Cloud droplet activation mechanisms of amino acid aerosol particles: insight from molecular dynamics simulations

Figures & data

Fig. 1 Chemical structures of the l-amino acids under study.

Table 1 DFAAs detected in air collected between 32.5 to 36.6°N and 132.7 and 133.0°W

DFAA	Type of side chain – R
GLY*	Hydrophobic
ALA*	Hydrophobic
THR	Polar
GLU	Negative charge
ASP	Negative charge
CYS	Polar
PRO	Hydrophobic
SER*	Polar
LYS	Positive charge
TYR	Hydrophobic
VAL*	Hydrophobic
LEU	Hydrophobic
ILE	Hydrophobic
HIS	Positive charge
PHE*	Hydrophobic
MET*	Hydrophobic

The DFAAs are shown in decreasing order of abundance (Malder et al., 2004): THR: threonine, GLU: glutamic acid, ASP: asparagine, CYS: cysteine, LYS: lysine, TYR: tyrosine, LEU: leucine, ILE: iso-leucine, HIS: histidine.

Fig. 2 Axial number densities of the six types of amino acids in planar interface simulations.

Table 2 Cartesian components of pressure tensor (in bar), length of simulation box in z-direction (in nm), surface tension σ (in mJ m⁻²), and surface tension-concentration slope dσ/dC for planar liquid-gas interface together with reference data (in mJ m⁻² L mol⁻¹)

	(P xx +P yy )/2	P zz	σ	dσ/dC	dσ/dC (ref.)
Water	−128.12	0.10	65.63	0.00
SER	−125.19	0.18	66.33	1.25	0.76^a
GLY	−125.48	0.20	66.17	0.96	1.12^a, 0.92^b
ALA	−126.96	0.15	67.09	2.61	0.96^a, 0.58^b
VAL	−121.26	0.14	64.96	−1.20	−3.74^a
MET	−120.94	0.15	65.03	−1.07	−3.01^a
PHE	−118.36	0.16	64.01	−2.89	−17.28^a

^aBull and Breese (1974).

^bPappenheimer et al. (1936).

Fig. 3 (a) Snapshots for water droplets containing different amino acids. (b) Hydrogen bond network in phenylalanine molecules on droplet surface.

Fig. 4 (a) Radial number densities of different amino acids in droplets containing 5000 water molecules. (b) Molecular orientation of SER and VAL in droplets.

Fig. 5 Distribution of angle θ and order parameters.

Table 3 Order parameter of amino acids in the spherical clusters

Compound	Order parameter
SER	0.0006
GLY	0.0027
ALA	0.0145
VAL	0.4421
MET	0.4319
PHE	0.5209

Table 4 Density of liquid phase ρ _α (in nm⁻³), radius of equimolar dividing surface R _e (in nm), surface tension σ (in mJ m⁻²), and individual contributions to work of formation W (in 10⁻¹⁹ J) for spherical droplets consisting of 5000 water molecules and different amino acids

	σ	W K	W U	ΔW LJ	ΔW QQ
Water	70.12	102.86	−71.49	0.00	0.00
SER	69.10	103.88	−71.93	189.23	−189.67
GLY	72.16	103.87	−70.89	161.10	−160.50
ALA	69.76	103.88	−71.78	166.37	−166.66
VAL	81.41	103.88	−66.97	177.14	−172.62
MET	87.08	103.88	−64.34	212.96	−205.82
PHE	85.57	103.88	−65.14	205.18	−198.83

Fig. 6 Different effects of a pair of repulsive force on the surface tension of planar and spherical interfaces. Here, i and j denotes the two interacting particles, f _ij is the force exerted on particle j, O denotes centre of mass of droplet, and r _i and r _j are the O→i and O→j vectors, respectively.

Fig. 7 Curvature dependence of surface tension of droplets containing 5000 water molecules and different amino acid molecules.

Fig. 8 The relation between solubility and surface tension.

Table 5 Solubility and surface tensions for the DFAAs used in this study

DFAA	Bulk solubility^a (g kg^−1)	Spherical surface tension^b (mN m^−1)	Planar surface tension^b (mN m^−1)
SER	250	69.1	66.33
GLY	250.2	72.16	66.17
ALA	166.9	69.76	67.09
VAL	88	81.41	64.96
MET	56	87.08	65.03
PHE	27.9	85.57	64.01

^aLide, D.R., (ed.). 2005. CRC Handbook of Chemistry and Physics. 86th ed. Boca Raton, FL: CRC Press.

^bThis study.

Fig. 9 Comparison between (a) calculated GF according to Biskos et al. (2006) and (b) the empirical relation. In (a) the GF–RH curves with surface tension correction taken into account were shown as dashed lines.

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