Atmospheric aerosols significantly impact human health and Earth's radiative balance, influencing cloud properties and causing millions of premature deaths annually. New particle formation (NPF) is a crucial source of atmospheric aerosols, but the underlying mechanisms are not fully understood. While sulfuric acid is a key component, its atmospheric concentration is insufficient to explain observed nucleation rates, highlighting the need to identify additional compounds. Studies have shown that base compounds like ammonia or amines, and highly oxygenated molecules (HOMs) from VOC oxidation, can stabilize sulfuric acid clusters and contribute to NPF. Agriculture significantly impacts air quality and climate through emissions of ammonia, greenhouse gases, VOCs, and aerosols. Although agricultural emissions, particularly in regions like the eastern USA and Europe, are a substantial contributor to PM2.5, the contribution of secondary organic aerosols (SOAs) from agricultural activities remains poorly understood. The recycling of organic waste products (OWPs) in agriculture is increasing, offering benefits such as improved soil fertility but also posing potential environmental risks, including atmospheric emissions. While some studies have investigated VOC emissions from organic fertilizers, their contribution to SOAs remains largely unquantified. This research focuses on identifying the molecules involved in new particle formation from sewage sludge, a common OWP used as fertilizer, and elucidating the underlying mechanism. This is, to the best of the authors' knowledge, the first study to observe and quantify such aerosol formation phenomena in an agricultural system.

The literature extensively discusses the importance of atmospheric aerosols and their impact on climate and human health. Numerous studies have focused on new particle formation (NPF) and the role of sulfuric acid, ammonia, amines, and highly oxygenated molecules (HOMs) in this process. However, the understanding of NPF mechanisms remains incomplete, particularly concerning the contribution of agricultural sources. Existing research on agricultural emissions has primarily concentrated on ammonia and greenhouse gases, with limited understanding of secondary organic aerosol (SOA) formation from agricultural precursor gases. Although studies have investigated VOC emissions from organic fertilizers, their impact on SOAs remains poorly characterized. There is a notable gap in our knowledge regarding the specific VOCs emitted from different organic waste products (OWPs) and their role in atmospheric NPF. This research attempts to address this knowledge gap by focusing on the contribution of sewage sludge, a commonly used OWP in agriculture.

Experiments were conducted in a Teflon-coated reaction chamber (0.03 m³). Sewage sludge samples from a sewage treatment plant were introduced into the chamber and exposed to ambient levels of ozone. Gas-phase VOC concentrations were measured using a high-resolution proton transfer reaction time-of-flight mass spectrometer (HR-Q-PTR-ToF-MS), gas chromatography coupled to mass spectrometry (GC-MS), and ultra-high-performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS). Particle number concentration and size distribution were measured using a scanning mobility particle sizer (SMPS). The chemical composition of the generated aerosols was analyzed using two-step laser mass spectrometry (L2MS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), and UHPLC-HRMS. Experiments were also performed using commercially available skatole to test the NPF hypothesis. Ozone, SO2, and NH3 concentrations were monitored throughout the experiments. The data was then analyzed using appropriate software and techniques to determine the key variables influencing particle formation and growth.

Upon exposure to ozone, sewage sludge samples released a significant amount of skatole (C₁₁H₉N), which rapidly reacted with ozone, leading to the formation of oxygenated and nitrogen-containing gas-phase molecules. Particle formation occurred almost instantaneously after ozone injection, reaching a maximum concentration of 10⁶ particles cm⁻³ within 2 minutes, corresponding to a nucleation rate of up to 1.1 × 10⁶ cm⁻³ s⁻¹. The particle number concentration remained relatively constant or slightly decreased at longer reaction times. Ozone was not completely consumed during the experiment, indicating that only a small amount was needed for particle formation. Skatole was identified as the primary VOC responsible for particle formation. Two oxygenated products, m/z 136.075 (C₇H₇NOH⁺) and m/z 164.070 (C₉H₉NO₂H⁺), were observed in the gas phase. Gas chromatography and liquid chromatography analysis identified m/z 164.070 as 2-acetyl phenyl formamide. Experiments with non-SO₂-emitting sewage sludge showed no particle formation, highlighting the crucial role of SO₂ in the NPF process. A binary reaction mechanism involving skatole oxidation products and SO₂, leading to sulfuric acid formation, is proposed. Chemical analysis of the generated aerosols revealed various oxidized and bifunctional species. Experiments using commercially available skatole and ozone confirmed the importance of skatole oxidation products in the NPF process. Adding SO₂ to the skatole-ozone reaction promptly triggered NPF. The addition of water vapor did not significantly affect the aerosol number concentration but influenced the ratio of different gas-phase products. In contrast, adding NH3 did not lead to particle formation. An estimated 0.94 tons of particles are generated annually in France from sewage sludge spreading, representing ~0.03% of total PM₁₀ emissions from agriculture and forestry. SOA yields were estimated to average 2.45%.

The findings demonstrate that skatole, emitted from sewage sludge, plays a key role in NPF in the presence of ozone and SO₂. This study reveals a new pathway for SOA formation from agricultural activities that does not involve ammonia, unlike previously assumed mechanisms. The proposed binary mechanism involving skatole oxidation products and SO₂, leading to sulfuric acid formation, explains the observed NPF. The significant nucleation rates observed in the experiments suggest that, despite the relatively low overall contribution to total PM10 emissions, this source could significantly impact local and regional air quality during the short period of sewage sludge spreading. The fact that this NPF mechanism does not rely on ammonia suggests that previous models might have underestimated the contribution of agricultural sources to atmospheric aerosol concentrations. Further research is needed to determine the broader atmospheric implications of this finding, including the impact on cloud formation and climate.

This study highlights the significant contribution of skatole, a compound released from sewage sludge, to new particle formation (NPF) in the presence of ozone and SO2. This previously unrecognized pathway for secondary organic aerosol (SOA) formation from agricultural sources indicates a potential for underestimation of agricultural contributions to atmospheric aerosol levels. Further research is needed to refine the understanding of this mechanism and its impact on air quality and climate. The relatively low contribution to total PM10 emissions in comparison to other sources should not diminish its impact on local air quality during sewage sludge spreading. Future research could focus on field measurements to validate the findings under real atmospheric conditions, as well as explore the impact of other organic waste products on NPF.

The study is based on laboratory experiments conducted under controlled conditions, which may not fully replicate real-world atmospheric complexities. The estimate of annual particle emissions in France is a rough approximation based on several assumptions and could potentially vary. The chamber experiments may have overestimated particle formation rates due to the confined environment and limited residence time. More research is needed to determine the full extent of the atmospheric implications of the findings and validate the results under more realistic conditions.