Assessing the benefit of sample drying for AE33 aethalometers: evidence from long‑term field observations

Abstract. Aethalometers measuring equivalent black carbon (eBC) are widely used in both research and regulatory air quality monitoring networks. However, uncertainties regarding the influence of moisture in eBC measurements persist. While many networks recommend or require sample drying, there is limited long‑term field evidence quantifying the benefits of humidity control for aethalometers operated under real‑world conditions. Here we present a multi‑year field intercomparison of aethalometers (AE33, Aerosol Magee Scientific) operated with and without sample drying at an urban background site in Sydney, Australia. Three extended tests (December 2015 – August 2016, November 2016 – June 2017, January 2018 – March 2019) were conducted using four instruments in total. Instruments were co‑located within the same climate‑controlled enclosure and configured identically, differing only in sample humidity conditioning (membrane dryer). Across all tests, wavelengths and averaging periods, strong agreement was observed between dried and non‑dried instruments, with correlation coefficients (R²) consistently exceeding 0.98. For the 880 nm channel, regression slopes for non-dried vs dried sampling ranged from 0.97 – 1.07 across the three tests. Similar results were obtained for 1‑, 2‑, 5‑, and 10‑minute averages, although variability increased at shorter averaging intervals. Stratification by ambient relative humidity, dew‑point temperature, and dew‑point deficit showed no systematic degradation in agreement between hourly averages at high humidity, and moisture‑related effects were small compared with test‑to‑test variability. These results indicate that, for hourly eBC measurements in a temperate, high‑humidity coastal environment, the absence of sample drying introduces only small differences in measured concentrations when compared with humidity‑controlled sampling. While short‑term variability and instrument‑specific effects remain important considerations, these results suggest that long‑term, network‑scale eBC measurements from AE33 aethalometers can remain highly comparable even without sample drying. This study provides quantitative evidence to inform guidance on humidity control for aethalometers in monitoring networks.