Monitoring personal exposure to particle matter (PM_2.5) in ambient air requires performing measurements using portable monitors. In this work, the portable nephelometer SidePak™ AM510 Personal Aerosol Monitor manufactured by TSI Inc. was evaluated against a Tapered Element Oscillating Microbalance (TEOM) equipped with a Filter Dynamics Measurements System (FDMS). Conventionally, the SidePak is calibrated with respect to the Arizona Road Test Dust and then multiplied by an environmental calibration factor to yield mass concentration. To adapt this calibration to specific field conditions, the authors present an implementation of this calibration by introducing a growing factor correction which takes into account relative humidity and the dry and wet portions of the refractive index estimated from TEOM-FDMS measurements. PM_2.5 sampling with several SidePaks AM510 was carried out in background and rural sites in the Po Valley (Italy). Modeled SidePak data were plotted vs. reference TEOM-FDMS data which show a good agreement.