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Wet cooling towers evaporate water to transfer waste heat to the environment. In the process, they also emit liquid water in the form of spray drift. These emissions dry in the atmosphere to form aerosol particles composed of impurities in the water. Emission factor guidance for cooling towers is provided by EPA AP42. Total emissions are a combination of spray drift and ambient aerosol passing through the tower. Net emissions take into account the scrubbing of ambient particles in the tower. We directly measured tower particulate emissions and ambient concentrations to determine the contribution of spray drift to tower emissions as well as scrubbing effect on ambient particles. Two companion papers describe the instrumentation and the spray drift measurements. Copyright © 2025 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Cloud droplet number concentration (Nd) can be retrieved through passive satellite observation. These retrievals are useful due to their wide spatial and temporal coverage. However, the accuracy of the retrieved values is not well understood. In this paper, we compare satellite Nd derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument with in situ aircraft measurements using a phase Doppler interferometer onboard three flight campaigns sampling marine stratocumulus clouds. Intercomparison of Nd values shows that the discrepancy between retrieved and in situ Nd can be ±50 % or more. In the mean, there is evidence of an overestimation bias by MODIS retrievals, although the sample size is insufficient for statistical certainty. We find that MODIS Nd is best interpreted as representative of the mid-cloud region, as there is almost always a greater discrepancy from in situ values near cloud top and cloud base. We also find evidence of cases where Nd is accurately retrieved, but effective radius is not, presumably due to offsetting errors in other retrieval parameters. Vertical profiles of extinction coefficient β , liquid water content L , and effective radius re measured during sawtooth-pattern flight legs through cloud top are also compared to implicit MODIS retrieval profiles. For the one case with Nd agreement, all profiles match well. For seven cases with significant disagreement, there is no consistent underlying cause. The discrepancy originates from either: (a) discrepancy in the re profile, (b) discrepancy in the β and L profiles, or (c) discrepancy in both. [ABSTRACT FROM AUTHOR]

Abstract Data from three research flights, conducted over water near the California coast, are used to investigate the boundary between stratocumulus cloud decks and clearings of different sizes. Large clearings exhibit a diurnal cycle with growth during the day and contraction overnight and a multiday life cycle that can include oscillations between growth and decay, whereas a small coastal clearing was observed to be locally confined with a subdiurnal lifetime. Subcloud aerosol characteristics are similar on both sides of the clear–cloudy boundary in the three cases, while meteorological properties exhibit subtle, yet important, gradients, implying that dynamics, and not microphysics, is the primary driver for the clearing characteristics. Transects, made at multiple levels across the cloud boundary during one flight, highlight the importance of microscale (~1 km) structure in thermodynamic properties near the cloud edge, suggesting that dynamic forcing at length scales comparable to the convective eddy scale may be influential to the larger-scale characteristics of the clearing. These results have implications for modeling and observational studies of marine boundary layer clouds, especially in relation to aerosol–cloud interactions and scales of variability responsible for the evolution of stratocumulus clearings.

Cloud droplet formation depends on the condensation of water vapor on ambient aerosols, the rate of which is strongly affected by the kinetics of water uptake as expressed by the condensation (or mass accommodation) coefficient, αc. Estimates of αc for droplet growth from activation of ambient particles vary considerably and represent a critical source of uncertainty in estimates of global cloud droplet distributions and the aerosol indirect forcing of climate. We present an analysis of 10 globally relevant data sets of cloud condensation nuclei to constrain the value of αc for ambient aerosol. We find that rapid activation kinetics (αc > 0.1) is uniformly prevalent. This finding resolves a long-standing issue in cloud physics, as the uncertainty in water vapor accommodation on droplets is considerably less than previously thought.

Wet cooling towers are used in industrial processes, power plants, and heating, ventilation, and air conditioning systems. Their purpose is to reject waste heat from a system through evaporative cooling of process water. Due to their design, a small fraction of process water is emitted from the tower exit, termed "spray drift." Dissolved solids contained within spray drift droplets are regulated as PM10 emissions by EPA AP-42. Here, we use a novel instrumentation package to measure particulate emissions from drift droplets at two cooling towers located at separate power plants. We find that drift droplet properties depend on the sampling position over the tower so multiple positions need to be sampled to accurately estimate emissions. We estimate 11 and 3.3 grams of PM10 are emitted from each tower per hour, with 78% and 87% of PM10 emissions also qualifying as PM2.5. AP-42 overestimates PM10 emissions by a factor of 2.4 for one tower and nearly 300 for the other. These results suggest that AP-42 may require revision to both accurately reflect lower PM10 emissions and account for unregulated PM2.5, which constitute the majority of emissions. Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The mechanism responsible for formation of rain in warm clouds has been debated for over six decades. Here, the authors analyze new measurements of shallow cumulus made with a phase Doppler interferometer during the Rain in Cumulus over the Ocean (RICO) experiment. These observations show that drops sufficiently large (>55-[micro]m diameter) to initiate precipitation (termed collision-coalescence initiators or CCIs) are found preferentially at cloud top, tend to cluster with each other, and are found in environments that are thermodynamically, dynamically, and microphysically distinct from those of smaller drops. The CCI environments exhibit cloud spectra that are shifted to larger sizes, with enhanced broadening toward larger drop sizes. Increased entrainment is also associated with CCIs, suggesting that it is an important process in CCI production. A simple model combining inhomogeneous mixing and condensation is inadequate to explain these observations. It is hypothesized that CCIs are produced in cloud-top regions where turbulence generated by entrainment mixing locally enhances collision-coalescence rates.

Decades of cloud microphysical research have not provided conclusive understanding of the physical processes responsible for droplet spectral broadening. Numerous mechanisms have been proposed--for example, entrainment mixing, vortex shedding, giant cloud condensation nuclei (CCN), chemical processing of CCN, and radiative cooling--all of which are likely candidates under select conditions. In this paper it is suggested that variability in the composition of CCN, and in particular, the existence of condensation inhibiting compounds, is another possible candidate. The inferred potential abundance of these amphiphilic film-forming compounds (FFCs) suggests that their effect may be important. Using a cloud parcel model with a simplified treatment of the effect of FFCs, it is shown that modest concentrations of FFCs (on the order of 5% of the total aerosol mass) can have a marked effect on drop growth and can cause significant increases in spectral dispersions. Moreover, it is shown that FFCs may, in some cases, reduce the number concentration of cloud droplets, with implications for cloud-climate feedbacks. This trend is at least in qualitative agreement with results from a recent field campaign.

Measuring emissions from stacks is challenging due to accessibility and safety concerns, and requires techniques to address a broad range of conditions and measurement challenges. One way to facilitate such measurements is to build an instrument package and then use a crane to hold the package over the emissions source. Here we describe such an instrument package that is used to characterize both wet droplet and dried aerosol emissions from cooling tower spray drift. In this application, the instrument package characterizes the velocity, size distribution and concentration of the wet droplet emissions and the mass concentration and elemental composition of the dried PM2.5 and PM10 emissions. Subsequent papers will present and analyze the wet and dried emissions from individual towers. [ABSTRACT FROM AUTHOR]

This study uses airborne data from two field campaigns off the California coast to characterize aerosol size distribution characteristics in the entrainment interface layer (EIL), a thin and turbulent layer above marine stratocumulus cloud tops, that separates the stratocumulus-topped boundary layer (STBL) from the free troposphere (FT). The vertical bounds of the EIL are defined in this work based on considerations of buoyancy and turbulence using thermodynamic and dynamic data. Aerosol number concentrations are examined from three different probes with varying particle diameter (Dp) ranges: > 3 nm, > 10 nm, 0.11-3.4 µm. Relative to the EIL and FT layers, the sub-cloud (SUB) layer exhibited lower aerosol number concentrations and higher surface area concentrations. High particle number concentrations between 3 and 10 nm in the EIL is indicative of enhanced nucleation, assisted by high actinic fluxes, cool and moist air, and much lower surface area concentrations than the STBL. Slopes of number concentration versus altitude in the EIL were correlated with the particle number concentration difference between the SUB and lower FT layers. The EIL aerosol size distribution was influenced by varying degrees from STBL aerosol versus subsiding FT aerosol depending on the case examined. These results emphasize the important role of the EIL in influencing nucleation and aerosol-cloud-climate interactions. [ABSTRACT FROM AUTHOR]

Precipitation forms in warm clouds via collision-coalescence. This process is difficult to observe directly in situ and its implementation in numerical models is uncertain. We use aircraft observations of the drop-size distribution (DSD) near marine stratocumulus tops to estimate collision-coalescence rates. Marine stratocumulus is a useful system to study collisional growth because it is initiated near the cloud top and the clouds evolve slowly enough to obtain statistically useful data from aircraft. We compare rate constants estimated from observations with reference rate constants derived from a collision-coalescence box model, the result of which is termed the enhancement factor (EF). We evaluate two hydrodynamic collision-coalescence kernels, one quiescent and one including the effects of small-scale turbulence. Due to sampling volume limitations, DSDs must be averaged over length-scales much greater than those relevant to the underlying physics, such that we also examine the role of averaging length-scale with respect to process representation. Averaging length-scales of 1.5 and 30 km are used, corresponding roughly to the horizontal grid lengths of cloud-resolving models and high-resolution climate models, respectively. EF values range from 0.1 to 40, with the greatest EFs associated with small mode diameter cases and a generally decreasing trend with drop size. For any given drop size or averaging length-scale, there is about an order of magnitude variability in EFs. These results suggest that spatial variability on length-scales smaller than 1.5 km prevents accurate retrieval of rate constants from large-scale average DSDs. Large-scale models must therefore account for small-scale variability to represent cloud microphysical processes accurately. The turbulent kernel reduces EFs for all drop sizes, but can only account for at most half of the calculated EFs in marine stratocumulus. [ABSTRACT FROM AUTHOR]