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Brown carbon aerosols warm the air more than previously thought.

Brown carbon aerosols warm the air more than previously thought

10 August 2023

Some organic particulates found in wildfire smoke absorb visible solar radiation but are unaccounted for in climate models.

The orange skies of New York City in June 2023.
New York City’s air quality reached unsafe levels for several days in June because of smoke from Canadian wildfires. Credit: Aelthemplaer, Wikimedia Commons/CC BY 4.0

The 2023 Canadian wildfire season has become the most destructive in the country’s history. Some 13 million hectares of forest have burned, which is six times as much as the long-term seasonal average. The smoke from those wildfires has led to unhealthy air quality across the US and even as far as Europe this summer. It’s composed of about 95% organic carbon, made of various hydrocarbons produced from low-temperature combustion, and just 3% black carbon—a pure elemental particulate produced from high-temperature combustion.

Black carbon is relatively nonreactive and lingers in the atmosphere for a few days to a few weeks, long enough for it to absorb incoming shortwave solar radiation and heat the atmosphere on a global scale over the industrial era by about 1 W/m2. (In comparison, the total radiative forcing by people over the same time period is about 2.7 W/m2.) Less is known about organic carbon. Most of it is photoreactive and stays in the atmosphere for hours. So it contributes a negligible amount of atmospheric warming.

So far, organic carbon has usually been ignored in climate models. But recently, atmospheric chemists have discovered through lab experiments that 5–15% of the organic carbon aerosols, which are also called brown carbon aerosols, are nonvolatile and insoluble, which means they could persist long enough in the atmosphere to absorb visible radiation.

Now Rajan Chakrabarty and Rohan Mishra, of Washington University in St Louis, and their colleagues have analyzed measurements of brown carbon aerosols from many wildfire smoke plumes across the western US. They found that the particles are long-lived enough to absorb solar radiation at visible wavelengths, which has led them to conclude that climate models should be revised to account for the additional source of warming.

The data come from the Fire Influence on Regional to Global Environments and Air Quality field campaign, sponsored by NASA and NOAA to collect bulk and particle-scale measurements of smoke plumes from the 2019 wildfire season using ground-based instruments and an aircraft. At blue (405 and 488 nm) and red (664 nm) wavelengths, the organic aerosols absorb roughly two-thirds to three-quarters of the radiation. The microscopy image below shows what one of the brown carbon aerosols looks like, although how they form in combustion systems remains a mystery.

A gray ball that is a particle of brown carbon under the microscope.
Credit: R. K. Chakrabarty et al., Nat. Geosci. 16, 683 (2023)

The researchers tested the reactivity of the brown carbon aerosols by oxidizing them with hydroxyl and nitrate radicals using an oxidation flow reactor, which mimics the diurnal cycle of atmospheric aerosols. After the equivalent of three days, the researchers found no decrease in light absorption relative to baseline conditions. The photochemical reactions that remove the other types of organic carbon found in wildfire smoke, therefore, appear to have a limited effect on the brown carbon. The researchers suspect that the relatively high viscosity of brown carbon, relative to black carbon, limits its reactivity with oxidants in the atmosphere.

Quantifying the responses of various aerosols to photochemical reactions is one of the challenges of accurately estimating the total radiative forcing of aerosols on Earth’s climate. The findings by Chakrabarty, Mishra, and their colleagues offer a bit more information on the longevity of some organic aerosols, and incorporating them into future climate-model simulations should help tease out what their relative contribution may be to the total radiative forcing on the climate. (R. K. Chakrabarty et al., Nat. Geosci. 16, 683, 2023.)

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