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This article presents the ambient air pressure effects on a balloon borne optical particle counter (an aerosol sonde: AS) equipped with a laser as the light source, and the relevant measures to overcome these effects. To investigate the effects of ambient air pressure varying from 1013 to 10 hPa (from ground level to an altitude of about 30 km) and estimate the general performance of the AS, a novel versatile pressure-variable test chamber was constructed equipped with a built-in nebulizer system. To overcome the direct effect of ambient air pressure on the sensing zone occurring when an open cavity laser (an external mirror-type laser) is used, a flat parallel window was adopted in place of the Brewster window, and in addition, only the laser tube was sealed in an aluminum tube under normal atmospheric pressure. Consequently, the laser power change was suppressed to within ±0.5% for the pressure variation range. To overcome the large dependence in the aerosol sampling flow rate on the ambient air pressure, a new flow rate ratio (flow rate at low pressure divided by that at 1013 hPa) was defined to fall within the ±0.5% variation, as measured using a newly developed technique for measuring the flow rate ratio, owing to an incorporated gear pump system devised to be speed controlled through a pressure sensor. The nonlinear increase of the noise component with decreasing ambient air pressure is discussed, and shown to be overcome electrically, confirming the presumption that this increase is ascribable to the corona discharge caused by high voltage. Thus, for polystyrene latex spheres 0.1 μm in diameter, the developed AS maintained signal-to-noise ratio larger than 2–3 for the pressure variation, as revealed from analysis of the histograms obtained with a multichannel analyzer. Finally, actual field measurements were performed at Bandong, Indonesia, and the results were subjected to a cross-check with those obtained almost simultaneously at - the same location using an established conventional sonde, confirming the satisfactory accuracy and reliability of the developed AS. © 2003 American Institute of Physics.