R. M. Purvis

Ozone production from the 2004 North American boreal fires

We examine the ozone production from boreal forest fires based on a case study of wildfires in Alaska and Canada in summer 2004. The model simulations were performed with the chemistry transport model, MOZART-4, and were evaluated by comparison with a comprehensive set of aircraft measurements. In the analysis we use measurements and model simulations of carbon monoxide (CO) and ozone (O3) at the PICO-NARE station located in the Azores within the pathway of North American outflow. The modeled mixing ratios were used to test the robustness of the enhancement ratio ΔO3/ΔCO (defined as the excess O3 mixing ratio normalized by the increase in CO) and the feasibility for using this ratio in estimating the O3 production from the wildfires. Modeled and observed enhancement ratios are about 0.25 ppbv/ppbv which is in the range of values found in the literature and results in a global net O3 production of 12.9 ± 2 Tg O3 during summer 2004. This matches the net O3 production calculated in the model for a region extending from Alaska to the east Atlantic (9–11 Tg O3) indicating that observations at PICO-NARE representing photochemically well aged plumes provide a good measure of the O3 production of North American boreal fires. However, net chemical loss of fire-related O3 dominates in regions far downwind from the fires (e.g., Europe and Asia) resulting in a global net O3 production of 6 Tg O3 during the same time period. On average, the fires increased the O3 burden (surface −300 mbar) over Alaska and Canada during summer 2004 by about 7–9% and over Europe by about 2–3%.

Chemical composition observed over the mid-atlantic and the detection of pollution signatures far from source regions

The atmospheric composition of the central North Atlantic region has been sampled using the FAAM BAe146 instrumented aircraft during the Intercontinental Transport of Ozone and Precursors (ITOP) campaign, part of the wider International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). This paper presents an overview of the ITOP campaign. Between late July and early August 2004, twelve flights comprising 72 hours of measurement were made in a region from approximately 20 to 40°W and 33 to 47°N centered on Faial Island, Azores, ranging in altitude from 50 to 9000 m. The vertical profiles of O3 and CO are consistent with previous observations made in this region during 1997 and our knowledge of the seasonal cycles within the region. A cluster analysis technique is used to partition the data set into air mass types with distinct chemical signatures. Six clusters provide a suitable balance between cluster generality and specificity. The clusters are labeled as biomass burning, low level outflow, upper level outflow, moist lower troposphere, marine and upper troposphere. During this summer, boreal forest fire emissions from Alaska and northern Canada were found to provide a major perturbation of tropospheric composition in CO, PAN, organic compounds and aerosol. Anthropogenic influenced air from the continental boundary layer of the USA was clearly observed running above the marine boundary layer right across the mid-Atlantic, retaining high pollution levels in VOCs and sulfate aerosol. Upper level outflow events were found to have far lower sulfate aerosol, resulting from washout on ascent, but much higher PAN associated with the colder temperatures. Lagrangian links with flights of other aircraft over the USA and Europe show that such signatures are maintained many days downwind of emission regions. Some other features of the data set are highlighted, including the strong perturbations to many VOCs and OVOCs in this remote region.

Droplet impact on a thin fluid layer

The initial stages of high-velocity droplet impact on a shallow water layer are described, with special emphasis given to the spray jet mechanics. Four stages of impact are delineated, with appropriate scalings, and the successively more important influence of the base is analysed. In particular, there is a finite time before which part of the water in the layer remains under the droplet and after which all of the layer is ejected in the splash jet.

Forest fire plumes over the North Atlantic: p‐TOMCAT model simulations with aircraft and satellite measurements from the ITOP/ICARTT campaign

[1] Intercontinental Transport of Ozone and Precursors (ITOP) (part of International Consortium for Atmospheric Research on Transport and Transformation (ICARTT)) was an intense research effort to measure long-range transport of pollution across the North Atlantic and its impact on O3 production. During the aircraft campaign plumes were encountered containing large concentrations of CO plus other tracers and aerosols from forest fires in Alaska and Canada. A chemical transport model, p-TOMCAT, and new biomass burning emissions inventories are used to study the emissions long-range transport and their impact on the troposphere O3 budget. The fire plume structure is modeled well over long distances until it encounters convection over Europe. The CO values within the simulated plumes closely match aircraft measurements near North America and over the Atlantic and have good agreement with MOPITT CO data. O3 and NOx values were initially too great in the model plumes. However, by including additional vertical mixing of O3 above the fires, and using a lower NO2/CO emission ratio (0.008) for boreal fires, O3 concentrations are reduced closer to aircraft measurements, with NO2 closer to SCIAMACHY data. Too little PAN is produced within the simulated plumes, and our VOC scheme’s simplicity may be another reason for O3 and NOx modeldata discrepancies. In the p-TOMCAT simulations the fire emissions lead to increased tropospheric O3 over North America, the north Atlantic and western Europe from photochemical production and transport. The increased O3 over the Northern Hemisphere in the simulations reaches a peak in July 2004 in the range 2.0 to 6.2 Tg over a baseline of about 150 Tg.

Two high-speed, portable GC systems designed for the measurement of non-methane hydrocarbons and PAN: results from the Jungfraujoch High Altitude Observatory.

Near real-time measurements of light non-methane hydrocarbons (NMHCs) and peroxyacetyl nitrate (PAN) have been performed in the free troposphere using two fast gas chromatography (GC) instruments designed for use on aircraft. A GC-helium ionisation detector (HID) system measured 15 C(2)-C(5) hydrocarbons with 5 min time resolution and a dual channel GC-Electron Capture Detector (ECD) measured PAN with 90 s resolution. Both instruments had low parts per trillion by volume (pptV) detection limits and ran continuously at the remote Jungfraujoch (JFJ) research station in the Swiss Alps (46.55[degree]N, 7.98[degree]E), 3580 m above mean sea level (AMSL), during February/March 2003. Carbon monoxide, ozone, nitrogen oxide and nitrogen dioxide and all odd nitrogen species (NO(y)) were also measured continuously. Hydrocarbons and CO were strongly correlated in all air-masses whilst PAN exhibited both positive and negative correlations with respect to O(3), dependent on age and origin of the air-mass sampled. PAN was found to contribute [similar]20% to the NO(y) sampled on average. The experiment, as well as providing interesting datasets from this remote location, also demonstrated that when optimised, GC techniques have the potential to measure at a time resolution significantly greater than is traditionally considered, with high sensitivity and low uncertainty.

Alkyl nitrates in outflow from North America over the North Atlantic during Intercontinental Transport of Ozone and Precursors 2004

This paper is based on alkyl nitrate measurements made over the North Atlantic as part of the International Consortium for Research on Atmospheric Transport and Transformation (ICARTT). The focus is on the analysis of air samples collected on the UK BAe-146 aircraft during the Intercontinental Transport of Ozone and Precursors (ITOP) project, but air samples collected on board the NASA DC-8 and NOAA WP-3D aircraft as part of a Lagrangian experiment are also used. The ratios between the alkyl nitrates and their parent hydrocarbons are compared with those expected from chemical theory. Further, a box model is run to investigate the temporal evolution of the alkyl nitrates in three Lagrangian case studies and compared to observations. The air samples collected during ITOP do not appear to be strongly influenced by oceanic sources, but rather are influenced by emissions from the N.E. United States and from Alaskan fires. There also appears to be a widespread common source of ethyl nitrate and 1-propyl nitrate other than from their parent hydrocarbons. The general agreement between the alkyl nitrate data and photochemical theory suggests that during the first few days of transport from the source region, photochemical production of alkyl nitrates, and thus ozone, had taken place. The observations in the more photochemically processed air masses are consistent with the alkyl nitrate production reactions no longer dominating the peroxy radical self/cross reactions. Further, the results also suggest that the rates of photochemical processing in the Alaskan smoke plumes were small.

Air-water interactions near droplet impact

The effects of the thin air layer entering play when a water droplet impacts on otherwise still water or on a fixed solid are studied theoretically with special attention on surface tension and on post-impact behaviour. The investigation is based on the small density and viscosity ratios of the two fluids. In certain circumstances, and in particular for droplet Reynolds numbers below a critical value which is about ten million, the air-water interaction depends to leading order on lubricating forces in the air coupled with potential flow dynamics in the water. The nonlinear integro-differential system for the evolution of the interface and induced pressure is studied for pre-impact surface tension effects, which significantly delay impact, and for post-impact interaction phenomena which include significant decrease of the droplet spread rate. Above-critical Reynolds numbers are also considered.

Droplet impact on water layers: post-impact analysis and computations

An inviscid model of droplet impact into a water layer is examined analytically for small times just after impact and computationally for order-one distortion times. Various layer depths are considered, as are surface-tension effects. Good correspondence is found between analysis, computation and experiments.

Fluid flow through various branching tubes

In this part-review part-new work, studies on branching tube flows are described. These are based on modelling for increased flow rates as well as on direct numerical simulations and are motivated by applications to the cardiovascular system, lung airways and cerebral arteriovenous malformations. Small pressure differentials acting across a multiple branching are considered first, followed by substantial pressure differentials in a side branching, multiple branching or basic three-dimensional branching. All cases include a comparison of results between the modelling and the direct simulations. Wall shear, pressure variation, influence lengths, and separation or its suppression are examined, showing in particular sudden spatial adjustment of the pressure between mother and daughter tubes, nonunique flow patterns and a linear increase of flow rate with increasing number of daughters, dependent on the specific conditions. The agreement between modelling and direct simulations is generally close at moderate flow rates, suggesting their combined use in the biomedical applications.

Air cushioning in droplet impacts with liquid layers and other droplets

Air cushioning of a high-speed liquid droplet impact with a finite-depth liquid layer sitting upon a rigid impermeable base is investigated. The evolution of the droplet and liquid-layer free-surfaces is studied alongside the pressure in the gas film dividing the two. The model predicts gas bubbles are trapped between the liquid free-surfaces as the droplet approaches impact. The key balance in the model occurs when the depth of the liquid layer equals the horizontal extent of interactions between the droplet and the gas film. For liquid layer depths significantly less than this a shallow liquid limit is investigated, which ultimately tends towards the air-cushioning behavior seen in droplet impact with a solid surface. Conversely, for liquid layer depths much deeper than this, the rigid base does not affect the air-cushioning of the droplet. The influence of compressibility is discussed and the relevant parameter regime for an incompressible model is identified. The size of the trapped gas bubble as a fu...