FAQs AMS Data Analysis

The purpose of this page is to serve as a repository of FAQs of AMS data analysis.

How do you account for variable CO2 in the sampled air during analysis ?

• The AMS samples particles 10 million times (1e7) more efficiently than gases, due to the lens and differential pumping system. E.g. N2 is about 1 kg/m3 of air, but in the AMS we measure it as 100 ug/m3 of equivalent aerosol signal, or 1e-7 of the actual concentration.

• A typical CO2 concentration is of the order of 400 ppm of air, which is of the order of 400 mg/m3, which creates to about 40 ng m-3 of equivalent aerosol signal. This average value is generally subtracted with the Squirrel frag table and the HR frag table, as discussed in this section of the ToF-AMS Field Data Analysis Guide.

• If CO2 levels in your experiment are pretty constant AND the amount subtracted is a small fraction of the aerosol CO2+ signal, that constant subtraction may be sufficient.

• If CO2 levels vary a lot in your experiment, or reach much higher values (as when sampling concentrated combustion exhaust), AND/OR if the gas-phase CO2+ subtraction is a substantial (> 10%, or whichever criterion you establish based on your desired precision and accuracy), then you need to implement a time-dependent subtraction based on a CO2 gas-phase measurement.

• If the CO2 gas-phase concentrations are very high (percent) you may be able to measure them with the AMS PToF mode, which also may allow separation of the gas vs particle phase components.