.718 for PTPA. However, in the write-up for PTPA, you cited a radi ative forcing value of 0.71. Which is correct?

3M Response: In 2002 the radiative forcing was lis ted as 0.718. Later work was done using two different FTIR spectra and the Pinnock method t o get two new values that more values and their average is 0.7125 and was rounded down to 0.71, (0.706 +0.719).

4. In your write-up for PTBA, you state that the proce sses governing its atmospheric lifetime are reaction with OH and O3. However, our understandin g from our conversation on February 8 was that UV photolysis was the primary break-down m echanism for this compound, distinguishing it from the other amines. Would you like to update your submission for PTBA to
reflect this?

3M Response: We added the following to the write-u p: “The authors did note some degradation of PTBA in control experiments and hypo thesized that direct photolysis may occur at a wavelength near 222 nm. The actual atm ospheric lifetime of PTBA may be further limited by this reaction mechanism .”

5. For PIPM, does "adjusted radiative forcing" mean th at you have applied the cloudy-sky adjustment factor?

3M Response: Yes, Adj. Rad. Forcing = 1.07 * Inst. Rad. Forcing. The reason for this is described in more detail in the response to questio n 6.

6. Where you have used the Pinnock et al approximation method, have you consistently applied a cloudy-sky adjustment factor? If not, please expla in why not.

3M Response: When calculating GWPs, 3M attempts to approximate t he adjusted cloudysky radiative forcing. For many compounds this can be estimated by increasing the instantaneous forcing by 7% as described on page 23 ,237 of Pinnock et al., 1995 The adjusted forcing is the forcing adopted by IPCC to calculate GWPs. This forcing is the net change in irradiance at the tropopause that would o ccur once the stratospheric temperature profile has adjusted to the perturbatio n caused by the introduction of the compound, but the troposphere and surface has not c ome to equilibrium. (See page 353 Climate Change 2001). This simple adjustment is no t made for compounds that, like HFC-41, have absorbance that strongly overlaps that of ozone (between 1000 to 1100 cm -1 ). According to Pinnock et al., p23,233, [12] compound s with strong absorbance in this wavenumber range would absorb earth-radiated IR in the troposphere that would normally be absorbed by ozone in the stratosphere. Thus, the y would reduce stratospheric heating caused by the O 3 absorption of this IR. This cooling decreases the adjusted radiative forcing. The procedure to make this estimate invol ves making 25 evenly spaced measurements of the cross section [10 -18 cm 2 molecule -1 ] of the given compound between 1000 and 1100 cm -1 and averaging them. For example the resulting ave rage cross section in this band is 0.59 [10 -18 cm 2 molecule -1 ] for HFC-41. The cut-off that 3M uses for strong absorption between 1000 and 1100 cm -1 is 1/4 th of this roughly calculated average cross section of HFC-41 in this band. This 1/4 th value was chosen because, if one assumes that this average cross section in this band decreases a djusted forcing in proportion to its magnitude, then the 7% correction applied to a comp ound with an average cross section in this band greater than 1⁄4 that of HFC-41 would gi ve a "corrected" adjusted value that would be a less accurate approximation of the actua l adjusted radiative forcing than the instantaneous radiative forcing calculated by the P innock method. Thus, the adjusted radiative forcing, determined by multiplying the in stantaneous radiative forcing by 1.07, is used in GWP calculations for compounds with average cross sections in this 1000 to 1100

cm -1 band that are < 0.148 [10 -18 cm 2 molecule -1 ], ( < 1⁄4 that of HFC-41). The instantaneous radiative forcing is used in GWP calculations for c ompounds with average cross sections > 0.148 [10 -18 cm 2 molecule -1 ] in this band. Therefore, if the average cross se ction in the 1,000 to 1,100 cm -1 band is < 0.148 [10 -18 cm 2 molecule -1 ]:

Adj. Rad. Forcing = 1.07 * Inst. Rad. Forcing

7. What radiative forcing value did you calculate for HFE-7500?

3M Response: We updated the HFE 7500 write-up to i nclude the following: 

### The instantaneous forcing was calculated using the appr oximation method developed by
Pinnock et al. [4] as 0.37 Wm -2 

### ppbv -1 

### . This calculation assumes that there is a constant concentration profile of the compound at all altitu des. To correct for this, the value was multiplied by a scaling factor as suggested by Sihr a et al. [5] to account for the uneven vertical distribution of this compound in the atmos phere, resulting in a final radiative forcing value of 0.30 Wm -2 

### ppbv -1



### .