As ice plays a critical role in various aspects of life, from food preservation to ice sports and ecosystem, it is desirable to protect ice from melting, especially under sunlight. The fundamental reason for ice melt under sunlight is related to the imbalanced energy flows of the incoming sunlight and outgoing thermal radiation. Therefore, radiative cooling, which can balance the energy flows without energy consumption, offers a sustainable approach for ice protection. Here, we demonstrate that a hierarchically designed radiative cooling film based on abundant and eco-friendly cellulose acetate molecules versatilely provides effective and passive protection to various forms/scales of ice under sunlight. This work provides inspiration for developing an effective, scalable, and sustainable route for preserving ice and other critical elements of ecosystems.

Abstract. Organic aerosol (OA) has been considered as one of the most important uncertainties in climate modeling due to the complexity in presenting its chemical production and depletion mechanisms. To better understand the capability of climate models and probe into the associated uncertainties in simulating OA, we evaluate the Community Earth System Model version 2.1 (CESM2.1) configured with the Community Atmosphere Model version 6 (CAM6) with comprehensive tropospheric and stratospheric chemistry representation (CAM6-Chem) through a long-term simulation (1988–2019) with observations collected from multiple datasets in the United States. We find that CESM generally reproduces the interannual variation and seasonal cycle of OA mass concentration at surface layer with a correlation of 0.40 compared to ground observations and systematically overestimates (69 %) in summer and underestimates (−19 %) in winter. Through a series of sensitivity simulations, we reveal that modeling bias is primarily related to the dominant fraction of monoterpene-formed secondary organic aerosol (SOA), and a strong positive correlation of 0.67 is found between monoterpene emission and modeling bias in the eastern US during summer. In terms of vertical profile, the model prominently underestimates OA and monoterpene concentrations by 37 %–99 % and 82 %–99 %, respectively, in the upper air (> 500 m) as validated against aircraft observations. Our study suggests that the current volatility basis set (VBS) scheme applied in CESM might be parameterized with monoterpene SOA yields that are too high, which subsequently results in strong SOA production near the emission source area. We also find that the model has difficulty in reproducing the decreasing trend of surface OA in the southeastern US probably because of employing pure gas VBS to represent isoprene SOA which is in reality mainly formed through multiphase chemistry; thus, the influence of aerosol acidity and sulfate particle change on isoprene SOA formation has not been fully considered in the model. This study reveals the urgent need to improve the SOA modeling in climate models.

The radiative forcing associated with aerosol-cloud interactions, traditionally referred to as aerosol indirect effects, indirectly by modifying the microphysical properties of clouds, affecting their reflectivity and persistence, contributes the largest uncertainty to total radiative forcing estimates (Boucher et al., 2013). For liquid clouds, reducing droplet size and increasing reflectance of clouds due to increased droplet number for a constant liquid water path, namely the "Twomey" effect (Twomey, 1977), is relatively well understood (Christensen et al., 2020; Diamond et al., 2020; Liu & Li, 2019). However, aerosol effects on the amount of boundary layer clouds that cover large areas of the oceans and strongly reflect incoming solar radiation are still not well-documented (Bellouin et al., 2020), especially the magnitude of the aerosol influence on cloud fraction (CF) (Ghan et al., 2016; Gryspeerdt et al., 2016). Understanding how aerosols affect cloud cover helps to reduce the considerable uncertainty of the radiative forcing associated with aerosol-cloud interactions (Fan et al., 2016) because of the strong correlation of CF to other cloud properties and their large impact on radiation. The long-term satellite observations provide excellent opportunities for quantifying cloud-mediated aerosol radiative effects (

Observed surface organic aerosols (OA) concentrations slightly increased in the western US (WUS) but significantly decreased in the eastern US (EUS) in summer, and continuously decreased in winter over the US region. To understand the driving factors for the long-term surface OA trend, we apply a revised version of the Community Atmosphere Model version 6 with comprehensive tropospheric and stratospheric chemistry representation, considering the heterogeneous formation of isoprene-epoxydiol-derived secondary organic aerosols (SOAIE) and fast photolysis rate of monoterpene-derived secondary organic aerosols (MTSOA) to diagnose the OA evolution in 1988-2019. Compared to older versions, the revised model better reproduces the climatology, seasonal cycle, and long-term trend of surface OA as evaluated against the Interagency Monitoring of Protected Visual Environments measurements. We find the decrease in EUS summertime OA is likely attributed to the interplay between SOAIE and MTSOA. With anthropogenic emissions reduction, primary organic aerosols (POA) declined, SOAIE decreased along with sulfate, while MTSOA increased along with biogenic emissions driven by a warming climate. POA from wildfires with a significant trend of 2.9% yr −1 and considerable interannual variation of 62.8% drive the statistically insignificant but increasing WUS summertime OA, while anthropogenic POA dominates the decreasing wintertime OA in the US. Through sensitivity experiments, we find MTSOA show linear responses to the increasing monoterpenes emissions and negligible responses to NO x emissions reduction due to the mutual offsets between MTSOA components from different oxidation pathways. This study reveals the increasingly important role of MTSOA in summertime OA under a warming climate. Plain Language SummaryAs the major components of fine particles, organic aerosols (OA) increased in the western United States and decreased in the eastern United States in the summer, and kept decreasing in the winter in the past decades. The driving factors for the long-term trend of OA and their components remain unclear and are investigated by conducting a series of long-term simulations. We find the isoprene-epoxydiol-derived secondary organic aerosols decrease with sulfate emission controls, which is partly offset by the increasing monoterpene-derived secondary organic aerosols (MTSOA) under global warming and the statistically insignificant increase of primary organic aerosols driven by wildfires in summer. In winter, anthropogenic emissions dominate the declining surface OA. We also find MTSOA are more sensitive to increasing biogenic emissions than anthropogenic emissions reduction. Our results reveal the important role of MTSOA in total summertime OA under a warming climate.LIU ET AL.

Large uncertainties remain in the key physical processes associated with aerosol-cloud interactions (ACI) in models. With the help of A-Train satellite observations, the Weather Research and Forecasting Model with chemistry (WRF-Chem) model with two microphysical schemes, Morrison (MOR) and Lin (LIN), is evaluated by quantifying the susceptibilities of cloud properties, precipitation characteristics, and warm rain process to aerosols for marine stratocumulus over the Southeast Pacific. We reduced the meteorological control on clouds by stratifying them using cloud geometric thickness. Our results show that while the cloud fraction increases with increasing cloud droplet number concentration (N d) in observation and simulations, the susceptibility of cloud fraction to N d in simulations are only half of that in the observation. The cloud liquid water path increases with N d in simulations but decreases slightly in the observation. Compared with the observations, the warm rain in WRF-Chem simulations is generally less suppressed by aerosols, and it initiates at a much smaller cloud droplet effective radius (R e). The conversion from cloud to rain is substantially faster in simulations compared to satellite observations. The conversion rate accelerates at R e ≈ 13 μm in observations and at R e ≈ 9 μm in simulations.