LAS-AIR OPTICAL PARTICLE COUNTER

AUTHOR:
Cynthia Twohy
Ocean Admin 104
Oregon State University
Corvallis, OR 97331-5503
Phone: (541) 737-5690  Fax: (541) 737-2540 
twohy@coas.oregonstate.edu ; http://www.oce.orst.edu/ats/twohy/

OVERVIEW:  The LAS-AIR is a commercial instrument (PMS Model 1001) that measured the 
size distribution of particles between about 0.1 to > 3 microns diameter in 8 size channels.  
During DYCOMS, the LAS-AIR was one of a number of instruments sampling off the solid 
diffuser inlet (SDI) which was mounted on the right side of the C-130 aircraft. To dry the sample 
and prevent coincidence at high concentrations, the LAS-AIR sample stream was diluted with 
dry nitrogen in a 5:1 ratio (5 parts N2 to 1 part sample). All flow rates were monitored and 
maintained with constant volume flow controllers. The LAS-AIR samples over a 6 sec interval, 
so the data rate is once every 6 seconds. Rather than using the manufacturer's calibration, size 
limits were calibrated using monodisperse ammonium sulfate aerosol. Additional information on 
the LAS-AIR itself can be found on the PMS website at 
http://www.pmeasuring.com/particle/products/aerosol/docs/lasair .

DATA FILES: The following variables in the space-delimited ascii data files relate to the LAS-
AIR:

END-TIME: end time of sample period in UTC seconds after midnight. For example, 10:30 UTC 
would equal 37800 sec (10*3600+30*60).
CONCTOT: total number concentration in the size range of the instrument. Units are cm^-3 at 
ambient temperature and pressure.

CONC1-CONC8: number concentrations in each size range. Lower limits of the 8 size channels, 
in micrometers diameter, are 0.13, 0.25, 0.34, 0.45, 0.62, 0.83, 1.1, 2.4 (no specified upper limit). 
Units are cm^-3 at ambient temperature and pressure.

OPCD: particle geometric mean diameters based on the channel midpoints. Units micrometers.

OPCV: total particle volume based on the channel midpoints. Units micrometer^3 cm^-3. 

OPCDF: amount the sample was diluted with N2 to dry the sample and to avoid coincidence 
(usually set at 6.0).

FLAG: Flag to designate flow rate condition. Value of 0 means the sample is dried and all flow 
rates were as they should be. Value of 1 means dilution flow is off (sample is not dried). If 
accompanied  by -99999s for the concentration data, there were other flow problems (LAS-AIR 
flow less than 0.20 lit min^-1 or dilution flows not properly matched.)
. 

DATA COLLECTION AND ANALYSIS: The LAS-AIR count data were collected on a 
standard PC over its serial port, and flow rates were recorded on the RAF aircraft data system as 
OPC1, OPC2, and OPC3. Data were integrated and converted to concentrations with a Matlab 
program.

DATA PROBLEMS: Flights with missing or problem data are given below:

R1 and R2 (10 July and 11 July 2002): LAS-AIR inoperative.

R3 (13 July 2002): Two short periods with flow problems, FLAG=1, concentrations -99999s. 

R4 (17 July 2002): Sample not dried after 25180s, FLAG=1. 

R5 (18 July 2002): Sample not dried after 27400s, FLAG=1. 

R6 (20 July 2002): Sample not dried between 23570-25810s, bad flows 25810-28420s. 

R9 (27 July 2002): One period with bad flows at beginning (68430-69110s), FLAG=1, 
concentrations -99999. 

R10 (28 July 2002): Three short periods with bad flows, FLAG=1, concentrations –99999.