A sensor for sensing airborne particles larger than 10 nm, comprising a passage (110, 112, 114) with open ends, the passage (110, 112, 114) having disposed in it: - a discharge electrode for generating airborne unipolar ions in the passage (110, 112, 114); - an air-pervious counter electrode (124, 324) located downstream of the discharge electrode and adapted to attract said airborne unipolar ions; - an air-pervious screening electrode (128; 372, 374) located downstream of the air-pervious counter electrode (124, 324); and - a particle sensing section for sensing electrically charged airborne particles in the passage (110, 112, 114), the particle sensing section being located downstream of the air-pervious screening electrode (128; 372, 374), wherein the air-pervious screening electrode (128; 372, 374) is adapted to subject airborne ions to an electrostatic force in a second direction, distinct from the downstream direction, for separating airborne ions from the airflow; characterized in that: the air-pervious counter electrode (124, 324) is located upstream of the air-pervious screening electrode (128; 372, 374) in the passage (110, 112, 114); the geometry of the air-pervious counter electrode (124, 324) is adapted to cause a downstream net flow of airborne ions substantially in the downstream direction of the passage (110, 112, 114) such that the motion of airborne ions from the discharge electrode towards the counter electrode (124, 324) generates an ionic wind that can create and sustain an airflow through the passage (110, 112, 114) without having to rely on other means for generating airflow; wherein a path of the airborne ions drawn from the discharge electrode to the counter electrode (124, 324) is such that paths of all parts of the airflow cross the path of the ions, so that no airborne particles, or at least a limited portion of the airborne particles, can escape exposure to airborne ions; and wherein the counter electrode (124, 324) causes little or no flow resistance. - a discharge electrode for generating airborne unipolar ions in the passage (110, 112, 114); - an air-pervious counter electrode (124, 324) located downstream of the discharge electrode and adapted to attract said airborne unipolar ions; - an air-pervious screening electrode (128; 372, 374) located downstream of the air-pervious counter electrode (124, 324); and - a particle sensing section for sensing electrically charged airborne particles in the passage (110, 112, 114), the particle sensing section being located downstream of the air-pervious screening electrode (128; 372, 374), the air-pervious counter electrode (124, 324) is located upstream of the air-pervious screening electrode (128; 372, 374) in the passage (110, 112, 114); the geometry of the air-pervious counter electrode (124, 324) is adapted to cause a downstream net flow of airborne ions substantially in the downstream direction of the passage (110, 112, 114) such that the motion of airborne ions from the discharge electrode towards the counter electrode (124, 324) generates an ionic wind that can create and sustain an airflow through the passage (110, 112, 114) without having to rely on other means for generating airflow; wherein a path of the airborne ions drawn from the discharge electrode to the counter electrode (124, 324) is such that paths of all parts of the airflow cross the path of the ions, so that no airborne particles, or at least a limited portion of the airborne particles, can escape exposure to airborne ions; and wherein the counter electrode (124, 324) causes little or no flow resistance.