Abstract. Ambient measurements combined with theoretical simulations have shown evidence that the tropospheric degradation end-products of Freon alternatives, trifluoroacetic acid (TFA), one of the most important and abundant atmospheric organic substances, can enhance the nucleation process based on sulfuric acid (SA) and dimethylamine (DMA) in urban environments. However, TFA is widespread all over the world under different atmospheric conditions, such as temperature and nucleation precursor concentration, which are the most important factors potentially influencing the atmospheric nucleation process and thus inducing different nucleation mechanisms. Herein, using the density functional theory combined with the Atmospheric Cluster Dynamics Code, the influence of temperature and nucleation precursor concentrations on the role of TFA in the SA–DMA nucleation has been investigated. The results indicate that the growth trends of clusters involving TFA can increase with the decrease in temperature. The enhancement on particle formation rate by TFA and the contributions of the SA–DMA–TFA cluster to the cluster formation pathways can be up to 227-fold and 95 %, respectively, at relatively low temperature, low SA concentration, high TFA concentration, and high DMA concentration, such as in winter, at the relatively high atmospheric boundary layer, or in megacities far away from industrial sources of sulfur-containing pollutants. These results provide the perspective of the realistic role of TFA in different atmospheric environments, revealing the potential influence of the tropospheric degradation of Freon alternatives under a wide range of atmospheric conditions.

Abstract. Ambient measurements combined with theoretical simulations have shown evidence that the tropospheric degradation end-products of Freon alternatives, trifluoroacetic acid (TFA), one of the most important and abundant atmospheric organic substances, can enhance the process of sulfuric acid (SA) – dimethylamine (DMA) – based nucleation process in urban environments. However, TFA is widespread all over the world with different atmospheric conditions, such as temperature and nucleation precursor concentration, which are the most important factors potentially influencing the atmospheric nucleation process and thus inducing different nucleation mechanisms. Herein, using the Density Functional Theory combined with the Atmospheric Cluster Dynamics Code, the influence of temperature and nucleation precursor concentration on the role of TFA in the SA-DMA nucleation has been investigated. The results indicate that the growth trends of clusters involving TFA can increase with the decrease of temperature. The enhancement of particle formation rate by TFA and the contributions of SA-DMA-TFA cluster to the cluster formation pathways can be up to as much as 227 times and 95 %, respectively, at relatively low temperature, low SA concentration, high TFA concentration and high DMA concentration, such as in winter or at relatively high atmospheric boundary layer and in megacities far away from industrial sources of sulfur-containing pollutants. These results provide the perspective of the realistic role of TFA in different atmospheric environments, revealing the potential influence of the tropospheric degradation of Freon alternatives under a wide range of atmospheric conditions.

Table S1. Gibbs free formation energies (ΔG, kcal/mol) of clusters in the present study at different temperatures. Clusters ΔG (kcal/mol) 280 K (Lu et al., 2020) 260 K (TFA)2 -5.76 -6.51 (TFA)3 -3.79 -5.24 (SA)1• (TFA)1 -7.83 -8.53 (SA)1• (TFA)2 -13.49 -15.03 (SA)2• (TFA)1 -14.68 -16.27 (DMA)1• (TFA)1 -8.65 -9.31 (DMA)1• (TFA)2 -19.06 -20.57 (DMA)1• (TFA)3 -23.56 -26.06 (DMA)2• (TFA)2 -34.55 -36.88 (SA)1• (DMA)1• (TFA)1 -27.21 -28.67 (SA)2• (DMA)1• (TFA)1 -42.13 -44.48 (SA)1• (DMA)2• (TFA)1 -44.92 -47.27 (SA)1• (DMA)2• (TFA)2 -52.41 -55.74 (SA)2• (DMA)2• (TFA)1 -61.91 -65.25 (SA)1• (DMA)1• (TFA)2 -36.32 -38.57 (SA)2 -9.04 -9.72 (SA)3 -15.33 -16.87 (SA)2• (DMA)1 -36.30 -37.85 (SA)2• (DMA)2 -56.95 -59.26 (SA)3• (DMA)1 -50.06 -52.44 (SA)3• (DMA)2 -73.77 -76.90 (SA)3• (DMA)3 -94.24 -98.18 (SA)1• (DMA)1 -15.28 -15.98 (SA)2• (DMA)3• (TFA)1 -79.52 -83.64 (SA)1• (DMA)3• (TFA)2 -68.39 -72.51 (DMA)3• (TFA)3 -54.18 -58.31 (DMA)2• (TFA)3 -43.13 -46.42 Table S2. Collision coefficients (β, cm 3 s -1 ) for each cluster in the present study. 5 Collisions Collision coefficients (cm 3 s -1 ) 280 K (Lu et al., 2020) 260 K DMA+SA 2.45×10 -10 2.36×10 -10 (SA)1• (DMA)1+SA 2.67×10 -10 2.57×10 -10 (SA)2• (DMA)1+SA2.61×10 -10 2.52×10 -10 (SA)2• (DMA)1+DMA3.91×10 -10 3.77×10 -10 (SA)2• (DMA)2+SA3.05×10 -10 2.94×10 -10 (SA)3• (DMA)2+DMA5.82×10 -10 5.61×10 -10 DMA+TFA 2.85×10 -10 2.75×10 -10 (SA)1• (DMA)1+TFA2.95×10 -10 2.84×10 -10 (SA)2• (DMA)1+TFA2.84×10 -10 2.74×10 -10 DMA+(SA)1• (DMA)1• (TFA)1 4.39×10 -10 4.23×10 -10 (SA)2• (DMA)2+TFA3.28×10 -10 3.16×10 -10 DMA+(SA)2• (DMA)2• (TFA)1 4.90×10 -10 4.72×10 -10 (DMA)1•( TFA)1+TFA3.26×10 -10 3.15×10 -10 TFA+(SA)13.50×10 -10 3.38×10 -10 (DMA)2•( TFA)3+DMA 4.69×10 -10 4.52×10 -10 TFA+TFA 2.25×10 -10 2.17×10 -10 (TFA)2+TFA3.35×10 -10 3.23×10 -10 SA+TFA 1.97×10 -10 1.90×10 -10 (SA)1•( TFA)1+TFA3.04×10 -10 2.93×10 -10 (SA)2+TFA 2.80×10 -10 2.69×10 -10 SA+SA 1.70×10 -10 1.64×10 -10 (SA)2+SA 2.55×10 -10 2.46×10 -10 (TFA)2+SA3.11×10 -10 3.00×10 -10 (TFA)3+SA3.38×10 -10 3.25×10 -10 (SA)1•( TFA)1+SA2.79×10 -10 2.69×10 -10 (SA)1•( TFA)2+SA 4.54×10 -10 4.37×10 -10 (SA)2•( TFA)1+SA3.86×10 -10 3.72×10 -10 (DMA)1•( TFA)1+SA 2.99×10 -10 2.88×10 -10 (DMA)1•( TFA)2+SA 2.99×10 -10 2.87×10 -10 (DMA)1•( TFA)3+SA3.11×10 -10 3.00×10 -10 (DMA)2•( TFA)2+SA3.28×10 -10 3.16×10 -10 (DMA)3•( TFA)3+SA3.32×10 -10 3.20×10 -10 (DMA)2•( TFA)3+SA3.06×10 -10 2.95×10 -10 (SA)1• (DMA)1• (TFA)1+SA 2.93×10 -10 2.83×10 -10 (SA)2• (DMA)1• (TFA)1+SA3.95×10 -10 3.80×10 -10 (SA)1• (DMA)2• (TFA)1+SA3.29×10 -10 3.17×10 -10

Tetranuclear Cp4M4(CO)4 clusters have been synthesized for iron and vanadium but not for the intermediate first-row transition metals manganese and chromium. All of the low-energy structures of these “missing” Cp4M4(CO)4 (M = Mn, Cr) species are shown by density functional theory to consist of a central M4 tetrahedron with each of the four faces capped by a μ3-CO group. The individual low-energy structures differ in their spin states and in their formal metal–metal bond orders along the six edges of their central M4 tetrahedra. The two low-energy Cp4Mn4(μ3-CO)4 structures are a triplet structure with all Mn–Mn single bonds and a singlet structure with one MnMn triple bond and five Mn–Mn single bonds along the six tetrahedral edges. Related low-energy Cp4Cr4(μ3-CO)4 structures include a quintet structure with all Cr–Cr single bonds and a singlet structure with two CrCr triple bonds and four Cr–Cr single bonds. However, the potential energy surface of the Cp4Cr4(CO)4 system is complicated by three other structures of comparable energies including two triplet structures and one quintet structure with various combinations of single, double, and triple chromium–chromium bonds in the central Cr4 tetrahedron.