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A fate for organic acids, formaldehyde and methanol in cloud water: their biotransformation by micro-organisms

The interactions between microbial and chemical contents of cloud water were investigated. First, we observe that the bulk cloud water solution provides a substantial environment where bacteria can develop significantly. Then, a total number of 60 microbial strains originating from seven distinct samples of cloud water and affiliated to various taxonomic groups were examined for their ability to degrade some of the main atmospheric carboxylic compounds: formate, acetate, lactate, succinate, as well as formaldehyde and methanol. Biodegradation tests show that all these compounds can be transformed when used as single carbonaceous substrates, with activities depending on both the strain and the compound. The highest capacities of biodegradation are observed towards formaldehyde, formate and acetate, which are also the more concentrated compounds typically measured in cloud water. Hence, analyses by 1H NMR permitted to establish for instance that compounds like pyruvate or fumarate can be produced and released in the media in relation to the transformation of lactate or succinate. In addition, utilization of 13C labelled formaldehyde showed that it can be transformed through many metabolic pathways, similar to those induced by photochemistry and leading to the production of formate and/or methanol. These results suggest that microorganisms of cloud water can have various behaviours towards the chemical compounds present in the atmosphere: they can represent either a sink or source for organic carbon, and may have to be considered as actors of cloud chemistry.
The relevance of aerosol optical depth to cumulus fraction changes: a five-year climatology at the ACRF SGP site

The objective of this study is to investigate, by observational means, the magnitude and sign of the actively discussed relationship between cloud fraction N and aerosol optical depth τa. Collocated and coincident ground-based measurements and Terra/Aqua satellite observations at the Atmospheric Radiation Measurement (ARM) Climate Research Facility (ACRF) Southern Great Plains (SGP) site form the basis of this study. The N–τa relationship occurred in a specific 5-year dataset of fair-weather cumulus (FWC) clouds and mostly non-absorbing aerosols. To reduce possible contamination of the aerosols on the cloud properties estimation (and vice versa), we use independent datasets of τa and N obtained from the Multi-filter Rotating Shadowband Radiometer (MFRSR) measurements and from the ARM Active Remotely Sensed Clouds Locations (ARSCL) value-added product, respectively. Optical depth of the FWC clouds τcld and effective radius of cloud droplets re are obtained from the MODerate resolution Imaging Spectroradiometer (MODIS) data. We found that relationships between cloud properties (N,τcld, re) and aerosol optical depth are time-dependent (morning versus afternoon). Observed time-dependent changes of cloud properties, associated with aerosol loading, control the variability of surface radiative fluxes. In comparison with pristine clouds, the polluted clouds are more transparent in the afternoon due to smaller cloud fraction, smaller optical depth and larger droplets. As a result, the corresponding correlation between the surface radiative flux and τa is positive (warming effect of aerosol). Also we found that relationship between cloud fraction and aerosol optical depth is cloud size dependent. The cloud fraction of large clouds (larger than 1 km) is relatively insensitive to the aerosol amount. In contrast, cloud fraction of small clouds (smaller than 1 km) is strongly positively correlated with τa. This suggests that an ensemble of polluted clouds tends to be composed of smaller clouds than a similar one in a pristine environment. One should be aware of these time- and size-dependent features when qualitatively comparing N–τa relationships obtained from the satellite observations, surface measurements, and model simulations.
Heterogeneous oxidation of saturated organic aerosols by hydroxyl radicals: uptake kinetics, condensed-phase products, and particle size change

The kinetics and reaction mechanism for the heterogeneous oxidation of saturated organic aerosols by gas-phase OH radicals were investigated under NOx-free conditions. The reaction of 150 nm diameter Bis(2-ethylhexyl) sebacate (BES) particles with OH was studied as a proxy for chemical aging of atmospheric aerosols containing saturated organic matter. An aerosol reactor flow tube combined with an Aerodyne time-of-flight aerosol mass spectrometer (ToF-AMS) and scanning mobility particle sizer (SMPS) was used to study this system. Hydroxyl radicals were produced by 254 nm photolysis of O3 in the presence of water vapour. The kinetics of the heterogeneous oxidation of the BES particles was studied by monitoring the loss of a mass fragment of BES with the ToF-AMS as a function of OH exposure. We measured an initial OH uptake coefficient of γ0=1.3 (±0.4), confirming that this reaction is highly efficient. The density of BES particles increased by up to 20% of the original BES particle density at the highest OH exposure studied, consistent with the particle becoming more oxidized. Electrospray ionization mass spectrometry analysis showed that the major particle-phase reaction products are multifunctional carbonyls and alcohols with higher molecular weights than the starting material. Volatilization of oxidation products accounted for a maximum of 17% decrease of the particle volume at the highest OH exposure studied. Tropospheric organic aerosols will become more oxidized from heterogeneous photochemical oxidation, which may affect not only their physical and chemical properties, but also their hygroscopicity and cloud nucleation activity.
Insights into the role of soot aerosols in cirrus cloud formation

Cirrus cloud formation is believed to be dominated by homogeneous freezing of supercooled liquid aerosols in many instances. Heterogeneous ice nuclei such as mineral dust, metallic, and soot particles, and some crystalline solids within partially soluble aerosols are suspected to modulate cirrus properties. Among those, the role of ubiquitous soot particles is perhaps the least understood. Because aviation is a major source of upper tropospheric soot particles, we put emphasis on ice formation in dispersing aircraft plumes. The effect of aircraft soot on cirrus formation in the absence of contrails is highly complex and depends on a wide array of emission and environmental parameters. We use a microphysical-chemical model predicting the formation of internally mixed, soot-containing particles up to two days after emission, and suggest two principal scenarios: high concentrations of original soot emissions could slightly increase the number of ice crystals; low concentrations of particles originating from coagulation of emitted soot with background aerosols could lead to a significant reduction in ice crystal number. Both scenarios assume soot particles to be moderate ice nuclei relative to cirrus formation by homogeneous freezing in the presence of few efficient dust ice nuclei. A critical discussion of laboratory experiments reveals that the ice nucleation efficiency of soot particles depends strongly on their source, and, by inference, on atmospheric aging processes. Mass and chemistry of soluble surface coatings appear to be crucial factors. Immersed soot particles tend to be poor ice nuclei, some bare ones nucleate ice at low supersaturations. However, a fundamental understanding of these studies is lacking, rendering extrapolations to atmospheric conditions speculative. In particular, we cannot yet decide which indirect aircraft effect scenario is more plausible, and options suggested to mitigate the problem remain uncertain.
A case study on biomass burning aerosols: effects on aerosol optical properties and surface radiation levels

In spring 2006, biomass burning aerosols from eastern Europe were transported extensively to Finland, and to other parts of northern Europe. They were observed as far as in the European Arctic. In the first part of this paper, temporal and spatial evolution and transport of these biomass burning aerosols are monitored with MODIS retrieved aerosol optical depth (AOD) imagery at visible wavelengths (0.55 μm). Comparison of MODIS and AERONET AOD is conducted at Tõravere, Estonia. Then trajectory analyses, as well as MODIS Fire Mapper products are used to better understand the type and origin of the air masses. During the studied four-week period AOD values ranged from near zero up to 1.2 at 0.55 μm and the linear correlation between MODIS and AERONET was very high (~0.97). Temporal variability observed within this four-week period was also rather well explained by the trajectory analysis in conjunction with the fire detections produced by the MODIS Rapid Response System. In the second part of our study, the surface measurements of global and UV radiation at Jokioinen, Finland are used to study the effect of this haze episode on the levels of surface radiation. We found reductions up to 35% in noon-time surface UV irradiance (at 340 nm) as compared to typical aerosol conditions. For global (total solar) radiation, the reduction was always smaller, in line with the expected wavelength dependence of the aerosol effect.
Ambient aerosol concentrations of sugars and sugar-alcohols at four different sites in Norway

Sugars and sugar-alcohols are demonstrated to be important constituents of the ambient aerosol water-soluble organic carbon fraction, and to be tracers for primary biological aerosol particles (PBAP). In the present study, levels of four sugars (fructose, glucose, sucrose, trehalose) and three sugar-alcohols (arabitol, inositol, mannitol) in ambient aerosols have been quantified using a novel HPLC/HRMS-TOF (High Performance Liquid Chromatography in combination with High Resolution Mass Spectrometry – Time of Flight) method to assess the contribution of PBAP to PM>sub>10 and PM2.5. Samples were collected at four sites in Norway at different times of the year in order to reflect the various contributing sources and the spatial and seasonal variation of the selected compounds. Sugars and sugar-alcohols were present at all sites investigated, underlining the ubiquity of these highly polar organic compounds. The highest concentrations were reported for sucrose, reaching a maximum concentration of 320 ng m−3 in PM10 and 55 ng m−3 in PM2.5. The mean concentration of sucrose was up to 10 times higher than fructose, glucose and the dimeric sugar trehalose. The mean concentrations of the sugar-alcohols were typically lower, or equal, to that of the monomeric sugars and trehalose. Peak concentrations of arabitol and mannitol did not exceed 30 ng m−3 in PM10, and for PM2.5 all concentrations were below 6 ng m−3. Sugars and sugar-alcohols were associated primarily with coarse aerosols except during wintertime at the suburban site in Elverum, where a shift towards sub micron aerosols was observed. It is proposed that this shift was due to the intensive use of wood burning for residential heating at this site during winter, confirmed by high concurrent concentrations of levoglucosan. Elevated concentrations of sugars in PM2.5 were observed during spring and early summer at the rural background site Birkenes. It is hypothesized that this was due to ruptured pollen.
Mixing between a stratospheric intrusion and a biomass burning plume

Ozone, carbon monoxide, aerosol extinction coefficient, acetonitrile, nitric acid and relative humidity measured from the NOAA P3 aircraft during the TexAQS/GoMACCS 2006 experiment, indicate mixing between a biomass burning plume and a stratospheric intrusion in the free troposphere above eastern Texas. Lagrangian-based transport analysis and satellite imagery are used to investigate the transport mechanisms that bring together the tropopause fold and the biomass burning plume originating in southern California, which may affect the chemical budget of tropospheric trace gases.
Vertical profiles of lightning-produced NO2 enhancements in the upper troposphere observed by OSIRIS

The purpose of this study is to perform a global search of the upper troposphere (z≥10 km) for enhancements of nitrogen dioxide and determine their sources. This is the first application of satellite-based limb scattering to study upper tropospheric NO2. We have searched two years (May 2003–May 2005) of OSIRIS (Optical Spectrograph and Infrared Imager System) operational NO2 concentrations (version 2.3/2.4) to find large enhancements in the observations by comparing with photochemical box model calculations and by identifying local maxima in NO2 volume mixing ratio. We find that lightning is the main production mechanism responsible for the large enhancements in OSIRIS NO2 observations as expected. Similar patterns in the abundances and spatial distribution of the NO2 enhancements are obtained by perturbing the lightning within the GEOS-Chem 3-dimensional chemical transport model. In most cases, the presence of lightning is confirmed with coincident imagery from LIS (Lightning Imaging Sensor) and the spatial extent of the NO2 enhancement is mapped using nadir observations of tropospheric NO2 at high spatial resolution from SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) and OMI (Ozone Monitoring Instrument). The combination of the lightning and chemical sensors allows us to investigate globally the role of lightning to the abundance of NO2 in the upper troposphere (UT). Lightning contributes 60% of the tropical upper tropospheric NO2 in GEOS-Chem simulations. The spatial and temporal distribution of NO2 enhancements from lightning (May 2003–May 2005) is investigated. The enhancements generally occur at 12 to 13 km more frequently than at 10 to 11 km. This is consistent with the notion that most of the NO2 is forming and persisting near the cloud top altitude in the tropical upper troposphere. The latitudinal distribution is mostly as expected. In general, the thunderstorms exhibiting weaker vertical development (e.g. 11≤z≤13 km) extend latitudinally as far poleward as 45° but the thunderstorms with stronger vertical development (z≥14 km) tend to be located within 33° of the equator. There is also the expected hemispheric asymmetry in the frequency of the NO2 enhancements, as most were observed in the northern hemisphere for the period analyzed.
Sensitivity of PM2.5 to climate in the Eastern US: a modeling case study

The individual effects of various meteorological parameters on PM2.5 concentrations in the Eastern US are examined using the PMCAMx chemical transport model so that these effects and their relative magnitudes can be better understood. A suite of perturbations in temperature, wind speed, absolute humidity, mixing height, cloud cover, and precipitation are imposed individually on base case conditions corresponding to periods in July 2001 and January 2002 in order to determine the sensitivities of PM2.5 concentrations and composition to these separate meteorological parameters. Temperature had a major effect on average PM2.5 in January (−170 ng m−3 K−1) due largely to the evaporation of ammonium nitrate and organic aerosol at higher temperatures; increases in sulfate production with increased temperature counteracted much of this decrease in July. Changes in mixing height also had major effects on PM2.5 concentrations: 73 ng m−3 (100 m)−1 in January and 210 ng m−3 (100 m)−1 in July. Changes in wind speed (30 to 55 ng m−3 %−1) and absolute humidity (15 to 20 ng m−3 %−1) also had appreciable effects on average PM2.5 concentrations. Precipitation changes had large impacts on parts of the domain (a consequence of the base case meteorology), with sensitivities to changing area of precipitation in July up to 100 ng m−3 %−1. Perturbations in cloud cover had the smallest effects on average PM2.5 concentrations. The changes in PM2.5 concentrations resulting from changing all eight meteorological parameters simultaneously were approximately within 25% or so of the sum of the changes to the eight individual perturbations. The sensitivities of PM2.5 concentrations to changes in these meteorological parameters indicate that changes in climate could potentially have important impacts on PM2.5 concentrations.
An overview of snow photochemistry: evidence, mechanisms and impacts

It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3–4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NOx flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.