Ammonia Emerges as Key Driver of Climate-Warming Brown Carbon in China’s Haze

Deep in the winter haze that blankets the North China Plain, an invisible chemical transformation is taking place that could reshape how scientists think about air pollution and climate. A team of Chinese researchers has now shown that nitrogen-containing organic compounds, formed largely through reactions driven by ammonia, are the pivotal light-absorbing molecules governing the optical behavior of atmospheric brown carbon across this densely populated region. The finding, published in Atmospheric Chemistry and Physics, carries a striking policy implication: cutting ammonia emissions, long treated as a secondary concern compared with sulfur dioxide and nitrogen oxides, may be essential for finally clearing the region’s stubborn winter haze and reducing its climate impact.

Brown carbon, often abbreviated BrC, is the enigmatic cousin of black carbon. While black carbon is the familiar soot that absorbs sunlight across the entire visible spectrum, brown carbon is a complex brew of organic molecules that absorbs light selectively, with its absorbance climbing steeply toward ultraviolet wavelengths. This wavelength dependence makes brown carbon both scientifically fascinating and difficult to model. Recent modeling studies suggest that brown carbon accounts for more than twenty percent of the direct radiative effect caused by carbonaceous aerosols, and in some regions, such as the remote tropical upper troposphere, it may even dominate that effect. Beyond its climatic role, brown carbon can suppress the ultraviolet sunlight that drives tropospheric photochemistry, slowing the photolysis of ozone and radicals, and some of its chromophores, including polycyclic aromatic and nitro-heterocyclic compounds, carry significant oxidative potential with consequences for human health.

The new study, led by Can Wu and Gehui Wang of East China Normal University together with colleagues, set out to resolve a puzzle. Over the past decade, China has implemented stringent emission controls in the North China Plain, shifting residential heating from coal to natural gas and electricity, eliminating small coal-fired industrial boilers, and banning straw burning. Given that primary emissions from incomplete combustion are a major source of brown carbon, its levels should have plummeted. Yet strongly light-absorbing brown carbon kept appearing in high concentrations across the region. To understand why, the team conducted simultaneous measurements at five sites during the winter of 2023, from mid-November through the end of December: the cities of Beijing, Tianjin, Handan and Jinan, plus a rural station near Luancheng free from significant industrial influence. At each site, fine particulate matter smaller than 2.5 micrometers was collected every twelve hours onto prebaked quartz filters mounted on rooftops fifteen to twenty meters above the ground.

The spatial patterns the team uncovered were revealing. The average light absorption coefficient at 365 nanometers, a standard metric for water-soluble brown carbon, was highest at the rural Luancheng site, reaching 8.0 plus or minus 4.7 megapercubic megameter, roughly 1.1 to 3.5 times higher than at the urban sites. Beijing recorded the lowest value at just 2.3 plus or minus 1.9. Across the four urban sites, average light absorption had declined by about forty-five percent between 2018 and 2023, and in Beijing the value was only sixteen percent of what it had been a decade earlier, when the city’s average fine particle concentration stood at 158 micrograms per cubic meter compared with roughly a quarter of that during this campaign. The clean air actions, in other words, are visibly working. Yet multi-day haze episodes with hourly peaks reaching 203 micrograms per cubic meter, nearly three times the national grade-II standard of 75, still struck all five sites simultaneously, confirming that regional pollution remains a persistent winter challenge, particularly in the south.

When the researchers normalized absorption by the amount of water-soluble organic carbon, producing the mass absorption efficiency, a distinctly different spatial picture emerged. Jinan recorded the strongest light absorptivity of any site, at 1.40 plus or minus 0.02 square meters per gram, a value on par with severely polluted regions such as Xingtai, Xi’an and Delhi, India, and squarely within the range documented for brown carbon emitted from residential bituminous coal combustion. Tianjin, Luancheng and Handan shared values near 1.1 square meters per gram, closely matching biomass-burning-derived brown carbon, and concentration-weighted trajectory analysis linked their high absorption loadings to regions dense with satellite-detected fire hotspots. Beijing, by contrast, showed the weakest absorptivity at 0.71 square meters per gram, likely a combination of vehicle emissions, which typically produce weakly absorbing brown carbon, and atmospheric aging, since the ratio of the stable benzo[e]pyrene to its easily degraded counterpart benzo[a]pyrene indicated Beijing’s aerosol was the most chemically aged, and aging is known to bleach brown carbon’s color over time.

The pivotal insight came when the team compared clean periods, when fine particle concentrations stayed below 75 micrograms per cubic meter, with haze periods above that threshold. At most sites, the light absorptivity of water-soluble brown carbon increased as haze developed, and this enhancement tracked a parallel rise in the ratio of nitrogen to carbon within the water-soluble organic fraction. A positive correlation between that nitrogen-to-carbon ratio and light absorption held across the entire region, and a high nitrogen-to-carbon ratio also coincided with an elevated ratio of water-soluble to total organic carbon, a fingerprint of secondary formation. Together, these patterns pointed to nitrogenous organic compounds, or NOCs, as the crucial chromophores modulating brown carbon’s optical properties, and to secondary atmospheric production, rather than direct emissions, as their dominant source. A tracer-based apportionment confirmed the point: secondary formation explained roughly 64 plus or minus 21 percent of the water-soluble organic nitrogen in Beijing, comparable to other urban sites and about 1.2 times the rural fraction, and the secondary share grew as pollution intensified.

To identify what drives the formation of this secondary organic nitrogen, the researchers turned to machine learning. A random forest model, trained on seventy percent of the dataset and validated with ten-fold cross-correlation, attributed roughly forty-eight percent of the variance in secondary water-soluble organic nitrogen to ammonium and about twenty-eight percent to aerosol liquid water content, with a generalized additive model independently confirming both factors. The statistical evidence aligned with chemistry: reactions of ammonia or ammonium with atmospheric dicarbonyls such as glyoxal and methylglyoxal are a well-established pathway to light-absorbing nitrogen heterocycles like imidazoles, and laboratory work has shown that the uptake of these dicarbonyls, and the resulting yields of nitrogen-containing products and their oligomers, are significantly higher on neutral ammonium sulfate seeds than on acidic ammonium bisulfate. Thermodynamic calculations indicated the region’s aerosols were indeed dominated by ammonium sulfate and ammonium nitrate. During a humid haze episode in late December, offline aerosol mass spectrometry detected imidazole-related fragment ions that correlated strongly with ammonium, with a coefficient of determination of 0.93, but not with combustion tracers, and the ratio of these fragments to organic nitrogen climbed dramatically as the episode evolved, all pointing to ammonia-driven aqueous chemistry as the dominant formation route under moist conditions.

Aerosol acidity added a further layer of nuance. The mass fraction of secondary organic nitrogen correlated negatively with aerosol pH, suggesting that mildly acidic conditions favor nitrogenous chromophore formation, plausibly because ammonia partitions more readily into acidic particles, and because carbonyl-ammonia reactions are generally acid catalyzed. The team’s partitioning calculations showed that the fraction of ammonia present in the gas phase at pH 3.0 was an order of magnitude higher than at pH 5.0, consistent with this picture. Gas-phase photochemistry contributed too: nitrogen dioxide and ozone together explained over fourteen percent of the variance in secondary organic nitrogen, and nine quantified nitro-aromatic compounds, dominated by 4-nitrophenol and 4-nitrocatechol, were formed roughly forty-six to sixty-four percent by secondary oxidation, correlating with nitrogen dioxide rather than with biomass-burning tracers and displaying a temperature dependence consistent with gas-to-particle partitioning theory.

The broader implications stretch well beyond the North China Plain. A recent modeling study cited by the authors suggests that nitrogen-containing compounds dominate global organic aerosol absorption, accounting for eighteen percent of the direct radiative effect induced by carbonaceous aerosols, and previous work by the same group has shown that ammonia-driven aqueous reactions can generate brown carbon even in lifting air masses aloft. If ammonia is one of the key factors sustaining high loadings of strongly absorbing brown carbon across China’s boundary layer, then the persistently high ammonia loadings from insufficiently controlled agricultural emissions represent a blind spot in current air quality strategy. As the authors conclude, targeted ammonia emission control may prove indispensable for further mitigating wintertime haze and brown carbon pollution in ammonia-rich regions, especially during moist haze episodes when abundant aerosol liquid water opens the door to the aqueous chemistry that turns invisible organic vapors into climate-warming, light-absorbing particles.

Subject of Research: Light absorption and formation mechanisms of water-soluble brown carbon aerosols in the North China Plain

Article Title: Light absorption properties and composition of water-soluble brown carbon in North China Plain: implication for an enhancing role of nitrogenous organic compounds

Article References: Light absorption properties and composition of water-soluble brown carbon in North China Plain: implication for an enhancing role of nitrogenous organic compounds. (n.d.). https://doi.org/10.5194/acp-26-14229-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-14229-2026

Keywords: brown carbon, ammonia, nitrogenous organic compounds, haze, North China Plain, aerosol optics, secondary organic aerosol, aqueous-phase chemistry, imidazoles, nitro-aromatic compounds, air quality, climate forcing