New particle formation from agricultural recycling of organic waste products

The study addresses whether and how gaseous emissions from agricultural recycling of organic waste products (OWPs), particularly sewage sludge, contribute to atmospheric new particle formation (NPF) and secondary organic aerosol (SOA). Aerosols impact health and climate by acting as cloud condensation nuclei and affecting radiation. VOCs from anthropogenic and biogenic sources contribute to ozone and SOA formation. NPF is a dominant source of particle number but is mechanistically uncertain: sulfuric acid is key yet insufficient alone; bases (NH3/amines) and highly oxygenated molecules (HOMs) can stabilize clusters and even form particles without H2SO4. Agriculture is a major emitter of NH3, greenhouse gases, VOCs, and particles, with substantial regional PM2.5 contributions, but SOA formation from agricultural VOCs is poorly quantified. The authors hypothesize that VOCs emitted during land application of sewage sludge, notably indolic compounds such as skatole, together with co-emitted SO2, can drive NPF under atmospherically relevant oxidant levels, independent of ammonia.

Prior work shows: (1) NPF commonly involves sulfuric acid with stabilization by ammonia, amines, or HOMs, and can occur without H2SO4 under certain conditions. (2) Ozonolysis of monoterpenes produces HOMs and SOA with quantified yields; models include these processes but uncertainties remain in organic aerosol formation and aging. (3) Agriculture substantially influences air quality through NH3, VOCs, and aerosols; in some regions agricultural sources dominate PM2.5. VOC emissions from organic fertilizers and waste management (e.g., composting) include sulfur compounds, terpenes, nitrogen-containing compounds, ketones, aldehydes, and aromatics; however, their roles in SOA from agricultural activities are under-studied. (4) Indoles (e.g., skatole) are prominent odorous compounds from animal wastes and sewage treatment and are reactive toward ozone; aqueous-phase ozonation kinetics of skatole are fast. (5) Stabilized Criegee intermediates (SCI) from alkene ozonolysis can oxidize SO2 to H2SO4 significantly, influencing NPF in VOC-rich environments. Existing agricultural aerosol studies focus mainly on ammonia-driven nucleation; contributions from organic waste-derived VOCs remain largely unquantified.

Experiments were conducted in a Teflon-lined poly(methyl methacrylate) simulation chamber (0.03 m^3; 0.55 × 0.27 × 0.2 m) at 293 K and ~40% RH with 5 min residence time, supplied with high-purity dry air (filtered for oxygen, moisture, hydrocarbons). Sewage sludge samples from a wastewater treatment plant (Colmar, France) were placed in a stainless steel tray inside the chamber. After chamber flushing (<1 particle cm−3), VOC emissions were allowed to stabilize, then ozone (~55 ppb) was introduced (no OH/Criegee scavengers; no seed aerosol). Fifteen sewage sludge experiments and two with commercial skatole (Sigma Aldrich, >98%) were performed. Gas-phase measurements: PTR-ToF-MS (HR-Q-PTR-ToF-MS) for VOCs (1 s resolution, m/z 10–510; E/N 132 Td; calibrated), GC-MS (thermal desorption on Tenax TA, Agilent 7890B/5977A), and UHPLC-HRMS for product identification (Orbitrap Q-Exactive, positive ESI). O3 (UV photolysis generator; monitored at inlet/outlet), NO, SO2, NH3, CO2, and H2O were continuously measured with calibrated analyzers. Particle number and size distributions (2.64–64 nm electrical mobility diameter) were measured using an SMPS (TSI 3938 with DMA 3085 and CPC 3788). Wall and tube loss corrections (up to 5% at smallest sizes) were applied. Nucleation rates were estimated following Kulmala et al. Freshly formed aerosols were collected on quartz filters (3–5 h) for surface and bulk chemical analyses: TOF-SIMS (positive/negative ion modes; Bi+ primary, 25 keV; static regime) and L2MS (two-step laser desorption/ionization; Nd:YAG 532/266 nm; m/Δm ~8000). UHPLC employed an Acquity BEH C18 column with acetonitrile/H2O/0.1% formic acid gradient at 0.3 mL min−1, 30 °C. Control experiments tested skatole ozonolysis in an empty chamber and the effects of introducing SO2, NH3, and water vapor (RH up to 90%). Additional experiments used sewage samples that did not emit SO2.

• Sewage sludge exposed to ~55 ppb O3 produced immediate NPF with particles spanning 2–64 nm and particle number concentrations up to ~10^6 cm−3 within <2 min. Derived nucleation rates reached up to 1.1 × 10^6 cm−3 s−1 during NPF (elsewhere in the paper also stated as up to 1.1 × 10^7 cm−3 s−1).
• VOC monitoring identified skatole (m/z 132.087, C8H9NH+) as the major reactive emission; upon O3 exposure its signal rapidly decreased, while oxygenated nitrogen-containing products increased (e.g., m/z 136.075 C8H9NOH+, m/z 164.070 C9H9NO2H+). Gas/particle analyses identified 2-acetyl phenyl formamide as a principal ozonolysis product.
• Estimated gas-phase reactivity of skatole with O3 corresponds to a rate constant on the order of 5 × 10^−15 (units as reported) and an atmospheric lifetime of ~3 min at typical O3, indicating high reactivity compared with indole and α-pinene.
• SO2 is necessary for NPF: sewage samples not emitting SO2 showed no particle formation despite skatole ozonolysis. In skatole-only chamber tests, O3 consumption occurred but no particles formed until SO2 was introduced, at which point NPF began promptly.
• Mechanism: Ozonolysis produces Criegee intermediates that oxidize SO2 to SO3, rapidly yielding H2SO4; sulfuric acid nucleation is enhanced by basic oxidation products of skatole (and skatole itself) that can stabilize acid clusters. SIMS detected sulfate-related ions (SO3−, HSO4−) on particles and in sewage sludge.
• Water vapor (RH increase to 90%) altered product distributions (e.g., increased m/z 136.075 and 164.070; decreased stabilized Criegee signal at m/z 146.060) but did not change particle number concentrations. NH3 addition during skatole ozonolysis did not induce NPF.
• Particle mass and number in chamber: ~2.9 μg m−3 mass and ~5 × 10^6 cm−3 number. Average SOA mass yield from skatole ozonolysis was ~2.45% under these conditions.
• Estimated skatole emissions from land application are on the order of ~50 μg m−2 min−1 (consistent with literature values 4.9–8.3 μg m−2 min−1 for other slurries).
• National-scale implication (France): using measured emission fluxes, spread area, and a 48 h incorporation time, annual particle production from sewage sludge spreading is ~0.94 tons, ~0.03% of total PM10 from agriculture and forestry; because spreading occurs on a few mid-summer days, localized air quality impacts can be significant.

The experiments demonstrate that VOCs emitted during sewage sludge application, specifically skatole, can drive new particle formation when oxidized by ozone in the presence of SO2 co-emitted from sludge. This addresses the gap in understanding agricultural contributions to NPF beyond ammonia: ammonia alone did not trigger NPF in these tests, whereas the organic–SO2 pathway did. The proposed mechanism involves Criegee intermediates formed from skatole ozonolysis oxidizing SO2 to H2SO4, with sulfuric acid clusters stabilized by basic nitrogen-containing oxidation products (and possibly skatole itself), leading to nucleation and initial growth. Chemical analyses of the aerosol phase corroborate sulfate involvement and the incorporation of oxidized skatole products, indicating continued growth via further oxidation and partitioning. The findings imply that emissions from OWP spreading can locally enhance nucleation and particle numbers under tropospherically relevant ozone, potentially affecting cloud condensation nuclei populations, cloud properties, and air quality during spreading events. Though chamber concentrations and time scales differ from ambient conditions, the mechanistic evidence supports inclusion of indole–SO2–ozone chemistry in models of agricultural aerosol formation.

This work identifies sewage sludge as a previously unaccounted source of NPF precursors, showing that skatole emissions, when oxidized by ozone in the presence of SO2, produce rapid nucleation and growth of new particles. The authors elucidate product distributions and propose a mechanistic pathway involving Criegee intermediates oxidizing SO2 to H2SO4, with nitrogenous organic products stabilizing acid clusters. Chamber measurements indicate high nucleation rates, significant particle numbers, measurable SOA yields (~2.45%), and a national-scale particle mass contribution of ~0.94 tons per year in France from sludge spreading, with potentially important short-term local impacts. Future research should include quantum chemical studies of sulfuric acid cluster stabilization by skatole and its oxidation products, larger-scale and longer-residence-time chamber studies to capture multi-generation oxidation and growth, and field measurements during spreading campaigns to quantify ambient impacts and constrain model parameterizations.

• Chamber constraints: small volume and 5 min residence time limit oxidation time and may not capture multi-generational chemistry and growth seen in the atmosphere.
• Concentrations in the chamber can exceed diluted ambient plumes; emissions were constrained by chamber volume, complicating direct comparison to ambient formation rates.
• No seed aerosol or scavengers were used; wall losses assumed constant and corrected but residual uncertainties remain.
• Affinity and stabilization mechanisms of skatole/its products with sulfuric acid are inferred; dedicated quantum chemical calculations were not performed.
• SOA yield estimates assumed particle density of 1.0 g cm−3 and did not include potential heterogeneous reactions on sewage surfaces; yields may be sensitive to mass loading (~3 μg m−3) and reaction time.
• The estimated national particle mass contribution is approximate, based on laboratory fluxes, spread area, and legislated incorporation time; real-world variability (meteorology, sludge composition, practices) may alter emissions.