An apparatus for measuring aerosol particles, the apparatus comprising: - a charging unit to form charged particles by charging particles of an aerosol sample flow by diffusion charging, - a collecting unit to provide an electric current (I 1 (t)) by collecting charges from the charged particles by diffusion of the charged particles, the electric current (I 1 (t)) being indicative of number density (n 0 (t)) of aerosol particles of the aerosol sample flow, - a pressure reducing unit to reduce pressure of the aerosol sample flow, and - a suction unit to draw the aerosol sample flow via the charging unit to the collecting unit, wherein the collecting unit is positioned downstream of the pressure reducing unit, wherein a charging efficiency function (η 1 (d p)) is indicative of an efficiency of the diffusion charging to charge the particles as a function of particle size (d p), wherein the charging efficiency function (η 1 (d p)) is approximated by a first exponential function having a first exponent (α), wherein a collecting efficiency function (η 2 (d p)) is indicative of an efficiency for collecting the charges by diffusion of the charged particles as a function of particle size (d p), wherein the collecting efficiency function (η 2 (d p)) is approximated by a second exponential function having a second exponent (-β), characterized in that a pump (PUMP1) of the suction unit is positioned downstream of the collecting unit, wherein the pressure reducing unit and the suction unit are arranged to keep an internal pressure (p 2) of the collecting unit at a selected pressure value (p SET) such that a negative slope (Δη 2 /Δd p) of the collecting efficiency function (η 2 (d p)) at least partly compensates a positive slope (Δη 1 /Δd p) of the charging efficiency function (η 1 (d p)) at least in the particle size range of 40 nm to 200 nm, such that the sum of the first exponent (α) and the second exponent (-β) is in the range of 0 to 0.65, the selected pressure value (p SET) being smaller than or equal to 80 kPa, wherein the first exponent (α) is in the range of 1.05 to 1.50, wherein the second exponent (-β) is in the range of -1.00 to -0.85, wherein the suction unit comprises the pump (PUMP1) for drawing a flow from the collecting unit, wherein the pump (PUMP1) and/or an adjustable valve of the suction unit is arranged to control a flow rate (Q 1) via the suction unit, wherein the suction unit comprises a pressure sensor (PSEN1) for monitoring the internal pressure (p 2) of the collecting unit, and a control unit for adjusting the flow rate (Q 1) of the suction unit, so as to keep the internal pressure (p 2) of the collecting unit substantially equal to the selected pressure value (p SET), wherein the control unit is arranged to provide a control signal for controlling operation of the valve and/or for controlling operation of the pump (PUMP1) based on a signal (S P) of the pressure sensor (PSEN1). - a charging unit to form charged particles by charging particles of an aerosol sample flow by diffusion charging, - a collecting unit to provide an electric current (I 1 (t)) by collecting charges from the charged particles by diffusion of the charged particles, the electric current (I 1 (t)) being indicative of number density (n 0 (t)) of aerosol particles of the aerosol sample flow, - a pressure reducing unit to reduce pressure of the aerosol sample flow, and - a suction unit to draw the aerosol sample flow via the charging unit to the collecting unit,