Weak Aerosol Hygroscopicity Measured over the Southern Tibetan Plateau: Implication for Cloud Activation

Cloud activation over the Tibetan Plateau (TP) plays a pivotal role in regional cloud-precipitation processes and, by extension, global climate. However, its characteristics remain elusive due to the absence of observations in the TP. Leveraging the Second Tibetan Plateau Scientific Expedition and Research Program, we conducted a ground in-situ aerosol-cloud-precipitation experiment in the southern TP (GACPE-STP) from August to October 2023, thereby unveiling, for the first time, the aerosol activation characteristics in this crucial region. Our findings reveal a discernibly weak aerosol activation capacity, with mean cloud condensation nuclei number concentration (N_CCN) ranging from 24 to 483 cm^-3 and activation fraction from 2% to 48% at the supersaturation (SS) range from 0.07% to 0.7%. Through multi-method measurements of aerosol hygroscopicity (k), including derivation from both dry and humidified particle number size distribution (PNSD) and scattering coefficients, along with calculations based on N_CCN(SS) and dry PNSD, we consistently observe low hygroscopicity with mean values below 0.1. This contrasts starkly with the recommended continental k value of 0.3, a departure that may be linked to unique surface characteristics and local fuel-usage practices in the TP region. As the dry aerosol diameter (D) increases, k exhibits an initial rise followed by a decline, adhering to a Gaussian distribution. The resulting k(D) fitting serves as a parameterization for predicting cloud activation in this region. Notably, utilizing the recommended continental κ value of 0.3 leads to a significant overestimation of cloud droplet number concentration (77% to 426%), subsequently contributing to an overestimation of cloud optical thickness and an underestimation of cloud-rain autoconversion. This cascade effect results in a substantial overestimation of the aerosol indirect effects. Employing the k(D) parameterization can significantly enhance the precision of cloud activation predictions in this region. These findings peel back a layer of mystery surrounding cloud activation in the TP region. To construct a comprehensive understanding, we advocate for additional in-situ experiments, including ice nuclei measurements, crucial for a nuanced depiction of cloud activation in the TP region.