Global warming, exacerbated by increasing carbon emissions, is causing more frequent and intense extreme weather events. While particulate matter (PM) from human activities pollutes the air, it also affects Earth's radiation budget, counteracting the warming effect of greenhouse gases by scattering shortwave solar radiation. Previous studies have established that anthropogenic PM, including secondary inorganic aerosols (SIAs) such as sulfate and nitrate, primarily exert a cooling effect. However, these studies often focused on the scattering of shortwave radiation and may have underestimated the absorption of longwave radiation. Natural sources of PM, like mineral dust, have been shown to enhance the greenhouse effect, with recent research highlighting the underestimation of the warming effect of coarse dust particles in climate models. The composition of atmospheric PM is complex, often involving mixtures of different particle types, such as mineral dust coated with sulfates. These mixtures and the sizes of particles significantly influence their radiative properties. This study aims to investigate the thermal radiation properties of fine sulfate particles and their mixtures with silica (SiO2), focusing on their absorption and emission behavior in the atmospheric infrared (IR) window regions (8-13 µm and 16-24 µm), where Earth's thermal radiation is emitted to space. Understanding these effects is crucial for accurately assessing the net radiative forcing of aerosols and improving climate models.

Existing literature demonstrates that anthropogenic PM, primarily secondary inorganic aerosols (SIAs) such as sulfates and nitrates, generally exerts a cooling effect by scattering shortwave solar radiation. However, studies have shown that the absorption of longwave radiation by sulfate is not negligible. The impact of natural sources of PM, like mineral dust, on the greenhouse effect has also been investigated, revealing that coarse dust particles cause a significant warming effect. Recent research has highlighted the underestimation of coarse dust in current climate models, underscoring the need for improved representation of dust size distribution, vertical distribution, and infrared optical properties. The mixing of different PM types in the atmosphere, for example, mineral dust coated with sulfates, leads to changes in their radiative impact. Transmission electron microscopy (TEM) studies provide valuable insights into the size, structure, and composition of individual fine particles, which are vital for understanding their climatic effects. Theoretical analyses of the relationship between PM mixtures, particle sizes, absorption coefficients, and wavelengths are necessary for accurate climate modeling. The atmospheric window, where the atmosphere is transparent to thermal radiation, is crucial for the Earth's heat dissipation. The impact of PM on the absorption and scattering within this atmospheric window needs further exploration.

This research employed Mie scattering theory and three-dimensional finite-difference time-domain (3D-FDTD) simulations to investigate the thermal radiation properties of sulfate and SiO2/sulfate core/shell mixture particles within the atmospheric IR window. The study focused on the electric dipole (ED) and magnetic dipole (MD) resonance behaviors of these particles. The 3D-FDTD method was used to simulate the electromagnetic fields inside and around PM particles of various sizes and core/shell structures. For simplification, the calculations of thermal radiation power density changes used World Health Organization air quality guidelines values, neglecting geographical and seasonal variations. The study considered (NH4)2SO4 as a representative SIA due to its strong absorption band and scattering in the IRW1. Mie theory was used to calculate the absorption cross-section (Ca) and absorption efficiency (Qa) for (NH4)2SO4 spheres with diameters ranging from 0.1 to 10 µm and wavelengths from 4 to 20 µm. The analysis considered the particle number effect (Ep) to account for the number of particles at the same mass concentration, using Ep = Dₚ⁻³. To analyze the impact of core-shell structures, the 3D-FDTD method simulated the electromagnetic field and absorption rates for SiO2 cores coated with (NH4)2SO4 shells, with various core/shell ratios and particle sizes. The scattering characteristics of sulfate particles were also calculated using Mie theory, considering both forward and backward scattering efficiencies. The radiative transfer model was used to estimate the absorptance of PM in the atmosphere and the warming effect (Patm) by calculating the thermal radiation with and without PM. A conceptual laboratory experiment was conducted to confirm the thermal radiation behavior of (NH4)2SO4 using samples with different particle sizes and shielding areas, measuring the cooling rates and thermal emissivity.

The study found that sulfate exhibits strong IR absorption, particularly in the atmospheric window (8-13 µm), leading to positive radiative forcing and a warming effect. (NH4)2SO4 particles, with their strong absorption bands, behave like "greenhouse PM." The absorption efficiency (Qa) is significantly affected by particle size, with particles around 2 µm diameter showing the maximum Qa (2.46 for homogeneous (NH4)2SO4 and 2.54 for SiO2/sulfate core/shell) at approximately 9 µm wavelength. Considering both particle size and number, the strongest absorption capacity was observed for (NH4)2SO4 particles of approximately 1.4 µm diameter. Particle size also impacts scattering efficiency; smaller particles exhibit greater backward scattering, potentially decreasing thermal emission to space. The 3D-FDTD simulations of SiO2/sulfate core-shell particles demonstrated that the optical properties are primarily determined by the (NH4)2SO4 shell, with the SiO2 core exhibiting minimal impact due to the high absorption and low penetration depth of (NH4)2SO4. The analysis of the atmospheric transmittance showed a decrease in transmittance in the IRW1 with increasing PM concentration, especially for sulfate and core-shell mixtures, emphasizing their absorption of outgoing longwave radiation. The conceptual laboratory experiment confirmed the increased temperature of surroundings due to PM thermal radiation absorption. The thermal emissivity is significantly affected by the (NH4)2SO4 particle size.

This study's findings challenge the prevailing view of fine sulfate particles solely as a cooling agent. The strong IR absorption of sulfate, especially in the atmospheric window, indicates its potential to warm the atmosphere. The size-dependent absorption and scattering behavior highlights the importance of accurate particle size distribution in climate modeling. The similar absorption efficiency of homogeneous and core-shell particles for a diameter of 2 µm suggests that the simplified homogeneous model may be adequate for certain particle sizes. However, the core-shell model is crucial for understanding the behavior of aged PM. The results emphasize the need to refine climate models to accurately incorporate the warming effects of fine sulfate and mixed PM particles. Further research should consider the interaction of PM with clouds and other atmospheric constituents to gain a more comprehensive understanding of their overall climate impact. This work provides a valuable contribution to the ongoing debate about the role of aerosols in climate change, suggesting that their effect is more complex than previously thought.

This study demonstrates the significant IR absorption of sulfate aerosols, particularly in the atmospheric window, causing a warming effect. The size of the particles plays a critical role in both absorption and scattering. While smaller particles scatter radiation more effectively, those around 2 µm diameter display the strongest absorption. The use of both Mie theory and 3D-FDTD simulations provides a comprehensive understanding of the optical properties of both homogeneous and core-shell particles. Future research should focus on a more comprehensive modeling of aerosol mixtures and incorporating these findings into climate models to improve the accuracy of climate change projections.

The study's simplified approach to modeling PM mixtures (homogeneous and core-shell) and the use of simplified air quality guideline values, neglecting geographical and seasonal variations, represent limitations. The laboratory experiment is conceptual, lacking the complexity of real atmospheric conditions. While (NH4)2SO4 was chosen as a representative SIA, it might not fully capture the variability of real-world SIA compositions. Further research including more complex models, incorporating more realistic atmospheric conditions, and a broader range of PM species is needed to strengthen the conclusions. The study primarily focused on the IR resonance properties of SIA particles on LWPMRE without comparison with a climate model; comparing with climate models is important but beyond the scope of this work.