Impactors are used to provide information about the size of the sampled aerosol particles. The sample flow always passes through a 10 micrometer impactor. Every 6 minutes a valve switches and the flow is sent thorough a 1 micrometer impactor for 6 minutes. (Different switching times are used for some systems with auxilliary instrumentation like a humidograph system.) The impactor provides a convoluted path for the air to flow through in order to remove particles above a certain size by inertial impaction. The impactor box can be arranged in a rack drawer or in a space within a rack.
This is a front view of the impactor box. The 10 um impactor is on the left and the 1um impactor is in the center. The big yellow Whitey valve switches flow between 10 and 1um impactors. The impactors should be cleaned periodically (station specific schedule). In the top center of the picture (inside the pink rectangle) is the mass flow controller (MFC) with a white hepa filter attached to the right side of it.
This is the back of the impactor box. This is where the nephelometer(s) and PSAP get their sample aerosol. After going through the impactorsi, the airflow is split to go (a) to the nephelometer(s) and (b) to the PSAP. If a system no longer utilizes a PSAP, the PSAP port is capped. Sites with CLAPs typically pick off the CLAP sample flow downstream of the nephelometer sample volume.
This schematic shows one way (the normal way) air can flow through the impactor box. If a span check is happening or the system is bypassed then different there are different flow paths.
CPDlive controls the switching of the electronic ball valve and also what solenoid is open or closed.
The zero check port is connected to a 3-way valve. After one cleans the impactors one can use the leak test procedure described in the manual to see whether changing the impactors induced a leak in the system.