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7 July 2013

Size Distribution of Sedimentary Haze Particulates via Monochromatic Diffraction of Light

Part of this article was adapted from my unpublished scientific paper. It was not submitted for publication due to an unfortunate turn in my methodology which yielded inconclusive results during analysis.

1°34'18.9"N, 103°37'14.9", 20 June 2013, 1014 local time

Recent agricultural burning and peat fires in the Sumatera and Riau islands has produced copious amount of carbonized dust and organic aerosols where its drift direction follows low level atmospheric dynamics. In mid June, the wind direction has brought Malaysia and Singapore the phenomena of haze where certain parts of Malaysia has reported record of 700 units in the Air Pollution Index (API). While heavy concentration of particulates within the atmosphere typically brings negative impact to the biological and ecological system to the affected region and thus population; the size and composition distribution determines the aerosol kinematics and consequently its duration in air.

The haze coincide with the occurrence of "super moon". Scattering caused the moon to appear redder than it should.  

According to Max-Planck Institute of Meteorology, atmospheric suspending particles range in size from 2 μm to 10 mm depending on different processes of particulate formation and its agglomeration. Ultrafine particles with diameter less than 1 mm has been characterized to have a high tendency of reaching into the lungs thus causing internal inflammation while particulate matter less than 2.5 mm can trigger respiratory diseases. 

In 2000, Emmanuel quantifies the Sumatran haze constitutes 94% particulates with diameter lesser than 2.5 μm using a scanning electron microscope. The immediate concern suspending matter in this size is that it could easily bypass biological defence mechanism, penetrating deep into the alveoli. Such fine particulate has been recognized by ASEAN haze action online as the result from soot-like material coming from wood-stoves, fireplaces and forest burning.

What I have been trying to do was essentially to measure the size distribution of the dust particles suspended in air which constitutes haze. Some time ago, I was informed that the industry have been using integrated systems incorporating optical diffraction for quick analysis of powdered particle sizes for quality sampling particularly in manufacturing processes such as the production of powdered milk. A quick thought came to me when the haze occurred so I went to my sketch book and started designing a simple method to measure the size of the haze particulates using my camera sensor as the image detector.

Figure obtained from

Optical diffraction occurs upon both apertures and opaque masks when they are irradiated by light. A monochromatic source should give rise to concentric rings diffraction patterns (figure above) upon a central airy disc emerged from uniformly illuminated opaque mask, i.e. a dust particle. This phenomena of optical diffraction is mathematically formulated under far-field (Fraunhofer) theory which for the case of a spherical (our assumption is that the haze particulates are taking a spherical geometry) dust, its radial size can be obtained by computing the approximation formula when the distance between the centre to the first minima of the Airy pattern, focal distance and wavelength of light are known.

So, adapting a modified glass body cap (photo above) from my solar pinhole experiment, I left it outdoor exposed to the haze for 2 hours on 22 June. Heavier dust particles tend to "settle down" when there is no wind current to carry it further in the air. After the collection, casual naked-eye observation of the glass plate shows numerous dust particles. In fact, even without purposely collecting it, my car windshield has been covered by a layer of dust coming from the haze. 

The body cap is then attached onto my camera (lens removed) and I allowed a beam of laser light, expanded using a cheap acrylic lens, to shine through the dust laden body cap into the sensor. Because laser has a relatively fixed wavelength, I used a 532nm DPSS (green) laser as my monochromatic light source and the distance between the glass lens-cap to the sensor has already been calculated from my running solar pinhole experiment so the only thing left was to measure the size of the Airy disc. 

The Airy patterns that was formed on my camera image plane was shown in the photograph above. Notice the green background which is the effect of using a DPSS light source. Each particulate should, form only one Airy pattern; collectively, the particles create diffraction patterns and they are scattered all over in the photo. 

So we took a sample of 150 Airy patterns by measuring their width and then crunch in the numbers with Excel. The histogram above shows the size distribution of the particulates. It should be noted that the size are quite large compared to the average "colloidal" particles suspended in haze. At a glance, it seems to agree with the particles seen on the body cap because it was essentially macroscopic! However, later data acquisition using the same body cap (washed and scrubbed with alcohol and organic solvents) still produced similar diffraction patterns, rendering this statistics unreliable. 

Putting failure aside, I still managed to capture some "souvenir" pictures of the haze related to optical sciences. In particular, it is scattering. Tiny dust particles can scatter blue light away, leaving the red part transmitting through. This was the essence of my previous "Scattering in Milk" demonstration and we now can see why haze is also a form of colloid, similar to milk. Because blue light has been scattered, when we look into the haze, it should appear slightly bluish against a dark background. This was exactly what I did with the dust particles. 

Shining light (white L.E.D) horizontally on welding glass with haze particulates 

Parallel to the diffraction experiment, I placed alongside a piece of welding glass next to my glass lens cap for dust collection. The welding glass provides a dark background when dust which sits on it was illuminated with a white L.E.D across its surface. The scattered blue light was obvious and the result was remarkable as I have never seen how something as sickly as the constitution of haze, capable of producing a painting of stars.  

Careful configuration of the light source using two L.E.D shining the dust from both horizontal sides to the welding glass produce a picture that is similar to a milky-way masterpiece from an astrophotographer. This effect is largely contributed by Rayleigh scattering and to remove the "stars", simply change the angle of illumination. The photograph below shows how the effect "turns off" when the welding glass was illuminated by the same light but parallel to the surface.

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