A DECISION TREE FOR GLOBAL ENVIRONMENTAL IMPACT SCREENING 

Robert E. Tapscott and Marcela G. Olivares-Sooley 

Center for Global Environmental Technologies 

The University of New Mexico 

Albuquerque, NM 87106-4339 

OVERVIEW 

Global environmental impacts-Ozone Depletion Potential (ODP), Global Warming Potential 

(GWP), and Atmospheric Lifetime-are of major importance in the development of acceptable 

halon (and CFC) replacements. In past reports and papers, we have described methods for 

estimating impacts for tropodegradable halocarbons [ 1, 21. Here these methods are expanded to 

allow evaluations for all materials, and a decision tree is proposed for screening new chemicals. 

Although experimentation is needed in making final determinations, the process permits 

relatively reliable estimations of acceptability without resorting to expensive and time-consuming 

evaluations. 

The overall procedure evaluates in turn (1) atmospheric release, (2) hydrolysis, (3) physical 

removal, (4) photolysis, (5) reaction with hydroxyl free radicals, (6) reaction with tropospheric 

ozone, (7) other reactions, (8) stratospheric ozone depletion, and (9) global warming, allowing 

decision points at each stage. Criteria, which may change as the regulatory environment changes, 

are also considered. 

DECISION TREE 

The overall methodology for the decision tree can be diagrammed as shown in Figure 1, with 

details shown in Figure 2. The meaning of the diagram symbols are shown in Figure 3. 

Figure 1. Abbreviated Decision Tree. 

Halon options Technical Wohing Conlerence 12-14May1998 315 
NO 1 I I 

Information 

Figure 2. Detailed Decision Tree. 

31 6 Halon Options Technical Working Conference 12-14 May 1998 
7 Input Information or Data - ____, Decision Flow 

Calculation 

Decision Point 

n End Point 

Figure 3. Diagram Symbols. 

METHODOLOGY 

Decisions are based on whether a calculated (or estimated) atmospheric lifetime (t),* Ozone 

Depletion Potential (ODP), or Global Warming Potential (GWP) is less than or greater than some 

maximum allowable value (t,,,,, ODP,,, GWP,,,). A major problem is the assignment of 

maximum allowable values, and this is determined not only by current international environmental regulations, but also by (1) what future regulations are likely, (2) national and local 

restrictions, (3) public perception, and (4) amount of chemical to be produced, which depends on 

application. Thus, maximum allowable values are not fixed and are subject to debate. The only 

values that would be acceptable to everyone under all circumstances are zero. Nevertheless, we 

might make some educated guesses here. 

It appears that the atmospheric lifetime of HFC-245fa (6.6 years [3]) may be acceptable to most 

of the environmental regulatory community. If this is the case, one might take t,,, to be, 

perhaps, 10 years. HCFC-123, which has the lowest calculated ODP for the HCFCs, has an ODP 

of 0.014 based on CFC-11 [4]. Since ODPs of this order of magnitude are of concern (HCFCs 

are regulated), ODP,,, must be lower than 0.014. An arbitrary value of 0.005 might, therefore, 

be assigned for ODP,,. This may, however, be too high in view of ever increasing regulatory 

measures. The lowest rigorously calculated GWP for any HCFC, a family of chemicals about 

which some global warming concern has been expressed, is 90 for a 100-year time horizon based 

on COz [3]. Thus, one might estimate perhaps 50 for GWP-. These maximum allowable 

values are not recommended values, but are only taken as a basis for discussion in this paper. 

As noted earlier, production quantity is extremely important in regulatory action, and chemicals 

expected to be used in small quantities are likely to have much larger cutoffs than chemicals 

expected to be used in large quantities. 

* To simplify certain equations, we will take all atmospheric lifetimes to be “e-folding” lifetimes, 

i.e., the time required for the concentration of a compound to decrease to lle (approximately 

0.369) of its initial value. 

Halon Options Technical Working Conference 12-14May1998 317 
Release 

The first step is to determine whether the vapor pressure (Py,) is sufficient (less than some 

maximum allowable value Pmm) to permit release. It has been assumed that anything with a 

vapor pressure that is measurable using standard techniques (i.e., anything that could evaporate 

within a finite period of time) is releasable. This includes sublimable solids. Conservatively, 

this means that only ionic compounds, polymers, and very high molecular weight or certain 

highly polar solids (e.g., sugars) should be considered as nonreleasable, although compounds that 

could be readily cleaned up and removed (destroyed) after spillage or use could also be 

considered nonreleasable. 

Hydrolysis 

Compounds that are readily hydrolyzed in the atmosphere will have low atmospheric lifetimes. 

The rate for hydrolytjc reaction of a compound C with a concentration [C] in a homogeneous 

reaction with water with a concentration [HzO] is given by Equation 1, where khyd is the 

hydrolysis rate constant.7 In this case, the e-folding atmospheric lifetime is given by Equation 2. 

One can then 

Effective hydrolysis requires a heterogeneous reaction involving water droplets, since 

homogeneous hydrolysis is very slow [5]. Thus, hydrolysis is closely related to rainout, a 

physical removal process discussed below. Some compounds are, however, sufficiently 

susceptible to hydrolysis that even homogeneous hydrolysis provides effective removal. For 

example, the carbonyl halides readily undergo hydrolysis to form carbon dioxide and hydrogen 

halides (Reaction 1). Even in this case, however, the final reactions may be heterogeneous with 

the carbonyl halides and the HX reaction products acting as nuclei for water droplet formation. 

O=CXz + Hz0 -+ COz + 2HX (1) 

Physical Removal 

Physical removal processes include (1) rainout (absorption by water droplets), (2) aerosol 

scavenging, and (3) solution into oceans [6]. The science of physical removal lags far behind 

that of the other tropospheric removal processes, and much is qualitative. The input data 

required for determination (or estimation) of physical removal are cc for rainout, P, for aerosol 

scavenging, and p and D for solution into oceans. These parameters are defined below. 

?Throughout this paper, equation numbers are inserted in brackets, [I, and chemical reaction 

numbers are placed in parentheses, (). 

31 8 Halon Options Technical Working Conference 12-14 May 1998 
Rainout 

Rainout is believed to be a very effective tropospheric removal mechanism for highly polar 

compounds such as hydrogen halides (HX, where X represents a halogen atom) and carbonyl 

halides (O=CX2). Absorption by water droplets, “rainout,” often occurs with concurrent 

hydrolysis [7]. For highly hydrophilic compounds such as nitric acid (HN03). the global time 

constant for rainout is about 7 days [81. As the polarity of a compound decreases, physical 

removal by water absorption becomes less and less effective. For the relatively nonpolar CFCs, 

HCFCs, HFCs, and PFCs, tropospheric rainout is essentially nonexistent. 

Equation 3 has been proposed to estimate the atmospheric lifetime for rainout [9]. Here, a is the 

solubility coefficient defined by Equation 4. The saturation vapor density is the density of vapor 

in equilibrium with liquid at a particular temperature (which can be taken as 298 K). Rainout 

could be significant for highly water-soluble compounds (large values of a) such as alcohols, 

amines, and carbonyl compounds (esters, carboxylic acids, aldehydes, ketones); however, 

substitution with fluorine will greatly decrease the solubility and, consequently, the rainout. For 

example, due to the very low polarity, perfluorornines such as tris(trifluoromethy1)amine 

(N(CF3)3) are likely to have little if any significant removal by rainout. 

solubility (a) 

saturation vapor density (gL) 

a= [41 

Aerosol Scavenging 

Aerosol scavenging involves adsorption of a chemical onto solid particulates. Equation 5 has 

been advanced to predict the atmospheric lifetime due to aerosol scavenging [9]. Here, Pvap is the 

vapor pressure of a compound in torr at 298 K. Solving for the Pvap required to achieve a 10-yr 

atmospheric lifetime (3.15 x lo8 sec), one obtains Pvap = 3 x 10.’ torr. For a 1000-yr lifetime 

(which is certain to be unacceptable), the required vapor pressure would be 3 x 10” torr. Thus, 

aerosol scavenging is ineffective for any compound with any significant volatility. 

[51 61 tile = tphys = 10 (10 Pvap +I) sec 

Solution Into Oceans 

The lower limit for the tropospheric lifetime due to absorption (solution) into oceans can be 

estimated by Equation 6 [lo]. Here p is the (ocean) water solubility of a gas in moles/m3-atm 

(essentially, a Henry’s law constant), D is the molecular diffusivity of a gas in water in m2/year 

[ll], and z is the film thickness in the classical stagnant film model for gas exchange. The film 

thickness is approximately 30 x lo6 m, and an upper limit on diffusivity at 298 K (giving a lower 

limit on the lifetime) is approximately 6 x cm2/sec (1.9 x 10.’ m2/year) [lo]. This gives an 

upper limit on D/z of 6 x lo3 dyr and a lifetime with a lower bound given by Equation 7. This 

is a lower bound not only because a particularly large value was placed on D/z, but also because 

of release from the ocean back into the atmosphere, a process not taken into account in this 

approximation. To obtain a lifetime of less than 10 yrs by this process, one needs compounds 

Halon Options Technical Working Conference 12-14May1998 319 
with a water solubility greater than 0.5 moles/m3-atm. The water solubilities of most HCFCs and 

HFCs are larger than this value; PFCs have water solubilities lower than this value. 

Photolvsis 

Photodissociation in the troposphere requires ... it a compound absorb radiation in the wave.-ngth - range from 299 nm to 700 nm [6]. The photodissociation rate constant kphoto is determined by 

Equation 8, where o(h) is the absorption cross section as a function of the wavelength h, I(h) is 

the solar intensity as a function of h, and q(h) is the photodissociation quantum yield. Thus, a 

chemical compound has two characteristics that determine the importance of photolysis: (1) the 

absorption cross section (how well it absorbs electromagnetic radiation encountered in the 

troposphere) and (2) the quantum yield. Both of these are a function of the electromagnetic 

radiation wavelength. The atmospheric lifetime for photodissociation only is then given by 

Equation 9. 

One might first estimate the possibility for photolysis from (Tmid, the absorption cross section at 

the tropospheric window midpoint for solar radiation, and om,, the maximum allowable value 

for this cross section that will result in significant photolysis assuming a quantum yield of 1. If 

photolysis is possible, then a more rigorous calculation using o(h) and q(h) as input is needed. 

Reaction with Tropomheric *OH 

Tropospheric hydroxyl free radicals, *OH, may react by abstraction of a hydrogen atom or by 

addition at a point of unsaturation. If neither hydrogen atoms nor unsaturation is present, one can 

bypass consideration of reaction with tropospheric *OH as an effective removal mechanism. If 

reaction is possible, a more detailed calculation is needed. 

Rates for hydroxyl free radical reactions with a compound C follow the second-order rate law in 

Equation 10. Integration gives Equation 11, which can be used to estimate lifetimes by assuming 

pseudo first-order behavior and using a globally averaged hydroxyl free radical concentration 

([.OH])of 9.7 x lo5 molecules/cm3 [12]. 

- 

d[Cl dt = -kOH [C ][*OH] 

320 Halon Cptlons Technical Wohlng Conference 12-14 May 1998 

~ ____ __- ~ 
Reaction with Tropospheric Ozone 

The analytical procedure for reaction with tropospheric ozone (03) is similar to that used for 

tropospheric hydroxyl free radicals. For halocarbons, ozone reacts almost solely with alkenes. 

For compounds other than alkenes, one can bypass reaction with tropospheric ozone as an 

effective removal mechanism. If reaction is possible, a more detailed calculation is needed. 

Rates for reactions of ozone with a compound C follow the second-order rate law in Equation 12. 

Integration gives Equation 13, which can be used to estimate lifetimes by assuming pseudo firstorder behavior and using a globally averaged tropospheric ozone concentration ([03]) of 5.0 x 

10" molecules/cm3 [6]. 

ODP and GWP 

The final step, if needed, is to estimate the ODP and GWP and compare the numbers obtained 

with ODP,,,, and GWP,,, the maximum allowable values. Estimation methods for GWP are 

nearly nonexistent; however, some estimation methods for ODP have been proposed (in 

particular, see Reference 6). 

A method for estimating ODPs for non-bromine-containing one- and two-carbon halocarbons has 

been advanced [ 131. The method relates the ODP to two factors, one a reactivity factor related to 

the number of chlorine atoms in the molecule and the other a survival factor giving the fraction 

of molecules reaching the stratosphere (and, therefore, related to the atmospheric lifetime). The 

proposed relationship is shown in Equation 14, where n is the number of chlorine atoms, t is the 

atmospheric lifetime, and c1, cz, and c3 are parameters to be fit. Here, nc2 is the reactivity 

factor, e-('@) is the survival factor, and c1 is a normalizing constant. This equation gives 

values that are relatively close to the rigorously calculated ODPs used in fitting the parameters. 

Since the values for the rigorously calculated ODPs and atmospheric lifetimes used in the 

parameter fitting have changed since the equation was originally proposed, the reported 

parameters are not be given in this paper. 

In its present form, Equation 14 is not applicable to bromine-containing compounds. It has, 

however, been reported that the ODPs for bromine-containing compounds can be estimated from 

Equation 10 [14]. Here, Pis the photolysis factor, which is set equal to 1.0 if there are no special 

structural features that make the molecule subject to tropospheric photolysis. Otherwise, P = 

0.180 for a bromine geminal to a chlorine (e.g., Br-C-CI), P = 0.015 for geminal bromine atoms 

Halon Options Technical Working Conference 12-14 May 1998 321 
(Br-C-Br), or 0.370 for vicinal bromine atoms (Br-C-C-Br). A is the normalizing constant 

(0.446), B is 0.740, C is 32.000, and D is 1.120. The hydrogen factor E is 1.000 if there are no 

hydrogen atoms; otherwise it is 0.0625. #C, #Br, and #C1 are the numbers of carbon, bromine, 

and chlorine atoms, respectively. 

ODP = AEP[(#CI)’ + C(#Br)]D‘#C-” [I51 

Use of Equation 10 gives, at best, rough approximations, even if the equation is restricted to 

simple saturated halocarbons. The trends, however, are preserved. It may be that a better fit of 

parameters could be obtained with a larger set of more recent data as a basis. 

ACKNOWLEDGMENT 

This work was sponsored, in part, by The Department of Defense Next-Generation Fire 

Suppression Technology Program, funded by the DoD Strategic Environmental Research and 

Development Program (SERDP) under Contract MDA972-97-M-0011 [15]. 

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