Articles | Volume 10, issue 7
Research article
13 Jul 2017
Research article |  | 13 Jul 2017

Bias caused by water adsorption in hourly PM measurements

Gyula Kiss, Kornélia Imre, Ágnes Molnár, and András Gelencsér

Abstract. Beta-attenuation monitors are used worldwide to monitor PM mass concentration with high temporal resolution. Hourly PM10 and PM2. 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 PM2. 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 %.

Short summary
As a result of multiple efforts the PM10 mass concentration has been reduced in many developed countries in the past 2 decades. However, as the PM10 mass concentrations are decreasing, the potential relative bias caused by water interactions is likely becoming more significant. This is simply due to the fact that the bias caused by adsorption or desorption of water on and from the filter medium becomes proportionally more significant relative to the aerosol mass deposited on the filter.