The Alphasense optical particle counter (OPC) provides a low-cost and lightweight solution for measurements of the size and number concentration of airborne particulate matter (PM). The micro fan with which it is originally equipped cannot, however, achieve a high enough pressure differential for maintaining an adequate flow rate when connected to sampling/pretreatment aerosol lines, limiting its use for air quality monitoring. Here, we propose a simple modification on the sample flow system that enables the connection of the Alphasense OPC with sampling/pretreatment lines (e.g., dryers) commonly employed on ground observational sites, as well as its use onboard Unmanned Aerial Systems (UASs). Tests of the modified OPC using monodisperse polystyrene spherical (PS) particles having sizes between 0.8 and 2.5 μm show that both the sizing and counting performance of the modified instrument is in agreement with that of a calibrated reference OPC at concentrations from ca. 50 to 800 #/ml, which are typically encountered in the atmosphere in that size range. For particle number concentrations below or above this range, we observed a concentration-dependent counting efficiency, which can be corrected using a polynomial function derived from our measurements. Tests conducted under reduced pressure and temperature conditions demonstrate the capability of the modified OPC for accurately (i.e., within 13% deviation from the reference measurements) determining the size and number concentration of the sampled particles. The tests reported in this work show that the proposed modification can qualify the Alphasense OPC for use in both ground and aerial observations without affecting its performance, and at the same time maintaining its strongly desired characteristics (i.e., cost effectiveness and high portability).@en