Varuntida Varutbangkul

Chamber studies of secondary organic aerosol growth by reactive uptake of simple carbonyl compounds

Recent experimental evidence indicates that heterogeneous chemical reactions play an important role in the gas-particle partitioning of organic compounds, contributing to the formation and growth of secondary organic aerosol in the atmosphere. Here we present laboratory chamber studies of the reactive uptake of simple carbonyl species (formaldehyde, octanal, trans,trans-2,4-hexadienal, glyoxal, methylglyoxal, 2,3-butanedione, 2,4-pentanedione, glutaraldehyde, and hydroxyacetone) onto inorganic aerosol. Gas-phase organic compounds and aqueous seed particles (ammonium sulfate or mixed ammonium sulfate/sulfuric acid) are introduced into the chamber, and particle growth and composition are monitored using a differential mobility analyzer and an Aerodyne Aerosol Mass Spectrometer. No growth is observed for most carbonyls studied, even at high concentrations (500 ppb to 5 ppm), in contrast with the results from previous studies. The single exception is glyoxal (CHOCHO), which partitions into the aqueous aerosol much more efficiently than its Henrys law constant would predict. No major enhancement in particle growth is observed for the acidic seed, suggesting that the large glyoxal uptake is not a result of particle acidity but rather of ionic strength of the seed. This increased partitioning into the particle phase still cannot explain the high levels of glyoxal measured in ambient aerosol, indicating that additional (possibly irreversible) pathways of glyoxal uptake may be important in the atmosphere.

Gas‐phase products and secondary aerosol yields from the photooxidation of 16 different terpenes

The photooxidation of isoprene, eight monoterpenes, three oxygenated monoterpenes, and four sesquiterpenes were conducted individually at the Caltech Indoor Chamber Facility under atmospherically relevant HC:NO_x ratios to monitor the time evolution and yields of SOA and gas-phase oxidation products using PTR-MS. Several oxidation products were calibrated in the PTR-MS, including formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, nopinone, methacrolein + methyl vinyl ketone; other oxidation products were inferred from known fragmentation patterns, such as pinonaldehyde; and other products were identified according to their mass to charge (m/z) ratio. Numerous unidentified products were formed, and the evolution of first- and second-generation products was clearly observed. SOA yields from the different terpenes ranged from 1 to 68%, and the total gas- plus particle-phase products accounted for ∼50–100% of the reacted carbon. The carbon mass balance was poorest for the sesquiterpenes, suggesting that the observed products were underestimated or that additional products were formed but not detected by PTR-MS. Several second-generation products from isoprene photooxidation, including m/z 113, and ions corresponding to glycolaldehyde, hydroxyacetone, methylglyoxal, and hydroxycarbonyls, were detected. The detailed time series and relative yields of identified and unidentified products aid in elucidating reaction pathways and structures for the unidentified products. Many of the unidentified products from these experiments were also observed within and above the canopy of a Ponderosa pine plantation, confirming that many products of terpene oxidation can be detected in ambient air using PTR-MS, and are indicative of concurrent SOA formation.

Gas‐phase products and secondary aerosol yields from the ozonolysis of ten different terpenes

The ozonolyses of six monoterpenes (α-pinene, β-pinene, 3-carene, terpinolene, α-terpinene, and myrcene), two sesquiterpenes (α-humulene and β-caryophyllene), and two oxygenated terpenes (methyl chavicol and linalool) were conducted individually in Teflon chambers to examine the gas-phase oxidation product and secondary organic aerosol (SOA) yields from these reactions. Particle size distribution and number concentration were monitored and allowed for the calculation of the SOA yield from each experiment, which ranged from 1 to 54%. A proton transfer reaction mass spectrometer (PTR-MS) was used to monitor the evolution of gas-phase products, identified by their mass to charge ratio (m/z). Several gas-phase oxidation products, formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, and nopinone, were identified and calibrated. Aerosol yields, and the yields of these identified and calibrated oxidation products, as well as many higher m/z oxidation products observed with the PTR-MS, varied significantly between the different parent terpene compounds. The sum of measured oxidation products in the gas and particle phase ranged from 33 to 77% of the carbon in the reacted terpenes, suggesting there are still unmeasured products from these reactions. The observations of the higher molecular weight oxidation product ions provide evidence of previously unreported compounds and their temporal evolution in the smog chamber from multistep oxidation processes. Many of the observed ions, including m/z 111 and 113, have also been observed in ambient air above a Ponderosa pine forest canopy, and our results confirm they are consistent with products from terpene + O_3 reactions. Many of these products are stable on the timescale of our experiments and can therefore be monitored in field campaigns as evidence for ozone oxidative chemistry.

New particle formation from photooxidation of diiodomethane (CH2I2)

Photolysis of CH_2I_2 in the presence of O_3 has been proposed as a mechanism leading to intense new particle formation in coastal areas. We report here a comprehensive laboratory chamber study of this system. Rapid homogeneous nucleation was observed over three orders of magnitude in CH_2I_2 mixing ratio, down to a level of 15 ppt (∼4 × 10^8 molec. cm^(−3)) comparable to the directly measured total gas-phase iodine species concentrations in coastal areas. After the nucleation burst, the observed aerosol dynamics in the chamber was dominated by condensation of additional vapors onto existing particles and particle coagulation. Particles formed under dry conditions are fractal agglomerates with mass fractal dimension, D_f ∼ 1.8–2.5. Higher relative humidity (65%) does not change the nucleation or growth behavior from that under dry conditions, but results in more compact and dense particles (D_f ∼ 2.7). On the basis of the known gas-phase chemistry, OIO is the most likely gas-phase species to produce the observed nucleation and aerosol growth; however, the current understanding of this chemistry is very likely incomplete. Chemical analysis of the aerosol using an Aerodyne Aerosol Mass Spectrometer reveals that the particles are composed mainly of iodine oxides but also contain water and/or iodine oxyacids. The system studied here can produce nucleation events as intense as those observed in coastal areas. On the basis of comparison between the particle composition, hygroscopicity, and nucleation and growth rates observed in coastal nucleation and in the experiments reported here, it is likely that photooxidation of CH_2I_2, probably aided by other organic iodine compounds, is the mechanism leading to the observed new particle formation in the west coast of Ireland.

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

inorganic ions (including SO4� ) 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 SO4� when sampling through both inlets in clouds. Predictions from a chemical cloud parcel model considering the aqueous-phase production of dicarboxylic acids and SO4� show good agreement for the relative magnitude of SO4� 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.

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.

Aerosol-cloud drop concentration closure in warm cumulus

Our understanding of the activation of aerosol particles into cloud drops during the formation of warm cumulus clouds presently has a limited observational foundation. Detailed observations of aerosol size and composition, cloud microphysics and dynamics, and atmospheric thermodynamic state were collected in a systematic study of 21 cumulus clouds by the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft during NASAs Cirrus Regional Study of Tropical Anvils and Cirrus Layers–Florida Area Cirrus Experiment (CRYSTAL-FACE). An “aerosol-cloud” closure study was carried out in which a detailed cloud activation parcel model, which predicts cloud drop concentration using observed aerosol concentration, size distribution, cloud updraft velocity, and thermodynamic state, is evaluated against observations. On average, measured droplet concentration in adiabatic cloud regions is within 15% of the predictions. This agreement is corroborated by independent measurements of aerosol activation carried out by two cloud condensation nucleus (CCN) counters on the aircraft. Variations in aerosol concentration, which ranged from 300 to 3300 cm^(−3), drives large microphysical differences (250–2300 cm^(−3)) observed among continental and maritime clouds in the South Florida region. This is the first known study in which a cloud parcel model is evaluated in a closure study using a constraining set of data collected from a single platform. Likewise, this is the first known study in which relationships among aerosol size distribution, CCN spectrum, and cloud droplet concentration are all found to be consistent with theory within experimental uncertainties much less than 50%. Vertical profiles of cloud microphysical properties (effective radius, droplet concentration, dispersion) clearly demonstrate the boundary layer aerosols effect on cloud microphysics throughout the lowest 1 km of cloud depth. Onboard measurements of aerosol hygroscopic growth and the organic to sulfate mass ratio are related to CCN properties. These chemical data are used to quantify the range of uncertainty associated with the simplified treatment of aerosol composition assumed in the closure study.

Toward aerosol/cloud condensation nuclei (CCN) closure during CRYSTAL‐FACE

concentrations were 233 cm 3 (at S = 0.2%) and 371 cm 3 (at S = 0.85%). Three flights during the experiment differed from this general trend; the aerosol sampled during the two flights on 18 July was more continental in character, and the observations on 28 July indicate high spatial variability and periods of very high aerosol concentrations. This study also includes a simplified aerosol/CCN closure analysis. Aerosol size distributions were measured simultaneously with the CCN observations, and these data are used to

Aerosol-cloud drop concentration closure for clouds sampled during the International Consortium for Atmospheric Research on Transport and Transformation 2004 campaign

This study analyzes 27 cumuliform and stratiform clouds sampled aboard the CIRPAS Twin Otter during the 2004 International Consortium for Atmospheric Research on Transport and Transformation (ICARTT) experiment. The data set was used to assess cloud droplet closure using (1) a detailed adiabatic cloud parcel model and (2) a state-of-the-art cloud droplet activation parameterization. A unique feature of the data set is the sampling of highly polluted clouds within the vicinity of power plant plumes. Remarkable closure was achieved (much less than the 20% measurement uncertainty) for both parcel model and parameterization. The highly variable aerosol did not complicate the cloud droplet closure, since the clouds had low maximum supersaturation and were not sensitive to aerosol variations (which took place at small particle sizes). The error in predicted cloud droplet concentration was mostly sensitive to updraft velocity. Optimal closure is obtained if the water vapor uptake coefficient is equal to 0.06, but can range between 0.03 and 1.0. The sensitivity of cloud droplet prediction error to changes in the uptake coefficient, organic solubility and surface tension depression suggest that organics exhibit limited solubility. These findings can serve as much needed constraints in modeling of aerosol-cloud interactions in the North America; future in situ studies will determine the robustness of our findings.

Cloud condensation nuclei activity, closure, and droplet growth kinetics of Houston aerosol during the Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS)

In situ cloud condensation nuclei (CCN) measurements were obtained in the boundary layer over Houston, Texas, during the 2006 Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) campaign onboard the CIRPAS Twin Otter. Polluted air masses in and out of cloudy regions were sampled for a total of 22 flights, with CCN measurements obtained for 17 of these flights. In this paper, we focus on CCN closure during two flights, within and downwind of the Houston regional plume and over the Houston Ship Channel. During both flights, air was sampled with particle concentrations exceeding 25,000 cm^(−3) and CCN concentrations exceeding 10,000 cm^(−3). CCN closure is evaluated by comparing measured concentrations with those predicted on the basis of measured aerosol size distributions and aerosol mass spectrometer particle composition. Different assumptions concerning the internally mixed chemical composition result in average CCN overprediction ranging from 3% to 36% (based on a linear fit). It is hypothesized that the externally mixed fraction of the aerosol contributes much of the CCN closure scatter, while the internally mixed fraction largely controls the overprediction bias. On the basis of the droplet sizes of activated CCN, organics do not seem to impact, on average, the CCN activation kinetics.