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Interpretation of Aura satellite observations of CO and aerosol index related to the December 2006 Australia fires

Enhanced Carbon Monoxide (CO) in the upper troposphere (UT) is shown by collocated Tropospheric Emission Spectrometer (TES) and Microwave Limb Sounder (MLS) measurements near and down-wind from the known wildfire region of SE Australia from 12–19 December 2006. Enhanced UV aerosol index (AI) derived from Ozone Monitoring Instrument (OMI) measurements correlate with these high CO concentrations. HYSPLIT model back trajectories trace selected air parcels to the SE Australia fire region as their initial location, where TES observes enhanced CO in the upper and lower troposphere. Simultaneously, they show a lack of vertical advection along their tracks. TES retrieved CO vertical profiles in the higher and lower southern latitudes are examined together with the averaging kernels and show that TES CO retrievals are most sensitive at approximately 300–400 hPa. The enhanced CO observed by TES at the upper (215 hPa) and lower (681 hPa) troposphere are, therefore, influenced by mid-tropospheric CO. GEOS-Chem model simulations with an 8-day emission inventory, as the wildfire source over Australia, are sampled to the TES/MLS observation times and locations. These simulations only show CO enhancements in the lower troposphere near and down-wind from the wildfire region of SE Australia with drastic underestimates of UT CO. Although CloudSat along-track ice-water content curtains are examined to see whether possible vertical convection events can explain the high UT CO values, sparse observations of collocated Aura CO and CloudSat along-track ice-water content measurements for the single event precludes any conclusive correlation. Vertical convection that uplift fire-induced CO (i.e. most notably referred to as pyro-cumulonimbus, pyroCb) may provide an explanation for the incongruence between these simulations and the TES/MLS observations of enhanced CO in the UT. Future GEOS-Chem simulations are needed to validate this conjecture as the the PyroCb mechanism is currently not incorporated in GEOS-Chem.
Technical Note: Detection and identification of radical species formed from α-pinene/ozone reaction using DMPO spin trap

The reactions of ozone with monoterpenes proceed via the formation of multiple oxygen- and carbon-centered free radical species. These radical species are highly reactive and thus, have generally not been measureable. A method for their detection and characterization is needed to preserve these radicals for a sufficiently long time to permit analyzes to be performed. Radical-addition reactions, also called spin trapping techniques, allow the detection of short-lived radicals. This approach has been applied to products from the α-pinene/ozone reaction. Secondary organic aerosol (SOA) from a reaction chamber was collected on quartz fiber filters and extracted with a solution of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) (spin trap) followed by analysis with ion-trap tandem mass spectrometry (MSn) using electrospray ionization (ESI) in the positive scan mode. The DMPO adducts with radical species appear as positive ions [DMPO−R+H]+, [DMPO−OR+H]+ and [DMPO−O−OR+H]+ in full MS spectra of the samples. Tandem mass spectrometry (MS2) was performed to identify the radical species. The DMPO adducts with the C-centered radical species [DMPO−R+H]+ are characterized by m/z 114 [DMPO+H]+ in the MS2 spectra and with peaks that represent the loss of [DMPO+H]+. The DMPO adducts with O-centered radical species (RO· and ROO·) are identified by m/z 130 [DMPO−OH+H]+ and m/z 146 [DMPO−O−OH+H]+, respectively, and with peaks that correspond to the loss of those adducts. DMPO was also able to capture OH radicals from the particle phase, and the product ion fragmentation confirmed DMPO/OH structure providing evidence for particle-bound OH radicals.
What drives the observed variability of HCN in the troposphere and lower stratosphere?

We use the GEOS-Chem global 3-D chemistry transport model to investigate the relative importance of chemical and physical processes that determine observed variability of hydrogen cyanide (HCN) in the troposphere and lower stratosphere. Consequently, we reconcile ground-based FTIR column measurements of HCN, which show annual and semi-annual variations, with recent space-borne measurements of HCN mixing ratio in the tropical lower stratosphere, which show a large two-year variation. We find that the observed column variability over the ground-based stations is determined by a superposition of HCN from several regional burning sources, with GEOS-Chem reproducing these column data with a positive bias of 5%. GEOS-Chem reproduces the observed HCN mixing ratio from the Microwave Limb Sounder and the Atmospheric Chemistry Experiment satellite instruments with a mean negative bias of 20%, and the observed HCN variability with a mean negative bias of 7%. We show that tropical biomass burning emissions explain most of the observed HCN variations in the upper troposphere and lower stratosphere (UTLS), with the remainder due to atmospheric transport and HCN chemistry. In the mid and upper stratosphere, atmospheric dynamics progressively exerts more influence on HCN variations. The extent of temporal overlap between African and other continental burning seasons is key in establishing the apparent bienniel cycle in the UTLS. Similar analysis of other, shorter-lived trace gases have not observed the transition between annual and bienniel cycles in the UTLS probably because the signal of inter-annual variations from surface emission has been diluted before arriving at the lower stratosphere (LS), due to shorter atmospheric lifetimes.
An elevated large-scale dust veil from the Taklimakan Desert: Intercontinental transport and three-dimensional structure as captured by CALIPSO and regional and global models

An intense dust storm occurred during 19–20 May 2007 over the Taklimakan Desert in northwestern China. Over the following days, the space-borne lidar CALIOP tracked an optically thin, highly elevated, horizontally extensive dust veil that was transported intercontinentally over eastern Asia, the Pacific Ocean, North America, and the Atlantic Ocean. A global aerosol transport model (SPRINTARS) simulated the dust veil quite well and provided a three-dimensional view of the intercontinental dust transport. The SPRINTARS simulation revealed that the dust veil traveled at 4–10 km altitudes with a thickness of 1–4 km along the isentropic surface between 310 and 340 K. The transport speed was about 1500 km/day. The estimated dust amount exported to the Pacific was 30.8 Gg, of which 65% was deposited in the Pacific and 18% was transported to the North Atlantic. These results imply that dust veils can fertilize open oceans, add to background dust, and affect the radiative budget at high altitudes through scattering and absorption. The injection mechanism that lifts dust particles into the free atmosphere is important for understanding the formation of the dust veil and subsequent long-range transport. We used a regional dust transport model (RC4) to analyze the dust emission and injection over the source region. The RC4 analysis revealed that strong northeasterly surface winds associated with low pressures invaded the Taklimakan Desert through the eastern corridor. These winds then formed strong upslope wind along the high, steep mountainsides of the Tibetan Plateau and blew large amounts of dust into the air. The updraft lifted the dust particles farther into the upper troposphere (about 9 km above mean sea level, MSL), where westerlies are generally present. The unusual terrain surrounding the Taklimakan Desert played a key role in the injection of dust to the upper troposphere to form the dust veil.
Hydrogen soil deposition at an urban site in Finland

Hydrogen deposition velocities (vd) were estimated by field chamber measurements and model simulations. A closed-chamber method was used for soil deposition studies in Helsinki, Finland, at an urban park inhabited by broad-leaved trees. Radon tracer method was used to estimate the vd in nighttime when photochemical reactions were minimal and radon gas was concentrated in the shallow boundary layer due to exhalation from soil. A two-dimensional atmospheric model was used for the calculation of respective vd values and radon exhalation rates. The vd and radon exhalation rates were lower in winter than in summer according to all methods. The radon tracer method and the two-dimensional model results for hydrogen deposition velocity were in the range of 0.13 mm s−1 to 0.93 mm s−1 (radon tracer) and 0.12 mm s−1 to 0.61 mm s−1 (two-dimensional). The soil chamber results for vd were 0.00 mm s−1 to 0.70 mm s−1. Both models and chamber measurements revealed a relation between one week cumulative rain sum and deposition velocity. When precipitation events occurred a few days before the chamber measurements, lower vd values were observed. A snow cover also lowered vd.
Impact of solar radiation on aerosol-cloud interactions in thin stratocumulus clouds

This study examines the role of solar radiation in the effect of aerosols on liquid-water path (LWP) in thin, marine stratocumulus clouds with LWP of ~50 g m−2 or less by performing four sets of simulations with different solar radiation. Each set is composed of a simulation with present-day (PD) aerosols and a simulation with preindustrial (PI) aerosols. As solar radiation increases, decoupling within the marine boundary layer (MBL) becomes stronger, leading to less condensation and less LWP and thus the absence of the surface precipitation. This enables the evaporation of rain to affect the cloud-base instability. As rain evaporation increases due to more conversion of cloud liquid to rain in the PI case, the cloud-base instability increases and thus updrafts increase which leads to larger LWP in the PI case than in the PD case. In the cases with no surface precipitation, when solar radiation decreases and thus decoupling becomes weaker, rain evaporation and cloud-base instability become larger, which increases the LWP more with PI aerosols than with PD aerosols. As solar radiation decreases further, condensation and, thus, the LWP increase, which leads to the presence of the surface precipitation. This stabilizes the entire MBL and thus prevents the interactions that cause the evaporation of rain to enhance the cloud-base instability. In cases with the surface precipitation, the in-cloud interactions among cloud droplet number concentration (CDNC), supersaturation, and updrafts play an important role in the effect of aerosols on the LWP; these in-cloud interactions produce larger LWP with the PD aerosols than with the PI aerosols. In a case with lower solar radiation and with surface precipitation, weaker decoupling induces stronger in-cloud interactions, which results in larger increases in LWP with PD aerosols compared to PI aerosols than that in a case with higher solar radiation. The results of this study demonstrate that solar radiation can act as an important environmental factor by inducing a large variation in the LWP and by changing the sign of aerosol effects on the LWP of thin stratocumulus clouds. Hence, the effect of solar radiation on decoupling and thus on the feedbacks between microphysics and dynamics needs to be included in climate models for a better prediction of the effect of aerosols on clouds and thus climate.
Generation of free convection due to changes of the local circulation system

Eddy-covariance and Sodar/RASS experimental measurement data of the COPS (Convective and Orographically-induced Precipitation Study) field campaign 2007 are used to investigate the generation of near-ground free convection conditions (FCCs) in the Kinzig valley, Black Forest, Southwest Germany. The measured high-quality turbulent flux data revealed that FCCs are initiated near the ground in situations where moderate to high buoyancy fluxes and a simultaneously occurring drop of the wind speed were present. The minimum in wind speed – observable by the Sodar measurements through the whole vertical extension of the valley atmosphere – is the consequence of a thermally-induced valley wind system, which changes its wind direction from down to up-valley winds in the morning hours. Buoyancy then dominates over shear within the production of turbulence kinetic energy near the ground. These situations are detected by the stability parameter (ratio of the measurement height to the Obukhov length) when the level of free convection, which starts above the Obukhov length, drops below that of the sonic anemometer. An analysis of the scales of turbulent motions during FCCs using wavelet transform shows the occurrence of large-scale turbulence structures. Regarding the entire COPS measurement period, FCCs in the morning hours occur on about 50% of all days. Enhanced surface fluxes of latent and sensible heat are found on these days.
Impact of nucleation on global CCN

Cloud condensation nuclei (CCN) are derived from particles emitted directly into the atmosphere (primary emissions) or from the growth of nanometer-sized particles nucleated in the atmosphere. It is important to separate these two sources because they respond in different ways to gas and particle emission control strategies and environmental changes. Here, we use a global aerosol microphysics model to quantify the contribution of primary and nucleated particles to global CCN. The model considers primary emissions of sea spray, sulfate and carbonaceous particles, and nucleation processes appropriate for the free troposphere and boundary layer. We estimate that 45% of global low-level cloud CCN at 0.2% supersaturation are secondary aerosol derived from nucleation (ranging between 31–49% taking into account uncertainties in primary emissions and nucleation rates), with the remainder from primary emissions. The model suggests that 35% of CCN (0.2%) in global low-level clouds were created in the free and upper troposphere. In the marine boundary layer 55% of CCN (0.2%) are from nucleation, with 45% entrained from the free troposphere and 10% nucleated directly in the boundary layer. Combinations of model runs show that primary and nucleated CCN are non-linearly coupled. In particular, boundary layer nucleated CCN are strongly suppressed by both primary emissions and entrainment of particles nucleated in the free troposphere. Elimination of all primary emissions reduces global CCN (0.2%) by only 20% and elimination of upper tropospheric nucleation reduces CCN (0.2%) by only 12% because of the increased contribution from boundary layer nucleation.
Assessment of vertically-resolved PM10 from mobile lidar observations

We investigate in this study the vertical PM10 distributions from mobile measurements carried out from locations along the Paris Peripherique (highly trafficked beltway around Paris), examine distinctions in terms of aerosol concentrations between the outlying regions of Paris and the inner city and eventually discuss the influence of aerosol sources, meteorology, and dynamics on the retrieved PM10 distributions. To achieve these purposes, we combine in situ surface measurements with active remote sensing observations obtained from a great number of research programs in Paris area since 1999. Two approaches, devoted to the conversion of vertical profiles of lidar-derived extinction coefficients into PM10, have been set up. A very good agreement is found between the theoretical and empirical methods with a discrepancy of 3%. Hence, specific extinction cross-sections at 355 nm are provided with a reasonable relative uncertainty lower than 12% for urban (4.5 m2 g−1) and periurban (5.9 m2 g−1) aersols, lower than 26% for rural (7.1 m2 g−1) aerosols, biomass burning (2.6 m2 g−1) and dust (1.1 m2 g−1) aerosols The high spatial and temporal resolutions of the mobile lidar (respectively 1.5 m and 1 min) enable to follow the spatiotemporal variability of various layers trapping aerosols in the troposphere. Appropriate specific extinction cross-sections are applied in each layer detected in the vertical heterogeneities from the lidar profiles. The standard deviation (rms) between lidar-derived PM10 at 200 m above ground and surface network stations measurements was ~14μg m−3. This difference is particularly ascribed to a decorrelation of mass concentrations in the first meters of the boundary layer, as highlighted through multiangular lidar observations. Lidar signals can be used to follow mass concentrations with an uncertainty lower than 25% above urban areas and provide useful information on PM10 peak forecasting that affect air quality.
Source apportionment of fine organic aerosols in Beijing

Fine particles (PM2.5, i.e., particles with an aerodynamic diameter of ≤2.5 μm) were collected from the air in August 2005, August–September 2006, and January–February 2007, in Beijing, China. The chemical compositions of particulate organic matter in the ambient samples were quantified by gas chromatography/mass spectrometry. The dominant compounds identified in summertime were n-alkanoic acids, followed by dicarboxylic acids and sugars, while sugars became the most abundant species in winter, followed by polycyclic aromatic hydrocarbons, n-alkanes, and n-alkanoic acids. The contributions of seven emission sources (i.e., gasoline/diesel vehicles, coal burning, wood/straw burning, cooking, and vegetative detritus) to particulate organic matter in PM2.5 were estimated using a chemical mass balance receptor model. The model results present the seasonal trends of source contributions to organic aerosols. Biomass burning (straw and wood) had the highest contribution in winter, followed by coal burning, vehicle exhaust, and cooking. The contribution of cooking was the highest in summer, followed by vehicle exhaust and biomass burning, while coal smoke showed only a minor contribution to ambient organic carbon.