Aerosol mobility imaging
A system and method to measure a size distribution of particles based on their electrical mobility. The method includes: introducing, via a sheath flow inlet, a particle free sheath flow into a chamber formed by two parallel walls which are separated by a gap, the chamber having a width and a length, the sheath flow having a direction along the length of the chamber and flowing a laminar manner; introducing an aerosol sample flow into the chamber downstream of the sheath inlet such that the aerosol sample flow joins the particle free sheath flow in a laminar manner; applying an electric field between the two parallel walls of the chamber, the field having a strength which varies across the width of the chamber; extracting an output aerosol flow through a first outlet downstream of the sample inlet; and outputting an excess flow equal to a sum of the sheath flow and aerosol sample flow minus the output aerosol flow. The method may also include: passing the output aerosol flow through a growth cell in a laminar manner, the growth cell having a region of wetted walls with two or more temperature regions such that the particles within the output aerosol flow grow by condensation to form droplets, and such that relative positions of droplets are indicative of particle electrical mobility; and counting and capturing a spatial position of individual droplets exiting the growth cell.
1 . A method to measure a size distribution of particles based on their electrical mobility comprising:
introducing, via a sheath flow inlet, a particle free sheath flow into a chamber formed by two parallel walls which are separated by a gap, the chamber having a width and a length, the sheath flow having a direction along the length of the chamber and flowing a laminar manner;
introducing an aerosol sample flow into the chamber at a sample inlet downstream of the sheath inlet such that the aerosol sample flow joins the particle free sheath flow in a laminar manner;
applying an electric field between the two parallel walls of the chamber, the field having a strength which varies across the width of the chamber;
extracting an output aerosol flow through a first outlet downstream of the sample inlet;
outputting an excess flow equal to a sum of the sheath flow and aerosol sample flow minus the output aerosol flow;
passing the output aerosol flow through a focusing growth cell in a laminar manner, the growth cell having a converging channel and a region of wetted walls with two or more temperature regions such that the particles within the output aerosol flow grow by condensation to form droplets, and such that relative positions of droplets are indicative of particle electrical mobility; and
counting and capturing a spatial position of individual droplets exiting the focusing growth cell.
2 . The method of claim 1 further including creating an image of the individual droplets and mapping of the position of the droplets that are imaged onto the electrical mobility of particle at the sample inlet.
3 . The method of claim 2 wherein the mapping of the position of the droplets that are imaged onto the electrical mobility of particle at the sample inlet is accomplished through calibration with particles of known electrical mobility.
4 . The method of claim 2 wherein the mapping of the position of the droplets that are imaged onto the electrical mobility of particle at the sample inlet is accomplished through model calculations of the flow and electric fields.
5 . The method of claim 4 wherein the excess flow is recirculated through an air mover and filter and reintroduced at the sheath flow inlet.
6 . The method of claim 1 further including calculating a distribution in particle electrical mobilities based on the counting and mapping, and based on a distribution in electrical mobility and an electrical charge distribution of the aerosol at the sample inlet.
7 . The method of claim 1 further including passing the aerosol sample flow through a bipolar ion source prior to introduction at the sample inlet.
8 . The method of claim 1 wherein the electrical field is applied by actuating a voltage on a plurality of conductive traces on a printed circuit board, such traces extending in straight lines along a portion of the length of the chamber.
9 . The method of claim 8 wherein the actuating comprises increasing the voltage applied to at least a portion of the plurality of conductive traces exponentially with trace position across the width of the chamber.
10 . The method of claim 8 wherein the actuating comprises applying the voltage at a first polarity for approximately one-half of the plurality of conductive traces and at an opposite polarity for the other half of the conductive traces.
11 . The method of claim 1 where a magnitude of the sheath flow is at least three times higher than a magnitude of aerosol sample flow.
12 . The method of claim 1 where a magnitude of the aerosol sample flow and a magnitude of output aerosol flow are equal.
13 . The method of claim 1 further including controlling relative humidity of the sheath flow.
14 . The method of claim 1 where a charge distribution on the sampled aerosol is inferred through comparison of negative and positive mobility distributions.
15 . An aerosol mobility imaging system, comprising:
a chamber formed by a first wall and a second wall, the first and second walls being parallel and separated by a gap, the chamber having a width and a length;
a sheath flow inlet to the chamber, the sheath flow inlet configured to receive a particle free sheath flow;
an aerosol sample inlet configured to receive an aerosol sample flow, the sample inlet located in the first wall, downstream of the sheath flow inlet, such that the aerosol sample flow joins the particle free sheath flow in a laminar manner;
an electrode configured to provide electric field between the first wall and the second wall, the electric field having a strength which varies across the width of the chamber an aerosol flow extraction outlet provided in the second wall and downstream of the sample inlet;
an excess flow outlet downstream of the aerosol flow extraction outlet;
a growth cell coupled to the aerosol flow extraction outlet to receive an extracted aerosol flow, the growth cell including a converging channel and a region of wetted walls with two or more temperature regions such that the particles within the extracted aerosol flow grow by condensation to form droplets within the growth cell, the growth cell configured to maintain laminar flow such that relative positions of droplets exiting the growth cell are indicative of particle electrical mobility, the growth cell having an output; and
an imaging system coupled to the output of the growth cell, the imaging system configured to count and capture a spatial position of individual droplets as they exit the growth cell.
16 . The system of claim 15 further including a system controller, the controller including instructions causing a processor to:
map a position of the droplets that are imaged onto the electrical mobility of particle at the sample inlet;
calculate a distribution in particle electrical mobilities from the count and mapping, wherein
calculate a size distribution of particles based on the distribution in electrical mobilities and an electrical charge distribution of the aerosol at the sample inlet.
17 . The system of claim 15 further including a bipolar ion source coupled to the sample inlet and configured to receive the aerosol sample flow prior to the aerosol sample flow passing through the sample inlet.
18 . The system of claim 15 wherein the electrode is formed by multiple traces on an printed circuit board, such traces extending in straight lines along a portion of the length of the chamber.
19 . The system of claim 15 where a voltage applied each of at least a portion of individual traces increases exponentially with position across the width of the chamber.
20 . The system of claim 15 where the excess flow outlet is coupled to an air mover and filter, and excess flow exiting the outlet is reintroduced at the sheath flow inlet.