Separating the Twomey effect and the semi-direct effect in absorbing aerosol environments through the cloud-aerosol mixing ratio

Abstract. Air masses carrying absorbing aerosols play a dual role by altering ambient supersaturation and acting as cloud condensation nuclei (CCN), thus contributing to both the semi-direct and indirect aerosol effects. However, in real cloud development processes, aerosol influences can simultaneously act to suppress cloud formation or enhance droplet production, resulting in microphysical characteristics that remain difficult to describe. In the study, an aerosol and cloud microphysics experiment, focusing on warm cloud events strongly coupled with biomass-burning aerosols transported from the Southeast Asia Peninsula, was conducted in the spring of 2024 at the Lulin Atmospheric Background Station (LABS, 2862 m a.s.l.) in Taiwan. One-minute resolution of in-situ data was used in the data analysis, allowing exploration of the microphysical responses under varying mixing states of absorbing aerosol concentration and cloud liquid water content. This study applies the cloud-aerosol mixing ratio (e.g., mass concentration mixing ratio (MCr), log⁡(LWC×106PM2.5)) in conjunction with the ACI index (ACINd=13(∂ln⁡Nd∂ln⁡α)) to describe the behavior of aerosol-cloud interactions. Results identify two distinct responses: (1) under high MCr conditions (MCr>4.5), clouds exhibit features of the Twomey effect (ACINd≈0.08); (2) under low MCr conditions (MCr=3–4.5), not only is the ACINd less than −0.06, but the high aerosol loading also coincides with elevated temperatures, lower relative humidity, and a reduction in small droplets (<10 µm), consistent with a semi-direct effect. This study provides a comprehensive explanation of how absorbing aerosols influence cloud systems over East Asia and highlights the critical role of the cloud-aerosol mixing ratio in characterizing the microphysical responses associated with the Twomey effect and the semi-direct effect.