Cloud activation properties of aerosol particles in a continental Central European urban environment

• Main Authors: I. Salma, W. Thén, M. Vörösmarty, A. Z. Gyöngyösi
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://acp.copernicus.org/articles/21/11289/2021/acp-21-11289-2021.pdf

<p>Collocated measurements using a condensation particle counter, differential mobility particle sizer and cloud condensation nuclei counter were realised in parallel in central Budapest from 15 April 2019 to 14 April 2020 to gain insight into the cloud activation properties of urban aerosol particles. The median total particle number concentration was 10.1 <span class="inline-formula">×</span> 10<span class="inline-formula">³</span> cm<span class="inline-formula">⁻³</span>. The median concentrations of cloud condensation nuclei (CCN) at water vapour supersaturation (<span class="inline-formula"><i>S</i></span>) values of 0.1 %, 0.2 %, 0.3 %, 0.5 % and 1.0 % were 0.59, 1.09, 1.39, 1.80 and 2.5 <span class="inline-formula">×</span> 10<span class="inline-formula">³</span> cm<span class="inline-formula">⁻³</span>, respectively. The CCN concentrations represented 7–27 % of all particles. The CCN concentrations were considerably larger but the activation fractions were systematically substantially smaller than observed in regional or remote locations. The effective critical dry particle diameters (<span class="inline-formula"><i>d</i>_c,eff</span>) were derived utilising the CCN concentrations and particle number size distributions. Their median values at the five supersaturation values considered were 207, 149, 126, 105 and 80 nm, respectively; all of these diameters were positioned within the accumulation mode of the typical particle number size distribution. Their frequency distributions revealed a single peak for which the geometric standard deviation increased monotonically with <span class="inline-formula"><i>S</i></span>. This broadening indicated high time variability in the activating properties of smaller particles. The frequency distributions also showed fine structure, with several compositional elements that seemed to reveal a consistent or monotonical tendency with <span class="inline-formula"><i>S</i></span>. The relationships between the critical <span class="inline-formula"><i>S</i></span> and <span class="inline-formula"><i>d</i>_c,eff</span> suggest that urban aerosol particles in Budapest with diameters larger than approximately 130 nm showed similar hydroscopicity to corresponding continental aerosol particles, whereas smaller particles in Budapest were less hygroscopic than corresponding continental aerosol particles. Only modest seasonal cycling in CCN concentrations and activation fractions was seen, and only for large <span class="inline-formula"><i>S</i></span> values. This cycling likely reflects changes in the number concentration, chemical composition and mixing state of the particles. The seasonal dependencies of <span class="inline-formula"><i>d</i>_c,eff</span> were featureless, indicating that the droplet activation properties of the urban particles remained more or less the same throughout the year. This is again different from what is seen in non-urban locations. Hygroscopicity parameters (<span class="inline-formula"><i>κ</i></span> values) were computed without determining the time-dependent chemical composition of the particles. The median values for <span class="inline-formula"><i>κ</i></span> were 0.15, 0.10, 0.07, 0.04 and 0.02, respectively, at the five supersaturation values considered. The averages suggested that the larger particles were considerably more hygroscopic than the smaller particles. We found that the <span class="inline-formula"><i>κ</i></span> values for the urban aerosol were substantially smaller than those previously reported for aerosols in regional or remote locations. All of these characteristics can be linked to the specific source composition of particles in cities. The relatively large variability in the hygroscopicity parameters for a given <span class="inline-formula"><i>S</i></span> emphasises that the individual values represent the CCN population in ambient air while the average hygroscopicity parameter mainly corresponds to particles with sizes close to the effective critical dry particle diameter.</p>