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Control and Conditioning Components

Size Control

NOAA has designed a rack-mounted impactor drawer to control size cut switching of the aerosol sample. There are a 1 µm and 10 µm within the rack-mounted impactor drawer upstream of the nephelometer and PSAP. The impactors ensure that only particles smaller than the impactor size cut, are measured. The 10 µm impactor is always in line, while every 6 minutes (for humidograph systems, the impactor switching is set to every 30 minutes) the 1 µm impactor is switched in line and the submicrometer (PM1) aerosol are measured for 6 minutes (or 30 minutes). Particles smaller than 1 µm are important as they are the dominant contributor to light scattering. Measuring light scattering by particles smaller than 10 µm (PM10) is useful as that tends to encompass all the scattering by particles.

Sample Air Humidity Control

Basic measurements of the aerosol are made at low RH (<40%) conditions. If necessary, the sample is heated using humidity controllers and heaters upstream of the aerosol instrumentation to maintain the relative humidity in the reference nephelometer at less than 40%.

A number of calibrated Vaisala T/RH sensors are used to monitor temperature and RH throughout the system. These measurements are also used to ensure the reference measurements are at the correct humidity ~50%, and they provide a check that there are no leaks in the system, i.e., dewpoint should remain constant unless water vapor is being added or diluted in the aerosol air stream. The nephelometers also have built in T/RH sensors.

Sample Dilution System

At some sites with high humidity and/or high concentrations of aerosol, a dilution system is used to lower humidity and/or aerosol concentrations to prevent damage to instruments by water condensation or particle deposition.

Stack/Inlet system

NOAA/GML have designed an aerosol stack and inlet system that is relatively inexpensive to build and provides excellent sampling efficiency for sub-10µm (PM10) aerosol . It is less effective for ultra-fine aerosol. The high sample flow (150 lpm) allows for multiple instruments to get their sample from the same inlet.

Pumpbox

NOAA/GML pumpbox design includes the actual box containing the pumps as well as pumps, a filter for pump exhaust and various electronic and analog indicators for diagnosing pump status.