Development of eddy accumulation technique for aerosol particle number concentration and ammonia flux measurements

Eddy accumulation technique provides an opportunity to quantify the turbulent exchange processes of properties for which fast response measuring devices are not available. The signals of slow response instrument are separated based on the up- (c₊) and down-flowing (c_–) air concentration by using a long-term averaging process (20-30 minutes). The difference in the average concentration of the up- and down-flow, multiplied by the standard deviation of the vertical wind is proportional to the turbulent flux of the given property. Two applications are presented.

i) The particle number concentration was measured by a condensation particle counter (CPC, type A30, Airmodus, Finland) at the ELTE BpART Lab (http://salma. elte.hu/BpART/) in summer 2023. The instrument uses butanol as a working fluid, and detects particles with a diameter >7 nm in single particle counting mode with coincidence correction up to 150  000 cm^–3 with a time resolution of 1 s. Fast response (10 Hz) turbulence measurements were accomplished using an EC150 Open-Path CO₂/H₂O gas analyser combined with CSAT-3 sonic anemometer. The time lag between the two instruments due to air intake was considered. The autocovariance function of the vertical velocity and the particle number concentration were also calculated. The concentration within the 1-s averaging time was i) taken as constant, and ii) the tenth-second values were calculated by linear interpolation. A 10-Hz data series were generated, knowing that the covariance [cov(w,c)] is underestimated. In each case, 30-min averaging period was used. As a check of the method, the flux calculations based on relaxed eddy accumulation (REA) were compared with eddy covariance (EC) measurements for raw vertical fluxes of sonic temperature, moisture, and CO₂. The fluxes calculated in these two ways were in good agreement (R² > 0.83).

ii) Another methodological development is a novel approach that involves combination of a photoacoustic ammonia sensor with a CSAT-3 sonic anemometer. The photoacoustic instrument, which is currently in the development phase, yields reliable concentration values with an averaging time of approximately 10 s. Similar to wind parameters, the output signals from the ammonia sensor were collected at a frequency of 10 Hz, acknowledging the presence of significant white noise. These measurements were started in a sunflower field even before sowing and fertilization during spring 2024. Our analysis encompassed: a) assessing the reliability of ammonia concentration measurements, b) studying the daily concentration patterns, c) investigating concentration differences detectable between upstream and downstream air flows. Initial findings from this new field campaign are summarised on the posters.