The effect of rapid relative humidity changes on fast filter-based aerosol particle light absorption measurements: uncertainties and correction schemes

Measuring vertical profiles of the particle light absorption coefficient by using absorption photometers may face the challenge of fast changes in relative humidity. These absorption photometers determine the particle light absorption coefficient due to a change in light attenuation through a particle-loaded filter. The filter material, however, takes up or releases water with changing relative humidity (rh in %), influencing thus the light attenuation. A sophisticated set of laboratory experiments was therefore conducted to investigate the effect of fast rh changes (drh/dt) on the particle light absorption coefficient (σabs in Mm−1) derived with two absorption photometers. The rh dependency was examined based on different filter types and filter loadings with respect to loading material and loading areal density. Different filter material was used in the two examined instruments. The Single Channel Tri-Color Absorption Photometer (STAP; Brechtel Manufacturing Inc, 1789 Addison Way, Hayward, CA 94544, USA) relies on quartz-fiber filter (PALL LifeScience, Pallflex Membrane Filters Type E70-2075W) and the microAeth® MA200 (AethLabs, 1640 Valencia St, Suite 2C, San Francisco, CA 94110, USA) is based on a Polytetrafluoroethylene (PTFE) filter band. Furthermore, three cases were investigated: clean filter, filter loaded with black carbon (BC) and filter loaded with ammonium sulfate. The filter loading areal densities (ρ*) ranged from 3.1 to 99.6 mg m−2 in the case of the STAP and ammonium sulfate, 1.2 to 37.6 mg m−2 considering the MA200. Investigating BC loaded cases, ρ*BC was in the range of 2.9 to 43.0 and 1.1 to 16.3 mg m−2 for the STAP and MA200, respectively. In addition, the effect of a silica-bead based diffusion on the rh effect was investigated. Both instruments revealed opposing responses to relative humidity changes (Δrh) with different amplitudes. Whereas the STAP shows a linear dependence to relative humidity changes, the MA200 is characterized by an exponential recovery after its filter was exposed to relative humidity changes. At a wavelength of 624 nm and for the default 60 second average output, the STAP reveals an absolute change in σabs per absolute change of rh (Δσabs/Δrh) of 0.14 Mm−1 %−1 in the clean case, 0.29 Mm−1 %−1 in the case of BC loaded filters, and 0.21 Mm−1 %−1 considering filters loaded with ammonium sulfate. The 60-second running average of the particle light absorption coefficient at 625 nm measured with the MA200 revealed response of around −0.4 Mm−1 %−1 for all three cases. Whereas the response of the STAP varies over the different loading materials in contrast the MA200 was quite stable. The minimum and maximum response was for the STAP 0.17 Mm−1 %−1 and 0.24 Mm−1 %−1 considering ammonium sulfate loading and in the BC loaded case 0.17 Mm−1 %−1 and 0.62 Mm−1 %−1, respectively. The minimum response shown by the MA200 was −0.42 Mm−1 %−1 and −0.36 Mm−1 %−1 at maximum for ammonium sulfate and −0.42 Mm−1 %−1 and −0.37 Mm−1 %−1 in case of BC loading, respectively. Using the aerosol dryer upstream, the STAP did not change the behavior, but the amplitude of the observed effect was reduced by a factor of up to three. A linear correction function for the STAP was developed here. It is provided by correlating recalculated particle light absorption coefficients at 1 Hz time resolution against the change rate of rh. The linear response is estimated with 10.08 Mm−1 s−1 %−1 and can be used to correct for bias induced to rh changes at this time resolution. A correction approach for the MA200 is also provided, however, the behavior of the MA200 is more complex. Further research and multi-instrument measurements have to be conducted to fully understand the underlying processes, since the correction approach resulted in different correction parameters across various experiments. However, the exponential recovery after the filter of the MA200 experienced a rh change could be reproduced. Due to our findings, we recommend to use an aerosol dryer upstream of absorption photometers to reduce the rh effect significantly. Furthermore, when absorption photometers are used in vertical measurements, the ascending or descending speed through layers of large rh gradients has to be low to minimize the observed rh effect. Additionally, recording the rh of the sample stream allows correcting for the bias during post processing of the data. This data correction leads to reasonable results, according the given example in this study.