What is a Micro-Aethalometer; how do portable Black Carbon (BC) Monitors work?
Following @ Friddy's comment, I started reading Aethlabs AE51 FAQ and was quite surprised!
While a conventional aethalometer use a long tape and move it to the new position after each measurement (which could be as fast as one measurement per minute), the compact device shown in the image in question uses a single filter position, and just keeps measuring as it gets darker and darker as black carbon collects.

Q: Do all the microAeth use the same filters?
A: No. The microAeth AE51 uses a T60 Teflon-coated borosilicate glass fiber filter strip which has 1 sampling location and must be manually replaced. The microAeth MA200 uses a roll of Polytetrafluoroethylene (PTFE) filter material housed in a filter tape cartridge which has 15 sampling locations that can automatically advance. The microAeth MA300 and MA350 use a roll of Polytetrafluoroethylene (PTFE) filter material that is housed in a filter tape cartridge which has 85 sampling locations that can automatically advance. Filter tape cartridges have to be manually replaced much less often than a single use filter strip.

Also:

[...] A guide for measurement limits regarding saturation can be found on the AE51 Specifications Sheet. For example, filters need to be changed according to the following BC concentration/flow rate ranges:

 - Avg.   5 µg BC/m3   for 24 hours    @ 100 ml/min
 - Avg. 100 µg BC/m3   for  3 hours    @ 50 ml/min
 - Avg.   1 mg BC/m3   for 15 minutes  @ 50 ml/min

Compare this to a conventional aethalometer:

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