Particulate matter warming the atmosphere: A story about black and brown carbon
Particulate matter (PM) is often mentioned in the news due to its health effects; worldwide, it is causing at least 7 million premature deaths per year and it's becoming a major public health issue. PM consists of small particles suspended in the air. The smaller fraction of these particles (diameter below 2.5 microns) are called PM2.5. Tackling its emission would not only benefit public health but would also help mitigate global warming.
Light-absorbing aerosol particles
Black and brown carbon, two kinds of particles included in PM2.5, are a by-product of incomplete combustion. They are emitted by fossil fuel vehicles, thermoelectric power plants, and by open vegetation fires, either agricultural or from forests. Black carbon (BC) is the one with highest warming potential (radiative forcing of 1.1 W/m²) and gets the silver medal on global warming, being second place just after CO2. However, brown carbon (brC) can also warm the atmosphere and can't be ignored. Formed later in the atmosphere, brown carbon's chemistry is more complicated but what we know its largest source are wildfires and avoiding them is critical to reduce its emission.
Figure 1. Aerosol absorption properties: Mass absorption cross-section and absorption Ångström exponent. Figure by Jorge Saturno. An extended version can be found at https://doi.org/10.6084/m9.figshare.5993377.v1.
In figure 1, the absorption properties of different kinds of aerosol particles are presented. Additionally to BC and brC, dust can also absorb radiation but in a much smaller scale. Two properties are crucial and I would like to mention them. The mass absorption cross section (MAC), which is the amount of radiation that 1 g of the aerosol can absorb; and the other is the absorption Ångström exponent (AAE), which is related of how much this radiation depends of the wavelength of light. The closer AAE gets to 1.0, the more independent is absorption of the light wavelength. That is the case of BC, which can absorb in the whole visible spectrum -and also in the infrared-, therefore the black color. The case of brC is different, it can absorb more efficiently in the shorter wavelengths, having the brown color, and having an AAE of up to 7. The warming potential of these particles increases with MAC, (see Y axis in Fig. 1). BC particles coated with other material like sulfates, nitrates or organics, exhibit a higher MAC value because more radiation is focused toward the BC part of the particle — especially if it is located in the center.
Observations in the Amazon rain forest
In 2012, the Amazon Tall Tower Observatory (ATTO), a Brazil-Germany cooperation research facility in the middle of central Amazonia, was built to measure gases and aerosols over the forest. The aerosol absorption measurements were done to understand the impact of agricultural and wildfire emissions over the forest and also to investigate the forest background conditions in terms of absorption. In the beginning, we found that during the fire season aerosol absorption increased significantly compared to the "clean" or rainy season. Nevertheless, the characteristics of the dark aerosol arriving during the fire season were not so trivial and we found some surprises. First, the aerosol had very small amount of brC (< 20%) and BC was predominant. We realized the air masses were so old that brC was being bleached on its way, becoming white and therefore losing its absorption properties — losing its brown color. There is strong evidence that brC is quickly photo-oxidized by ozone and OH radicals in the atmosphere. Another interesting fact was that those old air masses were not only containing BC from fires in Brazil, but were rather a mixture of Brazilian agricultural fire and African savanna fire emissions. African emissions were arriving early in the polluted season and weeks later were masked by the Brazilian emissions, which tended to predominate at the end of the season.
Figure 2. Black and brown carbon contribution to aerosol absorption at 370 nm wavelength measured at the ATTO site from 2012 to 2017.
El Niño messed it up
Our measurements drastically changed in 2015. El Niño Southern Oscillation (ENSO) caused dry and warmer conditions for several months that spanned even during part of what was supposed to be the rainy season of 2016. The "normal" trend we observed of aerosol absorption in the past years was gone. Instead, much higher numbers were measured (see. Fig. 2). However, the most striking observation was brC not being less than 20% contribution anymore but increasing up to 47% for some episodes. Fire emissions were likely close to home. The brC that was oxidized on the way to ATTO the years before, was this time there — it was freshly emitted. Satellite observations confirmed a higher occurrence of fire in Amazonia. Soon after El Niño was gone, aerosol absorption came back to its normal trend. El Niño 2015 served as a window to the future: A warmer and drier future that could take place if we continue warming our planet. A higher abundance of aerosol in the atmosphere would block a larger part of the incoming radiation and therefore reduce forest primary production. This, added to a warming climate, will affect the resilience of the Amazonian ecosystems. The question is how long we have to avoid that tragedy. According to the IPCC, just a little more than a decade.
Reference
Saturno, J., Holanda, B. A., Pöhlker, C., Ditas, F., Wang, Q., Moran-Zuloaga, D., Brito, J., Carbone, S., Cheng, Y., Chi, X., Ditas, J., Hoffmann, T., Hrabe de Angelis, I., Könemann, T., Lavrič, J. V., Ma, N., Ming, J., Paulsen, H., Pöhlker, M. L., Rizzo, L. V., Schlag, P., Su, H., Walter, D., Wolff, S., Zhang, Y., Artaxo, P., Pöschl, U., and Andreae, M. O.: Black and brown carbon over central Amazonia: long-term aerosol measurements at the ATTO site, Atmos. Chem. Phys., 18, 12817-12843, https://doi.org/10.5194/acp-18-12817-2018, 2018.
