| 05 Aug 2026
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.
Review report for ”Assessing the benefit of sample drying for AE33 aethalometers: evidence from long‑term field observations”
In their study, Riley et al. investigate whether drying the sampled air is critical in Aethalometer (model AE33) measurements. As AE33 instruments are widely used in monitoring black carbon (BC) concentration and as BC is an emerging air quality parameter, the number of active AE33 measurements will likely increase even further. Aiming to keep the measurements comparable, drying the sample is recommended by several research infrastructures. However, setting up a dryer is not always possible or the drying capacity is not sufficient to keep the relative humidity (RH) under the recommended 40 %. This study raises an important point by studying whether we can still get comparable results for black carbon even though the aerosol sample has not been dried. I suggest accepting the manuscript after changes, which I have suggested below.
General comments
The manuscript does not define the RH range observed for the dry or the ambient sample. That would be important to define so that they could be compared to other measurements. Would be good to show also the RH for the dried sample as a background data. Was it measured or has it been tested if the drying capacity was enough (e.g., did it stay below 40 % as recommended by GAW)? How big fraction of the ambient sample was below RH < 40 %, which could be considered dry aerosol by GAW and is not expected to affect the measurements so much.
The manuscript focuses on equivalent black carbon measurements, which are conducted on wavelength 880 nm, which is of course useful in the viewpoint of air quality monitoring. Comparison at other wavelengths is also shown but not discussed so much. This could be done with absorption Ångström exponent (AAE), which is a very commonly derived parameter from AE33 data and used in the “aethalometer model” also mentioned in the manuscript. Therefore, by extending the analysis on AAE, the scientific impact of the manuscript would be improved.
Detailed comments
Abstract: Please mention what were the RH ranges for dry and ambient samples to define what is meant with dried and ambient samples.
L179: What was the shed temperature on average and how much it varied? I think stating this shortly would be good background information for the reader to have an idea of the differences in outdoor and indoor temperatures (as it affects the sample RH).
L195-197: It would be good to mention here the materials of the filters and if they can they be affected humidity in a different manner?
L233-234: Indeed, it is unfortunate that the instruments were not compared. Having even a short comparison period before each deployment at the station either with ambient or dried sample would have make the results more reliable. But I assume that having a regular maintenance and testing for the instruments gives good enough basis here. Performing the neutral density tests should raise an issue, if the instrument would measure too high or low concentrations. I suggested mentioning the unit-to-unit variability (Cuesta-Mosquera et al., 2021) also here to remind the reader about expected agreement between the instruments.
Figure 2: Please mention that the T, RH, and dew point are ambient outdoor measurements to avoid misunderstandings. RH from indoor could be included in the RH panel to show the variation between indoor and outdoor air. RH is more practical parameter and easier to understand than DPD. If I understood correctly, the indoor RH was measured with the Vaisala sensor (or it could be estimated based on the outdoor T and RH, and indoor T).
L262-263: I do not understand why run the tests for ambient outdoor RH as that does not represent the measurement conditions, especially not in winter if the sampled air warms up to shed temperature and the sample RH decreases. This is also questioned in the manuscript on L389-394 and in the discussion. I would recommend running the tests for indoor (or sample) RH, which is typically monitored at stations or by aerosol instruments themselves and therefore the results would be easier to interpret.
L356-359 Isn’t this expected, as those are the months when the indoor ambient sample RH is the highest because there is no “passive drying” due to the indoor and outdoor temperatures being fairly similar.