Aerosol mixing state, new particle formation, and cloud droplet number concentration in an urban environment

• Published in: The Science of the total environment 2024-11, Vol.951 (C), p.175307, Article 175307
• Main Authors: Kasparoglu, Sabin, Meskhidze, Nicholas, Petters, Markus D.
• Format: Article
• Language: English
• Subjects: Aerosol hygroscopic growth; Aerosol mixing state; Aerosol modal growth; Aerosol-cloud condensation nuclei-cloud droplet number concentration closure; Humidified tandem differential mobility analyzer
• ISSN: 0048-9697; 1879-1026; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.175307

Anthropogenically derived aerosols have been hypothesized to influence convective precipitation by increasing the available pool of cloud condensation nuclei. Here, we present a synthesis of aerosol size distribution and subsaturated hygroscopicity measurements between 15 and 250 nm diameter particles during the TRacking Aerosol Convection interactions ExpeRiment (TRACER). We found that the aerosol is externally mixed and can be described by a quasi-two-component description comprising a more and less hygroscopic mode. The mean hygroscopicity parameters for these modes across all sizes were 0.03 ± 0.04 and 0.22 ± 0.08 with no significant dependence on particle size. The number fraction of the more hygroscopic mode is 40 % for particles between 15 and 40 nm and gradually increases to ~70 % for particles >100 nm. Winds from the southerly direction feature particles with larger hygroscopicity parameters and have a larger fraction of particles in the more hygroscopic mode. The hygroscopicity parameter exhibits diurnal cycles that are consistent with condensation of a species with a hygroscopicity parameter ~0.1 which corresponds to values expected for secondary organic aerosol. We also identified nine small particle events that were attributed to particle formation by nucleation. The data are consistent with new particle formation having occurred aloft, followed by downward mixing with daytime turbulence. The species that are responsible for modal growth had hygroscopicity parameters varying between 0.05 and 0.34. These values systematically depended on the wind sector, suggesting that the chemical composition of the precursors differed. Hourly cloud condensation nuclei (CCN) and cloud droplet number concentration (CDNC) values derived from the aerosol size distribution, subsaturated hygroscopicity measurements, and adiabatic parcel model simulations showed a dynamic range of a factor of 2–3 in CDNC depending on the wind sector, with lower values associated with southerly onshore flow. [Display omitted] •The hygroscopicity parameter is independent of size between 15 and 250 nm.•Aerosol is externally mixed and has a more and less hygroscopic mode.•The hygroscopicity parameter of daytime condensing species is ~0.1.•Urban new particle formation likely occurred in the upper boundary layer.•Estimated cloud droplet number varies by a factor of 3, depending on wind direction.