H. A. Scheeren

Tracer correlations in the northern high latitude lowermost stratosphere: Influence of cross-tropopause mass exchange

We present an analysis of trace gas correlations in the lowermost stratosphere. In-situ aircraft measurements of CO, N(2)O, NO(y) and O(3) Obtained during the STREAM 1997 winter campaign, have been used to investigate the role of cross-tropopause mass exchange on tracer-tracer relations. At altitudes several;kilometers above the local tropopause, undisturbed stratospheric air was found with NO(y)/NO(y)* ratios close to unity, NO(y)/O(3) about 0.003 - 0.006 and CO mixing ratios as low as 20 ppbv (NO(y)* is a proxy for total reactive nitrogen derived from NO(y)-N(2)O relations measured in the stratosphere). Mixing of tropospheric-air into the lowermost stratosphere has been identified by enhanced ratios of NO(y)/NO(y)* and NO(y)/O(3), and from scatter plots of CO versus O(3). The enhanced NO(y)/O(3) ratio in the lowermost stratospheric mixing zone points to a reduced efficiency of O(3) formation from aircraft NO(x) emissions.

Continuing emissions of methyl chloroform from Europe

The consumption of methyl chloroform (1,1,1-trichloroethane), an industrial solvent, has been banned by the 1987 Montreal Protocol because of its ozone-depleting potential. During the 1990s, global emissions have decreased substantially and, since 1999, near-zero emissions have been estimated for Europe and the United States. Here we present measurements of methyl chloroform that are inconsistent with the assumption of small emissions. Using a tracer transport model, we estimate that European emissions were greater than 20 Gg in 2000. Although these emissions are not significant for stratospheric ozone depletion, they have important implications for estimates of global tropospheric hydroxyl radical (OH) concentrations, deduced from measurements of methyl chloroform. Ongoing emissions therefore cast doubt upon recent reports of a strong and unexpected negative trend in OH during the 1990s and a previously calculated higher OH abundance in the Southern Hemisphere compared to the Northern Hemisphere.

Isoprene and Its Oxidation Products Methyl Vinyl Ketone, Methacrolein, and Isoprene Related Peroxides Measured Online over the Tropical Rain Forest of Surinam in March 1998

Airborne measurements of volatile organic compounds (VOC) were performed overthe tropical rainforest in Surinam (0–12 km altitude,2°–7° N, 54°–58° W) using the proton transferreaction mass spectrometry (PTR-MS) technique, which allows online monitoringof compounds like isoprene, its oxidation products methyl vinyl ketone,methacrolein, tentatively identified hydroxy-isoprene-hydroperoxides, andseveral other organic compounds. Isoprene volume mixing ratios (VMR) variedfrom below the detection limit at the highest altitudes to about 7 nmol/molin the planetary boundary layer shortly before sunset. Correlations betweenisoprene and its product compounds were made for different times of day andaltitudes, with the isoprene-hydroperoxides showing the highest correlation.Model calculated mixing ratios of the isoprene oxidation products using adetailed hydrocarbon oxidation mechanism, as well as the intercomparisonmeasurement with air samples collected during the flights in canisters andlater analysed with a GC-FID, showed good agreement with the PTR-MSmeasurements, in particular at the higher mixing ratios.Low OH concentrations in the range of 1–3 × 105molecules cm-3 averaged over 24 hours were calculated due to lossof OH and HO2 in the isoprene oxidation chain, thereby stronglyenhancing the lifetime of gases in the forest boundary layer.

Overview of the trace gas measurements on board the Citation aircraft during the intensive field phase of INDOEX

During the intensive field phase of the Indian Ocean Experiment (INDOEX), measurements of the atmospheric chemical and aerosol composition over the Indian Ocean were performed from a Cessna Citation aircraft. Measurements were performed during February and March 1999 over the northern Indian Ocean from 70 degreesE to 80 degreesE, and from 8 degreesN tb 8 degreesS in the 0-13 km altitude range. An overview of the trace gas-measurements is presented. In the lowest 3 km the highest levels of pollution were found during February 1999, mostly originating from northeastern India and southeastern Asia. Lower levels of pollution were detected in March 1999, when the sampled air mostly originated from the Arabian Sea region. The mixing ratios of a number of trace compounds, indicative of biomass burning, were well correlated. The pollutant emission factors inferred from the measurements are consistent with literature values for fire plumes, confirming that the residential use of biofuels in Asia is a major source of gaseous pollutants to the atmosphere over the Indian Ocean, in accord with emission databases. The removal of reactive trace gases was studied over an extended area without interfering local emissions, and is shown to be governed by photochemical processes rather than by mixing and deposition. At intermediate altitudes of 3-8 km the mixing ratios of all trace gases other than ozone were generally lower, and the measurements suggest that the Photochemical processing of these air masses is much more extensive than in the 0-3 kin range. In the 8-13 km altitude range some evidence is obtained for the importance of convective cloud systems in the transport of gaseous pollutants to the upper troposphere.

High acetone concentrations throughout the 0-12 km altitude range over the tropical rainforest in Surinam

Airborne measurements of acetone were performed overthe tropical rainforest in Surinam(2°–7° N, 54°–58° W, 0–12 kmaltitude) during the LBA-CLAIRE campaign in March1998, using a novel proton transfer reaction massspectrometer (PTR-MS) that enables the on-linemonitoring of volatile organic compounds (VOC) with ahigher proton affinity than water. The measuredacetone volume mixing ratios ranged from ∼0.1 nmol/molup to ∼8 nmol/mol with an overall average of 2.6nmol/mol and a standard deviation of 1.0 nmol/mol. Theobserved altitude profile and correlations with CO,acetonitrile, propane and wind direction are discussedwith respect to potential acetone sources. No linearcorrelation between acetone and CO mixing ratios wasobserved, at variance with results of previousmeasurement campaigns. The mean acetone/CO ratio(0.022) was substantially higher than typical valuesfound before. The abundance of acetone appears to beinfluenced, but not dominated, by biomass burning,thus suggesting large emissions of acetone and/oracetone precursors, such as possibly 2-propanol, fromliving plants or decaying litter in the rainforest.

Chlorine activation and ozone destruction in the northern lowermost stratosphere

We report aircraft measurements from the Stratosphere-Troposphere Experiments by Aircraft Measurements (STREAM) II campaign, performed during February 1995 from Kiruna, northern Sweden, near 67 degrees N latitude. We have measured trace species, e.g., O-3, nitrogen compounds, HCl, hydrocarbons, CO, ice particles. and aerosols, to characterize the chemical conditions of the lowermost stratosphere at altitudes up to similar to 12.5 km. From the observation of anomalously high CO/C2H6 ratios, caused by enhanced C2H6 breakdown, we derive that heterogeneous chlorine activation has occurred. This has liberated a diurnal mean Cl concentration up to 1.0 x 10(4) atoms cm(-3), which efficiently destroys ozone. We also infer that much of the Cl activation has taken place on ice crystals above the tropopause. The measured crystal number densities were typical of thin cirrus. Model calculations suggest that heterogeneous chlorine conversion on ice crystals, in addition to that on liquid aerosols, may contribute significantly to ozone destruction in the middle- and high-latitude lowermost stratosphere.

Methyl chloride and other chlorocarbons in polluted air during INDOEX

Methyl chloride (CH 3 Cl) is the most abundant, natural, chlorine-containing gas in the atmosphere, with oceans and biomass burning as major identified sources. Estimates of global emissions suffer from large uncertainties, mostly for the tropics, partly due to a lack of measurements. We present analyses of whole-air canister samples for selected nonmethane hydrocarbons and chlorocarbons. The samples were collected from an aircraft during the Indian Ocean Experiment (INDOEX) campaign over the northern Indian Ocean in February and March 1999. The CH 3 Cl results are correlated to selected nonmethane hydrocarbons and in situ measurements of carbon monoxide (CO) and acetonitrile (CH 3 CN). We relate high mixing ratios of ∼750 pmol mol -1 of CH 3 Cl to biomass burning, as observed in polluted air masses from India and Southeast Asia. We infer a relatively high enhancement ratio relative to CO, ΔCH 3 Cl/ΔCO ≃ 1.74 ± 0.21 x 10 -3 mol mol -1 . The CH 3 Cl levels relate to the extensive biofuel use in India and Southeast Asia, notably the burning of agricultural waste and dung with a comparatively high chlorine content. It appears that CH 3 Cl emissions from biofuel consumption in India and Southeast Asia have been underestimated in the past. Furthermore, we observed enhanced dichloromethane (CH 2 Cl 2 ) and trichloromethane (CHCl 3 ) levels, correlating with high CO, acetylene (C 2 H 2 ) and CH 3 Cl, indicating that biomass burning is a small but significant source of these species.

The temporal evolution of the ratio HNO3/NOy in the Arctic lower stratosphere from January to March 1997

Aircraft-based measurements of HNO3, NOy, N2O, and O3 have been performed in the Arctic lower stratosphere in January (POLSTAR I) and March (STREAM III) of 1997. The two projects employed different aircraft platforms. In addition, NOy and O3 were measured using different instruments in the two campaigns. HNO3 and NOy were found strongly correlated with correlation coefficients of 0.84 (POLSTAR I) and 0.69 (STREAM III), respectively. The fraction of HNO3 within NOy decreased from 96% in January to 59% in March. The decrease is consistent with the lifetime of HNO3 due to photolysis after polar sunrise. The relationship of NOy and HNO3 with N2O shows that in January NOy and HNO3 values were markedly higher than expected, which may indicate nitrification by PSC-II particle sedimentation and evaporation. Contradictory, the ratios NOy/O3 observed in January are only slightly elevated. In March, NOy-N2O and NOy-O3 relations agree well with others reported in the literature. The difference between the NOy-O3 and NOy-N2O relationships is partly explained by an observed O3 decrease of about 30% between January and March.

Measurement of aerosol sulfuric acid 2. Pronounced layering in the free troposphere during the second Aerosol Characterization Experiment (ACE 2)

Measurements of aerosol sulfuric acid in the free troposphere were performed in the vicinity of Tenerife, Canary Islands (28degreesN, 16degreesW), in July 1997. These measurements were carried out on board a Dutch Cessna Citation 11 research aircraft within the framework of the second Aerosol Characterization Experiment (ACE 2). We used the Volatile Aerosol Component Analyzer for the detection of sulfuric acid (H2SO4). Vertical profiles between 2 and 13 km altitude were obtained. Typically, H2SO4 mixing ratios ranged between 10 and 120 pptv. Between 4 and 6 km altitude a distinct H2SO4 aerosol layer was observed repeatedly with H2SO4 mixing ratios of up to 550 pptv. The measurements are in agreement with total aerosol mass concentrations inferred from simultaneous measurements of aerosol size distributions using two condensation particle counters, a differential mobility analyzer, and an optical aerosol counter. At altitudes above 4 km the predominant aerosol component was sulfuric acid, frequently correlated with ozone, suggesting photochemical air pollution as a common source. Sulfur dioxide measured by chemical ionization mass spectrometry technique revealed typical mixing ratios between 10 and 60 pptv at altitudes above 6 kin and up to 200 pptv in the lower troposphere.

JRC Ispra EMEP - GAW Regional Station for Atmospheric Research: 2010 Report

The aim of the JRC-Ispra station for atmospheric research (45°49’N, 8°38’E) is to monitor atmospheric parameters (pollutant concentrations and fluxes, atmospheric particle chemical composition, number size distribution and optical properties) to contribute in assessing the impact of European policies on air pollution and climate change. The station has been operated continuously since November 1985, with a gap in gas phase data due to a severe breakdown of the data acquisition system in 2003 though. The measurements performed in 2007 led to annual averages of ca. 32 μg m O3, 0.8 μg m SO2, 21 μg m NO2 and 30 μg m PM10. Carbonaceous species (organic matter plus elemental carbon) are the main constituents of PM2.5 (> 55 %) followed by NH4NO3 (20-30 %) and (NH4)2SO4 (10-20 %). The measurements confirmed the seasonal variations observed over the previous years, mainly driven by meteorology rather than by changes in emissions. Aerosol physical and optical properties were also measured in 2007. The average particle number (from 10 nm to 10 μm) was about 9200 cm in 2007. The mean (close to dry) aerosol single scattering albedo (0.79) was low compared to the values generally observed in Europe, which means that the cooling effect of aerosols is reduced in our region compared to others. Long-term trends (over 20 years) show consistent decreases in sulfur concentrations and deposition, PM mass concentration (-0.9 μg m yr) and in extreme ozone value occurrence frequency. The decreasing trends in oxidised and reduced nitrogen species are much less pronounced. However, historical minimum in NO3, NH4, (and SO4) wet deposition, as well as in O3 pollution indicators (AOT40 and SOMO35) were observed in 2007. How to obtain EU publications Our priced publications are available from EU Bookshop (http://bookshop.europa.eu), where you can place an order with the sales agent of your choice. The Publications Office has a worldwide network of sales agents. You can obtain their contact details by sending a fax to (352) 29 29-42758.