Mass concentration determination of particles smaller than 2.5 microns in air
A method for determining airborne concentrations of an ensemble of particulate matter below the size fraction 2.5 μm in aerodynamic diameter (PM 2.5 ). The method is to be composed of a sharp cut PM 2.5 cyclone, optical sensor capable of producing scattering coefficients dependent on particle size, temperature and humidity sensors for adapting to changes in the environment, and a processing system for applying logic-based algorithm to convert raw sensor values into PM concentrations. The optical sensor is a dual wavelength integrating nephelometer capable of yielding particle characteristic scattering coefficients at two wavelengths. The particle scattering ratios between two wavelengths are used in conjunction with temperature and humidity in the method's algorithm to reduce particle growth bias and to ultimately yield PM 2.5 concentrations.
1 . A method for real-time monitoring of total airborne particulate matter, the method comprising:
volumetrically controlling a flow rate into a sharp cut cyclone using a flow sensor, an internal pump, a pressure sensor, and a temperature probe affixed to an inlet of the sharp cut cyclone to generate an air sample;
measuring relative humidity and temperature of the air sample for determining a dewpoint correction factor;
maintaining a relative humidity threshold of the air sample with a sample conditioning system;
determining a first scattering coefficient of airborne particles in the air sample for a first optical wavelength via an optical sensor;
determining a second scattering coefficient of airborne particles in the air sample for a second optical wavelength, via the optical sensor;
determining a scattering Ångström exponent (SAE) using at least the first scattering coefficient and the second scattering coefficient; and
determining an airborne particle concentration for a range of particle sizes based at least on the SAE, the first scattering coefficient further used outside of the determination of the SAE, and the dewpoint correction factor.
2 . The method of claim 1 , wherein the optical sensor is a nephelometer.
3 . The method of claim 2 , wherein the nephelometer is an integrating nephelometer.
4 . The method of claim 2 , wherein the nephelometer is a multi-wavelength nephelometer.
5 . The method of claim 4 , wherein the nephelometer has a truncated integrating range from 10-171°.
6 . The method of claim 1 , wherein the first optical wavelength is in a first range from approximately 410 nm to approximately 490 nm, and the second optical wavelength is in a second range from approximately 590 nm to approximately 680 nm.
7 . The method of claim 6 , wherein the first optical wavelength is 635 nm, and the second optical wavelength is 450 nm.
8 . The method of claim 1 , wherein the sharp cut cyclone is a PM 2.5 sharp cut cyclone.
9 . The method of claim 8 , wherein the flow rate through the PM 2.5 sharp-cut cyclone is volumetrically controlled specific to the chosen sharp-cut cyclone.
10 . The method of claim 1 , wherein an air mass sample heater is included to reduce humidity, so that relative humidity is controlled to be <35%.
11 . The method of claim 1 , wherein the determining the airborne particle concentration for a range of particle sizes is further based on a function of the SAE that is based at least in part on a curve fit of empirical mass scattering efficiency data plotted against SAE as a surrogate to particle size.
12 . The method of claim 11 , wherein the function is broken up into segments based on ranges of the SAE.
13 . The method of claim 1 , wherein the determining the SAE further uses the first optical wavelength and the second optical wavelength.