Center For Interdisciplinary Remotely-Piloted Aircraft Studies Research Articles

Droplet inhomogeneity in shallow cumuli: the effects of in-cloud location and aerosol number concentration

Abstract. Aerosol–cloud interactions are complex, including albedo and lifetime effects that cause modifications to cloud characteristics. With most cloud–aerosol interactions focused on the previously stated phenomena, there have been no in situ studies that focus explicitly on how aerosols can affect large-scale (centimeters to tens of meters) droplet inhomogeneities within clouds. This research therefore aims to gain a better understanding of how droplet inhomogeneities within cumulus clouds can be influenced by in-cloud droplet location (cloud edge vs. center) and the surrounding environmental aerosol number concentration. The pair-correlation function (PCF) is used to identify the magnitude of droplet inhomogeneity from data collected on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft, flown during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS). Time stamps (at 10−4 m spatial resolution) of cloud droplet arrival times were measured by the Artium Flight phase-Doppler interferometer (PDI). Using four complete days of data with 81 non-precipitating cloud penetrations organized into two flights of low-pollution (L1, L2) and high-pollution (H1, H2) data shows enhanced inhomogeneities near cloud edge as compared to cloud center for all four cases. Low-pollution clouds are shown to have enhanced overall inhomogeneity, with flight L2 being solely responsible for this enhanced inhomogeneity. Analysis suggests cloud age plays a larger role in the amount of inhomogeneity experienced than the aerosol number concentration, with dissipating clouds showing increased inhomogeneities as compared to growing or mature clouds. Results using a single, vertically developed cumulus cloud demonstrate enhanced droplet inhomogeneity near cloud top as compared to cloud base.

High-Definition Sounding System (HDSS) for Atmospheric Profiling

AbstractThe High-Definition Sounding System (HDSS) is an automated system deploying the expendable digital dropsonde (XDD) designed to measure wind and pressure–temperature–humidity (PTH) profiles, and skin sea surface temperature (SST) within and around tropical cyclones (TCs) and other high-impact weather events needing high sampling density. Three experiments were conducted to validate the XDD.On two successive days off the California coast, 10 XDDs and 14 Vaisala RD-94s were deployed from the navy’s Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft over offshore buoys. The Twin Otter made spiral descents from 4 km to 60 m at the same descent rate as the sondes. Differences between successive XDD and RD-94 profiles due to true meteorological variability were on the same order as the profile differences between the spirals, XDDs, and RD-94s. XDD SST measured via infrared microradiometer, referred to as infrared skin SST (SSTir), and surface wind measurements were within 0.5°C and 1.5 m s−1, respectively, of buoy and Twin Otter values.A NASA DC-8 flight launched six XDDs from 12 km between ex-TC Cosme and the Baja California coast. Repeatability was shown with good agreement between features in successive profiles. XDD SSTir measurements from 18° to 28°C and surface winds agreed well with drifting buoy- and satellite-derived estimates.Excellent agreement was found between PTH and wind profiles measured by XDDs deployed from a NASA WB-57 at 18-km altitude offshore from the Texas coast and NWS radiosonde profiles from Brownsville and Corpus Christi, Texas. Successful XDD profiles were obtained in the clear and within precipitation over an offshore squall line.

Aerosols, clouds, and precipitation in the North Atlantic trades observed during the Barbados aerosol cloud experiment – Part 1: Distributions and variability

Abstract. Shallow marine cumulus clouds are by far the most frequently observed cloud type over the Earth's oceans; but they are poorly understood and have not been investigated as extensively as stratocumulus clouds. This study describes and discusses the properties and variations of aerosol, cloud, and precipitation associated with shallow marine cumulus clouds observed in the North Atlantic trades during a field campaign (Barbados Aerosol Cloud Experiment- BACEX, March–April 2010), which took place off Barbados where African dust periodically affects the region. The principal observing platform was the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft, which was equipped with standard meteorological instruments, a zenith pointing cloud radar and probes that measured aerosol, cloud, and precipitation characteristics.The temporal variation and vertical distribution of aerosols observed from the 15 flights, which included the most intense African dust event during all of 2010 in Barbados, showed a wide range of aerosol conditions. During dusty periods, aerosol concentrations increased substantially in the size range between 0.5 and 10 µm (diameter), particles that are large enough to be effective giant cloud condensation nuclei (CCN). The 10-day back trajectories showed three distinct air masses with distinct vertical structures associated with air masses originating in the Atlantic (typical maritime air mass with relatively low aerosol concentrations in the marine boundary layer), Africa (Saharan air layer), and mid-latitudes (continental pollution plumes). Despite the large differences in the total mass loading and the origin of the aerosols, the overall shapes of the aerosol particle size distributions were consistent, with the exception of the transition period.The TO was able to sample many clouds at various phases of growth. Maximum cloud depth observed was less than ∼ 3 km, while most clouds were less than 1 km deep. Clouds tend to precipitate when the cloud is thicker than 500–600 m. Distributions of cloud field characteristics (depth, radar reflectivity, Doppler velocity, precipitation) were well identified in the reflectivity–velocity diagram from the cloud radar observations. Two types of precipitation features were observed for shallow marine cumulus clouds that may impact boundary layer differently: first, a classic cloud-base precipitation where precipitation shafts were observed to emanate from the cloud base; second, cloud-top precipitation where precipitation shafts emanated mainly near the cloud tops, sometimes accompanied by precipitation near the cloud base. The second type of precipitation was more frequently observed during the experiment. Only 42–44 % of the clouds sampled were non-precipitating throughout the entire cloud layer and the rest of the clouds showed precipitation somewhere in the cloud, predominantly closer to the cloud top.

Impact of emissions from shipping, land, and the ocean on stratocumulus cloud water elemental composition during the 2011 E-PEACE field campaign

This study reports on cloud water chemical and pH measurements off the California coast during the July–August 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). Eighty two cloud water samples were collected by a slotted-rod cloud water collector protruding above the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter in boundary layer stratocumulus clouds impacted to varying degrees by ocean-derived emissions, ship exhaust, and land emissions. Cloud water pH ranged between 2.92 and 7.58, with an average of 4.46. Peak pH values were observed north of San Francisco, simultaneous with the highest concentrations of Si, B, and Cs, and air masses originating over land. The lowest pH values were observed south of San Francisco due to ship emissions resulting in the highest concentrations of sulfate, nitrate, V, Fe, Al, P, Cd, Ti, Sb, P, and Mn. Many of these species act as important agents in aqueous-phase reactions in cloud drops and are critical ocean micronutrients after subsequent wet deposition in an ocean system that can be nutrient-limited. E-PEACE measurements suggest that conditions in the California coastal zone region can promote the conversion of micronutrients to more soluble forms, if they are not already, due to acidic cloud water conditions, the ubiquity of important organic agents such as oxalic acid, and the persistence of stratocumulus clouds to allow for continuous cloud processing.

Reply to comment by Yangang Liu et al. on “Cloud droplet spectral width relationship to CCN spectra and vertical velocity”

Reply to comment by Yangang Liu et al. on “Cloud droplet spectral width relationship to CCN spectra and vertical velocity”

Vertical structure of aerosols, temperature, and moisture associated with an intense African dust event observed over the eastern Caribbean

AbstractAn unusually intense African dust event affected a large area of the western Atlantic and eastern Caribbean in early April 2010. Measurements made east of Barbados from the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter research aircraft are used to characterize particle size distributions; vertical distributions of aerosols, temperature, and moisture; and processes leading to the observed stratification in the boundary layer. The vertical profiles of various aerosol characterizations were similar on both days and show three layers with distinct aerosol and thermodynamic characteristics: the Saharan Air Layer (SAL; ~2.2 km ± 500 m), a subcloud layer (SCL; surface to ~500 m), and an intermediate layer extending between them. The SAL and SCL display well‐mixed aerosol and thermodynamic characteristics; but the most significant horizontal and vertical variations in aerosols and thermodynamics occur in the intermediate layer. The aerosol variability observed in the intermediate layer is likely associated with modification by shallow cumulus convection occurring sometime in the prior history of the air mass as it is advected across the Atlantic. A comparison of the thermodynamic structure observed in the event from its origin over Africa with that when it reached Barbados indicates that the lower part of the SAL was moistened by surface fluxes as the air mass was advected across the Atlantic. Mixing diagrams using aerosol concentrations and water vapor mixing ratios as conserved parameters provide insight into the vertical transports and mixing processes that may explain the observed aerosol and thermodynamic variability in each layer.

Eastern Pacific Emitted Aerosol Cloud Experiment

Aerosol–cloud–radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models in addressing these important questions but quantifying them correctly is nontrivial, thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft and the research vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. EPEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely 1) shipboard smoke-generated particles with 0.05–1-μm diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), 2) combustion particles from container ships with 0.05–0.2-μm diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and 3) aircraft-based milled salt particles with 3–5-μm diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large-eddy simulations of deeper clouds in ship tracks and aerosol– cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm-cloud microphysics.

Inorganic and black carbon aerosols in the Los Angeles Basin during CalNex

We evaluate predictions from the Community Multiscale Air Quality (CMAQ version 4.7.1) model against a suite of airborne and ground‐based meteorological measurements, gas‐ and aerosol‐phase inorganic measurements, and black carbon (BC) measurements over Southern California during the CalNex field campaign in May/June 2010. Ground‐based measurements are from the CalNex Pasadena ground site, and airborne measurements took place onboard the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Navy Twin Otter and the NOAA WP‐3D aircraft. BC predictions are in general agreement with observations at the Pasadena ground site and onboard the WP‐3D, but are consistently overpredicted when compared to Twin Otter measurements. Adjustments to predicted inorganic mass concentrations, based on predicted aerosol size distributions and the AMS transmission efficiency, are shown to be significant. Owing to recent shipping emission reductions, the dominant source of sulfate in the L.A. Basin may now be long‐range transport. Sensitivity studies suggest that severely underestimated ammonia emissions, and not the exclusion of crustal species (Ca2 +, K+, and Mg2 +), are the single largest contributor to measurement/model disagreement in the eastern part of the L.A. Basin. Despite overstated NOx emissions, total nitrate concentrations are underpredicted, which suggests a missing source of HNO3 and/or overprediction of deposition rates. Adding gas‐phase NH3 measurements and size‐resolved measurements, up to 10 μm, of nitrate and various cations (e.g. Na+, Ca2 +, K+) to routine monitoring stations in the L.A. Basin would greatly facilitate interpreting day‐to‐day fluctuations in fine and coarse inorganic aerosol.

Characterisation and airborne deployment of a new counterflow virtual impactor inlet

Abstract. A new counterflow virtual impactor (CVI) inlet is introduced with details of its design, laboratory characterisation tests and deployment on an aircraft during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE). The CVI inlet addresses three key issues in previous designs; in particular, the inlet operates with: (i) negligible organic contamination; (ii) a significant sample flow rate to downstream instruments (∼15 l min−1) that reduces the need for dilution; and (iii) a high level of accessibility to the probe interior for cleaning. Wind tunnel experiments characterised the cut size of sampled droplets and the particle size-dependent transmission efficiency in various parts of the probe. For a range of counter-flow rates and air velocities, the measured cut size was between 8.7–13.1 μm. The mean percentage error between cut size measurements and predictions from aerodynamic drag theory is 1.7%. The CVI was deployed on the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter for thirty flights during E-PEACE to study aerosol-cloud-radiation interactions off the central coast of California in July and August 2011. Results are reported to assess the performance of the inlet including comparisons of particle number concentration downstream of the CVI and cloud drop number concentration measured by two independent aircraft probes. Measurements downstream of the CVI are also examined from one representative case flight coordinated with shipboard-emitted smoke that was intercepted in cloud by the Twin Otter.

Black carbon aerosol over the Los Angeles Basin during CalNex

Refractory black carbon (rBC) mass and number concentrations were quantified by a Single Particle Soot Photometer (SP2) in the CalNex 2010 field study on board the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter in the Los Angeles (LA) Basin in May, 2010. The mass concentrations of rBC in the LA Basin ranged from 0.002–0.530μg m−3, with an average of 0.172 μg m−3. Lower concentrations were measured in the Basin outflow regions and above the inversion layer. The SP2 afforded a quantification of the mixing state of rBC aerosols through modeling the scattering cross‐section with a core‐and‐shell Mie model to determine coating thickness. The rBC particles above the inversion layer were more thickly coated by a light‐scattering substance than those below, indicating a more aged aerosol in the free troposphere. Near the surface, as the LA plume is advected from west to east with the sea breeze, a coating of scattering material grows on rBC particles, coincident with a clear growth of ammonium nitrate within the LA Basin and the persistence of water‐soluble organic compounds as the plume travels through the outflow regions. Detailed analysis of the rBC mixing state reveals two modes of coated rBC particles; a mode with smaller rBC core diameters (∼90 nm) but thick (>200 nm) coating diameters and a mode with larger rBC cores (∼145 nm) with a thin (<75 nm) coating. The “weekend effect” in the LA Basin results in more thickly coated rBC particles, coinciding with more secondary formation of aerosol.

Water-soluble organic aerosol in the Los Angeles Basin and outflow regions: Airborne and ground measurements during the 2010 CalNex field campaign

A particle-into-liquid sampler coupled to a total organic carbon analyzer (PILS-TOC) quantified particulate water-soluble organic carbon (WSOC) mass concentrations during the May 2010 deployment of the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter in the CalNex field study. WSOC data collected during 16 flights provide the first spatiotemporal maps of WSOC in the San Joaquin Valley, Los Angeles Basin, and outflow regions of the Basin. WSOC was consistently higher in concentration within the Los Angeles Basin, where sea breeze transport and Basin topography strongly influence the spatial distribution of WSOC. The highest WSOC levels were associated with fire plumes, highlighting the importance of both primary and secondary sources for WSOC in the region. Residual pollution layers enriched with WSOC are observed aloft up to an altitude of 3.2 km and the highest WSOC levels for each flight were typically observed above 500 m. Simultaneous ground WSOC measurements during aircraft overpasses in Pasadena and Riverside typically exhibit lower levels, especially when relative humidity (RH) was higher aloft suggestive of the influence of aerosol-phase water. This points to the underestimation of the radiative effects of WSOC when using only surface measurements. Reduced aerosol-phase water in the eastern desert outflow region likely promotes the re-partitioning of WSOC to the gas phase and suppression of processes to produce these species (partitioning, multiphase chemistry, photolytic production); as a result, WSOC is reduced relative to sulfate (but not as much as nitrate) as aerosol is advected from the Basin to the outflows.

A Mobile, Phased-Array Doppler Radar For The Study of Severe Convective Storms

A mobile X-band, phased-array Doppler radar was acquired from the U.S. Army by the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) at the Naval Postgraduate School and adapted for meteorological use by ProSensing, Inc. The radar was used during field experiments conducted in the Southern Plains by faculty and students from the School of Meteorology at the University of Oklahoma during the spring storm seasons of 2007 and 2008. During these field experiments, storm-scale, rapid-scan, volumetric, Doppler-radar observations were obtained in tornadic and nontornadic supercells, quasilinear mesoscale convective systems, and in both boundary layer–based and elevated ordinary convective cells. A case is made for the use of the radar for studies of convective weather systems and other weather phenomena that evolve on time scales as short as tens of seconds.

Comprehensive airborne characterization of aerosol from a major bovine source

Abstract. We report an extensive airborne characterization of aerosol downwind of a massive bovine source in the San Joaquin Valley (California) on two flights during July 2007. The Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed chemical composition, particle size distribution, mixing state, sub- and supersaturated water uptake behavior, light scattering properties, and the interrelationship between these parameters and meteorology. Total PM1.0 levels and concentrations of organics, nitrate, and ammonium were enhanced in the plume from the source as compared to the background aerosol. Organics dominated the plume aerosol mass (~56–64%), followed either by sulfate or nitrate, and then ammonium. Particulate amines were detected in the plume aerosol by a particle-into-liquid sampler (PILS) and via mass spectral markers in the Aerodyne C-ToF-AMS. Amines were found to be a significant atmospheric base even in the presence of ammonia; particulate amine concentrations are estimated as at least 14–23% of that of ammonium in the plume. Enhanced sub- and supersaturated water uptake and reduced refractive indices were coincident with lower organic mass fractions, higher nitrate mass fractions, and the detection of amines. The likelihood of suppressed droplet growth owing to kinetic limitations from hydrophobic organic material is explored. After removing effects associated with size distribution and mixing state, the normalized activated fraction of cloud condensation nuclei (CCN) increased as a function of the subsaturated hygroscopic growth factor, with the highest activated fractions being consistent with relatively lower organic mass fractions and higher nitrate mass fractions. Subsaturated hygroscopic growth factors for the organic fraction of the aerosol are estimated based on employing the Zdanovskii-Stokes Robinson (ZSR) mixing rule. Representative values for a parameterization treating particle water uptake in both the sub- and supersaturated regimes are reported for incorporation into atmospheric models.

Aerosol‐cloud relationships in continental shallow cumulus

Aerosol‐cloud relationships are derived from 14 warm continental cumuli cases sampled during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) by the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. Cloud droplet number concentration is clearly proportional to the subcloud accumulation mode aerosol number concentration. An inverse correlation between cloud top effective radius and subcloud aerosol number concentration is observed when cloud depth variations are accounted for. There are no discernable aerosol effects on cloud droplet spectral dispersion; the averaged spectral relative dispersion is 0.30 ± 0.04. Aerosol‐cloud relationships are also identified from comparison of two isolated cloud cases that occurred under different degrees of anthropogenic influence. Cloud liquid water content, cloud droplet number concentration, and cloud top effective radius exhibit subadiabaticity resulting from entrainment mixing processes. The degree of LWC subadiabaticity is found to increase with cloud depth. Impacts of subadiabaticity on cloud optical properties are assessed. It is estimated that owing to entrainment mixing, cloud LWP, effective radius, and cloud albedo are decreased by 50–85%, 5–35%, and 2–26%, respectively, relative to adiabatic values of a plane‐parallel cloud. The impact of subadiabaticity on cloud albedo is largest for shallow clouds. Results suggest that the effect of entrainment mixing must be accounted for when evaluating the aerosol indirect effect.

Statistical comparison of properties of simulated and observed cumulus clouds in the vicinity of Houston during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

We present statistical comparisons of properties of clouds generated by Large Eddy Simulations (LES) with aircraft observations of nonprecipitating, warm cumulus clouds made in the vicinity of Houston, TX during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), carried out in the summer of 2006. Aircraft data were sampled with the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter airplane. Five flights (days) that are most suitable for studying aerosol‐cloud interactions are selected from the 22 flights. The model simulations are initiated with observed environmental profiles. The simulations are used to generate an ensemble of thousands of cumulus clouds for statistically meaningful evaluations. Statistical comparisons focus on the properties of a set of dynamical and thermodynamical variables, sampled either in the cloud or the cloud updraft core. The set of variables includes cloud liquid water content (LWC), number mixing ratio of cloud droplets (Nd), cloud effective radius (re), updraft velocity (w), and the distribution of cloud sizes. In general, good agreement between the simulated and observed clouds is achieved in the normalized frequency distribution functions, the profiles averaged over the cloudy regions, the cross‐cloud averages, and the cloud size distributions, despite big differences in sample size between the model output and the aircraft data. Some unresolved differences in frequency distributions of w and possible differences in cloud fraction are noted. These comparisons suggest that the LES is able to successfully generate the cumulus cloud populations that were present during GoMACCS. The extent to which this is true will depend on the specific application.

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.

On the Source of Organic Acid Aerosol Layers above Clouds

During the July 2005 Marine Stratus/Stratocumulus Experiment (MASE) and the August-September 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS), the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter probed aerosols and cumulus clouds in the eastern Pacific Ocean off the coast of northern California and in southeastern Texas, respectively. An on-board particle-into-liquid sampler (PILS) quantified inorganic and organic acid species with < or = 5-min time resolution. Ubiquitous organic aerosol layers above cloud with enhanced organic acid levels were observed in both locations. The data suggest that aqueous-phase reactions to produce organic acids, mainly oxalic acid, followed by droplet evaporation is a source of elevated organic acid aerosol levels above cloud. Oxalic acid is observed to be produced more efficiently relative to sulfate as the cloud liquid water content increases, corresponding to larger and less acidic droplets. As derived from large eddy simulations of stratocumulus underthe conditions of MASE, both Lagrangian trajectory analysis and diurnal cloudtop evolution provide evidence that a significant fraction of the aerosol mass concentration above cloud can be accounted for by evaporated droplet residual particles. Methanesulfonate data suggest that entrainment of free tropospheric aerosol can also be a source of organic acids above boundary layer clouds.

The Marine Stratus/Stratocumulus Experiment (MASE): Aerosol‐cloud relationships in marine stratocumulus

The Marine Stratus/Stratocumulus Experiment (MASE) field campaign was undertaken in July 2005 off the coast of Monterey, California to evaluate aerosol‐cloud relationships in the climatically important regime of eastern Pacific marine stratocumulus. Aerosol and cloud properties were measured onboard the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft. One cloud that was clearly impacted by ship emissions as well as the ensemble of clouds observed over the entire mission are analyzed in detail. Results at both the individual and ensemble scales clearly confirm the Twomey effect (first indirect effect of aerosols) and demonstrate drizzle suppression at elevated aerosol number concentration. For the ship track impacted cloud, suppressed drizzle in the track led to a larger cloud liquid water path (LWP) at the same cloud thickness, in accord with the so‐called second indirect effect. Ensemble averages over all clouds sampled over the entire 13‐flight mission show the opposite effect of aerosol number concentration on LWP, presumably the result of other dynamic influences (e.g., updraft velocity and ambient sounding profile). Individual polluted clouds were found to exhibit a narrower cloud drop spectral width in accord with theoretical prediction (M.‐L. Lu and J. H. Seinfeld, Effect of aerosol number concentration on cloud droplet dispersion: A large‐eddy simulation study and implications for aerosol indirect forcing, Journal of Geophysical Research, 2006). This field experiment demonstrates both the indirect aerosol effect on ship track perturbed clouds, as well as the subtleties involved in extracting these effects over an ensemble of clouds sampled over a 1‐month period.

Update to “Reconciliation of coarse mode sea‐salt aerosol particle size measurements and parameterizations at a subtropical ocean receptor site” regarding the use of aerodynamic particle sizers in marine environments

In the January 2006 issue of Journal of Geophysical Research, Reid et al. presented their findings regarding the sometimes large biases found in the measurement of the size and inferred mass concentration of coarse mode sea‐salt particles. This was done on the R/P Floating Instrument Platform (FLIP) off of the coast of Oahu, Hawaii, where long fetches of the clean marine environment could be studied. As part of this analysis, data from optical particle counters and TSI aerodynamic particle sizers (APS) (model 3320) were compared to filter mass and chemistry. It was found that a ground‐based APS 3320 gave superior performance on measurement of sea‐salt properties relative to other size spectrometers when previously published correction factors were used. However, a calibration study of APS instruments by Volckens and Peters (2005) was published while Reid et al. (2006) was in press. Volckens and Peters (2005) found significant differences in counting efficiency between wet and dry particles that included evidence for particle impaction inside the APS's acceleration nozzle. This finding does not impact Reid et al.'s (2006) key results or conclusions. However, it does bear relevance to specific comments made regarding what was suggested as appropriate use of APS‐type instruments. It also has bearing on some unexplained anomalous behavior in the Center for Interdisciplinary Remotely‐Piloted Aircraft Studies (CIRPAS) wing‐mounted APS unit. In this short note we briefly review the APS performance bibliography, provide correction factors for size estimates presented in Reid et al. (2006), and discuss how the most current findings impact the study of coarse mode sea salt.

Oxalic acid in clear and cloudy atmospheres: Analysis of data from International Consortium for Atmospheric Research on Transport and Transformation 2004

Oxalic acid is often the leading contributor to the total dicarboxylic acid mass in ambient organic aerosol particles. During the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) field campaign, nine inorganic ions (including SO42−) and five organic acid ions (including oxalate) were measured on board the Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter research aircraft by a particle‐into‐liquid sampler (PILS) during flights over Ohio and surrounding areas. Five local atmospheric conditions were studied: (1) cloud‐free air, (2) power plant plume in cloud‐free air with precipitation from scattered clouds overhead, (3) power plant plume in cloud‐free air, (4) power plant plume in cloud, and (5) clouds uninfluenced by local pollution sources. The aircraft sampled from two inlets: a counterflow virtual impactor (CVI) to isolate droplet residuals in clouds and a second inlet for sampling total aerosol. A strong correlation was observed between oxalate and SO42− when sampling through both inlets in clouds. Predictions from a chemical cloud parcel model considering the aqueous‐phase production of dicarboxylic acids and SO42− show good agreement for the relative magnitude of SO42− and oxalate growth for two scenarios: power plant plume in clouds and clouds uninfluenced by local pollution sources. The relative contributions of the two aqueous‐phase routes responsible for oxalic acid formation were examined; the oxidation of glyoxylic acid was predicted to dominate over the decay of longer‐chain dicarboxylic acids. Clear evidence is presented for aqueous‐phase oxalic acid production as the primary mechanism for oxalic acid formation in ambient aerosols.