Possible warming effect of fine particulate matter in the atmosphere

The study addresses how fine particulate matter, especially secondary inorganic aerosols like sulfate, influences Earth’s radiation budget via longwave interactions. While anthropogenic aerosols are known to cool climate by scattering shortwave solar radiation, evidence suggests coarse mineral dust can warm the atmosphere in the infrared. The authors explore whether fine, nominally non-absorbing inorganic aerosols (e.g., ammonium sulfate) can also produce positive radiative forcing through infrared absorption resonances within atmospheric windows. Given that climate models may underestimate coarse dust and that ambient particles are chemically mixed (e.g., dust cores with sulfate coatings), understanding size- and composition-dependent radiative behavior is critical. The purpose is to quantify absorption and scattering of sulfate particles (homogeneous and SiO2/sulfate core/shell) across sizes, evaluate resonance effects in the 8–13 µm and 16–24 µm atmospheric windows, and estimate the resulting changes in atmospheric longwave power density and transmittance.

- Anthropogenic aerosols exert net negative radiative forcing via aerosol–radiation (~ -0.35 W m^-2) and aerosol–cloud (~ -0.45 W m^-2) interactions, whereas greenhouse gases contribute ~ +2.83 W m^-2. - Secondary inorganic aerosol (SIA), primarily sulfate and nitrate, constitutes major fractions of PM2.5/PM10. Previous work often concluded aerosol longwave effects are small due to decreasing opacity at longer infrared wavelengths. - Natural PM sources (marine aerosol, mineral/Saharan dust, smoke) can enhance greenhouse effect; recent studies show atmospheric dust is coarser and more abundant than represented in climate models, implying underestimated warming from coarse dust and a need to revise dust size and vertical distributions and IR optical properties. - Ambient particles are often internally mixed; TEM reveals mineral dust cores coated with sulfate and other soluble materials, potentially altering radiative properties versus homogeneous particles. - Atmospheric windows (8–13 µm, IRW1; 16–24 µm, IRW2) are key pathways for terrestrial radiative cooling; aerosols within these windows can perturb Earth’s thermal emission. Radiative cooling applications highlight the sensitivity of thermal radiation to spectral absorption in these bands. - This context motivates detailed, wavelength-resolved analysis of particle size, composition, and structure effects on longwave radiative transfer, beyond simplified k-distribution approaches that may miss resonance features.

- Optical data: High-resolution FTIR transmittance spectra for (NH4)2SO4, Na2SO4, NH4NO3, and NaNO3 obtained from the Sigma FT-IR library. Example optical constant for (NH4)2SO4 at 9 µm: 0.99 + 1.7i; SiO2 extinction coefficient at 9 µm k = 0.759. - Mie theory: Computed absorption cross-section (σa) and scattering cross-section (σs) for spherical (NH4)2SO4 particles with diameters 0.1–10 µm over wavelengths 4–20 µm using custom MATLAB code. Absorption efficiency Qa = σa normalized by particle geometric cross-section. Forward/backward scattering efficiencies (FWQ, BWQ) computed; BWQ/(BWQ+FWQ) analyzed versus particle size. - 3D-FDTD simulations: RSoft FullWAVE used to simulate electromagnetic fields and absorption rates inside/around particles for homogeneous sulfate and SiO2/sulfate core/shell spheres. Core/shell geometry: core/shell radius ratio 0.5; total diameters Dp = 0.04, 0.5, 2, 8 µm (core radii 0.01, 0.125, 0.5, 2 µm). Outputs include normalized electric field intensity (|E/E0|^2) and normalized absorption rate per unit volume (Pabs = 0.5 ω ε0 |E/E0|^2). - Particle number effect: For equal mass concentration, particle number ∝ Dp^-3 and surface area ∝ Dp^2. Define Ep = Dp^-3 to represent number effect; use Qa × Ep as a metric of size-dependent absorption capacity under the same mass burden. - Radiative transfer and cases: Line-by-line (LBL) radiative calculations used to evaluate aerosol absorption resonance effects across 4–20 µm and to estimate atmospheric longwave power density with and without PM. Six cases span homogeneous sulfate (cases 1–3) and SiO2/sulfate core/shell (cases 4–6) at concentrations 20, 50, and 1,180 µg m^-3 (WHO guideline-relevant levels and severe pollution). Thermal radiation power density in the atmosphere (Patm) and its change ΔPatm (W m^-2) computed. - Atmospheric transmittance: Modeled clear-sky transmittance and changes due to PM (homogeneous and core/shell) across wavelengths, highlighting impacts within IRW1 and IRW2. - Conceptual experiment: Prepared KBr tablets containing 0.1 wt% (NH4)2SO4 (particle sizes ~2–20 µm). Characterized by SEM; emissivity spectra (2.5–25 µm) measured via FTIR with emissivity adapter and calibrated using a blackbody furnace. Monitored cooling rates under different sample shielding areas using K-type thermocouples and a high-speed recorder to qualitatively assess thermal radiation absorption effects. - Assumptions: Clear atmosphere baseline; calculations preclude geographic/seasonal variations. Mixed particle states simplified to homogeneous sulfate and idealized SiO2/sulfate core/shell structures.

- Atmospheric windows and terrestrial emission: For surface temperatures 250, 300, and 350 K, λmax = 11.51, 9.50, and 8.25 µm, respectively. Corresponding fractions of thermal radiation are concentrated in IRW1 (8–13 µm): ~31%, 38%, 41%, and in IRW2 (16–24 µm): ~29%, 24%, 20%. - Sulfate absorption bands: Strong ν3(SO4^2−) absorption within IRW1 for (NH4)2SO4 and Na2SO4 indicates potential to block thermal emission in atmospheric windows. - Size-dependent absorption (homogeneous sulfate): Qa spectra exhibit three bands (~7, 9, 16 µm), with strong electric dipole resonances near 9 µm (Dp ~2 µm) and 16 µm (Dp ~4 µm). Maximum Qa in IRW1 occurs at Dp = 2 µm with Qa = 2.46 near 9 µm. Average Qa in IRW1 and IRW2 increases with particle size due to higher-order resonances and band broadening. - Mass-normalized absorption capacity: Considering particle number effect Ep = Dp^-3 at equal mass, Qa × Ep peaks at Dp ≈ 1.4 µm, indicating the overlapping 1–3 µm size range (between fine and coarse modes) contributes most to thermal IR absorption per unit mass. PM2.5 exerts greater absorption influence than PM10−2.5 under equal mass burden. - Core/shell mixtures (SiO2 core, sulfate shell): Shell dominates optical response; for Dp = 2 µm, Qa,max = 2.54 at 8.97 µm, similar to or slightly exceeding homogeneous sulfate (Qa = 2.46). For thin shells (Dp = 0.04–0.5 µm), fields partially penetrate into the SiO2 core, but lower SiO2 extinction yields small core absorption; for larger particles (Dp = 2–8 µm), penetration depth of sulfate is small, so absorption occurs mostly in the sulfate shell. - Scattering is minor in longwave: In the 4–20 µm regime, scattering is much smaller than absorption. Backward scattering fraction BWQ/(BWQ+FWQ) decreases with size; for Dp < 5 µm, BWQ is more important and tends to redirect thermal radiation back toward the surface, further reducing outgoing longwave emission. - Longwave warming effect (ΔPatm): Modeled increases in atmospheric longwave power density due to PM are positive across cases. Homogeneous sulfate: ΔPatm = 1.19, 2.83, 27.01 W m^-2 at 20, 50, 1,180 µg m^-3, respectively (Patm = 324.61, 326.24, 350.43). Core/shell: ΔPatm = 1.26, 3.02, 32.46 W m^-2 at 20, 50, 1,180 µg m^-3, respectively (Patm = 324.68, 326.43, 355.87). Severe pollution yields substantial longwave positive forcing, implying a greenhouse-like effect from sulfate-rich PM. - Atmospheric transmittance: Presence of sulfate reduces transmittance, especially around ~9 µm within IRW1; core/shell mixtures reduce transmittance more than homogeneous sulfate at the same high concentration (1,180 µg m^-3). - Conceptual experiment: Samples with larger sulfate-containing shielding area exhibited slower cooling, consistent with enhanced absorption of thermal radiation. Measured emissivity varied with particle size, corroborating size-dependent optical effects. - Overall: Even fine, non-light-absorbing inorganic aerosols like sulfate can induce warming by absorbing and re-emitting terrestrial radiation at resonance within atmospheric windows, challenging the view that fine sulfate solely cools via shortwave scattering.

The findings demonstrate that sulfate aerosols, despite weak visible/near-IR absorption, can significantly interact with terrestrial longwave radiation through resonant absorption within the 8–13 µm and 16–24 µm atmospheric windows. Size-dependent resonances, especially around Dp ~2 µm near 9 µm, maximize absorption efficiency and can yield positive longwave radiative forcing. When accounting for realistic mass burdens and particle number effects, particles in the 1–3 µm overlap region contribute disproportionately to thermal IR absorption per mass, implying that fine-mode sulfate can act as a greenhouse-like aerosol under certain conditions. Internal mixing (SiO2 cores with sulfate shells) further enhances or maintains strong longwave absorption due to the dominant sulfate shell, with reduced atmospheric transmittance relative to homogeneous particles. These mechanisms reduce the efficacy of the atmospheric windows as cooling channels, thereby warming the near-surface environment. The results suggest that climate impact assessments and geoengineering proposals involving sulfate must incorporate longwave aerosol-radiation interactions, resonance behavior, size distributions, and mixing states. Moreover, models that underestimate coarse and supermicron aerosol burdens may misrepresent the balance between shortwave cooling and longwave warming, necessitating improved aerosol size and optical property representations.

This work shows that sulfate aerosols can behave as “greenhouse particles,” producing positive longwave radiative forcing via resonant absorption within atmospheric windows. Homogeneous sulfate particles around 2 µm exhibit Qa ≈ 2.46 near 9 µm, and SiO2/sulfate core/shell particles show Qa ≈ 2.54, while mass-normalized absorption capacity peaks near 1.4 µm. Radiative transfer calculations indicate increased atmospheric longwave power density (ΔPatm up to ~32 W m^-2 under severe pollution), and atmospheric transmittance decreases markedly in IRW1 with increasing sulfate burden. Conceptual experiments support the thermal absorption and size dependence of emissivity. These results underscore the need to include longwave aerosol interactions, resonance effects, and mixing states in climate assessments and geoengineering evaluations. Future work should extend analysis to additional PM species and complex aging processes, and undertake quantitative comparisons with climate models to assess regional and global impacts.

- Simplified particle mixing states: Analyses assume homogeneous sulfate and idealized SiO2/sulfate core/shell geometries; real atmospheric particles have complex, variable mixing states and morphologies. - Model scope: Focus on IR resonance properties and longwave aerosol radiative effects without full coupling to climate models; quantitative climate impacts remain to be assessed. - Environmental conditions: Calculations assume clear atmosphere and do not account for geographic, vertical, or seasonal variability in aerosol distributions or meteorology. - Optical data and spectral treatment: Use of specific optical constants and line-by-line spectral methods captures resonances but broader uncertainties in optical properties (humidity, phase, temperature) and potential simplifications (e.g., k-distribution limitations) remain. - Experimental design: Laboratory “conceptual” experiments (KBr tablets with 0.1 wt% sulfate) qualitatively support findings but are not direct analogs of atmospheric conditions. - Size distribution and burden: While size effects and number scaling are considered (Ep = Dp^-3), real atmospheric size distributions and mixing with other species introduce additional uncertainties explored only briefly in supplementary analyses.