Practicality of stratospheric geoengineering withclimate.envsci.rutgers.edu/pdf/SootPracticality.pdf · KRAVITZ ET AL.: BC GEOENGINEERING PRACTICALITY X - 3 1. Introduction 22 Stratospheric

GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,

Practicality of stratospheric geoengineering with1

black carbon aerosols2

Ben Kravitz1

and Alan Robock2

Ben Kravitz, Department of Global Ecology, Carnegie Institution for Science, 260 Panama

Street, Stanford, CA 94305, USA. ([email protected]) (Corresponding Author)

Alan Robock, Department of Environmental Sciences, Rutgers University, 14 College Farm

Road, New Brunswick, NJ 08901, USA. ([email protected])

1Department of Global Ecology, Carnegie

Institution for Science, Stanford, California,

USA.

2Department of Environmental Sciences,

Rutgers University, New Brunswick, New

Jersey, USA.

D R A F T August 21, 2011, 8:52am D R A F T

X - 2 KRAVITZ ET AL.: BC GEOENGINEERING PRACTICALITY

We calculate costs and emissions factors for two methods of producing 13

Tg per year of black carbon aerosols in the stratosphere as a means of geo-4

engineering the climate. The cheapest investigated idea of combusting diesel5

fuel in the stratosphere to produce the aerosols would require an initial in-6

vestment of US $1.4 trillion and would incur an annual operating cost of ap-7

proximately US $540 billion, or 2.0% and 0.8% of worldwide GDP, respec-8

tively. Costs are reduced by 1-2 orders of magnitude if the aerosols are pro-9

duced on the ground and transported to the stratosphere. Using carbon black10

would be much cheaper, with both fixed and annual costs of approximately11

US $1 billion. Additional emissions dependent upon the means of produc-12

tion and transport would mostly be negligible, with the exception of large13

amounts of NOx from diesel combustion. The amount of NOx produced could14

cause catalytic ozone destruction by approximately 3-10%. The current world-15

wide oil production and refining capacity cannot support diesel fuel combus-16

tion to produce 1 Tg of black carbon aerosols, but producing this much car-17

bon black would be cheap and relatively easy with current technology. The18

cost of using black carbon for stratospheric geoengineering may not be a lim-19

iting factor, but there are other reasons why this technique is impractical,20

including massive ozone depletion.21


KRAVITZ ET AL.: BC GEOENGINEERING PRACTICALITY X - 3

1. Introduction

Stratospheric geoengineering with black carbon (BC) aerosols has been proposed as a22

potential means of alleviating some amount of anthropogenic warming [Crutzen, 2006;23

Kravitz et al., 2011]. Should this be deemed a viable geoengineering option, one of the24

most immediate questions is how one can produce a large amount of black carbon aerosols25

in the stratosphere. We adapt some of the methods and calculations of Robock et al. [2009],26

who performed similar investigations for sulfate aerosols.27

We look at two methods of BC aerosol production: diesel combustion and carbon black.28

Soot production is a particularly sensitive marker of diesel exhaust [Fruin et al., 2004].29

Diesel combustion has the significant advantage of a vast infrastructure currently in place,30

including transportation and regulation, which would lend this technology particularly31

well to geoengineering purposes. Carbon black results from furnace combustion of heavy32

fuel oil in low oxygen [Crump, 2000]. It is generally an agglomeration of mostly elemental33

carbon particles, whereas black carbon aerosols often have adsorbed organic particles, de-34

pending upon the source of the emission [Watson and Valberg , 2001], but the mechanisms35

of formation of the two compounds are quite similar [Medalia et al., 1983], so we can36

assume the particle density and refractive indices are similar. However, a typical radius37

of black carbon aerosol is approximately 0.1 µm [Rose et al., 2006], but a typical carbon38

black agglomerate can have a diameter on the order of millimeters [Gandhi , 2005]. A39

larger diameter means a greatly increased fall speed as well as a reduced radiative effi-40

ciency, so the same mass of carbon black might be significantly less effective, or possibly41

ineffective, for geoengineering.42



2. Logistics and costs of using diesel fuel

The black carbon emissions for heavy-duty diesel vehicles are approximately 1 g black43

carbon emitted per kg of fuel used [Kirchstetter et al., 1999], but the highest emitting 10%44

of all heavy-duty diesel trucks produce 42% of black carbon emissions [Ban-Weiss et al.,45

2009]. Assuming diesel engines could be tuned to produce 10 g black carbon per kg of fuel,46

the largest value reported by Ban-Weiss et al. [2009], and assuming an average density47

of 0.84 kg L−1 of diesel fuel [Brown, 1999], producing 1 Tg of black carbon would require48

combustion of 1.19 × 1011 L of diesel fuel, or approximately 10.8% of current worldwide49

production [EIA, 2010a]. Refineries in the United States are operating at approximately50

90% capacity [EIA, 2010b], so extrapolating this value worldwide implies geoengineering51

by combustion of diesel would require additional expansion of the current refining capacity,52

especially since this capacity is likely a theoretical maximum. We do not have estimates of53

cost for this expansion. We assume the cost of obtaining the oil, refining it into diesel, and54

transporting it to its desired destination, which would be the geoengineering deployment55

site, is included in the at-the-pump fuel cost. For each US $0.01 increase in the market56

price of diesel fuel, the annual cost of geoengineering increases by US $314 million.57

Industrial diesel engines are designed to run continuously at 100% capacity and need58

to be maintained relatively infrequently [USP&E , 2010]. As the central model for our59

calculations, we use specifications of the Caterpillar 3516B industrial engine [Caterpillar ,60

2010a]. Average costs for this particular engine are not available, but several auctions61

reported the sold price at US $395,000 (used), which we adopt as our price estimate.62

Assuming the engine is in operation for 2920 hours a year (365 days a year, 8 hours per63



day - the case for 24 hour per day operation is discussed in Table 1), this would require64

75,100 engines at a capital cost of approximately US $30 billion. We do not include65

estimates of the cost of replacing the engines when they reach the end of their operational66

lifetime, but average diesel engine lifespans are in the range of 10-22 years [MacKay &67

Co., 2003; Lyon, 2007].68

The maintenance costs are twofold: actual cost to maintain the engine and replacement69

engines to operate during the equipment’s downtime. Maintenance requirements for the70

Caterpillar 3516B mean each engine will be inoperative 3.4% of the time [Caterpillar ,71

2010c]. To meet the required black carbon production rate, an additional 255 engines are72

required at a capital cost of US $100 million.73

Maintenance estimates for the Caterpillar G3520 industrial gas engine are approximately74

US $0.008 per kW-h, which is likely more expensive than maintaining a diesel engine75

[USP&E , 2010]. Using this as an upper bound, given that the 3516B runs at a maximum76

of 1492 kW of power generation [Caterpillar , 2010a], the total annual maintenance cost77

is US $2.6 billion.78

A natural solution for getting the black carbon aerosols to the stratosphere is to place79

these diesel engines and diesel fuel in the cargo hold of airplanes and fly them to the80

stratosphere. Robock et al. [2009] evaluated several choices of aircraft that would be81

suitable for geoengineering (their cost estimates were similar to those of McClellan et al.82

[2010]), but we base our calculations here on the KC-10 Extender [USAF , 2010a]. It has83

a payload of 76,560 kg, a ceiling of 12.7 km, and a unit purchase price (2010 dollars) of84

US $116 million. This maximum altitude is only suitable for reaching the stratosphere at85



high latitudes, but because the aerosols self-loft, getting them to the upper troposphere86

is sufficient, resulting in rain-out of about 20% [Mills et al., 2008; Robock et al., 2007;87

Fromm et al., 2010].88

Each airplane is capable of carrying more than one engine, so we decompose our cal-89

culations into units consisting of an engine and 8 hours of diesel fuel. The 3516B engine90

weighs up to 8028 kg and can consume 4339.12 L of fuel in 8 hours for a total unit weight91

of 11,977.0 kg [Caterpillar , 2010b]. Each KC-10 Extender can hold 6 units per airplane,92

meaning 12,342 airplanes would be required at a total purchase price of US $1.4 trillion.93

Curtin [2003] gives an estimate of US $3.7 million in annual cost, based on 300 flying94

hours per year, for personnel, fuel, maintenance, modifications, and spare parts for the95

KC-135 airplane. As Robock et al. [2009] state, the KC-10 is a newer airplane and would96

likely be cheaper, so we use this value as an upper limit for our estimations. Scaling these97

maintenance costs, annual maintenance and personnel costs will be approximately US $3698

million per plane, for a total annual operating cost of approximately US $450 billion.99

This combination results in a fixed cost of US $1.4 trillion and an operating cost of100

approximately US $540 billion. This is the cheapest of the methods shown in Table 1,101

which include calculations for the Caterpillar 3406C engine [Caterpillar , 2010a], the KC-102

135 Stratotanker airplane [USAF , 2010b], and geoengineering in three 8-hour shifts per103

day instead of a single shift.104

The world gross domestic product (purchasing power parity) in 2009 was US $69.98105

trillion [CIA, 2010]. The initial investment for geoengineering would be 2.0% of worldwide106

GDP, with an additional 0.8% each year. Stern [2006] states the cost of climate change for107



2-3◦C of warming could be a permanent loss of up to 3% of GDP, so geoengineering with108

black carbon aerosols is slightly cheaper than the damage that would be caused by climate109

change and is vastly more expensive than geoengineering with sulfate aerosols [Robock et110

al., 2009]. IPCC [2007] calculates that mitigation to reach a stabilization of 535-590 ppm111

CO2-eq would result in a GDP reduction by 0.2-2.5%, with a median reduction of 0.6%112

and an annual reduction of GDP growth rate by less than 0.1%. Compared to the cost of113

black carbon geoengineering by diesel fuel combustion, mitigation either costs the same114

or is cheaper by as much as an order of magnitude.115

The largest source of cost in this method is using airplanes to fly the diesel fuel and116

engines up to the stratosphere. If the black carbon aerosols were produced on the ground,117

collected, and then flown into the stratosphere to be dispersed, the fixed costs could be118

reduced to about US $31 billion, and the annual costs could be reduced to about US $97119

billion [Robock et al., 2009].120

Thus far we have not considered the potential benefit of generation of a large amount of121

electricity from diesel fuel combustion. Using the Caterpillar 3516B engine would create122

approximately 330 TW-h of energy per Tg of BC aerosols produced, which could possibly123

be used to power the airplanes, reducing the associated costs and resources of operating124

the fleet. For comparison, in 2008, the worldwide energy consumption was approximately125

132,000 TW-h [British Petroleum, 2009].126

3. Logistics and costs of using carbon black

Carbon black feedstock is produced from fractional distillation of petroleum and is127

generally extracted as a heavy or residual fuel oil [Dow , 2010a, b; ICBA, 2004]. The128



yield of carbon black from the oil furnace process is 35-65%, depending upon the chosen129

feedstock and the desired particle size, with smaller particles resulting in lower yields130

[EPA, 1995]. Small particles are more efficient for geoengineering, so we use the lowest131

value in this range. Assuming an average density of carbon black feedstock of 1.08 kg L−1132

[Dow , 2010b] and that suitable feedstock (residual fuel oil) comprises approximately 8%133

of refinery yields [EIA, 2010b], producing 1 Tg of carbon black will require 3.18× 1010 L134

of oil, or an additional 0.8% of current worldwide production [EIA, 2010b].135

Available furnace black production capacity in the United States (1998) is 1.6 × 109136

kg, more than sufficient to produce 1 Tg [Crump, 2000]. Annual production costs for all137

carbon black produced in the United States (1998) was US $625 million, or approximately138

US $0.33 per kg, with the finest grade having a 1998 cost of approximately US $1.03 per139

kg. Therefore, using current infrastructure, producing 1 Tg of carbon black would cost140

approximately US $1 billion.141

The costs of ferrying 1 Tg of carbon black to the stratosphere are similar to those142

reported in Robock et al. [2009]. The cheapest transportation option has fixed costs of US143

$1 billion and annual costs of about US $320 million. Including the cost of manufacturing144

the carbon black, the total per-Tg cost of geoengineering with carbon black is US $1145

billion fixed and US $1.3 billion annually. We do not include the cost of transporting the146

carbon black to the geoengineering site in these estimates.147

4. Emissions factors

Table 2 summarizes the various emission factors for the most abundant products of148

aerosol production, some of which we examine in more detail.149



Carbon dioxide, the chief contributor to climate change, is the largest emissions factor150

in Table 2 [IPCC , 2007]. The total worldwide emissions of CO2 are approximately 30 Pg151

of CO2 per year [IEA, 2010], so this would constitute an additional 1.1% of emissions.152

The additional CO2 produced from jet fuel combustion as part of the geoengineering153

process would be less than 1% of current aviation emissions, which are already only 2-3%154

of current worldwide emissions [Enviro Aero, 2009].155

Diesel combustion produces NOx from high temperature dissociation of ambient nitro-156

gen [EPA, 1996]. Creating 1 Tg of black carbon aerosols from diesel combustion would157

produce 8.5×109 kg of NOx. Total worldwide NO emissions in 1990 were 49.6 Tg [Steven-158

son et al., 2004], so this source of NOx would be an additional 17%. NOx is an effective159

catalyst for destruction of stratospheric ozone [Crutzen, 1970]. This mechanism competes160

with reactions between other species, depending upon altitude [Finlayson-Pitts and Pitts ,161

2000]. Based on an experiment by Stolarski et al. [1995] simulating ozone destruction from162

a stratospheric fleet of high speed civil transport aircraft and extrapolating from a linear163

fit to their results, we roughly estimate that combustion of diesel fuel for geoengineering164

could cause a -10 to -3% change in total ozone column due to NOx alone.165

CO is by far the predominant emissions factor of carbon black manufacturing, although166

the CO emissions can be reduced by up to 99.8 percent by controlling with CO boilers,167

incinerators, or flares [EPA, 1995]. Without these controls, producing 1 Tg of carbon168

black would result in the emission of 1.4× 106 kg of CO. Diesel combustion would result169

in a larger amount of 1.8×109 kg of CO. Carbon monoxide is naturally produced at a rate170

of 5× 1012 kg annually in the troposphere [Weinstock and Niki , 1972], so the additional171



CO from producing this large amount of aerosols would be negligible. The stratosphere is172

a natural sink for carbon monoxide, due to reaction with the hydroxyl radical [Pressman173

and Warneck , 1970], so we anticipate this emissions factor would not cause any noticeable174

adverse effects.175

Sulfur compounds resulting from diesel fuel combustion are due to sulfur content of the176

fuel. During the combustion process, nearly all of the sulfur is oxidized to SO2, which is177

a precursor to sulfate aerosols [EPA, 1996]. Using the emissions factor given in Table 2,178

creating 1 Tg of black carbon aerosols would result in the production of approximately179

0.57 Tg of SO2, which would cause small climate effects but would still significantly impact180

the planetary radiation budget [Robock et al., 2008; Kravitz and Robock , 2011; Solomon181

et al., 2011]. It is an insufficient amount to produce damaging acid rain [Kravitz et al.,182

2009]. Since the reporting of the year 1996 emissions factors given in Table 2, ultra-low183

sulfur diesel has been introduced to the market and is the only readily available diesel184

fuel in the United States [EPA, 2009], so this emissions factor would likely be lower than185

the values reported here. The chemistry effects of this increase in SOx may not be trivial,186

especially when considering the effects on ozone. Simulations of 2 Tg a−1 injections of S187

into the stratosphere showed a delay in the recovery of the ozone hole by approximately 30188

years [Tilmes et al., 2009], suggesting the additional SOx from our diesel fuel combustion189

calculations has the potential to cause ozone destruction.190

Methane is a powerful greenhouse gas, 23 times more effective than CO2 on a 100-191

year time scale [IPCC , 2007]. From the emissions factor reported in Table 2, producing192

1 Tg of carbon black would result in the production of 2.5 × 104 kg of methane, or193



an increase in atmospheric concentrations by significantly less than 1 part per trillion.194

Current concentrations of methane are on the order of 1 ppm [IPCC , 2007], so this is a195

negligible contribution.196

5. Discussion and Conclusions

A comparison of all of the discussed means of geoengineering, as well as the calculations197

for SO2 performed by Robock et al. [2009] are in Figure 1. Based on our calculations, if198

black carbon aerosols were to be used for geoengineering, producing 1 Tg of aerosols via in199

situ diesel combustion would be prohibitively expensive and potentially greater than the200

cost of mitigation, which would actually directly address the problem of anthropogenic201

climate change. Producing the aerosols on the ground and transporting them to the202

stratosphere would be much cheaper.203

This study does not address side effects of black carbon geoengineering. The climate204

effects would likely be adverse and severe [Kravitz et al., 2011]. Moreover, diesel fuel,205

black carbon aerosols, carbon black, and their respective byproducts of manufacture and206

combustion are hazardous to human health [CDC , 1999; Baan et al., 2006]. This poses207

a risk to all those in the employ of the geoengineering program, as well as those affected208

by deposition of the particulate matter. In combustion of diesel fuel, a small portion of209

the emissions (2-3%) are through the crankcase instead of the exhaust, which could pose210

a hazard to the airplane pilots and the maintenance crews [EPA, 1996]. Despite these211

unknowns and negative impacts, most emissions factors from black carbon geoengineering212

would be negligible from a climate impact standpoint, although the chemical effects would213

need to be evaluated.214



The calculations presented here are one aspect of many that should be carefully eval-215

uated. Geoengineering is a complex problem with multiple facets, all of which must be216

considered before society decides whether to geoengineer.217

Acknowledgments. We thank Barbara Turpin, Ken Caldeira, Drew Shindell, Mark218

Miller, and Anthony Broccoli for comments. This work is supported by NSF grant ATM-219

070452.220

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Table 1. Fixed (one-time) and annual costs for geoengineering by combustion of diesel fuel

for each considered combination of diesel engine, airplane, and daily shift number. Included in

the annual costs are an estimate of fuel consumption with an at-the-pump price of US $3.00 per

gallon, for a total of US $94.3 billion. Values reported are rounded to two significant digits.

Engine/Airplane 2920 hours per year 8760 hours per yearFixed Annual Fixed Annual

3516B, KC-135 US $1.3 trillion US $990 billion US $460 billion US $2.8 trillion3516B, KC-10 US $1.4 trillion US $540 billion US $510 billion US $1.4 trillion3406C, KC-135 US $1.6 trillion US $1.2 trillion US $550 billion US $3.4 trillion3406C, KC-10 US $1.8 trillion US $650 billion US $600 billion US $1.7 trillion

Table 2. Significant emissions factors for diesel fuel combustion and carbon black production.

Emissions factors for diesel fuel are from EPA [1996, 2005] and for carbon black from EPA [1995].

All emissions factors are in total number of kg emitted for producing 1 Tg of black carbon aerosol

(diesel fuel combustion) or carbon black. 95% of all carbon black is created by the furnace black

process [Crump, 2000], so we use those emissions factors here. Only emissions factors which were

deemed to be significant products are included.

Emissions factor Diesel fuel Carbon blackCO2 3.2× 1011

NOx 8.5× 109

CO 1.8× 109 1.4× 106

organics (exhaust) 6.8× 108

organics (crankcase) 1.9× 107

PM-10 6.0× 108

SOx 5.7× 108

aldehydes 1.4× 108

H2S 3× 104

CS2 3× 104

OCS 1× 104

CH4 2.5× 104

C2H2 4.5× 104



Figure 1. Fixed and annual per-Tg costs of stratospheric geoengineering with black carbon

aerosols. Sulfur gas calculations are repeated from Robock et al. [2009] and are included for

comparison.


Documents

Practicality of stratospheric geoengineering withclimate.envsci.rutgers.edu/pdf/SootPracticality.pdf · KRAVITZ ET AL.: BC GEOENGINEERING PRACTICALITY X - 3 1. Introduction 22 Stratospheric