Aerosol mobility imaging for rapid size distribution measurements
A parallel plate dimensional electrical mobility separator and laminar flow water condensation provide rapid, mobility-based particle sizing at concentrations typical of the remote atmosphere. Particles are separated spatially within the electrical mobility separator, enlarged through water condensation, and imaged onto a CCD array. The mobility separation distributes particles in accordance with their size. The condensation enlarges size-separated particles by water condensation while they are still within the gap of the mobility drift tube. Once enlarged the particles are illuminated by a laser. At a pre-selected frequency, typically 10 Hz, the position of all of the individual particles illuminated by the laser are captured by CCD camera. This instantly records the particle number concentration at each position. Because the position is directly related to the particle size (or mobility), the particle size spectra is derived from the images recorded by the CCD.
1. An apparatus for measuring a size and number concentration of airborne particles, the apparatus comprising:
an elongated gap extending through the apparatus, being bounded on opposing sides by first and second substantially planar surfaces, wherein each of the planar surfaces has a width;
a sheath inlet admitting a sheath flow of fluid into the elongated gap, the sheath flow moving away from the sheath inlet in a first flow direction;
a mobility separator section downstream of the sheath inlet in the first flow direction, having an electrode forming at least part of the first planar surface of the elongated gap, and a ground plate forming at least part of the second planar surface of the elongated gap, the ground plate being opposed to the electrode;
a growth cell downstream of the mobility separator section in the first flow direction, the growth cell including a conditioning section that exposes the sheath flow to at or below ambient temperature;
an initiator section downstream of the conditioning section in the first flow direction, wherein the first and second planar surfaces of the elongated gap in the initiator section each comprises a water-saturated evaporative source, the initiator section further comprising a first heat source to expose the sheath flow to a temperature elevated above ambient; and
an equilibrator section downstream of the initiator section in the first flow direction, wherein the first and second planar surfaces of the elongated gap in the equilibrator section each comprises a water-saturated evaporative source, the equilibrator section comprising:
a first heat sink to cool the planar surfaces to lower the dew point of the sheath flow to a supersaturated state; and
an optical section downstream of the growth cell in the first flow direction, wherein the sheath flow is exposed to a collimated laser light plane substantially perpendicular to the direction of the sheath flow, and
an observation port through which aerosol particles in the sheath flow illuminated by the laser light are observable.
2. The apparatus according to claim 1 , wherein each of the water-saturated evaporative sources of the initiator section or the equilibrator section comprises a wick surface having a porosity of about 25%, a first side of the wick surface forming a part of the first or second planar surfaces of the elongated gap, the wick being saturated by water.
3. The apparatus according to claim 2 , wherein each wick surface comprises art alumina bisque of about 0.25-inch thickness.
4. The apparatus according to claim 1 , wherein each of the conditioning section, the initiator section, and the equilibrator section are thermally isolated from the apparatus.
5. The apparatus according to claim 4 , wherein the conditioning section and the initiator section, the initiator section and the equilibrator section and the equilibrator section and the optical section are each separated by an interface, and wherein the thermal apparatus further comprises a thermally insulating separator interposed in at least one of the interfaces between the conditioning section and the initiator section, between the initiator section and the equilibrator section, or between the equilibrator section and the optical section.
6. The apparatus according to claim 4 , wherein the conditioning section and the initiator section, the initiator section and the equilibrator section and the equilibrator section and the optical section are each separated by an interface, and wherein the thermal apparatus further comprises an air gap in the first and second planar surfaces interposed in at least one of the interfaces between the conditioning section and the initiator section, between the initiator section and the equilibrator section, or between the equilibrator section and the optical section.
7. The apparatus according to claim 1 , wherein the first and second planar surfaces of the elongated gap in the conditioning section each comprises a water-saturated evaporative source, and wherein the equilibrator section further comprises a second heat sink to cool the sheath flow below an ambient temperature.
8. The apparatus according to claim 7 , wherein each of the water-saturated evaporative sources of the conditioning section comprises a wick surface having a porosity of about 25%, a first side of the wick surface forming a part of the first or second planar surfaces of the elongated gap, the wick being saturated by water.
9. The apparatus according to claim 8 , wherein the wick surface comprises an alumina bisque of about 0.25-inch thickness.
10. The apparatus according to claim 1 , wherein the mobility separator section further comprises an aerosol inlet, including a slit substantially the width of either the first or second planar surface of the elongated gap, positioned upstream in the first direction with respect to the ground plate or the electrode.
11. The apparatus according to claim 1 , wherein the electrode comprises a two-dimensional electrode wherein voltage potential applied to the electrode may be varied in a direction transverse to the first flow direction.
12. The apparatus according to claim 1 , wherein the first heat sink comprises a thermoelectric cooler.
13. The apparatus according to claim 1 , wherein the first heat source comprises a cartridge heater.