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 PM_2.5 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, NO_x 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 PM_2.5 and 630 tonnes of SO₂ emissions. This resulted in an annual average reduction of ambient PM_2.5 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 PM_2.5 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)

Resources

• 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