Imaging of pH distribution inside individual microdroplet by stimulated Raman microscopy

Abstract

Aerosol microdroplets as microreactors for many important atmospheric reactions are ubiquitous in the atmosphere. pH largely regulates the chemical processes within them; however, how pH and chemical species spatially distribute within an atmospheric microdroplet is still under intense debate. The challenge is to measure pH distribution within a tiny volume without affecting the chemical species distribution. We demonstrate a method based on stimulated Raman scattering microscopy to visualize the three-dimensional pH distribution inside single microdroplets of varying sizes. We find that the surface of all microdroplets is more acidic, and a monotonic trend of pH decreasing is observed in the 2.9-μm aerosol microdroplet from center to edge, which is well supported by molecular dynamics simulation. However, bigger cloud microdroplet differs from small aerosol for pH distribution. This size-dependent pH distribution in microdroplets can be related to the surface-to-volume ratio. This work presents noncontact measurement and chemical imaging of pH distribution in microdroplets, filling the gap in our understanding of spatial pH in atmospheric aerosol.

Keywords: aerosol; imaging; microdroplet; pH distribution; stimulated Raman.

Fig. 1.

Calibration of [SO₄²⁻] and [HSO₄⁻] with pH via SRS. (A) The principle of SRS microscopy. (B) The SRS spectra of sulfates in standard solutions of Na₂SO₄ (955 to 1,015 cm⁻¹) and NaHSO₄ (910 to 1,130 cm⁻¹). (C) The SRS spectra of H₂O in the above Na₂SO₄ and NaHSO₄ solutions range from 3,214 to 3,584 cm⁻¹. (D) SRS intensity ratio of ν_s(SO₄²⁻)/v (H₂O) and ν_s(HSO₄⁻)/v(H₂O) as a function of the [SO₄²⁻] and [HSO₄⁻]. (E) Comparison of measured [SO₄²⁻] and [HSO₄⁻] by SRS and MRS. They were marked as [SO₄²⁻]_SRS, [HSO₄⁻]_SRS, [SO₄²⁻]_MRS, and [HSO₄⁻]_MRS, respectively. (F) Dependence of the $\mathrm{Log}\left(\frac{{\gamma }_{{\mathrm{SO}}_4^{2-}}}{{\gamma }_{{\mathrm{HSO}}_4^{-}}}\right)$ and pH on $\sqrt{4\times [{\mathrm{SO}}_4^{2-}]+[\mathrm{H}{\mathrm{SO}}_4^{-}]}$ . The log(γ_bisulfate/γ_sulfate) was solved using Eq. 1A and premeasured pH from a probe, and the estimated pH was calculated via Eqs. 1A and 1B and SRS spectra.

Fig. 2.

Stereoscopic pH distribution inside small aerosol microdroplet. (A–C) 2D distribution of SO₄²⁻, HSO₄⁻, and pH in XY plane of the microdroplet (d = ~2.9 μm). (A) SRS images of microdroplet at the Raman shifts of 3,420 cm⁻¹ (H₂O), 985 cm⁻¹ (SO₄²⁻), and 1,050 cm⁻¹ (HSO₄⁻). (B) The chemical imaging of pH, [SO₄²⁻], and [HSO₄⁻] within microdroplet. (C) The [SO₄²⁻], [HSO₄⁻], and pH inside microdroplet as a function of distance to central. The [SO₄²⁻] and [HSO₄⁻] calculated from four single line profiles (0°, 45°, 90°, and 135°) marked as circular scatters while the average [SO₄²⁻], [HSO₄⁻], and pH calculated from parallel line profiles (green box) marked as lines. Inset shows the diagram of the line profile and distribution zone of (bi)sulfates. (D and E) Molecular dynamics of nanodroplet. (D) Snapshots of a sulfate water nanodroplet (5 nm) vs. time. Sticks in red, orange, green, dark blue, and light blue represent H₃O⁺, HSO₄⁻, SO₄²⁻, Na⁺, and water molecules, respectively ([HSO₄⁻] = 3.8 M, [SO₄²⁻] = 2.3 M, [H⁺] = 1.047 M). (E) Number profiles of molecules from classical MD trajectories of the water nanodroplet along the radial direction. (F) Projections along Z of coarse-grained simulation data showing the number density distribution of H₃O⁺, HSO₄⁻, and SO₄²⁻ within nanodroplet.

Fig. 3.

Stereoscopic pH distribution inside bigger cloud microdroplet. (A) pH distribution in XY plane of the 27.0-μm microdroplet with different z values (droplet center is set as 0). (B) pH distribution in the XZ plane of the microdroplet based on line profiles. (C) The variation of average pH, [SO₄²⁻], and [HSO₄⁻] in horizontal (X and Y) or vertical (Z) direction based on the coordinate in B. Values in the horizontal and vertical directions are marked as hollow and solid scatter, respectively. (D) Schematic diagram of pH distribution difference between aerosol and cloud microdroplets, where an abnormal pH increase (marked green) occurs near the edge of larger microdroplets.

Fig. 4.

The effect of particle size on stereoscopic pH distribution inside microdroplets. (A) The pH distribution within 8 microdroplets as a function of ND to the center where the starting and ending points of the microdroplet diameter in the X-direction (Fig. 2) are normalized to −1 and 1, respectively, and the microdroplet center is defined as ND = 0. Symbol color corresponds to ΔpH of microdroplet along X directions ( $∆\mathrm{pH}={\mathrm{pH}}_{{\mathrm{x}}_{\mathrm{i}+1}}-{\mathrm{pH}}_{{\mathrm{x}}_{\mathrm{i}}}$ ). Microdroplet diameter and pixel size were marked as black and gray, respectively. (B) Δ[H⁺] as a function of average [H⁺] at the microdroplet edge. (C) The distribution of calculated [H⁺] via pH and simulated EF based on reported model within microdroplets of different size. (D) The normalized width of the layer containing only HSO₄⁻ (green), and zone mixed of HSO₄⁻ and SO₄²⁻ (orange) within microdroplets of different diameters indicates the effect of microdroplet size on the surface preference of (bi)sulfate. (E) Δ[H⁺] ([H⁺]_edge − [H⁺]_center) between the center and microdroplet edge as well as the width (ΔL) of the acidification zone as a function of normalized distance to the center. The [H⁺]_edge and [H⁺]_center were determined by the average of the pH values at the microdroplet edge. Symbol color corresponds to microdroplet size for ΔL.