NCAR-ACOM PSD (particle size distribution) instrument
John Ortega
January 19, 2017

The NCAR Particle Size Distribution (PSD) instrument is the super-position of two Scanning Mobility Particle Sizers (SMPS).  The same sample flow of ambient air is ionized with a Po210 charger, and then sent to both a nano-SMPS and a regular SMPS at flow
rates of 1.5 and 1 L/min respecively.  Sheath air and high voltage to the DMA is supplied using an NCAR-built blower box that continuously scans the voltage from 0 to ~ 8000V and back from 8000 V to 0 completing a cycle every 280 seconds followed by 20 seconds idle time.  A new cycle is started every 300 seconds starting at midnight and every 5 minutes after that resulting in 288 up/down scans per day.  The mono-disperse particles exiting the 2 DMAs are counted by TSI 3025 (nsmps) and a TSI 3760 (rsmps) condensation particle counters (CPC).  The nsmps counts particles with mobility diameters between ~4nm and ~80 nm, and the rsmps counts particles with mobility diameters between ~40nm and 300 nm.  The overlap region between the two distributions is merged during post processing.  Only singly charged particles are considered.  The raw data is inverted using a 1st principles technique where the raw counts are converted in to dN/dlogDp using the Fuch's charging efficiency, DMA penetration efficiency, diffusion losses, CPC counting
efficiency and DMA transfer function.  This summarized in Stolzenburgn and McMurry (Aerosol Science and Technology, 42: 421-432, 2008).  

The inlet for the instrument is ~3 meters above ground level at the Manitou Experimental Forest Observatory (near Woodland Park, CO).  It consists of a 3/8" copper tube protected against rain and large particles and insects and brought through a port into one of the climate-controlled trailers at the site.  Instrument control and data storage is done using a Labview code and the data inversion is done in a subsequent steps using a series of Matlab scripts.  

The data is saved in MS Excel worksheets, which can easily be read in by any standard data analysis program.  The first two rows are Dp in nm and dlogDp respectively.  The first column is dime of day (in decimal day; 0.25 = 6am), and subsequent columns are dN/dlogDp for each Dp listed in the first row followed by integrated particle number counts (N in cm^-3), Particle surface area in um2/cm3, volume in um3/cm3 and mass in ug/m3.  Time is Mountain standard time.