Aerosol hygroscopicity: Hygroscopic growth proxy based on visibility for low-cost PM monitoring

Visibility is a simple indicator of air quality, and its drastic decrease is often related to poor air quality resulting from an increase in aerosol (particulate matter PM) concentration. Visibility is also related to water vapor due to the hygroscopicity of aerosols. As a result, water vapor may considerably influence PM measurements especially in the case of low-cost PM sensors which typically measure the ambient (wet) size of PM. Several possibilities are available to eliminate the effect of aerosol hygroscopicity on PM10 measurements, and we aimed to discuss and compare three of the methods: gravimetry (mass change of aerosol filters due to RH variation), application of the AIM model (based on aerosol chemical composition) and estimation derived from visibility data. In this work, we discuss how hygroscopic growth factors, obtained from different methods, are related, as well as the relevance of the hygroscopic growth rate derived from visibility observations in PM10 measurements. Moreover, since in PM monitoring - including the low-cost PM sensors - a quasi-real time, appropriate and simple method would be desirable for consideration of aerosol hygroscopicity, we aimed to construct a proxy for this purpose. We found that the visibility-derived mass growth rate could serve as a simple basis for these requirements.