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The black carbon black box: Introduction to black carbon and short-lived climate pollutants

  • Oliver Emanuel
    Carbon Ratings Scientist
  • Matt Lavelle
    Senior Carbon Ratings Scientist

Here are some key takeaways

  1. Black carbon is a major cause of air pollution, contributing to high annual mortality in developing regions. Black carbon is often referred to in the voluntary carbon market (VCM) under SDG 3, ‘Good Health and Well-Being’, in relation to household air pollution from cookstoves.

  2. Cookstove projects can help to reduce black carbon emissions, but stove and fuel type, along with cooking conditions, may diminish true SDG impacts.

  3. At present, there are no projects that seek credits for black carbon emission reductions in the VCM.

Contents

Introduction

Cookstove projects account for greenhouse gas (GHG) reductions using improved efficiency or fuel-switch devices. Many also claim Sustainable Development Goal (SDG) impacts which are often based on the reduction of household air pollution (HAP) - where black carbon emissions are key. 

Approximately 2.4 billion people use solid fuels such as wood, charcoal, and fossil fuels for cooking globally.¹ The incomplete combustion of these fuels forms sooty deposits, known as black carbon. Globally, 25% of black carbon emissions are the result of biomass fuels in residential cookstoves. ²

Black carbon is released as particulate matter, which has a diameter of less than 2.5 micrometres, also known as PM2.5. It is one of several short-lived climate pollutants (SLCPs) emitted from low-efficiency cooking technologies. Black carbon persists from a few days to a couple of weeks (consequently termed ‘short-lived’). In contrast, GHGs can remain in the atmosphere for up to hundreds of years. Black carbon emissions can therefore have more immediate effects and as such are reported to contribute to up to 35% of global climate forcing, whereby elements on the earth’s surface or in the atmosphere change the global energy flux.³

To reduce HAP, many jurisdictions are implementing action plans and policies. For example, in Kenya, the five-year National Climate Change Action Plan aims to promote energy efficiency and transition toward cleaner cooking with the expectation of providing micro-financing options, increasing stove access and improving manufacturing. Furthermore, the plan targeted to reduce household mortality from biomass energy use from 49% to 20%, annually. 

In the voluntary carbon market (VCM), within the Household Devices sector - cookstove, or ICS projects, are the main focus of this insight. The concept of these projects is to replace inefficient traditional stoves with improved stoves which deliver a more complete combustion of fuel. These projects aim to decrease GHGs and improve HAP while concomitantly reducing black carbon.

Currently, there are no projects in the VCM that claim credits towards black carbon reductions, as physical particles are excluded from emission reductions. This is primarily because the conversion of black carbon to CO₂e would be specious at best, impacting credit fungibility. Although projects can claim benefits toward SDGs associated with reductions in HAP such as black carbon, we find that these proposed improvements in HAP and reductions of black carbon often remain uncertain, as they are largely assessed by qualitative methods. As part of our assessment of ICS projects under the BeZero Carbon Rating (BCR), we try to examine the extent to which projects may actually be reducing black carbon.

What is black carbon?

Black carbon has a detrimental impact on the climate, human health, and atmospheric conditions. Naturally, it can occur through wildfires, but the burning of the aforementioned fuels in cooking and heating is also a significant source. 

The ‘short-lived’ nature of black carbon means the impact is immediate, such as altering the albedo effect (the reflection of sunlight from ice and snow) and changing rainfall patterns. Evidence from an academic study suggests that black carbon is only second to CO2 in terms of its climate-forcing impact on the planet. This is where anthropogenic and/or natural factors impact the carbon dioxide levels and temperature within the atmosphere. ⁴ 

In addition to black carbon, other SLCPs can also be emitted during incomplete fuel combustion, contributing to differential warming and cooling effects. This is driven by their individual Global Warming Potentials (GWP) (Table 1). Although cooling can take place, most notably in the form of organic carbon, warming from black carbon is likely to outweigh any of these effects.

Table 1. The twenty-year global warming potential (GWP20) of short-lived climate pollutants from the IPCC 2013 ‘The Physical Science Basis’ report. In this case, a 20-year GWP is used as it focuses on pollutants that do not impact the climate after their short lifetime. Values displaying ‘-’ represent cooling effects.⁵

Household air pollution

In 2020, HAP was considered to be the cause of mortality for approximately 3.2 million people.⁶ Black carbon is a leading cause of HAP, and inefficient cooking technologies and methods are a major contributor to emissions. Therefore, reducing black carbon by using technologies and fuels that improve cooking conditions is a necessary measure in reducing HAP mortality.  

We assess air quality data in regions where ICS projects are most common, such as Sub-Saharan Africa which hosts over 500 projects in countries including Rwanda, Ethiopia, and Kenya (Figure 1).

Figure 1. ICS projects in the VCM are most common in Sub-Saharan Africa, followed by Central and Southern Asia, Eastern and Southeastern Asia and finally, Latin America and the Caribbean. Data as of July 2023.

The World Health Organisation’s (WHO) Air Quality Guidelines (AQGs) released in 2005 indicated that the average safe level of PM2.5 was 10 µg/m3. In 2021, this was updated to 5µg/m3. Figure 2 displays the PM2.5 from the aforementioned regions. 

In the region with the second most VCM cookstove projects, Central and Southern Asia, PM2.5 exceeded up to around five-fold the recommended level in 2019 with a concentration of 50 µg/m3. Similarly, in sub-Saharan Africa, these pollutant levels exceeded recommended guidelines by three-fold. In comparison, the highest PM2.5 levels during the same period for North America and Europe, and Australia and New Zealand were close to or under recommended concentrations at 13.69 and 9.09 µg/m3, respectively.

Across all regions, the temporal trends between 2010 and 2019 are relatively static. Sub-Saharan Africa does show some fluctuations, although only a reduction of 6 µg/m3 overall. This is likely due to population growth and urban expansion, which may mitigate the PM2.5 reductions that occur from cleaner and more efficient technologies.⁷

Figure 2. World Health Organisation (WHO) data on average PM2.5 concentrations for five geographical regions, and the WHO 2005 Air Quality Guidelines (AQG) for Particulate Matter (PM2.5). ACQ indicates the safe average level of PM2.5 which is based on all non-accidental mortality ⁸ and cause-specific mortality. Therefore, countries should be aiming for this level to reduce PM2.5-related deaths.

Accurately capturing black carbon avoidance in cookstove projects

In ICS carbon projects, it is assumed that HAP will be reduced, predominantly through black carbon reductions. However, this premise rests on a number of assumptions that do not always manifest, such as robust sampling and monitoring, as discussed in our webinar on Cookstove Projects.

Another aspect crucial to accurate black carbon avoidance is the design of improved cookstoves. For example, one of the most commonly distributed stoves rated by BeZero, rocket stoves, is identified as having minimal impact on  black carbon reduction, since there is no separate combustion chamber.⁹ Further, studies have indicated that black carbon emissions have increased in some cases compared to baseline scenarios.¹⁰ This may be because the amount of fuel has not significantly decreased in comparison to the quantity used by traditional stoves.¹¹

The Clean Cooking Alliance has a catalogue of ICS displaying various performance metrics. Table 2 shows several types of stoves in projects with a BeZero Carbon Rating (BCR), in addition to other stoves commonly cited in studies. In this instance, black carbon emissions fall under both ‘Overall Emissions’ and ‘Indoor HAP’. 

Table 2. International Workshop Agreement tiers for overall emissions, indoor emissions and energy efficiency for 10 stove types using charcoal and other biomass. Tier 0 defines the worst performance, tier 4 defines the best. A ‘-’ indicates where no data is available. ‘Overall Emissions’ indicates the performance of associated emission factors, ‘Indoor HAP’ indicates the performance of associated HAP.¹²

It can be seen that the three best-performing stoves are the ACE One Fan Gasifier and Natural Draft Top Lit, which use woody biomass, and the Kike Green Cook KD1, which uses bioethanol. A common modification across the biomass stoves are the two different air flows which allow for cleaner and more complete combustion through gasification. In contrast, the Jikokoa - an ICS used in several projects across sub-Saharan Africa - achieves a relatively low performance compared to others, albeit a high ‘Energy Efficiency’. This is likely down to the simplistic rocket design, which as aforementioned, does not necessarily reduce black carbon emissions. 

Beyond improved stove design, the opportunity to switch fuels has the potential to produce even lower black carbon emissions. The World Health Organisation (WHO)¹³ defines clean fuels and technology as electricity, biogas, natural gas, liquified petroleum gas (LPG), solar or ethanol. Fuel switch projects that transition to these sources generally confer greater confidence in emission reductions. For example, bioethanol stoves do not release black carbon emissions indicating that a complete fuel switch may be a more appropriate action than simply using an efficient ICS.¹⁴ In terms of black carbon emissions, the bioethanol Kike Green Cook KD1 is the highest-performing stove for HAP (table 1).

Methodologies for black carbon

In 2017, the first black carbon-focused methodology was created¹⁵ for projects to claim emission reductions based on accounting for black carbon and other SLCPs. These are issued as black carbon equivalent (BCe) credits, and are not equivalent to or fungible with carbon credits. Instead, BCe’s form part of certified SDG impact products, which projects can issue in order to monetise their impacts.¹⁶

As of June 2023, we have reviewed all projects using the TPDDTEC methodology to review BCe claims. Only carbon projects using this methodology are able to claim for BCe credits due to the quantification approach. We found that no issuance of BCe credits has taken place, nor have any projects used the methodology, although being eligible to do so.

Considering the release of the methodology in 2017, it is unclear why this has been the case. One reason may be because the trading system for SDG impacts is a relatively nascent concept. Another reason may be that demand from buyers is not yet sufficient to outweigh the costs of monitoring and accounting for BCe, especially as projects can claim SDG 3 recognition through other means. 

The most commonly claimed SDG by rated projects in the cookstove sub-sector is SDG 3 (with the exception of SDG 13: Climate Action), which aims to “Ensure healthy lives and promote well-being for all at all ages, targeting a reduction of air pollution”. This largely focuses on SDG indicator 3.9.1, reducing mortality rates from a reduction of HAP (Table 2). Read our insight on the robustness of SDG 3 and 7 claims in the VCM here. 

To achieve this, projects have to report verified qualitative and quantitative evidence (although this is not always the case and requirements vary across standards bodies). However, based on the ICS projects we have rated, this evidence stems primarily from end-user surveys, which can suffer from large uncertainties due to sample sizes and techniques. As black carbon is a major contributor to HAP, we would expect projects that claim for SDG 3 by reducing air pollution to provide explicit evidence pertaining to black carbon and PM2.5 reductions. This would involve the measurement of black carbon emission factors and other related SLCPs. However, we find that this is routinely not the case. 

The type of assessment that supports SDG claims varies, with one project using previous literature on the stove type, another using calculations related to stove variables, and the remaining six using an interview or survey with end users. The percentage of end users which experience an improvement in air pollution, according to interviews and surveys, is as high as 100%. In comparison, the techniques which are study or calculation driven achieve  lower values. 

Monitoring that takes place for interviews and surveys may suffer the same sampling over-crediting risks we find common in cookstove projects, as SDG 3 monitoring tends to be carried out simultaneously with usage and household surveys. However, it is also likely that the other two methods have limitations. For example, although previous studies may examine the same stove type, fuel and stove conditions may impact overall black carbon emissions. Furthermore, using stove efficiencies is also vulnerable to some risk due to the limitations of techniques used to test stove efficiency.

Table 3. Current BCR projects which have claimed SDG 3: Ensure healthy lives and promote well-being for all at all ages. The information noted in each of the columns is from the Project Design Document or Monitoring Report of each of the projects listed.

When assessing the reported improvements in air pollution in comparison to the IWA tiers for overall and indoor emissions in Table 1, we find discrepancies. For example, one Kenyan-based project uses a Jikokoa stove and states in the third Monitoring Report that smoke has been reduced by 96%, yet, according to the IWA, both emissions performance targets for this stove are in the second lowest tier (Table 1). This presents conflicting results as the design of the stove has limitations in regards to black carbon reductions, compared to other stove types which are reported to perform better. This highlights the risks associated with qualitative surveys and SDG claims in particular, as their reliability may be inconsistent with other forms of black carbon accounting. Credits with SDG 3 claims indicating the reduction of HAP are likely to be true, although the extent to which reductions actually happen is uncertain.

Conclusion

Projects do not currently account for black carbon reductions and instead often attribute this to co-benefits. However, our analysis indicates that different cooking conditions have different effects on black carbon, meaning reductions are not a given. The impact of improved cooking practices on black carbon is heterogeneous and requires robust, project-specific monitoring. 

Introduction of a specific methodology and rigorous monitoring for black carbon and other SLCP’s is likely to be more accurate, albeit some limitations do remain, such as monitoring and sampling techniques. This could also address co-benefit reporting by carbon projects which claim SDG impacts related to black carbon and air pollution. 


References

¹ World Health Organisation, n.d.

²  Garland et al., 2017; de la Sote et al., 2019

³ Bailis et al., 2015

 Bond et al., 2013

  IPCC, 2021

World Health Organization, n.d.

Lim et al., 2020

 World Health Organization, n.d.

Kar et al., 2012

¹⁰ Pantage et al., 2015; Garland et al., 2017; Rupakheti et al., 2019

¹¹ Garland et al., 2017

¹² Clean Cooking Alliance, n.d.

¹³ WHO

¹⁴ Armstrong et al., 2021

¹⁵ Gold Standard, n.d.

¹⁶ Gold Standard, n.d.