M. Kanakidou

Acetone in the atmosphere: Distribution, sources, and sinks

Acetone (CH3COCH3) was found to be the dominant nonmethane organic species present in the atmosphere sampled primarily over eastern Canada (0–6 km, 35°–65°N) during ABLE3B (July to August 1990). A concentration range of 357 to 2310 ppt (= 10−12 v/v) with a mean value of 1140±413 ppt was measured. Under extremely clean conditions, generally involving Arctic flows, lowest (background) mixing ratios of 550±100 ppt were present in much of the troposphere studied. Correlations between atmospheric mixing ratios of acetone and select species such as C2H2, CO, C3H8, C2C14 and isoprene provided important clues to its possible sources and to the causes of its atmospheric variability. Biomass burning as a source of acetone has been identified for the first time. By using atmospheric data and three-dimensional photochemical models, a global acetone source of 40–60 Tg (= 1012 g)/yr is estimated to be present. Secondary formation from the atmospheric oxidation of precursor hydrocarbons (principally propane, isobutane, and isobutene) provides the single largest source (51%). The remainder is attributable to biomass burning (26%), direct biogenic emissions (21%), and primary anthropogenic emissions (3%). Atmospheric removal of acetone is estimated to be due to photolysis (64%), reaction with OH radicals (24%), and deposition (12%). Model calculations also suggest that acetone photolysis contributed significantly to PAN formation (100–200 ppt) in the middle and upper troposphere of the sampled region and may be important globally. While the source-sink equation appears to be roughly balanced, much more atmospheric and source data, especially from the southern hemisphere, are needed to reliably quantify the atmospheric budget of acetone.

Organic aerosols in Eastern Mediterranean: components source reconciliation by using molecular markers and atmospheric back trajectories

Molecular markers, such as n-alkanes, hopanes, PAHs, n-alkanols, n-alkanals, n-alkan-2-ones and n-alkanoic acids, and atmospheric back trajectories have been conjointly used to reconcile Eastern Mediterranean marine organic aerosols with their emission sources. In the urban site, local inputs of polar and non polar lipids control the aerosol composition. In addition the presence of iso- and anteisoalkanes in the aliphatic fraction of all urban samples analysed, demonstrated the contribution of cigarette smoke to urban aerosols. The composition of the aliphatic and aromatic fractions demonstrated a clear petrogenic input. In the rural site the composition and concentrations of the PAH fraction were dependant on the origin of air masses, and showed a rather pyrolytic origin. They were higher for air masses from the north than for air masses originating from the south. Some molecular markers, such as 6,10,14-trimethylpentadecan-2-one and α,ω-dicarboxylic acids with Cn > C20, characterized rural aerosols corresponding to air masses with a pronounced marine origin.

Impact of Non-Methane Hydrocarbons on Tropospheric Chemistry and the Oxidizing Power of the Global Troposphere: 3-Dimensional Modelling Results

The impact of natural and anthropogenicnon-methane hydrocarbons (NMHC) on troposphericchemistry is investigated with the global,three-dimensional chemistry-transport model MOGUNTIA.This meteorologically simplified model allows theinclusion of a rather detailed scheme to describeNMHC oxidation chemistry. Comparing model resultscalculated with and without NMHC oxidation chemistryindicates that NMHC oxidation adds 40–60% to surfacecarbon monoxide (CO) levels over the continents andslightly less over the oceans. Free tropospheric COlevels increase by 30–60%. The overall yield of COfrom the NMHC mixture considered is calculated to beabout 0.4 CO per C atom. Organic nitrate formationduring NMHC oxidation, and their transport anddecomposition affect the global distribution of NOxand thereby O3 production. The impact of theshort-lived NMHC extends over the entire tropospheredue to the formation of longer-lived intermediateslike CO, and various carbonyl and carboxyl compounds.NMHC oxidation almost doubles the net photochemicalproduction of O3 in the troposphere and leads to20–80% higher O3 concentration inNOx-rich boundarylayers, with highest increases over and downwind ofthe industrial and biomass burning regions. Anincrease by 20–30% is calculated for the remotemarine atmosphere. At higher altitudes, smaller, butstill significant increases, in O3 concentrationsbetween 10 and 60% are calculated, maximizing in thetropics. NO from lightning also enhances the netchemical production of O3 by about 30%, leading to asimilar increase in the global mean OH radicalconcentration. NMHC oxidation decreases the OH radicalconcentrations in the continental boundary layer withlarge NMHC emissions by up to 20–60%. In the marineboundary layer (MBL) OH levels can increase in someregions by 10–20% depending on season and NOxlevels.However, in most of the MBL OH will decrease by10–20% due to the increase in CO levels by NMHCoxidation chemistry. The large decreases especiallyover the continents strongly reduce the markedcontrasts in OHconcentrations between land and oceanwhich are calculated when only the backgroundchemistry is considered. In the middle troposphere, OHconcentrations are reduced by about 15%, although dueto the growth in CO. The overall effect of thesechanges on the tropospheric lifetime of CH4 is a 15%increase from 6.5 to 7.4 years. Biogenic hydrocarbonsdominate the impact of NMHC on global troposphericchemistry. Convection of hydrocarbon oxidationproducts: hydrogen peroxides and carbonyl compounds,especially acetone, is the main source of HOx in theupper troposphere. Convective transport and additionof NO from lightning are important for the O3 budgetin the free troposphere.

Simultaneous global observations of glyoxal and formaldehyde from space

[1] The first global simultaneous observations of glyoxal (CHOCHO) and formaldehyde (HCHO) columns retrieved from measurements by the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) satellite instrument are presented and compared to model calculations. The global pattern of the distribution of CHOCHO is similar to that of HCHO. High values are observed over areas with large biogenic isoprene emissions (Central Africa, parts of South America, and Indonesia). Also regions with biomass burning and anthropogenic pollution exhibit elevated levels of CHOCHO. The ratio of the columns of CHOCHO to HCHO is generally of the order of 0.05 in regions having biogenic emissions, which is in reasonable agreement with the current understanding of the oxidation of hydrocarbons emitted by the biosphere. However and in contrast to our model, high values of both HCHO and CHOCHO are also observed over areas of the tropical oceans. This is tentatively attributed to outflow from the continents and local oceanic biogenic sources of the precursors of HCHO and CHOCHO. Citation: Wittrock, F., A. Richter, H. Oetjen, J. P. Burrows, M. Kanakidou, S. Myriokefalitakis, R. Volkamer, S. Beirle, U. Platt, and T. Wagner (2006), Simultaneous global observations of glyoxal and formaldehyde from space, Geophys. Res. Lett., 33, L16804, doi:10.1029/2006GL026310.

Reactive nitrogen and ozone over the western Pacific: Distribution, partitioning, and sources

Measurements of important reactive nitrogen species (NO, NO2, HNO3, PAN, PPN, NO3−, NOy), C1 to C6 hydrocarbons, O3, chemical tracers (C2Cl4, CO), and meteorological parameters were made in the troposphere (0 to 12 km) over the western Pacific (0°–50°N) during the Pacific Exploratory Mission-West A campaign (September–October 1991). Under clean conditions, mixing ratios of NO, NO2, NOy, and O3 increased with altitude and showed a distinct latitudinal gradient. PAN showed a midtropospheric maximum, while nitric acid mixing ratios were generally highest near the surface. Measured NOy concentrations were significantly greater than the sum of individually measured nitrogen species (mainly NOx, PAN, and HNO3), suggesting that a large fraction of reactive nitrogen present in the atmosphere is made up of hitherto unknown species. This shortfall was larger in the tropics (≈65%) compared to midlatitudes (≈40%) and was minimal in air masses with high HNO3 mixing ratios (>100 ppt). A global three-dimensional photochemical model has been used to compare observations with predictions and to assess the significance of major sources. It is possible that the tropical lightning source is much greater than commonly assumed, and both lightning source and its distribution remain a major area of uncertainty in the budgets of NOy and NOx. A large disagreement between measurement and theory exists in the atmospheric distribution of HNO3. It appears that surface-based anthropogenic emissions provide nearly 65% of the global atmospheric NOy reservoir. Relatively constant NOx/NOy ratios imply that NOy and NOx are in chemical equilibrium and the NOy reservoir may be an important in situ source of atmospheric NOx. Data are interpreted to suggest that only about 20% of the upper tropospheric (7–12 km) NOx is directly attributable to its surface NOx source, and free tropospheric sources are dominant. In situ release of NOx from the NOy reservoir, lightning, direct transport of surface NOx, aircraft emissions, and small stratospheric input collectively maintain the NOx balance in the atmosphere. It is shown that atmospheric ratios of reactive nitrogen and sulfur species, along with trajectory analysis, can be used to pinpoint the source of Asian continental outflow. Compared to rural atmospheres over North America, air masses over the Pacific are highly efficient in net O3 production. Sources of tropospheric NOx cannot yet be accurately defined due to shortcomings in measurements and theory.

Tetrachloroethylene as an indicator of low Cl atom concentrations in the troposphere

Tetrachloroethylene (C2Cl4), a largely man-made chemical pollutant, is known to react with Cl atoms at a rate that is some 300 times faster than with OH radicals (kCl/kOH = 365 at 275 K). Analysis of C2Cl4 data, with the help of a global 2-D model and in conjunction with the OH field derived from CH3CCl3 observations, has been used to provide a sensitive means for evaluating Cl atom abundance in the troposphere. In the “mean case” scenario, that employs best available measurements, emissions and kinetic parameters, it is found that OH oxidation is adequate to balance the C2Cl4 budget and significant removal by Cl is not indicated (Cl ≤10² molec. cm−3). An “upper limit” analysis that takes into account possible uncertainties in measurements, source emissions (man-made and natural), and reaction rates, is performed to estimate that annually averaged Cl atom concentrations in the troposphere are <5–10×10² molec. cm−3. If we assume that nearly all (80–100%) of the Cl atoms reside in the marine boundary layer (MBL), mean MBL concentrations are estimated to be <5–15×10³ molec. cm−3. This analysis implies that mean Cl concentrations in the MBL are below or near the lower end of the values inferred in recent studies (104-106 molec. cm−3). We conclude that despite their high reactivity, Cl atoms are too few to compete with OH radicals (≈ 106 molec. cm−3) in influencing the oxidizing capacity of the global troposphere.

Carboxylic acids in gas and particulate phase above the Atlantic Ocean

Mixing ratios of acetic (CH3COOH), formic (HCOOH), pyruvic (CH3COCOOH), and oxalic ((COOH)2) acids were measured both in gas and particulate phase in the marine boundary layer, over the Atlantic Ocean from 63°N to 39°S on board of the R/V Polarstern in October/November 1996. The average mixing ratios for gas phase CH3COOH, HCOOH, CH3COCOOH, and (COOH)2 were 291.2±151.9, 448.7±182.1, 1.1±1.0, and 6.1±5.4 parts per trillion by volume (pptv), respectively. The partitioning of these acids in the aerosol phase and the size distribution of their particulate form was also investigated. In the fine mode the mean mixing ratios for acetate, formate, pyruvate, and oxalate were 69.7±47.5, 32.5±39.4, 1.0±1.0, and 5.1±3.7 pptv, respectively. Elevated levels of all organic acids were encountered in the southern hemisphere (springtime) compared to the northern hemisphere (autumn), indicating a possible seasonal variation in their source strength and/or enhanced photochemical production. The observed distributions of formic and acetic acids have been compared to the results of a global chemistry/transport three-dimensional model. The model results show that acetic acid has mainly secondary photochemical sources (about 120 Tg CH3COOH/yr). On the opposite, the known chemical sources of HCOOH are quite weak (20 Tg HCOOH/yr) and insufficient to simulate the HCOOH levels observed in the marine atmosphere. A local marine source of HCOOH of about 50 Tg/yr on a global scale is required to reasonably simulate the observations in the marine atmosphere.

A two-dimensional study of ethane and propane oxidation in the troposphere

The oxidation chemistry of ethane (C2H6) and propane (C3H8) in the troposphere was studied using a global two-dimensional model, adopting observed surface volume mixing ratios of C2H6 and C3H8 as a function of latitude and season. From the calculated distribution of OH, C2H6, and C3H8 the source strengths, which compensate the chemical loss of these hydrocarbons in the atmosphere, are estimated at 16 Tg C2H6/a and 23 Tg C3 H8/a. Uncertainties involved in the calculations are discussed. The resulting seasonal and latitudinal distribution of various organic compounds, such as acetaldehyde, acetone, peroxyacetyl nitrate (PAN), peroxypropyl nitrate (PPN), and alkyl nitrates were derived. The contribution of various nitrogen species to the unidentified NOy observed during measurement campaigns was examined. C2−C3 alkyl nitrates and HNO4 formed at mixing ratios of a few tens of pptv could account only for some of the unidentified NOy. PAN is calculated to be the most abundant organic nitrate, with mixing ratios exceeding 100 pptv at mid-latitudes to high latitudes in spring in the northern hemisphere. These values are low compared to observations, however. Regionally, up to 10 times more odd nitrogen may be transported in the form of PAN than NOx. The influence of C2H6 and C3H8 chemistry on calculated mean tropospheric NOx mixing ratios and, subsequently, on O3 and OH concentrations is limited. Therefore major effects on global O3 and OH concentrations must be due to PAN formation in the low troposphere from NOx and reactive hydrocarbons other than C2H6 and C3H8. Such hydrocarbons are required to explain the observed high PAN mixing ratios.

Temporal variations of surface regional background ozone over Crete Island in the southeast Mediterranean

The first year-round observations of seasonal and diurnal variations of background ozone at a coastal site on Crete Island in the southeast Mediterranean area are presented here. They point out (1) the existence of a well-defined seasonal cycle with maximum during summer months, (2) the presence of elevated O 3 levels (up to 80 ppbv) during daytime and over time periods of several days, and (3) the dependence of O 3 mixing ratios on air mass origin. Comparison with three-dimensional chemistry transport model results shows that during summer the measured O 3 values exceed the calculated by 10-20 ppbv. Inclusion of biomass burning and biogenic volatile organic emissions in the model could partly offset the discrepancy between model results and observations.

Human-activity-enhanced formation of organic aerosols by biogenic hydrocarbon oxidation

Tropospheric aerosol can affect climate and the chemistry of the atmosphere. Organic particulates form a significant fraction of the atmospheric suspended matter over forested areas and may originate to a large extent from the oxidation of natural hydrocarbons. A three-dimensional global model of the troposphere is used to evaluate the contribution to the global organic aerosol (OA) source of the secondary organic aerosol (SOA) derived from the ozonolysis of biogenic volatile organic compounds (BVOC) and its evolution since preindustrial times. BVOC have been represented by a mixture of α- and β-pinenes, and their aerosol-forming parameters and chemical reactivities versus O3, OH, and NO3 have been estimated using laboratory information. An important factor in SOA formation is the deposition of condensable aerosol oxidation products onto preexisting organic aerosol, and this has been taken into account. The thus-calculated source of SOA is evaluated to have increased from 17–28 Tg/yr in preindustrial times to 61–79 Tg/yr at present. This threefold to fourfold enhancement of the formation of organic aerosol from natural BVOC is attributed to an increase in ozone and organic aerosol from anthropogenic sources. The main uncertainties involved in our calculations are related to the composition of BVOC emissions and the details of their aerosol formation capabilities.