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Analysis finds air-quality justification for CNG vehicle conversion in developing cities, despite negative climate impact

Impact pathway approach for modeling policy interventions in (a) air quality and (b) climate impacts. Credit: ACS, Zia and Tanzila. Click to enlarge.

An analysis by a team in Bangladesh found large air quality and associated health benefits accruing to the residents of Dhaka (the capital of Bangladesh) as a result of the rapid conversion of the motor vehicle fleet to CNG. Around 2,045 avoided premature deaths in greater Dhaka can be attributed to air quality improvements from the CNG conversion policy in 2010, resulting in a saving of around US$400 million, they found.

However, CNG conversion was apparently detrimental from a climate change perspective, as CH4 emissions increased. (There is some uncertainty over the impact of ultrafine particulates.) As the greenhouse gas impacts (costs or benefits) are much smaller than the health benefits, the conversion of petroleum vehicles to CNG can be justified on the basis of local air pollution benefits alone, they concluded. Their paper is published in the ACS journal Environmental Science & Technology.

Tornado charts for sensitivity of the monetary impacts due to uncertainties in inputs: (left) local air quality benefits and (right) GHG emissions costs. Note the difference in scales of the horizontal axes. Credit: ACS, Zia and Tanzila. Click to enlarge.

In developed countries, local air pollution problem from motor vehicles received attention decades ago, resulting in various strong policy measures (e.g., vehicle emissions standards, phasing out of leaded petrol, etc.), and a major concern now is the control of GHG emissions. The situation is reversed in most developing countries where local air quality is worsening, primarily because of increasing motor vehicle ownership resulting from a high economic growth and lax vehicle emissions control. While GHG emissions are also increasing and is of some concern, the priority to the policy makers in these countries or cities is reducing criteria pollutants from motor vehicles.

Replacing the existing petroleum vehicles with compressed natural gas (CNG) vehicles or retrofitting them to run on CNG offers large air quality benefits, and accordingly many of the megacities (Rio de Janeiro, Mexico City, Delhi, Mumbai, Karachi) have successfully introduced a varied number of CNG vehicles in their vehicle fleet. Most of the fleet conversion was particularly for heavy-duty diesel buses and trucks or two-stroke three-wheelers, which were the most polluting vehicles on the street in these cities.

—Zia and Tanzila (2013)

Dhaka is a densely populated megacity of 13 million with poor air quality; annual average PM2.5 concentration was 80.4 μg/m3 in 2009. By contrast, the US Environmental Protection Agency’s (EPA’s) annual standard is 12.0 μg/m3.

Motor vehicles were the major source of air pollution in Dhaka. Although the city was late to act on its poor air quality, it adopted a strategy of rapid conversion of its motor vehicle fleet—including personal vehicles—to CNG.

The government statistics show that around 40% of the vehicle fleet (117,000 vehicles)—excluding motorcycles—in Dhaka now run on CNG, while nationwide the number is around 200,000 (in 2010). Vehicle ownership in Bangladesh is small, which puts Bangladesh among the top five countries in the world in CNG penetration (∼18% of vehicle fleet). A spot survey of around 2,000 vehicles at different locations in Dhaka found almost 85% of all cars surveyed running on CNG in June 2010. The proportion of buses and minibuses running on CNG was above 75%.

Local emissions reductions because of CNG conversion of vehicles have been analyzed in other megacities (Rio De Janeiro, Mexico City, New Delhi), while the GHG benefits of such conversion for Delhi has also been studied. Reynolds and Kandlikar, however, showed that increased CH4 emissions from CNG vehicles can reduce the GHG benefits in Delhi. The GHG benefits in Delhi primarily arose through conversion from diesel vehicles which are large emitters of particulates and black carbon, which is more potent than CO2 or CH4 as a GHG. Large scale conversion of petrol vehicles, which are not as large an emitter of particulates and black carbon as diesel vehicles, thus runs the risk of reduced GHG benefit or even negative GHG impacts.

This paper aims to quantify such impacts for the widespread CNG conversion policy in Dhaka. In addition, air quality (AQ) benefits are modeled to understand the relative impacts of local air quality improvements and GHG emissions reduction. Unlike previous studies, focus is not on emissions reduction alone but on the impacts of the reduction and its monetization. To our knowledge, this is the first study to model both the local and global environmental co-benefits of a large scale CNG conversion policy in terms of economic benefits (costs).

—Zia and Tanzila (2013)

The main benefit of CNG conversion is reduced emissions of criteria air pollutants and the associated health benefits. CNG vehicles can have large reductions in CO, NOx and particulates compared to similar gasoline or diesel vehicles. For the study, the team developed a spatially disaggregated, gridded, bottom-up emissions inventory for road transport in Dhaka, with pre- and post-conversion emissions.

They fed these into an air quality model to determine the changes in ambient air quality (i.e., PM2.5 concentration) to which people are exposed. The modeled improvements in ambient air quality were coupled with spatial population distribution and epidemiological concentration−response (CR) functions of the health impacts to determine the avoided health impacts of different types.

On the greenhouse gas side, they concentrated on five global emissions—CO2, CH4, SO2, BC (black carbon), and OC (organic carbon)—before and after the conversion of the vehicles. They used the 100-year global warming potentials of each of these pollutants to normalize them to a common CO2 equivalent metric. (Global warming factors for OC and SO2 are negative—i.e., an increase in atmospheric concentration results in cooling.)

They then multiplied net CO2-equivalent emissions are multiplied by unit value of carbon to monetize the GHG impacts. (They used a social cost of carbon of US$45 in 2010 for the calculations.)

Among their findings:

  • CNG conversion resulted in a reduction of 2090 tonnes of PM2.5 and 630 tonnes of SO2 emissions. This resulted in an annual average reduction of ambient PM2.5 concentration of around 6.5 μg/m3 in the entire modeling domain and around 10.72 μg/m3 in Dhaka city area. The maximum annual reduction in a single grid was 35.63 μg/m3 at a traffic pollution hot spot.

    This improvement in PM2.5 concentrations has avoided around 1,665 premature deaths in Dhaka City Corporation (DCC) and about 2,045 premature deaths in greater Dhaka.

  • With a VSL (value of a statistical life) of US$200,000, the CNG conversion policy resulted in an air quality benefit of US$409 million in the greater Dhaka area. This is around 0.4% of the annual GDP of Bangladesh, “and thus the air quality benefits are clearly large.”

  • Impact pathway model runs for gasoline-to-CNG and diesel-to-CNG conversions show that around 186,000 gasoline to CNG conversions resulted in around 344 avoided premature deaths, while the 16,500 diesel to CNG conversions resulted in around 1,532 avoided deaths.

  • Considering CO2, the conversion policy is mildly beneficial (i.e., reduces warming) through a reduction in CO2 emissions of 140,000 tonnes. However, since emissions of CH4 from all vehicles increase due to conversion, and CH4 has a larger GWP than CO2, the combined impact of CO2 and CH4 is harmful for climate (i.e., warming impact increases).

  • Addition (subtraction) of the warming (cooling) impacts of aerosols and their precursors (SO2, BC, and OC) to the impacts of CO2 and CH4 counters the warming impacts of CH4 but still results in a mild net increase in warming emissions.

  • The net CO2-equivalent impact is a warming effect for gasoline-to-CNG conversions; however, large reductions in BC emissions from the diesel-to-CNG conversions make the net CO2-e impact beneficial for the diesel group.

The monetized costs (USD 17.7 Million) of damages because of global warming attributable to CNG conversion are relatively negligible compared to the health benefits. If the net increases in CO2-e emissions are balanced by buying carbon credits (i.e., if we use market price instead of social costs of carbon), the direct monetary loss is even less, around USD 5 Million.

… The personal vehicle fleet in Dhaka has added substantially new vehicles with better emissions performances over the past few years, despite lax emissions standards. This results from a ban on import of vehicles more than 5 years old. Considering the lower criteria emissions from these and future vehicles (especially particulates and BC), subsequent smaller air quality benefits, potential GHG costs of CNG conversion, potentially larger travel activities of CNG personal vehicles and large opportunity costs of CNG (natural gas fuels three-fourths of electricity generation in Bangladesh and current supply is constrained), the suitability of the CNG conversion policy, especially for the petrol vehicles, requires a critical revisit in a larger integrated framework including all these issues.

—Zia and Tanzila (2013)


  • Wadud Zia, Khan Tanzila (2013) “Air Quality and Climate Impacts Due to CNG Conversion of Motor Vehicles in Dhaka, Bangladesh,” Environmental Science & Technology doi: 10.1021/es402338b



Conversion of ICEVs from gasoline to CNG is not always
as positive as many may think it is?

Is there an easy way to reduce CH4 emissions from CNG leaks and combustion?

Roger Pham

Yes, the easy way to reduce CH4 emission is FCV bus and truck and cars. Convert NG to H2 and use it on vehicles with nearly double the efficiency. Smaller cars and motorcycles can run on battery electricity.


I agree that FCs + super caps may be one of the best solution for clean running larger future vehicles.

Regular size cars and motorcycles could soon run on improved batteries, specially where clean electricity is available. That's our case with lots/surplus clean Hydro (96%) and Wind (4%) low cost energy.

Ontario is not so fortunate. Repair and overhaul of two old Candu Nuke + replacing 4 others with NG plants will require a 33+% tariff increase in the next 3 years, starting with an 11% increase by the end of the month. The same problem with come again (2020 or so) when the other Candu will have to be overhauled or replaced at a very high cost.

Many nuclear power plants in USA will also have to be overhauled or replaced sooner or latter. The price tag will be higher than many expect.

Even France @ 80% nuclear is seriously thinking to reduce nuclear @ 50% by 2025 or so. Wind + solar + NG may replace older nuclear power plants.


The problem here is Bangladesh is a third world country. They have "lax emissions standards" for all vehicles (that includes those fueled by NG), poor maintenance and conversions that were not of the best workmanship: Of course they are going to leak.

On the plus side their CNG doesn't have to come from fossil sources. Bangladesh also has a waste disposal problem; farm waste, city waste & household waste is either not collected quickly enough or dumped into poorly maintained landfills. CH4 leaks from these too. The solution to this problem is the growing trend of household biodigestors which capture the CH4 and feed it into the home for cooking gas. (saves on deforestation as well) Farm and city biodigestors, with their larger input of biomass, could be used to fuel vehicles and hire better trained personal to maintain them to reduce leaks.

Henry Gibson

Combined cycle power plants can get the efficiency of fuel cells at lower capital costs. Cogeneration at home can burn the methane without any release of methane gas and charge battery electric vehicles and plug in hybrid vehicles. Cogeneration reduces carbon releases even with coal fuel and the effect is best with methane. Cogeneration at homes or businesses is the most cost effective way of reducing Carbon releases right now. The capital costs of electric energy delivered to the home is far more significant than the fuel costs. Fuel costs are less than 25 percent in almost all cases. Power companies could have lending banks to lend the capital costs of cogeneration units to developers of large facilities to expand generation faster and at lower end capital costs than central power stations plus the distribution infrastructure. Power electronics and computer control has eliminated the need for central power plants that burn fuels. Absorption cooling units have provided a market for waste heat year round. Very low maintenance microturbines with single moving parts and air lubrication and multiple unit redundancy makes self generation the most reliable source of electricity. Molten salt NGK batteries or GE batteries with very long lives can improve the reliability and may in the future with lower prices, lower the costs. All automobiles of the present and future should have facilities installed to burn CNG or LNG or LPG as part of the fuel use but rarely as the sole fuel; range anxiety is the main reason there are not more CNG vehicles. Recent reports of the underestimation of methane releases in oil production areas supports my past comments that coal may not be the dirtiest fuel when it comes to comparing gasoline made from oil or coal or using natural gas power stations instead of coal fired ones. There is one motor car that operates all the time without contributing to the earths CO2; it is on Mars where CO2 fills the cold sky. Considering all the computers on earth that monitor it, it puts more CO2 into Earth's skies than any other vehicle of the same weight or much larger. ..HG..

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