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Beta-attenuation monitors are used worldwide to monitor PM mass concentration with high temporal resolution. Hourly PM₁₀ and PM_2. 5 dry mass concentrations are publicly available with the tacit assumption that water is effectively removed prior to the measurement. However, as both the filter material of the monitor and the aerosol particles are capable of retaining a significant amount of water even at low relative humidities, the basic assumption may not be valid, resulting in significant bias in reported PM₁₀ and PM_2. 5 concentrations. Here we show that in PM₁₀ measurement, particle-free air can produce apparent hourly average PM concentrations in the range of −13–+21 µg m⁻³ under conditions of fluctuating relative humidity. Positive and negative apparent readings are observed with increasing and decreasing relative humidities, respectively. Similar phenomena have been observed when the instrument filter was previously loaded with atmospheric aerosol. As a result the potential measurement biases in hourly readings arising from the interaction with water may be in the range of −53… + 69 %.