Fog Droplet Size Distribution and the Interaction between Fog Droplets and Fine Particles during Dense Fog in Tianjin, China

Efficient Nitrate Formation in Fog Events Implicates Fog Interstitial Aerosols as Significant Drivers of Atmospheric Chemistry

Clouds and fogs, consisting of tiny water droplets formed by the condensation of water in supersaturated air, are vital in atmospheric chemistry, as they facilitate multiphase reactions. While measuring high-altitude cloud is challenging, fog as ground-level clouds offer a unique opportunity for direct observation. In this study, we explored radiation fogs in the North China Plain using an advanced aerosol-fog sampling system to measure the chemical and physical properties of both inactivated interstitial aerosols and activated fog droplet residues. Our findings revealed that efficient nitrate formation primarily occurred on fog interstitial aerosols rather than within fog droplets, with observed fog interstitial aerosol nitrate net production rates reaching up to 3.6 μg m-3 h-1. Box model simulations identified the hydrolysis of NO2 and N2O5 as key pathways for nitrate formation. NO2 hydrolysis was often overlooked in previous studies, contributing 40-79 and 57-76% to total nitrate production during nighttime and daytime fog periods. This oversight suggests that substantial nitrate formation through hydrolysis reactions involving interstitial aerosols may have been neglected. Our results highlight the need for further research into the chemistry of cloud and fog interstitial aerosols and their inclusion in atmospheric chemistry models.

Synergistically Utilizing a Liquid Bridge and Interconnected Porous Superhydrophilic Structures to Achieve a One-Step Fog Collection Mode

Conventional fog collection efficiency is subject to the inherent inefficiencies of its three constituent steps: fog capture, coalescence, and transportation. This study presents a liquid bridge synergistic fog collection system (LSFCS) by synergistically utilizing a liquid bridge and interconnected porous superhydrophilic structures (IPHS). The results indicate that the introduction of liquid bridge not only greatly accelerates water droplet transportation, but also facilitates the IPHS in maintaining rough structures that realize stable and efficient fog capture. During fog collection, the lower section of the IPHS is covered by a water layer, however due to the effect of the liquid bridge, the upper section protrudes out, while covered by a connective thin water film that does not obscure the microstructures of the upper section. Under these conditions, a one-step fog collection mode is realized. Once captured by the IPHS, fog droplets immediately coalesce with the water film, and are simultaneously transported into a container under the effect of the liquid bridge. The LSFCS achieves a collection efficiency of 6.5 kg m-2 h-1, 2.3 times that of a system without a liquid bridge. This study offers insight on improving fog collection efficiency, and holds promise for condensation water collection or droplet manipulation.

Development of a multi-stage fog droplet screening system based on the virtual impact principle

Accurately measuring fog droplet spectra is essential for understanding fog's formation, dissipation, and composition, which makes a challenge to the performance of droplet sampling and measurement systems. Standard particles such as glass beads are widely used to characterize their performance. However, the disparities between glass beads and fog droplets, including refractivity, size distribution, and composition, may lead to calibration errors. In this context, we developed a three-stage fog droplet screening system based on the virtual impact principle. We determined the Stokes number and the diameter of the acceleration nozzle through theoretical analysis. Subsequently, we utilized the computational fluid dynamics software Fluent to explore the influence of key system parameters on screening efficiency, including the diameter of the collection nozzle (D1) and the distance between the acceleration nozzle and the collection nozzle (S). The simulation results indicated that the screening efficiency improved with S. The best performance was achieved when D1 = 1.35 D0 and S = 1.90 D0 (where D0 represents the diameter of the acceleration nozzle), resulting in an average screening efficiency of 75.4%. Finally, we conducted experiments to validate the effectiveness of the screening system. The screening efficiency of each outlet was estimated at 42.2%, 66.1%, 84.0%, and 95.3%, with differences of 2.0%, 3.3%, 4.1%, and 4.7% compared to the simulations. The average screening efficiency was 71.9%, with a deviation of 3.5% from the simulation. These findings demonstrated that the screening system could provide an alternative technical apparatus for characterizing droplet sampling and measurement systems.

Severe photochemical pollution formation associated with strong HONO emissions from dew and guttation evaporation

The unknown daytime source of HONO has been extensively investigated due to unexplained atmospheric oxidation capacity and current modelling bias, especially during cold seasons. In this study, abrupt morning increases in atmospheric HONO at a rural site in the North China Plain (NCP) were observed almost on daily basis, which were closely linked to simultaneous rises in atmospheric water vapor content and NH3 concentrations. Dew and guttation water formation was frequently observed on wheat leaves, from which water samples were taken and chemically analyzed for the first time. Results confirmed that such natural processes likely governed the daily nighttime deposition and daytime release of HONO and NH3, which have not been considered in the numerous HONO budget studies investigating its large missing daytime source in the NCP. The dissolved HONO and NH3 in leaf surface water droplets reached 1.4 and 23 mg L-1 during the morning on average, resulting in averaged atmospheric HONO and NH3 increases of 0.89 ± 0.61 and 43.7 ± 29.3 ppb during morning hours, with relative increases of 186 ± 212 % and 233 ± 252 %, respectively. The high atmospheric oxidation capacity contained within HONO was stored in near surface liquid water (such as dew, guttation and soil surface water) during nighttime, which prevented its atmospheric dispersion after sunset and protected it from photodissociation during early morning hours. HONO was released in a blast during later hours with stronger solar radiation, which triggered and then accelerated daytime photochemistry through the rapid photolysis of HONO and subsequent OH production, especially under high RH conditions, forming severe secondary gaseous and particulate pollution. Results of this study demonstrate that global ecosystems might play significant roles in atmospheric photochemistry through nighttime dew formation and guttation processes.

Observation on the Droplet Ranging from 2 to 16 μm in Cloud Droplet Size Distribution Based on Digital Holography

Cloud droplets size distribution (DSD) is one of the significant characteristics for liquid clouds. It plays an important role for the aerosol–droplet–cloud mechanism and variation in cloud microphysics. However, the minuscule sampling space is insufficient for the observation of whole DSD when using high-magnification optical systems. In this paper, we propose an observation method for cloud droplets ranging from 2 to 16 μm, by which the balance relationship between sampling space and optical magnification is realized. The method combines an in-line digital holographic interferometer (DHI) with the optical magnification of 5.89× and spatial stitching technique. The minimum size in DSD is extended to 2 μm, which improves the integrity of size distribution. Simultaneously, the stability of DSD is enhanced by increasing the tenfold sampling volume of cloud droplets. The comparative experiment between the in-line DHI and fog monitor demonstrates that the DSD obtained by this method is reliable, which can be used for the analysis of microphysical parameters. In the Beijing Aerosol and Cloud Interaction Chamber (BACIC), the observation results show that the size of cloud droplets follows the Gamma distribution, which is consistent with the theoretical DSD. The results of cloud microphysical parameters indicate that each pair of parameters has a positive correlation, and then the validity of observation method is confirmed. Additionally, the high-concentration aerosol condition significantly mitigates the effect of random turbulence and enhances the robustness of the microphysical parameter data.

Review on Parameterization Schemes of Visibility in Fog and Brief Discussion of Applications Performance

Low visibility, associated with fog, severely affects land, marine, and air transportation. Visibility is an important indicator to identify different intensities of fog; therefore, improving the ability to forecast visibility in fog is an urgent need for social and economic development. Establishing a proper visibility parameterization scheme is crucial to improving the accuracy of fog forecast operation. Considering various visibility impact factors, including RH, Nd, D, LWC, the parameterization formula of visibility in fog, as well as their performance in meteorology operation, are reviewed. Moreover, the estimated ability of the visibility parameterization formulas combined with the numerical model is briefly described, and their advantages and shortcomings are pointed out.

Biomimicking spider webs for effective fog water harvesting with electrospun polymer fibers

Fog is an underestimated source of water, especially in regions where conventional methods of water harvesting are impossible, ineffective, or challenging for low-cost water resources. Interestingly, many novel methods and developments for effective water harvesting are inspired by nature. Therefore, in this review, we focused on one of the most researched and developing forms of electrospun polymer fibers, which successfully imitate many fascinating natural materials for instance spider webs. We showed how fiber morphology and wetting properties can increase the fog collection rate, and also observed the influence of fog water collection parameters on testing their efficiency. This review summarizes the current state of the art on water collection by fibrous meshes and offers suggestions for the testing of new designs under laboratory conditions by classifying the parameters already reported in experimental set-ups. This is extremely important, as fog collection under laboratory conditions is the first step toward creating a new water harvesting technology. This review summarizes all the approaches taken so far to develop the most effective water collection systems based on electrospun polymer fibers.

The importance of nanofiber hydrophobicity for effective fog water collection

To increase fog collection efficiency in a fiber system, controlled wetting properties are desirable. In this work, hydrophobic (PA11) and hydrophilic (PA6) polyamides were tested to verify the surface wetting effect on fog water collection rate. Highly porous fiber meshes were obtained from both polymer solutions. Randomly oriented fibers with average diameter of approximately 150 nm were observed with a scanning electron microscope (SEM). Despite the similar geometry and zeta potential of PA6 and PA11 meshes, it was shown that the hydrophobic PA11 nanofibers are more effective at water collection than hydrophilic PA6. These results indicate that wetting properties of electrospun nanofiber mesh have a significant effect on the process of draining from the mesh, as discussed in this paper. The results obtained are crucial for designing more efficient fog water collectors that include nanofibers in their construction.

Observation, Simulation and Predictability of Fog: Review and Perspectives

Fog affects human activities in various ways, but the societal impact of fog has significantly increased during recent decades due to increasing air, marine and road traffic [...]

The Microphysical Properties of a Sea-Fog Event along the West Coast of the Yellow Sea in Spring

The microphysics and visibility of a sea-fog event were measured at the Qingdao Meteorological Station (QDMS) (120°19′ E, 36°04′ N) from 5 April to 8 April 2017. The two foggy periods with low visibility (<200 m) lasted 31 h together. The mean value of the average liquid water content (LWC) was 0.057 g m−3, and the mean value of the number concentration (NUM) was 64.4 cm−3. We found that although large droplets only constituted a small portion of the total number of the concentration; they contributed the majority of the LWC and therefore determined ~76% of total extinction of the visibility. The observed droplet-size distribution (DSD) exhibited a new bimodal Gaussian (G-exponential) distribution function, rather than the well-accepted Gamma distribution. This work suggests a new distribution function to describe fog DSD, which may help to improve the microphysical parameterization for the Yellow Sea fog numerical forecasting.