IceNucleationbyLahoreSmogParticles
The first ice-nucleation characterisation of South Asian urban aerosol.
Scroll the story ↓
A sampler built from scratch
A custom rooftop high-volume sampler — welded steel frame, sheltered inlet, PTFE membrane filters. Running at 15 L/min for 24 h draws 21.6 m³ of air through each filter. It collected ~1.1 mg of mass — about 51 µg/m³ over 24 h, more than 3× the WHO 24-hour PM2.5 guideline — sampled through the November 2024 smog season.
Discrete particles, not a smear
Electron microscopy shows particles sitting individually on the filter fibres — not a continuous smear, which is what makes single-particle analysis possible. Soot primary particles run 15–75 nm; the largest mineral grains reach 3–5 µm. (FEI Nova NanoSEM, 10 kV.)
Full SEM/EDX result, size distribution & soot images →
Two signatures, two sources
Nine elements appear on the loaded filter and are absent from the control. Two tell a story: alumino-silicate mineral dust — with potassium pointing to feldspar, the most efficient known mineral ice nucleator — and sulphur from combustion as sulphate. Potassium also traces biomass burning; zinc traces traffic.
| Si | Silicon | Silicate mineral dust |
| Al | Aluminium | Alumino-silicate — feldspar |
| K | Potassium | Feldspar + biomass burning |
| Ca | Calcium | Crustal / road dust |
| Fe | Iron | Crustal dust |
| S | Sulphur | Combustion sulphate |
An in-house cold-stage freezing assay
Droplets are frozen on a temperature-controlled stage: a Linkam T95 with LNP95 liquid-nitrogen cooling, stable to ±0.1 °C. 1 µL droplets sit in a grid, 6–10 per run; freezing is caught optically as each droplet turns from transparent to opaque. A two-stage ramp — 20 °C/min to −10 °C, then 1 °C/min to −40 °C — and a 0.5% DMOAP silane coating for a stable hydrophobic surface.
Pure-water baseline, before vs after moving to a cleanroom.
The tell was the scatter, not the temperature
Early pure-water runs froze at −23.3 °C with ~7 °C of scatter. Moving the setup into a cleanroom dropped the baseline to −28.4 °C and cut the spread below 3 °C. That the reproducibility improved as much as the temperature was the clue: the early signal was ambient ice-nucleating particles from lab air contaminating the samples — not a property of the water. Diagnosing and removing that systematic error is what made the real result trustworthy.
Smog freezes water 10.7 °C warmer
Pure water froze at a median −28.4 °C (±1.1); Lahore smog at −17.7 °C (±0.9) — a reproducible 10.7 °C shift. Published values from Beijing, Delhi and Mexico City fall between −16 and −20 °C; Lahore sits right with them, and this is the first such measurement for South Asian urban aerosol.
My data: droplet-freezing spectrum
Fraction of 12 droplets frozen as they cool. Lahore-smog droplets freeze about 10.7 °C warmer than pure water — direct evidence of active ice-nucleating particles in the smog.
Source: Ice Nucleation Induced by Lahore Smog Particles — MS Physics thesis · LUMS · 2025 · Fig. 4.5. Pure water's homogeneous-freezing limit is ≈ -38 °C; a cleanroom move sharpened the pure-water result from -23.3 °C to -28.4 °C.
Microfabricated droplet arrays
To beat the 6–10 droplet statistical ceiling, I designed high-density arrays — eleven geometries on one photomask (deliberately redundant), including a 168-well array and a 42-well “crystal hotel”. The SU-8 process was refined to 19.8 µm against a 20 µm target with under 0.5 µm variation across the wafer (UV exposure cut to 4.6 s, staged soft bakes, controlled development). The recipe was validated; the full device was still in fabrication when the project ended.
Designed ice nucleators
With the Jonathan Bath group at the Clarendon Laboratory, Oxford, I'm exploring DNA origami as ice nucleators — where the geometry is chosen rather than inherited, making a designed nucleator a potentially better calibration standard. My contribution: folding by controlled thermal annealing, gel electrophoresis to confirm assembly, dynamic light scattering for size, and sonication to break up aggregates. Freezing measurements are in progress.