Mitigating black carbon emissions: Key drivers in residential and coke/brick productions

Black carbon (BC) is a short-lived climate forcing aerosol that can strongly absorb solar radiation, leading to climate warming. As a major component of fine particulate matter (PM_2.5), BC also adversely affects human health. Multiple epidemiological studies have shown that BC is significantly more toxic than other PM components. Reducing BC emissions is crucial for mitigating global warming and improving regional air quality. However, existing BC inventories have significant uncertainties due to data limitations and other factors.

This study improved China's BC emission inventory by updating a series of activity intensity data and emission factor data from field measurements. It identified and quantified the contributions of various driving factors of major BC emission sources in China. The inventory was extended to 2019, with emission source categories refined to 146 types, a spatial resolution of 0.1°×0.1°, and a monthly temporal resolution. Benefitted from the new information and structural decomposition analysis for critical factors driving long-term variation of BC emissions in the past and coming decades, the inventory has been substantially improved and will provide stronger scientific support to policy implementation in BC pollution mitigation.

As shown in Figure 1, which presents the temporal variation in BC emissions from major sources. The total annual emission was 1.11 (0.90-1.41) Tg in 1962, increased monotonically to a peak value of 3.03 (1.92-5.15) Tg in 1995, and declined thereafter to 1.02 (0.75-1.52) Tg in 2019. In 1962, residential solid fuel combustion dominated the BC emission sources, accounting for 83.2% of the total emissions. However, by the emission peak year of 1995, the relative contribution of the residential sector had significantly declined to 34.7%, while the industrial sector’s contribution had risen from 7.8% in 1962 to 56.4% in 1995, primarily due to the substantial increase in coke (32.0%) and brick (19.5%) productions. By 2019, the most significant sources of BC emissions were coke production (18.7%), residential coal consumption (15.4%), brick kilns (12.2%), residential biomass fuels (10.8%), and diesel vehicles (8.0%). In sum, the predominant contributors were the consumption of solid fuels in the residential sector and industrial coke and brick production. These three sources accounted for 89.4%, 83.6%, and 57.4% of the total emissions in 1962, 1995, and 2019, respectively.

This study quantitatively analyzed the driving factors for the three major emission sources mentioned above. For residential emissions, the dominant negative factors were residential energy mix transition and stove switching, particularly the transition from coal and biomass fuels to cleaner energy sources. While most factors influenced both rural and urban areas in the same direction, urbanization had a positive effect in urban areas and a negative effect in rural areas, leading to slightly negative net effects on the overall trend.

In 1996, the Coal Law was enacted, and beehive coke began to be banned. Despite beehive coke accounting for only 10.1% of the total production over these 60 years, its contribution to the total BC emissions from coking reached as high as 65.3% due to its extremely high emission factors and lack of control measures. Consequently, the overall temporal trends of coke production and BC emissions exhibited notable differences. The former depended primarily on industrial-scale coke production, while beehive coke ovens predominantly governed the latter. The major drivers behind the changes in BC emissions from coke production were quantified. The key drivers include coke production, the phasing out of beehive coke ovens, and various abatement measures.

There were two general peaks in brick production, with peak years around 1995 and 2016. The significant difference in the overall temporal pattern between the productions and emissions was the relatively low emissions during the second wave. This difference was caused by reduced EFs in recent years due to brick kiln upgrading and installing end-of-pipe dust removal facilities. The positive driver was the brick production on BC emissions. The most important negative driver was the replacement of annular kilns with tunnel kilns. In comparison, dust removal had a much smaller impact on emission reduction since end-of-pipe facilities cannot effectively mitigate strong BC emissions from fugitive sources.

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