Cloud Parcel Research Articles

Sensitivity of cloud droplet formation to the numerical treatment of the particle mixing state

The importance of the particle mixing state and hygroscopicity to cloud droplet activation was investigated using an adiabatic cloud parcel model. The particle mixing state was described in terms of the hygroscopic growth properties. Various aerosol types were considered, and the input parameters describing the particle size distribution and water uptake properties were taken from the field measurement data reported in the literature. A series of sensitivity studies were conducted where the treatment of the particle mixing state was simplified as compared to the reference model configuration that resolved particle populations both as a function of the size and hygroscopicity. The results show that describing the hygroscopicity and mixing state of an aerosol population by a single parameter induced differences of up to 12% compared to the reference case in the cloud droplet concentrations (CDNCs) in marine and continental background areas. In urban and rural environments where particles display a high degree of external mixing, the relative differences in CDNC were up to 35% if the particles were assumed to be internally mixed. It was also found that a computationally efficient and relatively accurate approach is to treat less and more hygroscopic particles separately using mean properties of each population. In this case, the relative differences in CDNC were generally below 20%. The model calculations also suggest that the mixing state of a particle distribution is reflected in the size‐resolved activation efficiency (fraction of particles activated into cloud droplets versus particle dry diameter).

Influence of multi-chemical-component aerosols on the microphysics of warm clouds in North China

An adiabatic bin-sized cloud parcel model is developed by incorporating the multi-chemical-component (MCC) aerosol effects into the UWyo single-chemical-component (SCC) parcel model. The effects of MCC aerosols on the warm cloud microphysics in North China are investigated with the model. The simulations are initialized using the data on chemical components and number size distribution of aerosols measured during the IPAC (Influence of Pollution on Aerosols and Cloud Microphysics in North China) campaign in spring 2006. It is found that the MCC aerosols in North China increase the cloud droplet number concentration (CDNC) and decrease the effective radius more efficiently than pure ammonium-sulfate aerosols. It is also shown that the MCC aerosols in North China can narrow the cloud droplet spectra (CDS) by increasing CDNC in small size and decreasing CDNC in large size. Our results indicate that aerosol chemical components and their size distributions can influence the microphysics of warm clouds, and thus affect atmospheric radiation and precipitation. This should attract more attentions in weather and climate change research in the future.

Cloud albedo increase from carbonaceous aerosol

Abstract. Airborne measurements from two consecutive days, analysed with the aid of an aerosol-adiabatic cloud parcel model, are used to study the effect of carbonaceous aerosol particles on the reflectivity of sunlight by water clouds. The measurements, including aerosol chemistry, aerosol microphysics, cloud microphysics, cloud gust velocities and cloud light extinction, were made below, in and above stratocumulus over the northwest Atlantic Ocean. On the first day, the history of the below-cloud fine particle aerosol was marine and the fine particle sulphate and organic carbon mass concentrations measured at cloud base were 2.4 μg m−3 and 0.9 μg m−3 respectively. On the second day, the below-cloud aerosol was continentally influenced and the fine particle sulphate and organic carbon mass concentrations were 2.3 μg m−3 and 2.6 μg m−3 respectively. Over the range 0.06–0.8 μm diameter, the shapes of the below-cloud size distributions were similar on both days and the number concentrations were approximately a factor of two higher on the second day. The cloud droplet number concentrations (CDNC) on the second day were approximately three times higher than the CDNC measured on the first day. Using the parcel model to separate the influence of the differences in gust velocities, we estimate from the vertically integrated cloud light scattering measurements a 6% increase in the cloud albedo principally due to the increase in the carbonaceous components on the second day. Assuming no additional absorption by this aerosol, a 6% albedo increase translates to a local daytime radiative cooling of ∼12 W m−2. This result provides observational evidence that the role of anthropogenic carbonaceous components in the cloud albedo effect can be much larger than that of anthropogenic sulphate, as some global simulations have indicated.

Ice Initiation by Aerosol Particles: Measured and Predicted Ice Nuclei Concentrations versus Measured Ice Crystal Concentrations in an Orographic Wave Cloud

Abstract The initiation of ice in an isolated orographic wave cloud was compared with expectations based on ice nucleating aerosol concentrations and with predictions from new ice nucleation parameterizations applied in a cloud parcel model. Measurements of ice crystal number concentrations were found to be in good agreement both with measured number concentrations of ice nuclei feeding the clouds and with ice nuclei number concentrations determined from the residual nuclei of cloud particles collected by a counterflow virtual impactor. Using lognormal distributions fitted to measured aerosol size distributions and measured aerosol chemical compositions, ice nuclei and ice crystal concentrations in the wave cloud were reasonably well predicted in a 1D parcel model framework. Two different empirical parameterizations were used in the parcel model: a parameterization based on aerosol chemical type and surface area and a parameterization that links ice nuclei number concentrations to the number concentrations of particles with diameters larger than 0.5 μm. This study shows that aerosol size distribution and composition measurements can be used to constrain ice initiation by primary nucleation in models. The data and model results also suggest the likelihood that the dust particle mode of the aerosol size distribution controls the number concentrations of the heterogeneous ice nuclei, at least for the lower temperatures examined in this case.

Effect of mixed-phase cloud on the chemical budget of trace gases: A modeling approach

A multiphase cloud chemistry model coupling a detailed chemical reactivity mechanism in gas phase and aqueous phase to a cloud parcel model with a two-moment microphysical scheme has been extended to include ice phase processes. This newly developed model is used to study the influence of the ice phase on HCOOH, HNO 3, H 2O 2 and CH 2O in a mixed-phase cloud. Microphysical processes are describing the interactions between the water vapor phase, the liquid phase (cloud and rain water) and the ice phase (pristine ice, snow and graupel) in the cloud and for soluble chemical species, their transfer by mixed-phase microphysical processes has been included. In addition to microphysical transfer between iced hydrometeors, the probable two main processes incorporating soluble chemical species in iced hydrometeors are the retention in ice phase as riming or freezing occurs and the burial in the ice crystal as the crystal grows by vapor diffusion. The model is applied to a cloud event describing a moderate precipitating mixed-phase cloud forming in a continental air mass in winter. The main features of the cloud are described and the evolution of key chemical species as function of time and temperature is discussed. Sensitivity tests are performed: a run without ice to highlight the influence of ice phase on the chemical gas phase composition of the cloud, a run without burial showing that it is a negligible process, a run assuming full retention in ice for all species and a run varying the ice crystal shapes. A detailed analysis of the microphysical rates and chemical rates linked to retention and burial effects show that for this cloud event, the effect of the ice phase on gas phase composition is driven by riming of cloud droplets onto graupels, which leads to retention or not of soluble chemical species in the ice phase. Finally, the impact of crystal geometry on the efficiency of collection is studied together with its impact on the riming of cloud droplets on graupels and also on the retention of chemical species in ice phase.

Relationships among properties of marine stratocumulus derived from collocated CALIPSO and MODIS observations

Collocated Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) 532 nm total attenuated backscatter coefficients were used to identify 50 km scale segments for ocean regions that had only a single layer of marine stratocumulus. On the basis of whether the underlying ocean surface was detected in the backscatter coefficients, the segments were separated into those for which all of the 1 km MODIS pixels were identified as being overcast (no surface detected) and those for which there were breaks in the cloud layer (surface detected). Cloud properties for the collocated MODIS pixels were obtained from the MODIS MOD06 cloud product and from a retrieval scheme designed to account for broken clouds within imager pixels. For the 50 km overcast segments the variations in optical depth, τ, and droplet effective radius, Re, obtained from the MOD06 and partly cloudy pixel retrievals were in agreement, and the variations in layer temperature were consistent with those inferred from the CALIPSO backscatter coefficients. In addition, the droplet effective radii retrieved separately using the 1.6, 2.1, and 3.7 μm MODIS reflectances were consistent with droplets growing as clouds thickened and mean values of dlnRe/dlnτ were close to 0.2, as predicted by adiabatic cloud parcel models. When the segments contained broken clouds, however, the droplet effective radii for the three near‐infrared wavelengths were inconsistent with droplets growing as clouds thickened and dlnRe/dlnτ departed from 0.2. This breakdown is thought to result from biases in the retrieved cloud properties caused by subpixel‐scale variations in liquid water amount and droplet radius.

Slower CCN growth kinetics of anthropogenic aerosol compared to biogenic aerosol observed at a rural site

Abstract. Growth rates of water droplets were measured with a static diffusion cloud condensation chamber in May–June 2007 at a rural field site in Southern Ontario, Canada, 70 km north of Toronto. The observations include periods when the winds were from the south and the site was impacted by anthropogenic air from the U.S. and Southern Ontario as well as during a 5-day period of northerly wind flow when the aerosol was dominated by biogenic sources. The growth of droplets on anthropogenic size-selected particles centred at 0.1 μm diameter and composed of approximately 40% organic and 60% ammonium sulphate (AS) by mass, was delayed by on the order of 1 s compared to a pure AS aerosol. Simulations of the growth rate on monodisperse particles indicate that a lowering of the water mass accommodation coefficient from αc=1 to an average of αc=0.04 is needed (assuming an insoluble organic with hygroscopicity parameter, κorg, of zero). Simulations of the initial growth rate on polydisperse anthropogenic particles agree best with observations for αc=0.07. In contrast, the growth rate of droplets on size-selected aerosol of biogenic character, consisting of >80% organic, was similar to that of pure AS. Simulations of the predominantly biogenic polydisperse aerosol show agreement between the observations and simulations when κorg=0.2 (with upper and lower limits of 0.5 and 0.07, respectively) and αc=1. Inhibition of water uptake by the anthropogenic organic applied to an adiabatic cloud parcel model in the form of a constant low αc increases the number of droplets in a cloud compared to pure AS. If the αc is assumed to increase with increasing liquid water on the droplets, then the number of droplets decreases which could diminish the indirect climate forcing effect. The slightly lower κorg in the biogenic case decreases the number of droplets in a cloud compared to pure AS.

Aerosol- and updraft-limited regimes of cloud droplet formation: influence of particle number, size and hygroscopicity on the activation of cloud condensation nuclei (CCN)

Abstract. We have investigated the formation of cloud droplets under pyro-convective conditions using a cloud parcel model with detailed spectral microphysics and with the κ-Köhler model approach for efficient and realistic description of the cloud condensation nucleus (CCN) activity of aerosol particles. Assuming a typical biomass burning aerosol size distribution (accumulation mode centred at 120 nm), we have calculated initial cloud droplet number concentrations (NCD) for a wide range of updraft velocities (w=0.25–20 m s−1) and aerosol particle number concentrations (NCN=200–105 cm−3) at the cloud base. Depending on the ratio between updraft velocity and particle number concentration (w/NCN), we found three distinctly different regimes of CCN activation and cloud droplet formation: (1) An aerosol-limited regime that is characterized by high w/NCN ratios (>≈10−3 m s−1 cm3), high maximum values of water vapour supersaturation (Smax>≈0.5%), and high activated fractions of aerosol particles (NCN/NCN>≈90%). In this regime NCD is directly proportional to NCN and practically independent of w. (2) An updraft-limited regime that is characterized by low w/NCN ratios (

Observations of marine stratocumulus microphysics and implications for processes controlling droplet spectra: Results from the Marine Stratus/Stratocumulus Experiment

During the Marine Stratus/Stratocumulus Experiment, cloud and aerosol microphysics were measured in the eastern Pacific off the coast of northern California on board Department of Energy Gulfstream‐1 in July 2005. Three cases with uniform aerosol concentration and minimal drizzle concentration were examined to study cloud microphysical behavior. For these three cases, the average droplet number concentration increased with increasing altitude, while the average interstitial aerosol concentration decreased with altitude. The data show enhanced growth of large droplets and spectral broadening in cloud parcels with low liquid water mixing ratio. Three mixing models, including inhomogeneous mixing, entity type entrainment mixing, and circulation mixing proposed in this study, are examined with regard to their influences on cloud microphysics. The observed cloud microphysical behavior is most consistent with the circulation mixing, which describes the mixing between cloud parcels with different lifting condensation levels during their circulations driven by evaporative and radiative cooling. The enhanced growth and spectrum broadening resulting from the circulation mixing reduce cloud albedo at the same liquid water path and facilitate the formation of precipitation embryos.

Parameterizing the competition between homogeneous and heterogeneous freezing in ice cloud formation – polydisperse ice nuclei

Abstract. This study presents a comprehensive ice cloud formation parameterization that computes the ice crystal number, size distribution, and maximum supersaturation from precursor aerosol and ice nuclei. The parameterization provides an analytical solution of the cloud parcel model equations and accounts for the competition effects between homogeneous and heterogeneous freezing, and, between heterogeneous freezing in different modes. The diversity of heterogeneous nuclei is described through a nucleation spectrum function which is allowed to follow any form (i.e., derived from classical nucleation theory or from observations). The parameterization reproduces the predictions of a detailed numerical parcel model over a wide range of conditions, and several expressions for the nucleation spectrum. The average error in ice crystal number concentration was −2.0±8.5% for conditions of pure heterogeneous freezing, and, 4.7±21% when both homogeneous and heterogeneous freezing were active. The formulation presented is fast and free from requirements of numerical integration.

Distinct CCN activation kinetics above the marine boundary layer along the California coast

The influence of aerosols on cloud properties remains one of the largest sources of uncertainty in estimates of the anthropogenic component of climate change. Here we report the rate of cloud droplet formation on particles sampled at a site near the California coast that is typically above the marine boundary layer. We observed persistent bimodal diameter spectra which are better explained by kinetic limitations than by differences in equilibrium properties. The slowly‐growing mode contained 10–25% of the total cloud condensation nuclei (CCN) and had apparent mass accommodation coefficients (α) 10–30 times smaller than that measured for ammonium sulfate. Cloud parcel modeling suggests that most of these slowly‐growing CCN will not form cloud droplets. The relatively small and narrow size distribution of the low‐α droplets suggest that a condensed film is a more likely cause of these limitations than slow dissolution.

Correlations of small cumuli droplet and drizzle drop concentrations with cloud condensation nuclei concentrations

Aircraft field measurements of cloud condensation nuclei (CCN) and cloud microphysics in maritime air masses showed ubiquitous influence of CCN. Flight averages of CCN concentrations and cloud droplet and drizzle drop concentrations were examined for as many as 17 flights during the Rain in Cumulus over the Ocean (RICO) project. CCN concentrations at only one supersaturation (S) of 1% measured at 100‐m altitude were compared with cloud droplet and drizzle drop concentrations at six altitude bands between 600 and 3000 m. High positive correlations (R) between these CCN concentrations and the small size threshold of the cumulative cloud droplet concentrations (i.e., total activated cloud droplets) were found at all altitudes. These high R values also persisted for cloud parcels with a wide span of liquid water contents (LWCs), most of which were far below adiabatic (unmixed) values. For all but the lowest LWC parcels, R was essentially constant. There was an even more consistent negative R between CCN and large cloud droplet and drizzle drop concentrations. There was a sharp transition from positive to negative R over a small size range. The size at which this R transition occurred increased with altitude and LWC as overall droplet sizes increased with altitude or LWC. Entrainment seemed to show an opposite effect on R, but this was only apparent at the highest altitudes where entrainment was greatest and only for the smallest droplet sizes. These results indicate that the effect of CCN concentrations on cloud microphysics was pervasive with altitude, LWC, cloud droplet, and drizzle drop concentrations. This indicates greater impact of the indirect aerosol effect (IAE) in both of its manifestations, first IAE cloud radiation and second IAE precipitation.

Parameterizing the competition between homogeneous and heterogeneous freezing in cirrus cloud formation – monodisperse ice nuclei

Abstract. We present a parameterization of cirrus cloud formation that computes the ice crystal number and size distribution under the presence of homogeneous and heterogeneous freezing. The parameterization is very simple to apply and is derived from the analytical solution of the cloud parcel equations, assuming that the ice nuclei population is monodisperse and chemically homogeneous. In addition to the ice distribution, an analytical expression is provided for the limiting ice nuclei number concentration that suppresses ice formation from homogeneous freezing. The parameterization is evaluated against a detailed numerical parcel model, and reproduces numerical simulations over a wide range of conditions with an average error of 6±33%. The parameterization also compares favorably against other formulations that require some form of numerical integration.

Cloud processing, cloud evaporation and Angström exponent

Abstract. With a cloud parcel model we investigate how cloud processing and cloud evaporation modify the size distribution and the Angström exponent of an aerosol population. Our study provides a new explanation for the observed variability of the aerosol optical thickness and Angström exponent in the vicinity of clouds. Cloud processing causes a decrease of aerosol particle concentrations, relatively most efficiently in the coarse mode, and reduces the relative dispersion of the aerosol distribution. As a result the Angström exponent of the aerosol increases. The Angström exponent is very sensitive for changes in relative humidity during cloud evaporation, especially between 90% and 100%. In addition, kinetic limitations delay evaporation of relatively large cloud drops, especially in clean and mildly polluted environments where the coarse mode fraction is relatively large. This hampers a direct relation between the aerosol optical thickness, the Angström exponent and the ambient relative humidity, which may severely complicate interpretation of these parameters in terms of aerosol properties, such as the fine mode fraction.

Cloud Formation in the Plumes of Solar Chimney Power Generation Facilities: A Modeling Study

The solar chimney power facility has the potential to become a valuable technology for renewable energy production. Its financial viability depends on a thorough understanding of the processes affecting its performance, particularly because of the large startup costs associated with facility design and construction. This paper describes the potential impacts on plant capacity resulting from cloud formation within or downwind of the solar chimney. Several proposed modifications to the basic concept of the solar chimney power facility have the potential to cause significant additions of water vapor to the air passing through the collector. As the air continues up through and out of the chimney, this excess water can condense to form cloud. This possibility is explored using a cloud parcel model initialized to simulate the range of expected operating conditions for a proposed solar chimney facility in southwestern Australia. A range of temperatures and updraft velocities is simulated for each of four seasonal representations and three levels of water vapor enhancement. Both adiabatic environments and the effects of entrainment are considered. The results indicate that for very high levels of water vapor, enhancement cloud formation within the chimney is likely; at more moderate levels of water vapor enhancement, the likelihood of plume formation is difficult to fully assess as the results depend strongly on the choice of entrainment rate. Finally, the impacts of these outcomes on facility capacity are estimated.

The influence of chemical composition and mixing state of Los Angeles urban aerosol on CCN number and cloud properties

Abstract. The relationship between cloud condensation nuclei (CCN) number and the physical and chemical properties of the atmospheric aerosol distribution is explored for a polluted urban data set from the Study of Organic Aerosols at Riverside I (SOAR-1) campaign conducted at Riverside, California, USA during summer 2005. The mixing state and, to a lesser degree, the average chemical composition are shown to be important parameters in determining the activation properties of those particles around the critical activation diameters for atmospherically-realistic supersaturation values. Closure between predictions and measurements of CCN number at several supersaturations is attempted by modeling a number of aerosol chemical composition and mixing state cases of increasing complexity. It is shown that a realistic treatment of the state of mixing of the urban aerosol distribution is critical in order to eliminate model bias. Fresh emissions such as elemental carbon and small organic particles must be treated as non-activating and explicitly accounted for in the model. The relative number concentration of these particles compared to inorganics and oxygenated organic compounds of limited hygroscopicity plays an important role in determining the CCN number. Furthermore, expanding the different composition/mixing state cases to predictions of cloud droplet number concentration in a cloud parcel model highlights the dependence of cloud optical properties on the state of mixing and hygroscopic properties of the different aerosol modes, but shows that the relative differences between the different cases are reduced compared to those from the CCN model.

Parameterization of cirrus cloud formation in large‐scale models: Homogeneous nucleation

This work presents a new physically based parameterization of cirrus cloud formation for use in large‐scale models which is robust, computationally efficient, and links chemical effects (e.g., water activity and water vapor deposition effects) with ice formation via homogenous freezing. The parameterization formulation is based on ascending parcel theory and provides expressions for the ice crystal size distribution and the crystal number concentration, explicitly considering the effects of aerosol size and number, updraft velocity, and deposition coefficient. The parameterization is evaluated against a detailed numerical cirrus cloud parcel model (developed during this study), the equations of which are solved using a novel Lagrangian particle‐tracking scheme. Over a broad range of cirrus‐forming conditions, the parameterization reproduces the results of the parcel model within a factor of 2 and with an average relative error of −15%. If numerical model simulations are used to constrain the parameterization, error further decreases to 1 ± 28%.

Combined Observational and Model Investigations of the Z–LWC Relationship in Stratocumulus Clouds

Abstract In situ measurements indicate the complexity and nonunique character of radar reflectivity–liquid water content (Z–LWC) relationships in stratocumulus and cumulus clouds. Parameters of empirical (statistical) Z–LWC dependences vary within a wide range. Respectively, the accuracy of retrieval algorithms remains low. This situation is partially related to the fact that empirical algorithms and parameters are often derived without a corresponding understanding of physical mechanisms responsible for the formation of the Z–LWC diagrams. In this study, the authors investigate the processes of formation of the Z–LWC relationships using a new trajectory ensemble model of the cloud-topped boundary layer (BL). In the model, the entire volume of the BL is covered by Lagrangian parcels advected by a turbulent-like velocity field. The time-dependent velocity field is generated by a turbulent model and obeys the correlation turbulent laws. Each Lagrangian parcel represents the “cloud parcel model” with an accurate description of processes of diffusion growth–evaporation of aerosols and droplets and droplet collisions. The fact that parcels are adjacent to each other allows one to calculate sedimentation of droplets and precipitation (drizzle) formation. The characteristic parcel size is 50 m; the number of parcels is 1840. The model calculates droplet size distributions (DSDs), as well as their moments (e.g., aerosol and drop concentration, mass content, radar reflectivity) in each parcel. In the course of the model integration, Z–LWC relationships are calculated for each parcel, as well as the scattering diagram including all parcels. The model reproduces in situ observed types of the Z–LWC relationships. It is shown that different regimes represent different stages of cloud evolution: diffusion growth, beginning of drizzle formation, and stage of heavy drizzle, respectively. The large scattering of the Z–LWC relationships is found to be an inherent property of any drizzling cloud. Different zones on the Z–LWC diagram are related to cloud volumes located at different levels within a cloud and having different DSD. This finding allows for improvement of retrieval algorithms.

The Effect of Dynamics on Mixed-Phase Clouds: Theoretical Considerations

Abstract A theoretical framework has been developed describing nonequilibrium formation and maintenance of mixed-phase clouds. The necessary and sufficient conditions required to activate liquid water within a preexisting ice cloud, and thus convert it to mixed phase, are considered for three scenarios: (i) uniform ascent, (ii) harmonic vertical oscillations, and (iii) turbulent fluctuations. The general conditions are the following:First necessary condition: The vertical velocity of an ice cloud parcel must exceed a threshold velocity to activate liquid water.Second necessary condition: The activation of liquid water within an ice cloud parcel, below water saturation, requires a vertical ascent above some threshold altitude to bring the vapor pressure of the parcel to water saturation. Only when the first and second conditions are true do these conditions become sufficient for the activation of liquid water in ice clouds. These required conditions for the generation of mixed-phase cloud are supported by parcel modeling results and analogous conditions for a harmonic oscillation concerning the amplitude and tangential velocity of the parcel motion are proposed. The authors do not assume steady-state conditions, but demonstrate that nonequilibrium evolution of cloud parcels can lead to long-term steady existence of mixed-phase cloud.

Particulate organic acids and overall water‐soluble aerosol composition measurements from the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

The Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter participated in the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) mission during August–September 2006. A particle‐into‐liquid sampler (PILS) coupled to ion chromatography was used to characterize the water‐soluble ion composition of aerosol and cloud droplet residual particles (976 5‐min PM1.0samples in total). Sulfate and ammonium dominated the water‐soluble mass (NH4++ SO42−= 84 ± 14%), while organic acids contributed 3.4 ± 3.7%. The average NH4+:SO42−molar ratio was 1.77 ± 0.85. Particulate concentrations of organic acids increased with decreasing carbon number from C9to C2. Organic acids were most abundant above cloud, presumably as a result of aqueous phase chemistry in cloud droplets, followed by subsequent droplet evaporation above cloud tops; the main product of this chemistry was oxalic acid. The evolution of organic acids with increasing altitude in cloud provides evidence for the multistep nature of oxalic acid production; predictions from a cloud parcel model are consistent with the observed oxalate:glyoxylate ratio as a function of altitude in GoMACCS cumuli. Suppressed organic acid formation was observed in clouds with relatively acidic droplets, as determined by high particulate nitrate concentrations (presumably high HNO3levels too) and lower liquid water content, as compared to other cloud fields probed. In the Houston Ship Channel region, an area with significant volatile organic compound emissions, oxalate, acetate, formate, benzoate, and pyruvate, in decreasing order, were the most abundant organic acids. Photo‐oxidation ofm‐xylene in laboratory chamber experiments leads to a particulate organic acid product distribution consistent with the Ship Channel area observations.