In a groundbreaking study set to transform our understanding of atmospheric chemistry, researchers have identified a novel photolytic process that significantly contributes to the formation of molecular chlorine (Cl2) in the atmosphere. The study, led by Li, S., Wang, Y., Liu, Y., and their team, reveals the photolytic oxidation of ammonium chloride (NH4Cl) as an unexpected and previously underappreciated source of Cl2. Published in Nature Communications in 2026, this discovery challenges long-held assumptions about chlorine chemistry’s contributions to air quality and atmospheric reactivity, with potentially far-reaching implications for environmental science and climate models.
The photolytic oxidation process detailed by the researchers sheds light on the intricate chemical interactions occurring in the lower atmosphere, where ammonium chloride—a common particulate matter component derived from natural and anthropogenic sources—undergoes transformation under the influence of sunlight. Ammonium chloride is prevalent in urban air, particularly in regions with significant industrial emissions combined with agricultural activities. Prior to this study, the role of NH4Cl in the atmospheric chlorine cycle had been largely speculative, with minimal mechanistic understanding.
At the core of this investigation is the revelation that ultraviolet (UV) radiation from sunlight triggers a photolysis reaction within atmospheric ammonium chloride particles. This process initiates the oxidation of NH4Cl, ultimately releasing molecular chlorine gas (Cl2) into the surrounding atmosphere. Cl2 is a highly reactive species known to participate in various photochemical cycles, including the destruction of ozone and the formation of reactive chlorine radicals (Cl·). The presence of Cl2 thus has profound implications for oxidative processes that govern air pollution and secondary aerosol formation.
To elucidate the underlying reaction mechanisms, the research team employed state-of-the-art spectroscopy techniques combined with atmospheric simulation chambers. These methods allowed for controlled replication of atmospheric conditions while enabling real-time measurements of chemical species evolution. The team observed that upon exposure to simulated solar radiation, NH4Cl exhibits a distinctive absorption feature triggering bond cleavage events. This results in the formation of reactive intermediates that subsequently recombine or react further, culminating in the evolution of Cl2 gas.
Previous models of atmospheric chlorine sources predominantly focused on sea salt aerosols and anthropogenic emissions such as industrial chlorine compounds. However, the identification of ammonium chloride as a photolytic Cl2 source introduces a paradigm shift, highlighting the need to revisit atmospheric chlorine budgets. This finding is especially critical in continental and urban environments where NH4Cl contributes significantly to particulate matter, potentially making these regions hotspots for chlorine radical-driven photochemistry.
From a chemical perspective, the reaction pathway involves the photodissociation of chloride ions (Cl−) associated with ammonium cations under UV influence. The photolytic energy effectively liberates chlorine radicals, which further oxidize in the presence of atmospheric oxidants, including ozone and hydroxyl radicals, eventually forming Cl2. Moreover, the researchers note that this process is sensitive to relative humidity and ambient temperature, factors that modulate particle phase state and reaction kinetics.
The influence of this photolytic oxidation on atmospheric oxidative capacity is substantial. Chlorine radicals generated from Cl2 photolysis can initiate chain reactions that degrade volatile organic compounds (VOCs), thereby contributing to the formation of ozone and fine particulate matter. This enhanced reactivity complicates urban smog formation and may exacerbate health risks associated with air pollution. Therefore, understanding this new Cl2 source is vital for accurate air quality forecasting and policy development aimed at emission reductions.
Beyond urban air quality, this discovery opens intriguing questions about broader climatic effects. Chlorine radicals play a known role in ozone depletion, particularly in polar stratospheric clouds. While the newly identified NH4Cl photolysis is primarily a tropospheric phenomenon, the additional chlorine source could influence regional ozone balances and reactive nitrogen chemistry. Researchers emphasize the necessity for integrating these photolytic processes into global atmospheric chemistry models to better predict climate feedbacks and pollutant dynamics under changing environmental conditions.
The implications for environmental monitoring are immediate and practical. Traditional satellite and ground-based sensors detect chlorine-containing compounds, but distinguishing their sources remains challenging. With the newfound knowledge of NH4Cl photolysis as a Cl2 generator, atmospheric chemists can refine interpretative models of observed chlorine data. Enhanced species-specific detection will aid in pinpointing pollution sources, allowing for more targeted environmental management strategies.
This study also underscores the importance of multidisciplinary research approaches in atmospheric sciences. By integrating physical chemistry, photophysics, and atmospheric modeling, the authors provide a comprehensive framework connecting molecular-level reactions to large-scale environmental phenomena. The research exemplifies how fundamental insights about seemingly simple compounds like ammonium chloride can unravel complex atmospheric behaviors with significant societal relevance.
Notably, the research calls attention to evolving atmospheric conditions influenced by anthropogenic activities, such as increased fertilizer use and fossil fuel combustion. These factors elevate atmospheric concentrations of ammonium and chloride ions, potentially amplifying the NH4Cl photolysis effect. Consequently, human-induced changes in emissions may inadvertently heighten reactive chlorine burdens and associated environmental consequences, emphasizing the urgency of sustainable emission practices.
Future research directions include quantifying geographic and seasonal variations of this photolytic process and its interaction with other atmospheric constituents. Further experimental campaigns in diverse atmospheric settings will validate the observed mechanisms and refine reaction rates for model integration. Additionally, exploring mitigation strategies to reduce ammonium chloride aerosol formation or transformation could provide new avenues to curb chlorine-induced air pollution.
In conclusion, the study by Li, S., Wang, Y., Liu, Y., and colleagues revolutionizes the understanding of atmospheric chlorine chemistry by unveiling photolytic oxidation of ammonium chloride as a critical source of molecular chlorine. This advancement challenges established chlorine source paradigms, emphasizing the dynamic and multifaceted nature of atmospheric chemical cycles. As the study’s findings ripple across environmental science and policy spheres, they herald a new chapter in battling air pollution and safeguarding climate integrity.
Subject of Research: Atmospheric chemistry; photolytic processes; formation of molecular chlorine; ammonium chloride oxidation; air pollution and reactive chlorine species.
Article Title: Photolytic oxidation of ammonium chloride as a source of Cl2 in the atmosphere.
Article References: Li, S., Wang, Y., Liu, Y. et al. Photolytic oxidation of ammonium chloride as a source of Cl2 in the atmosphere. Nat Commun (2026). https://doi.org/10.1038/s41467-026-70941-y
Image Credits: AI Generated
DOI: https://doi.org/10.1038/s41467-026-70941-y
Keywords: Photolytic oxidation, ammonium chloride, molecular chlorine, atmospheric chemistry, Cl2 source, reactive chlorine species, air pollution, ultraviolet radiation, aerosol photolysis, atmospheric reactive processes
