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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]

Two case studies of marine stratocumulus (one nocturnal and drizzling, the other daytime and nonprecipitating) are simulated by the UCLA large-eddy simulation model with bin microphysics for comparison with aircraft in situ observations. A high-bin-resolution variant of the microphysics is implemented for closer comparison with cloud drop size distribution (DSD) observations and a turbulent collision–coalescence kernel to evaluate the role of turbulence on drizzle formation. Simulations agree well with observational constraints, reproducing observed thermodynamic profiles (i.e., liquid water potential temperature and total moisture mixing ratio) as well as liquid water path. Cloud drop number concentration and liquid water content profiles also agree well insofar as the thermodynamic profiles match observations, but there are significant differences in DSD shape among simulations that cause discrepancies in higher-order moments such as sedimentation flux, especially as a function of bin resolution. Counterintuitively, high-bin-resolution simulations produce broader DSDs than standard resolution for both cases. Examination of several metrics of DSD width and percentile drop sizes shows that various discrepancies of model output with respect to the observations can be attributed to specific microphysical processes: condensation spuriously creates DSDs that are too wide as measured by standard deviation, which leads to collisional production of too many large drops. The turbulent kernel has the greatest impact on the low-bin-resolution simulation of the drizzling case, which exhibits greater surface precipitation accumulation and broader DSDs than the control (quiescent kernel) simulations. Turbulence effects on precipitation formation cannot be definitively evaluated using bin microphysics until the artificial condensation broadening issue has been addressed. [ABSTRACT FROM AUTHOR]

We analyse the cloud microphysical response to entrainment mixing in warm cumulus clouds observed from the CIRPAS Twin Otter during the GoMACCS field campaign near Houston, Texas, in summer 2006. Cloud drop size distributions and cloud liquid water contents from the Artium Flight phase-Doppler interferometer in conjunction with meteorological observations are used to investigate the degree to which inhomogeneous versus homogeneous mixing is preferred as a function of height above cloud base, distance from cloud edge and aerosol concentration. Using four complete days of data with 101 cloud penetrations (minimum 300 m in length), we find that inhomogeneous mixing primarily explains liquid water variability in these clouds. Furthermore, we show that there is a tendency for mixing to be more homogeneous towards the cloud top, which we attribute to the combination of increased turbulent kinetic energy and cloud drop size with altitude which together cause the Damköhler number to increase by a factor of between 10 and 30 from cloud base to cloud top. We also find that cloud edges appear to be air from cloud centres that have been diluted solely through inhomogeneous mixing. Theory predicts the potential for aerosol to affect mixing type via changes in drop size over the range of aerosol concentrations experienced (moderately polluted rural sites to highly polluted urban sites). However, the observations, while consistent with this hypothesis, do not show a statistically significant effect of aerosol on mixing type. [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-μm 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. [ABSTRACT FROM AUTHOR]

Discusses the influence of film-forming compound on droplet growth. Calculation of uptake water vapor in the prescribed rate of adiabatic ascent; Concentration of FFC materials near the modal radius surface; Effect of FFC on drop spectral broadening.

A new instrument for measuring cloud condensation nuclei (CCN) on board small aircraft is described. Small aircraft are attractive mainly because they are less costly, but they require instruments that are designed for minimum weight, volume, and power consumption; that are robust; and that are capable of autonomous operation and making measurements at a frequency appropriate for aircraft speeds. The instrument design combines the streamwise gradient technique previously reported by J. G. Hudson, and the alternating gradient condensation nuclei counter described by W. A. Hoppel et al. Field and laboratory measurements, and modeling studies show that this combination exhibits poor sensitivity for the measurement of CCN spectra; for the climatically important range of critical supersaturations, 0.03%--1%, the measured variable, droplet diameter, varies only by 30%. The ability to resolve CCN spectra using this method is therefore in question. Studies of this instrument in a fixed supersaturation mode show that it can measure CCN at a single supersaturation in the range of 0.1%--2%. Calibration and testing of the instrument in this mode is described. The instrument is capable of making accurate, high-frequency (.0.1 Hz) measurements of CCN at a fixed supersaturation, while satisfying the constraints for small aircraft. [ABSTRACT FROM AUTHOR]

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 PM 2.5 and PM 10 emissions. Subsequent papers will present and analyze the wet and dried emissions from individual towers. [ABSTRACT FROM AUTHOR]

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]

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]

This study examines the role of aerosol in mediating interactions between a warm trade cumulus cloud system and the environment that spawns it. Numerical simulations of the observed and well-studied Rain in Cumulus over the Ocean (RICO) field experiment are performed. The results draw on simulations of 34-h duration so as to avoid conclusions based on transients. Simulations show that, on average, aerosol-perturbed clouds are initially deeper and more vigorous but that after about 14 h there is a reversal in this trend, and unperturbed clouds deepen relative to the perturbed clouds. Differences in cloud depth are about 100 m, and differences in vertical velocity variance are about 30%. After about 20 h, most cloud fields are statistically similar with the exception of rain rate and optical depth, which are lower and higher, respectively, in the high-aerosol conditions. By sampling the model output at various points in the cloud system evolution, the mechanisms responsible for the initial differences and then convergence of most of the cloud field properties are addressed. Sensitivity tests indicate that responses are driven primarily by temperature profiles, rather than by humidity profiles, and that the general trend to homogenization of the bulk cloud field properties is robust for different forcings. Finally, the paper shows that even transient aerosol perturbations may endure beyond the duration of the perturbation itself, provided they persist long enough. Short-duration aerosol perturbations are unlikely to have much influence on the system. [ABSTRACT FROM AUTHOR]

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC), part of the larger experiment known as Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC 4 A), was held in the western Atlantic during the period 17 January–11 February 2020. This paper describes observations made during ATOMIC by the US National Oceanic and Atmospheric Administration's (NOAA) Lockheed WP-3D Orion research aircraft based on the island of Barbados. The aircraft obtained 95 h of observations over 11 flights, many of which were coordinated with the NOAA research ship R/V Ronald H. Brown and autonomous platforms deployed from the ship. Each flight contained a mixture of sampling strategies including high-altitude circles with frequent dropsonde deployment to characterize the large-scale environment, slow descents and ascents to measure the distribution of water vapor and its isotopic composition, stacked legs aimed at sampling the microphysical and thermodynamic state of the boundary layer, and offset straight flight legs for observing clouds and the ocean surface with remote sensing instruments and the thermal structure of the ocean with in situ sensors dropped from the plane. The characteristics of the in situ observations, expendable devices, and remote sensing instrumentation are described, as is the processing used in deriving estimates of physical quantities. Data archived at the National Center for Environmental Information include flight-level data such as aircraft navigation and basic thermodynamic information (, 10.25921/7jf5-wv54); high-accuracy measurements of water vapor concentration from an isotope analyzer (, 10.25921/c5yx-7w29); in situ observations of aerosol, cloud, and precipitation size distributions (, 10.25921/vwvq-5015); profiles of seawater temperature made with Airborne eXpendable BathyThermographs (AXBTs; , 10.25921/pe39-sx75); radar reflectivity, Doppler velocity, and spectrum width from a nadir-looking W-band radar (, 10.25921/n1hc-dc30); estimates of cloud presence, the cloud-top location, and the cloud-top radar reflectivity and temperature, along with estimates of 10 m wind speed obtained from remote sensing instruments operating in the microwave and thermal infrared spectral regions (, 10.25921/x9q5-9745); and ocean surface wave characteristics from a Wide Swath Radar Altimeter (, 10.25921/qm06-qx04). Data are provided as netCDF files following Climate and Forecast conventions. [ABSTRACT FROM AUTHOR]

This paper describes observations obtained during the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) by the US National Oceanic and Atmospheric Administration’s (NOAA) Lockheed WP-3D Orion research aircraft based on the island of Barbados during the period Jan 17 - Feb 11 2020. The aircraft obtained 95 hours of observations over eleven flights, many of which were coordinated with the NOAA research ship R/V Ronald H. Brown and autonomous platforms deployed from the ship. Each flight contained a mixture of sampling strategies including: high-altitude circles with frequent dropsonde deployment to characterize the large-scale environment; slow descents and ascents to measure the distribution of water vapor and its isotopic composition; stacked legs aimed at sampling the microphysical and thermodynamic state of the boundary layer; and offset straight flight legs for observing clouds and the ocean surface with remote sensing instruments and the thermal structure of the ocean with in situ sensors dropped from the plane. The characteristics of the in situ observations, expendable devices, and remote sensing instrumentation are described, as is the processing used in deriving estimates of physical quantities. Data archived at the National Center for Environmental Information include flight-level data such as aircraft navigation and basic thermodynamic information (doi:10.25921/7jf5-wv54); high-accuracy measurements of water vapor concentration from an isotope analyzer (doi:10.25921/c5yx-7w29); profiles of sea water temperature made with Airborne eXpendable BathyThermographs (AXBTs, doi:10.25921/pe39-sx75); profiles of radar reflectivity, Doppler velocity, and spectrum width from a nadir-looking W-band (94 GHz) radar (doi:10.25921/n1hc-dc30); estimates of cloud presence, the cloud top location, and the cloud-top radar reflectivity and temperature, along with estimates of 10-m wind speed obtained from remote sensing instruments operating in the microwave and thermal infrared spectral regions (doi:10.25921/x9q5-9745); and ocean surface wave characteristics from a Wide Swath Radar Altimeter (doi:10.25921/qm06-qx04). Data are provided as netCDF files following Climate and Forecast conventions. [ABSTRACT FROM AUTHOR]

Satellite retrievals of cloud effective radius re are frequently validated using aircraft in situ measurements. Past intercomparisons have found a significant bias toward larger remotely sensed re. Explanations for this bias have focused on retrieval algorithms and large‐scale heterogeneity, with in situ measurement uncertainty regarded as a minor factor. We compare Moderate Resolution Imaging Spectroradiometer re with in situ observations of marine stratocumulus clouds from three aircraft campaigns using a phase Doppler interferometer probe. Retrieved and in situ re typically agree within uncertainty in both nonprecipitating and drizzling conditions with no apparent systematic bias (mean bias of −0.22 μm, mean relative bias 3%). Agreement depends on the choice of in situ probe as well as microphysical context. We demonstrate that probes must adequately characterize the width of the drop size distribution to avoid systematic underestimation of re. Plain Language Summary: Satellite measurements provide the only global‐scale picture of cloud properties and are widely used in the research community. Since the first comparisons of satellite and aircraft measurements of characteristic cloud drop size, significant disagreement between the two has been found. Past studies assumed that disagreement is because of faults in how the satellite observations are handled as opposed to issues with the aircraft measurements. In this study we show that which aircraft instrument is chosen for comparison with satellite measurements can affect whether satellite and aircraft measurements agree and that an appropriate choice of aircraft instrument leads to robust agreement between satellite and aircraft measurements. Using a database of over 200,000 aircraft measurements in cloud, we show why some aircraft instruments are unable to accurately measure cloud properties in certain meteorological settings. Key Points: No systematic bias is found comparing MODIS cloud effective radius with in situ measurements taken by a phase Doppler interferometerThe lack of bias observed in precipitating conditions is consistent with modeling studiesAgreement between MODIS and in situ measurements depends on the ability of in situ probes to measure wide drop size distributions [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]

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. [ABSTRACT FROM AUTHOR]

Aerosols are suspensions of solid and//or liquid particles in the atmosphere and modify atmospheric radiative fluxes and chemistry. Aerosols move mass from one part of the earth system to other parts of the earth system, thereby modifying biogeochemistry and the snow surface albedo. This paper reviews our understanding of the impacts of aerosols on climate through direct radiative changes, aerosol-cloud interactions (indirect effects), atmospheric chemistry, snow albedo, and land and ocean biogeochemistry. Aerosols play an important role in the preindustrial (natural) climate system and have been perturbed substantially over the anthropocene, often directly by human activity. The most important impacts of aerosols, in terms of climate forcing, are from the direct and indirect effects, with large uncertainties. Similarly large impacts of aerosols on land and ocean biogeochemistry have been estimated, but these have larger uncertainties. [ABSTRACT FROM AUTHOR]