There is increasing concern about the health impacts of ambient Particulate Matter (PM) exposure. Recent studies have shown portable low-cost devices (e.g., optical particle counters, OPCs) can be utilised to determine the spatial-temporal characteristic of ambient PM.

However, their application under ambient conditions can be affected by high relative humidity (RH) conditions. Here, we show how, by exploiting the measured particle size distribution information rather than simply mass concentration[1], a correction can be derived which significantly improves sensor performance in different RH conditions while retaining fundamental information on particle composition.