An apparatus for measuring aerosol particles (P0), the apparatus comprising: - a charging unit to form charged particles (P1) by charging particles (P0) of an aerosol sample flow (FG1) by diffusion charging, - a collecting unit to provide an electric current (I 1 (t)) by collecting charges from the charged particles (P1) by diffusion of the charged particles (P1), the electric current (I 1 (t)) being indicative of number density (n 0 (t)) of aerosol particles (P0) of the aerosol sample flow (FG1), - a pressure reducing unit (PDU1) to reduce pressure of the aerosol sample flow (FG1), and - a suction unit (VAC1) to draw the aerosol sample flow (FG1) via the charging unit to the collecting unit , wherein the collecting unit is positioned downstream of the pressure reducing unit (PDU1), wherein a charging efficiency function (η 1 (d p )) is indicative of an efficiency of the diffusion charging to charge the particles (P1) 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 (P1) 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 (VAC1) is positioned downstream of the collecting unit , wherein the pressure reducing unit (PDU1) and the suction unit (VAC1) 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 (VAC1) comprises the pump (PUMP1) for drawing a flow (FG4) from the collecting unit , wherein the pump (PUMP1) and/or an adjustable valve (VAL1) of the suction unit (VAC1) is arranged to control a flow rate (Q 1 ) via the suction unit (VAC1), wherein the suction unit (VAC1) comprises a pressure sensor (PSEN1) for monitoring the internal pressure (p 2 ) of the collecting unit , and a control unit (CNT2) for adjusting the flow rate (Q 1 ) of the suction unit (VAC1), 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 (CNT2) is arranged to provide a control signal for controlling operation of the valve (VAL1) 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 (P1) by charging particles (P0) of an aerosol sample flow (FG1) by diffusion charging, - a collecting unit to provide an electric current (I 1 (t)) by collecting charges from the charged particles (P1) by diffusion of the charged particles (P1), the electric current (I 1 (t)) being indicative of number density (n 0 (t)) of aerosol particles (P0) of the aerosol sample flow (FG1), - a pressure reducing unit (PDU1) to reduce pressure of the aerosol sample flow (FG1), and - a suction unit (VAC1) to draw the aerosol sample flow (FG1) via the charging unit to the collecting unit , <EOS>