P. Hoor

Transport of biomass burning smoke to the upper troposphere by deep convection in the equatorial region

During LBA-CLAIRE-98, we found atmospheric layers with aged biomass smoke at altitudes >10 km over Suriname. CO, CO2, acetonitrile, methyl chloride, hydrocarbons, NO, O3, and aerosols were strongly enhanced in these layers. We estimate that 80-95% of accumulation mode aerosols had been removed during convective transport. Trajectories show that the plumes originated from large fires near the Brazil/Venezuela border during March 1998. This smoke was entrained into deep convection over the northern Amazon, transported out over the Pacific, and then returned to South America by the circulation around a large upper-level anticyclone. Our observations provide evidence for the importance of deep convection in the equatorial region as a mechanism to transport large amounts of pyrogenic pollutants into the upper troposphere. The entrainment of biomass smoke into tropical convective clouds may have significant effects on cloud microphysics and climate dynamics.

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.

High spatial and temporal resolution measurements of primary organics and their oxidation products over the tropical forests of Surinam.

Tropical forests with emissions greater than 10(15) g C of reactive hydrocarbons per year strongly affect atmospheric chemistry. Here we report aircraft-borne measurements of organics during March 1998 in Surinam, a largely unpolluted region which is optimally located to study chemical processes induced by tropical forest emissions. Isoprene and its degradation products methylvinyl ketone (MVK) and methacrolein (MACR) and possibly isoprene hydroperoxides (ISOHP), were measured in the nmol mol(-1) volume mixing ratio (VMR) range, consistent with estimated emissions and model calculations. In addition, high VMRs of some non-isoprene-derived organics were measured, such as acetone (approximate to 2-4 nmol mol(1) up to 12 km altitude), an important source of HO and HO(2) in the upper troposphere. Moreover, several masses were measured at significant mixing ratios which could not be identified by reference to previous field measurements or gas-phase isoprene chemistry. High VMRs, almost 0.4 nmol mol(-1), were also recorded for a compound which is most likely dimethyl sulphide (DMS). If so, boundary layer loss of HO by reactions with hydrocarbons and their oxidation products strongly prolongs the lifetime of DMS, allowing its transport deep into the Amazon forest south of the intertropical convergence zone (ITCZ). We postulate greater sulphate production and deposition north than south of the (ITCZ) with possible consequences for cloud and ecosystem properties

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.

Multimodel assessment of the upper troposphere and lower stratosphere: Extratropics

A multimodel assessment of the performance of chemistry-climate models (CCMs) in the extratropical upper troposphere/lower stratosphere (UTLS) is conducted for the first time. Process-oriented diagnostics are used to validate dynamical and transport characteristics of 18 CCMs using meteorological analyses and aircraft and satellite observations. The main dynamical and chemical climatological characteristics of the extratropical UTLS are generally well represented by the models, despite the limited horizontal and vertical resolution. The seasonal cycle of lowermost stratospheric mass is realistic, however with a wide spread in its mean value. A tropopause inversion layer is present in most models, although the maximum in static stability is located too high above the tropopause and is somewhat too weak, as expected from limited model resolution. Similar comments apply to the extratropical tropopause transition layer. The seasonality in lower stratospheric chemical tracers is consistent with the seasonality in the Brewer-Dobson circulation. Both vertical and meridional tracer gradients are of similar strength to those found in observations. Models that perform less well tend to use a semi-Lagrangian transport scheme and/or have a very low resolution. Two models, and the multimodel mean, score consistently well on all diagnostics, while seven other models score well on all diagnostics except the seasonal cycle of water vapor. Only four of the models are consistently below average. The lack of tropospheric chemistry in most models limits their evaluation in the upper troposphere. Finally, the UTLS is relatively sparsely sampled by observations, limiting our ability to quantitatively evaluate many aspects of model performance.

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.

Variability‐lifetime relationship for organic trace gases: A novel aid to compound identification and estimation of HO concentrations

In this study we report aircraft-borne measurements of organic species made during March 1998 in Surinam, an unpolluted region on the northeast coast of South America Measurements included the following: CO by tunable diode laser; a wide variety of organics including acetone, acetonitrile, and isoprene by proton transfer mass spectrometry (PTR-MS); and nonmethane hydrocarbon measurements by gas chromatography-flame ionization detection. Here we compare the standard deviation of the natural logarithm of the mixing ratio (Sigma_1nX) to the estimated lifetime of these species. This relationship has been used to support identification of masses measured by the PTR-MS; ascertain the consistency and quality of hydrocarbon measurement data; and to provide information concerning sinks of important trace species. A selection of the data is used to indirectly determine an average HO concentration of 2.0 X 10(5) molecules cm(-3) along the back trajectory for air encountered during the Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Cooperative LBA Airborne Regional Experiment (LBA-CLAIRE) measurement campaign between 0-1 km over the tropical rain forest. The lower than expected HO concentration derived could have been caused by significant atmospheric or oceanic photochemical production of acetone and MEK along the back trajectory.

Particle production in the lowermost stratosphere by convective lifting of the tropopause

Aircraft measurements of aerosol particles and trace gases were performed in the upper free troposphere and lower stratosphere during the Stratosphere and Troposphere Experiment by Aircraft Measurements (STREAM-96) campaign from Shannon airport, Ireland. During one measurement flight, ultrafine particle number densities up to 104 cm−3 (STP) were observed in the lowermost stratosphere. Concurrent with these very high number densities of ultrafine particles, high accumulation mode particle number densities were observed over the same geographical location in the free troposphere, which were attributed to convective transport in the troposphere. The observations suggest that adiabatic cooling of the stratospheric air, as a result of the convective transport in the troposphere that lifted the tropopause and the air in the lowermost stratosphere, was responsible for triggering the formation of new particles. However, also aircraft emissions could have contributed to the enhancement in ultrafine particles.

Assessing the effect of marine isoprene and ship emissions on ozone, using modelling and measurements from the South Atlantic Ocean

Environmental context. Air over the remote Southern Atlantic Ocean is amongst the cleanest anywhere on the planet. Yet in summer a large-scale natural phytoplankton bloom emits numerous natural reactive compounds into the overlying air. The productive waters also support a large squid fishing fleet, which emits significant amounts of NO and NO2. The combination of these natural and man-made emissions can efficiently produce ozone, an important atmospheric oxidant. Abstract. Ship-borne measurements have been made in air over the remote South Atlantic and Southern Oceans in January–March 2007. This cruise encountered a large-scale natural phytoplankton bloom emitting reactive hydrocarbons (e.g. isoprene); and a high seas squid fishing fleet emitting NOx (NO and NO2). Using an atmospheric chemistry box model constrained by in-situ measurements, it is shown that enhanced ozone production ensues from such juxtaposed marine biogenic and anthropogenic emissions. The relative impact of shipping and phytoplankton emissions on ozone was examined on a global scale using the EMAC model. Ozone in the marine boundary layer was found to be over ten times more sensitive to NOx emissions from ships, than to marine isoprene in the region south of 45°. Although marine isoprene emissions make little impact on the global ozone budget, co-located ship and phytoplankton emissions may explain the increasing ozone reported for the 40–60°S southern Atlantic region.

Detection of lightning-produced NO in the midlatitude upper troposphere during STREAM 1998

Simultaneous in situ measurements of NO, NOy, HNO3, CO, CO2, O-3, and aerosols were performed in the midlatitude upper troposphere (UT) and lower stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) 1998 summer campaign. The campaign focused on the region around Timmins in the Canadian province of Ontario (79.3 degreesW, 48.2 degreesN), close to the polar jet stream that rapidly transports trace species across the Atlantic Ocean. This paper focuses on the origin of total reactive nitrogen (NOy) in the UT, as our measurements show strong variations, which reflect large local sources. In situ production by lightning, stratospheric downdraft, aircraft emissions, and upward transport of polluted boundary layer air are discussed in two case studies as potential contributors. We use correlations among NO, NOy and CO to distinguish between transport from the boundary layer and in situ formations. Lightning production of NOx is found to be a strong contributor to the budget of NOy during high NOy episodes.