An active RH-controlled dry-ambient aerosol size spectrometer (DAASS) for the accurate measurement of ambient aerosol water content

A dry-ambient aerosol size spectrometer (DAASS) was previously developed to conduct measurements of aerosol water content at ambient relative humidity (RH) conditions. However, the RH difference of ambient RH and DAASS conditioning humidity (i.e., 10-20%, particularly when the DAASS operated at a high ambient RH) may introduce uncertainties in estimation of aerosol water content. In this study, we present a prototype of an RH-controlled DAASS. By employing an active RH controlling method, we resolve the discrepancy between the ambient RH and the DAASS conditioning RH, therefore extending the accurate measurement of aerosol water content to a wide RH range. The performance of this RH-controlled DAASS was characterized by both the laboratory evaluation and a 10-day field deployment. Results showed that the DAASS conditioning RH agreed well with the ambient RH (R2 of 0.993 when ambient RH up to 90%), with an average RH offset of less than 1.5%. The fraction of aerosol-bound water in the total ambient aerosol volume was -10% under RHs below 50% and increased to -50% when the ambient RH reached above 85%.

Automated summary

The study developed a prototype of an RH-controlled DAASS to improve the accuracy of aerosol water content measurement by resolving the discrepancy between ambient RH and DAASS conditioning RH. Experimental results showed that the RH-controlled DAASS performed well across a wide range of RH values and could be used for aerosol water content measurement in actual environmental conditions.