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

PTPA, and PTAA. The revisions significantly increased 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 would like to resolve these issues by March 5, before 

we make the GWPs and supporting information available 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 forcing of PTAA was estimated from the radiative 

forcing of PTPA plus the forcing of 6 perfluoropropane C-F bonds to yield a radiative forcing of 

0.913. In the second QSAR, the radiative forcing of PTAA was estimated from the radiative 

forcing of PTBA plus the forcing of 6 perfluoropropane 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 estimated 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 is 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 forcing 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 data for 10 PFCs. The new QSAR has a slope 

of 0.030. In addition we determined that the radiative forcing for PTPA should have been 0.71 

rather than 0.718 based on new data for PTPA. We have revised the GWP and write-up 

accordingly. 

2. For PTBA and PMM, you state that you based radiative forcing estimates on temperature 

changes per ppbV introduction that had been calculated using an internal model (AER 1-D RC, 

Ko and Chang, 1992). Our understanding from your submissions 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 radiative forcings, you multiplied the 

temperature change relative to CFC-11 by the radiative 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 temperature 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 older 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 producing 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 radiative forcing value of 0.71. Which is correct? 

3M Response: In 2002 the radiative forcing was listed as 0.718. Later work was done using 

two different FTIR spectra and the Pinnock method to 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 processes governing its atmospheric lifetime are 

reaction with OH and O3. However, our understanding from our conversation on February 8 

was that UV photolysis was the primary break-down mechanism 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-up: “The authors did note some 

degradation of PTBA in control experiments and hypothesized that direct photolysis may 

occur at a wavelength near 222 nm. The actual atmospheric lifetime of PTBA may be further 

limited by this reaction mechanism.” 

5. For PIPM, does "adjusted radiative forcing" mean that 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 question 6. 

6. Where you have used the Pinnock et al approximation method, have you consistently applied a 

cloudy-sky adjustment factor? If not, please explain why not. 

3M Response: When calculating GWPs, 3M attempts to approximate the 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 occur once the stratospheric 

temperature profile has adjusted to the perturbation caused by the introduction of the 

compound, but the troposphere and surface has not come to equilibrium. (See page 353 

Climate Change 2001). This simple adjustment is not 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] compounds 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, they would reduce stratospheric heating 

caused by the O3 absorption of this IR. This cooling decreases the adjusted radiative 

forcing. The procedure to make this estimate involves making 25 evenly spaced 

measurements of the cross section [10-18 cm2

 molecule-1] of the given compound between 

1000 and 1100 cm-1 and averaging them. For example the resulting average cross section 

in this band is 0.59 [10-18 cm2

 molecule-1] for HFC-41. The cut-off that 3M uses for strong 

absorption between 1000 and 1100 cm-1 is 1/4th of this roughly calculated average cross 

section of HFC-41 in this band. This 1/4th value was chosen because, if one assumes that 

this average cross section in this band decreases adjusted forcing in proportion to its 

magnitude, then the 7% correction applied to a compound with an average cross section 

in this band greater than 1⁄4 that of HFC-41 would give a "corrected" adjusted value that 

would be a less accurate approximation of the actual adjusted radiative forcing than the 

instantaneous radiative forcing calculated by the Pinnock method. Thus, the adjusted 

radiative forcing, determined by multiplying the instantaneous 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 cm2

 molecule-1], ( < 1⁄4 that of HFC-41). The instantaneous 

radiative forcing is used in GWP calculations for compounds with average cross sections 

> 0.148 [10-18 cm2 molecule-1] in this band. Therefore, if the average cross section in the 

1,000 to 1,100 cm-1 band is < 0.148 [10-18 cm2 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 include the following: The 

instantaneous forcing was calculated using the approximation method developed by 

Pinnock et al. [4] as 0.37 Wm-2ppbv-1. This calculation assumes that there is a constant

concentration profile of the compound at all altitudes. To correct for this, the value was 

multiplied by a scaling factor as suggested by Sihra et al. [5] to account for the uneven 

vertical distribution of this compound in the atmosphere, resulting in a final radiative 

forcing value of 0.30 Wm-2ppbv-1.