Multi-wavelength, size discriminating Aethalometer
During the last two decades, there has been an increasing realization of the importance of trace gases and aerosols. These play a significant role in virtually all areas of air quality related concerns, including photochemical smog, stratospheric ozone depletion, global climate change, indoor air quality, public and occupational health, and the genesis of atmospheric acidity and its effects. Aerosol composition measurement studies have confirmed that particulate air pollution can be associated with cardiopulmonary and lung cancer related mortality.
focuses on particle distribution studies on filter paper by an
aethalometer instrument (Fig.
1). An aethalometer measures the reduction in light transmission
through a filter paper as air is sampled through it. The transmission
measurement using aethalometer can be correlated to the mass of the
particulate matter of the atmospheric gas sample. Aethalometer
instrument that we have fabricated is made up of 11 LED’s and 512
photodiode arrays, as shown in schematic diagram of aethalometer. This
aethalometer is unique from others in that it is designed to separate
particles by their particle size and the absorbance is measured at
Fig. 1: Schematic diagram of aethalometer
dyes have been utilized towards the generation of aerosols. Different
particle sizes are generated from various dye solutions using a
vibrating orifice aerosol generator. The aerosol sample is collected on
the filter paper and the absorbance is measured. The sample filter
paper is as shown in Fig. 2.
2: Particle distribution of methylene blue dye on filter paper: A:
large particle size, B: medium particle size, C: small particle size.
Absorbance v/s Pixel of photodiode array
Dye particle size can be increased by increasing the sodium chloride concentration in the dye solution mixture fed into the VOAG. Different particle sizes (0.37 µm, 0.40 µm, 0.80 µm and 1.27 µm) of Metanil Yellow dye were generated for different sampling durations as shown in Fig. 3. Absorbance in all cases is maximum at pixel # 150 which corresponds to the nozzle position. Large size particles tend to deposit at the nozzle whereas smaller particles tend to follow streamlines around and lead to uniform distribution across the quartz filter paper. While all particles has the absorbance maximum at the nozzle position, the drop off is much sharper for higher particle size.
Fig 3: Absorbance spectra of different sizes of Metanil Yellow dye particles at different pixel numbers of 512 photodiode array detector.
Particle size and color distribution study
different colored dye particles of different particle sizes viz.,
absorption and particle distribution pattern was observed (Fig. 4). It is seen that even
for a complex mixture of four different colored dye particles of different particle sizes,
the data is easily discriminated on the basis of their size and color using aethalometer.
Fig. 4 Particle distribution pattern of various dye particles of different sizes obtained by aethalometer.
We also performed the additivity study using MS Excel Solver; the same color A aerosol in size 1 and color B aerosol in size 2 were deposited singly on two different filters and sequentially on the same filter to check that within the reproducibility offered by the Vibrating orifice aerosol generator, 1 + 2 = (1+2)