A modified liquid impingement unit is configured to automatically maintain a volume of buffer solution within a determined range, the buffer solution used to collect airborne particles from an impinging airflow. The liquid impingement unit includes a first end of an air nozzle and a first end of a vacuum tube sealed within collection vessel. A fluid delivery tube is positioned within the vacuum tube such that a first end of the fluid delivery tube is also positioned within the sealed collection vessel. External to the sealed collection vessel, the fluid delivery tube branches from the vacuum tube through an aperture in the vacuum tube. The vacuum tube is coupled to a vacuum pump and the fluid delivery tube is coupled to a fluid pump. A control module provides control signals to the fluid pump such that the fluid pump delivers a second volume of buffer solution from a buffer solution container to the collection vessel via the fluid delivery tube.