Tropospheric Boundary Layer Research Articles

Temperature and pressure dependent kinetics of the gas‐phase reaction of the hydroxyl radical with nitrogen dioxide

The reaction of OH with NO2 is pivotal in both stratospheric and tropospheric chemistry; in each case it is the dominant homogeneous mechanism for conversion of NOx to NOy. The rate constant is a strong function of pressure and temperature, and key portions of the pressure‐temperature domain are poorly or ambiguously covered by the available data. These include conditions typical of the tropospheric boundary layer and of the lower stratosphere. At surface conditions differences of 60% exist both in the literature data and between the major recommendations, while at lower stratospheric conditions there are few available data. Our High Pressure Flow kinetics system is ideally suited to studying this reaction, as we are able to scan both temperature and pressure while maintaining wall‐less conditions, eliminating the possible complications of heterogeneous chemistry. Here we report a temperature‐ and pressure‐dependent study (220–300 K, 50–150 torr) of this reaction; the measured rate constants are in excellent agreement with previously published values down to 240 K, but lie 10–20% lower than the historical data available below that temperature. An analysis of all available data motivates a large (∼ 20%) downward revision in the recommended rate constant below room temperature.

Hygroscopic growth of aerosol particles in the marine boundary layer over the Pacific and Southern Oceans during the First Aerosol Characterization Experiment (ACE 1)

The hygroscopic properties of submicrometer aerosol particles were studied with a Hygroscopic Tandem Differential Mobility Analyzer (H‐TDMA) in the remote marine tropospheric boundary layer (MBL) over the Pacific and Southern Oceans in connection with the southern hemisphere marine First Aerosol Characterization Experiment (ACE 1) in October–December 1995. The H‐TDMA was placed on board the ship R/V NOAA Discoverer and measured the hygroscopic diameter growth of individual aerosol particles when taken from a dry state to a relative humidity (RH) of 89–90%. Measurements were performed for the particles with dry diameters of 35, 50, 75, and 150 (or 165) nm. The natural aerosol present in the remote MBL largely consists of two types, a sea‐salt component and a non‐sea‐salt (nss) sulfate component. Since their hygroscopic behavior is significantly different, the H‐TDMA could clearly distinguish between these two types and thus make in situ measurements of the mixing state of the MBL aerosol. During the ACE 1 intensive campaign in the Southern Ocean south of Australia, the hygroscopic diameter growth factors at RH = 90% for the nss‐sulfate aerosol particles were 1.62, 1.66, and 1.78 at dry particle diameters of 35, 50, and 150 nm, respectively, and for time periods with remote marine air masses. These values exceed those normally found in continental polluted environments. The growth factors for the externally mixed sea‐salt particles were even higher (2.12 and 2.14 for 50 and 150 nm). The corresponding values for the Pacific Ocean (at RH = 89%) for the nss‐sulfate particles were 1.56, 1.59, 1.61, and 1.63 for 35, 50, 75, and 165 nm. Particle deliquescence and RH hysteresis between RH = 68–90% was only observed in air masses north of the South Pacific Gyre, and then only for the Aitken mode particles (particle diameters ∼20–80 nm). The occurrence of externally mixed sea‐salt particles could be linked to conditions with high wind speeds in connection with frontal passages or low pressure systems. Nevertheless, the number of externally mixed 150 nm sea‐salt particles was found to be poorly correlated with local wind speed, probably due to a rather long life‐time of these submicrometer particles. Particles with hygroscopic growth factors significantly less than those of the nsssulfate particles (denoted less hygroscopic particles) were only present during periods with anthropogenic influence.

Observations of the nitrate radical in the free troposphere at Izaña de Tenerife

This paper reports direct measurements of the nitrate radical (NO3) in the lower free troposphere. The measurements were carried out at Izaña de Tenerife (2300 m altitude) in the Canary Islands during May 1994, using the technique of differential optical absorption spectroscopy (DOAS). The average nighttime NO3 concentration in very clean air from the mid‐Atlantic was found to be 8 ppt, with a maximum observed concentration of about 20 ppt. Combining the NO3 data with ancillary measurements of NO2 and O3 in a model shows that there are no important NO3 scavengers in a part of the troposphere that is characterized by a very low relative humidity and aerosol particle count, and where the concentration of NO2 is too small for significant quantities of N2O5 to form. On occasion, the presence of a trace concentration of NO or an organic species such as α‐pinene is required to explain the observations. The lifetime of NO3 at night was in excess of 2 hours, much longer than measured hitherto in the tropospheric boundary layer.

Sub-gridscale effects in mesoscale deep convection: Initiation, organization and turbulence

Deep convection shows a tendency toward organized long-lived mesoscale cloud systems. These dynamically self-sustaining systems form an important element of atmospheric variability on the mesoscale and down to dissipative scales. As it is not possible to resolve all scales of motion involved in mesoscale cloud systems, the representation and modelling of organized mesoscale deep convection necessarily requires a rupture of the mesoscale spectrum and a separate representation of smaller scales. It is proposed here that a natural point for this rupture is the scale of initiation of deep convection, linked to the tropospheric scale height, O(10 km). Smaller scale motions that must be represented as sub-gridscale effects include the effects of turbulent motions; condensation and precipitation at sub-gridscales; the organization of the troposphere and planetary boundary layer on the convective scale. It will be shown that the initiation and organization of deep convection can be understood in a systematic manner by examining mechanisms by which convective-scale motions appear through transitions from forced motions at other scales.

Size distributions of sub-micron aerosols in the free troposphere and marine boundary layer of the sub-tropical North Atlantic

Size distributions of sub-micron aerosols in the free troposphere and marine boundary layer of the sub-tropical North Atlantic

Measurements of methyl chloride in the northwest Atlantic

Methyl chloride was measured directly in seawater using a purge and trap system and gas chromatography with electron capture detection. The results indicate that surface waters of the northwest Atlantic are a source of methyl chloride to the atmosphere during late spring/early summer. The average surface concentration was 271 pM (σ = 68 pM), supersaturated with respect to an assumed tropospheric boundary layer mixing ratio of 0.7 ppbv. Elevated concentrations were observed throughout the region in waters above the seasonal thermocline. Near‐surface maxima of differing thickness and intensity were also seen within this upper layer. Broad maxima within the 200–800m depth range were associated with water masses more recently subducted from the surface than the surrounding main thermocline waters. Coastal inputs of methyl chloride appear not to be an important source in the area of study. Although there is some indication of elevated concentrations associated with higher phytoplankton activity in shelf edge regions, the picture concerning phytoplankton production of methyl chloride is still unclear. Further work is required to look at direct and indirect mechanisms by which phytoplankton may influence the distribution of methyl chloride in the oceans. The global ocean‐to‐atmosphere flux estimated from this data set, 3.3 ‐ 4.8 × 1010 mol CH3Cl yr−1 (1.7 ‐ 2.4 × 1012 g CH3Cl yr−1) suggests that the oceans may contribute a smaller flux of methyl chloride to the atmosphere than previously thought.

Levels of polychlorinated biphenyls in the lower troposphere of the North- and South-Atlantic Ocean

Fourteen polychlorinated biphenyl (PCB) congeners were quantified in air samples of the tropospheric boundary layer of the Atlantic Ocean. The samples were taken on the German research vessel “Polarstern” during north-south cruises across the Atlantic Ocean (1990, 1991), and on the Capo Verde islands in the North Atlantic Ocean (1992). Values for the sum of PCB were between 48 pg/m3 (values for the seven indicator congeners were [in pg/m3]: PCB 28: 1.3; PCB 52: 5.0; PCB 101: 3.0; PCB 118: ≤0.5; PCB 138: 1; PCB 153: 1; PCB 180: <0.2) in the Westwind Belt of the eastern North Atlantic and 22 pg/m3 (values for the seven indicator congeners were [in pg/m3]: PCB 28: 2.3; PCB 52: 3.4; PCB 101: ≤0.5; PCB 118: <0.2; PCB 138: <0.2; PCB 153: <0.2; PCB 180: <0.2) in the Westwind Belt of the central South Atlantic. Up to 385 pg/m3 (values for the seven indicator congeners were [in pg/m3]: PCB 28: 2.6; PCB 52: 11.7; PCB 101: 28.4; PCB 118: 9; PCB 138: 21; PCB 153: 18; PCB 180: 5.5) were measured of the coast of South Patagonia. A difference depending on latitude and on terrestrial influenced air masses between the lower and the higher chlorinated congeners was observed. The levels of three- and tetrachlorinated congeners were highest in the Trade Wind regions. The contents of the higher chlorinated congeners had maxima in samples influenced by continental air masses. A correlation of the levels of the lower chlorinated congeners in air over the South Atlantic with the surface water temperature and thus with the temperature dependent gas/water partition coefficient Kgw was observed.

Spring 1989 observations of lower tropospheric chemistry in the Canadian high arctic

In the spring of 1989, airborne observations were made of C 2−C 4 hydrocarbons, ozone, and aerosols in the tropospheric boundary layer over a 96,000 km 2 area in the vicinity of Alert, NWT, Canada (82.5° N, 61.5° W). Samples were collected in stainless steel canisters and on filters. Aerial ozone measurements, particle counting measurements, and meteorological observations were also made. Analysis of the canister and filter samples has provided data on C 2−C 4 hydrocarbons and ionic species (Cl −, Br −, NO 3 −, SO 4 2−, Na +, K +, and NH 4 +). These data, together with observations of ozone, have provided further insight into the near-ground level ozone depletion during the Arctic spring. A positive correlation between the concentrations of ozone, ethane, propane, i-butane, n-butane, and acetylene was observed. In addition, there is an indication of a negative correlation between ozone concentrations and filterable bromine. An analysis of the relationship between the concentrations of ozone and hydrocarbons has provided evidence that chlorine atoms may be responsible for the observed depletion of hydrocarbons, but not ozone. The latter is more readily explained by reaction with bromine atoms.

Light hydrocarbons in the tropospheric boundary layer over tropical Africa

Fifty measurements of nonmethane hydrocarbons (NMHC) and several other trace gases were made over an equatorial rain forest in February 1988 as part of the DECAFE experiment. The measurements were made independently by two different laboratories. Each laboratory used its own sample containers, gas chromatographic measurement procedure, and calibration. Also, the altitudinal distribution of the samples differed. Apart from propene and i‐pentane for which a substantial difference in the absolute calibration existed between the two laboratories, the average results were very similar, and the vertical profiles were identical within the scatter of the data. For NMHC with longer atmospheric residence times (e.g., ethane and acetylene) the average results agreed within a few percent. In the boundary layer, only small gradients could be found. In all cases where a significant vertical gradient existed, there was an increase of the mixing ratios with increasing altitude. This can be explained by the different origin of the air masses at different altitudes. Above the boundary the trace gas mixing ratios decrease. The observed NMHC pattern can primarily be described as photochemically aged emissions from biomass burning. The observed depletion of the photochemically reactive NMHC also agrees with the occurrence of enhanced ozone levels in the boundary layer.

A numerical simulation of kinetic constraints upon achievement of the ammonium nitrate dissociation equilibrium in the troposphere

Various mechanisms and associated rate expressions for the reversible evaporation of volatile ammonium salts are presented, and experimental evidence for them discussed. The literature concerned with the existence of stable NH 4 + salt monomers in the gas phase in highlighted. Four kinetic mechanisms are presented: monomer formation; adsorption of one precursor species on the particle surface; biomolecular surface reaction; and transport-limited particle growth. No mechanism emerges as the rate-limiting process for all experimental studies. Rate constant ratios for the first two mechanisms are calculated from the literature and shown to differ greatly from the thermodynamic equilibrium constant. A simple box model is developed to determine the importance of kinetic constraints on aerosol growth and evaporation in the tropospheric boundary layer. No attempt is made to model actual tropospheric processes other than the daytime oxidation of NO 2 and the kinetics of aerosol growth itself, so that model output is representative rather than definitive. Substantial departures from equilibrium are seen, giving gas concentration products ranging from −200 to 5000% of the theoretical value, Ke. At low temperatures and high relative humidities (r.h.) a large proportion of the potential condensable NO 3 − remains in the gas phase. The behaviour is general over a range of initial aerosol loadings, temperatures, r.h. and model formulations. Both pure NH 4NO 3 aerosol and a hypothetical equimolar mixture with (NH 4) 2SO 4, conform to the general case.

Ozone in seawater and Lake Water: A reversible reservoir

Observations made at sea and examination of global ozone monitoring data from oceanic sites indicate that ozone mixing ratios over the sea surface often increase with increasing wind speed. However, the slope of the increase approaches zero at wind speeds above about 6 m/sec (13 kts). Subsequent laboratory experiments confirmed that ozone can be purged from any seawater sample whether freshly collected, stored, or from depth. Samples of Lake Michigan water are purged of ozone in less time than seawater and its capacity is less than that of seawater. The rate of evolution of ozone is increased by altering the pH, changing the ionic strength by the addition of salts or by evaporation. These results suggest that a significant portion of tropospheric boundary layer ozone mixing ratios could be maintained by a reversible equilibrium with the ocean and lake surfaces.

Un dispositif experimental utilisant des ballons plafonnants pour l'etude de la couche limite atmospherique

This paper presents a new system of constant volume balloons used to study the dynamical and thermodynamical properties of the tropospheric boundary layer. The system allows to simultaneously localize up to 30 balloons and to observe thermodynamical characteristics of the air during their flight inside the boundary layer. Each balloon is equipped with a radar reflector and a sounding system giving information each second on pressure, humidity and temperature by sequential emissions which are received at a ground station. The trajectories of balloons are obtained by a non-tracking S-band radar where a special hardware processing unit allows to real time cancellation of most ground clutter. A complete set of the balloons positions is obtained every two or three minutes. The system was tested in October 1984 during a field experiment in the south of France. Results of the experimental procedure and of the quality of the balloon, radar and sounding capabilities are given. The scientific use of constant volume balloons in order to study the atmospheric boundary layer is examined.