Demonstration of the New Aethalometer Model AE36s Performance and Equivalence with Its Predecessor, AE33

MARTIN RIGLER, Gašper Lavrič, Matic Ivančič, Viktor Pilko, Bálint Alföldy, Irena Ježek Brecelj, Asta Gregorič, Aerosol d.o.o.

     Abstract Number: 458
     Working Group: Instrumentation and Methods

Abstract
The Aethalometer is a widely used filter photometer capable of measuring the light-absorbing properties of aerosol particles. In this study, we present the performance of the new Aethalometer model AE36s (Aerosol Magee Scientific) and show equivalence with its predecessor AE33. The new model addresses two crucial areas that have emerged in the scientific community in recent years: (1) the sensitivity of filter photometers to rapid changes of relative humidity, (2) the need for improved characterization of the absorbing part of organic aerosols in blue and UV region, the so-called brown carbon (BrC). Despite technical changes and improvements of the instrument, continuity of data of absorption measurements is essential; thus, in this study, we demonstrate the equivalence with AE33 through a series of laboratory and ambient measurement campaigns.

Aerosol samples contain water vapor (represented by its relative humidity, RH), which can be adsorbed to the fibers or to the binding material of the filter tape used in filter photometers. It has been shown that rapid changes in humidity (dRH/dt), rather than absolute humidity itself, have a significant effect on the measurements made with the filter photometers. Water vapor can reach the filter through the sample inlet or enter through openings in the filter tape compartment, especially in environments where relative humidity changes rapidly (air-conditioned containers, mobile stations, etc.) While the RH changes in sample air are adequately decreased with a Sample Stream Drier (Aerosol Magee scientific), the problem of rapid RH changes in the vicinity of the filter tape is solved in the new Aethalometer model by air sealing the filter compartment. In this study, the AE36s was subjected to various tests, sampling particle-free air with stable RH, whereas the humidity in the surroundings of the instrument changed rapidly, simulating the fluctuation of RH introduced by air-conditioning.

Additionally, to improve the characterization of BrC share of absorption in blue and UV regions, two additional wavelengths were introduced in the new Aethalometer model AE36s with respect to the older models: the 400 nm increases the resolution of absorption in a large gap of AE33 channels between 370 nm and 470 nm, and 340 nm extends the measurement range in the UV spectrum.

The performance of AE36s with improvements mentioned above was evaluated and compared to its predecessor AE33 during a long-term field campaign at an urban background site in Ljubljana, Slovenia (winter-spring 2022 and winter 2023), and laboratory tests with an artificial soot generator. Slopes of the orthogonal regression for all common wavelengths are within 5% range relative to the unity with R2 > 0.99. Signal to noise ratio in the AE36s is also significantly improved, which results in more stable determination of the absorption Ångström exponent.