Studies focused on other air pollutants and secondary particles – particulate matter that is not directly emitted but formed through photochemistry in the atmosphere. This emphasis was spurred by the new understanding of how photochemistry formed the ozone in Los Angeles smog. Only as measurement techniques evolved were inconsistencies found in the carbon budget of air pollution. In the 1980s researchers revealed that black carbon was still a significant component of particulate matter (Novakov and Rosen, 2013). After that, until the introduction in 2005 of mandatory monitoring in the European Union for PM10 (particulate matter less than 10 microns in diameter) followed by PM2.5 monitoring in 2015, “black smoke”, a surrogate for black carbon, was a required measurement in nations such as Germany. There, an annual mean limit value of 14 µg m–3 for “black smoke” applied from 1995; from 1998 the limit was 8 µg m–3. Most monitoring of “black smoke” and black carbon stopped following the changes in regulations in 2005.

Black carbon’s contribution to climate forcing wasn’t revealed until the 1990s, when it was included in global climate models. Before that, only sunlight-scattering particles such as sulphate were allowed for, suggesting that particulate matter had a purely cooling effect. In 2004, Hansen and Nazarenko pointed out that the effect of black carbon on climate change had been underestimated (Hansen and Nazarenko, 2004) and in 2008, black carbon was ascribed the role of the most important climate forcer after carbon dioxide (Ramanathan and Carmichael, 2008). The current estimate of black carbon’s global impact on climate is +1.1 (+0.17 to +2.10) W m–2 (Bond et al., 2013), compared to +1.68 (± 0.35) W m–2 for carbon dioxide (Boucher et al., 2013).

Since black carbon has a lifespan of just one week or so, its distribution in the atmosphere is heterogeneous and concentrations are higher near large emission sources. This means that the importance of black carbon for climate warming varies on a regional scale. South of the Himalayas, for example, where emissions of black carbon are high due to the large numbers of people, its impact is likely to be as large as that from carbon dioxide (Ramanathan and Carmichael, 2008). What’s more, black carbon in this region can deposit onto mountain snow and ice, where it reduces their reflectivity (albedo), leading to absorption of more sunlight and warming of the surrounding air. This accelerates glacier melt, provoking changes in the hydrological cycle. This also happens in other regions like the Arctic to which air pollution, including black carbon, is transported over long distances.

When it comes to human health, impacts are much more local. The immediate exposure of people to black carbon is most relevant for cardiovascular and pulmonary problems (WHO, 2012). In many regions that have implemented air quality management plans, the situation has improved, while in others with increasing emissions the situation has worsened. The World Health Organization recommends that concentrations of PM2.5 should not exceed a daily average of 25 µg m–3 (note, there is no extra recommendation for black carbon). However, in Kathmandu 24-hour mean concentrations of black carbon alone can exceed 25 µg m–3 (Shakya et al., 2016).

Research is ongoing into exactly how much climate warming is due to black carbon and the precise mechanism causing the material’s health impacts. One source of uncertainty is the measurement of black carbon itself. “Black carbon” is a general term covering material with properties that are determined by several different methods (Petzold et al., 2013). Those techniques include the thermal–optical reflectance measurement of elemental carbon mass (EC); determination of inferred mass (known as equivalent black carbon) through optical measurements such as attenuated light transmission or multi-angle absorption; and quantification of inferred mass of refractory black carbon from laser-induced incandescence. The results of these methods can diverge significantly on the same sample (Petzold et al., 2013), or even when applying the same method under different protocols. For example, using thermal–optical reflectance measurement protocols as defined by the US Interagency Monitoring of Protected Visual Environments (IMPROVE), the US National Institute for Occupational Safety and Health (NIOSH) or the European Supersites for Atmospheric Aerosol Research (EUSAAR_2) can yield different outcomes (Cavalli et al., 2010).

Bond et al. (2013) highlighted further information gaps in the determination of black carbon’s climate effects, including:

  • Few measurements of the vertical distribution of black carbon;
  • Poor knowledge of the mixing state of black carbon – black carbon can either be present as a “pure” black carbon particle or can be coated by other components of particulate matter that change its optical properties;
  • Emissions data are uncertain so their use in climate modeling is challenging;
  • Estimating the present-day amount of anthropogenic black carbon climate forcing requires comparison with pre-industrial levels but these are not clear;
  • Atmospheric processes such as deposition and black carbon-cloud interactions are not well defined.

This implies that we need many more vertical- and spatially-distributed measurements of black carbon as well as more extensive measurements in snow. Unfortunately, satellites can’t easily monitor this pollutant, which is a shame as they could otherwise have rapidly provided a comprehensive picture of black carbon distribution and dispersion around the globe.

The uncertainties around the health, rather than climate, impacts of black carbon are even greater. We don’t yet know exactly how the material causes harm. However, in general, all health outcomes related to particulate matter are also linked to black carbon, in many cases with even greater significance. Studies of short-term effects are associated more robustly with black carbon than particulate matter; especially in the case of traffic emissions (WHO, 2012). Evidence from long-term exposure studies, on the other hand, is still inconclusive. Some studies suggest that effects from black carbon are stronger than those of particulate matter components such as sulphate, while others find the opposite (WHO, 2012).

Pollution policy

Given the current state of knowledge of black carbon, do we need new legislation? There have been suggestions that the mass of black carbon would be a better surrogate than particulate matter for air quality legislation (Grahame and Schlesinger, 2010). That said, there are many other components of particulate matter that also impair health. WHO has recommended we keep PM2.5 as the primary metric for human exposure to particulate air pollution. This, however, does not exclude the option to use black carbon as an additional indicator, especially in environments where black carbon emissions are high.

As black carbon is almost never emitted alone but in a mix of particulate and gaseous pollutants, consideration must be given to the following. Firstly, it is desirable to reduce emissions of PM in general because, as well as its negative health impacts, components of PM contribute to the acidification and eutrophication of ecosystems (Reis et al., 2012). So reductions in PM would lead to multiple benefits. Current policy in the European Union under the National Emissions Ceiling Directive requires no reduction of PM, but will include PM in the revised directive for 2020, with special attention given to emissions reductions from black carbon-rich sources. Secondly, whenever black carbon is emitted, carbon dioxide is emitted too, as both are linked to combustion. This calls for integrated solutions to reduce the climate effects of emissions from combustion. In this context, the use of alternative, renewable energy is particularly important, especially those technologies that eliminate both climate forcers at the same time.

What remains to be done? Currently, there is a great need both in the scientific and policy communities for more information on black carbon emissions and their effects. Efforts to harmonize black carbon measurement methods need to be sustained to make results comparable. One example of this is harmonisation of elemental and organic carbon (EC/OC) analysis under the Aerosols, Clouds, and Trace gases Research Infrastructure Network (ACTRIS), which resulted in the adoption of a new European standard (CEN/TC 264/WG 35). This is important not only for answering scientific questions, but also for monitoring black carbon as an air pollutant. Given its particular emphasis on reducing black carbon emissions, this standard needs to be evaluated through monitoring data. Ideally, implementing systematic black carbon monitoring on a regional scale would also help to improve emission estimates and, as a result, chemistry and climate models. In turn that would improve estimates of black carbon’s climate impact too.

Layers of black carbon found in ice originating from fossil fuel burning have been proposed as evidence for the new geological era, the Anthropocene, highlighting even further the importance of understanding the effects of black carbon and how much progress we still have to make.

References

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