Brian J. Bandy

Modeling OH, HO2, and RO2 radicals in the marine boundary layer: 1. Model construction and comparison with field measurements

An observationally constrained box model has been constructed in order to investigate the chemistry of the marine boundary layer at Mace Head, a remote location on the west coast of Ireland. The primary aim of the model is to reproduce concentrations of the hydroxyl (OH) and hydroperoxy (HO2) radicals measured by an in situ fluorescence assay by gas expansion (PAGE) instrument, and the sum of peroxy radicals ∑([HO2]+[RO2]) determined by a peroxy radical chemical amplification (PERCA) instrument. The model has been constructed based on observed concentrations of a suite of non-methane hydrocarbons, measured in situ by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism (MCM). This paper describes in detail the construction of the model, as well as the underlying approach. Comparisons of modeled and measured concentrations of radical species, from a recent field campaign held at the Mace Head Atmospheric Observatory during July and August 1996 (EASE 96), are also presented. For the limited OH data available from this campaign, the model tends to overestimate the observations by about 40%, although this discrepancy is within the uncertainties of the model (±31%, 2σ) and the PAGE measurements (±75% on average, 2σ). For HO2 the model reproduces the concentrations well on one day but less well on another. Low HOx concentrations compared to model results have been observed previously, with greater than expected heterogeneous losses invoked to explain the differences. Comparisons between measurements of peroxy radicals made by chemical amplification and model predictions show good agreement over a wide range of conditions.

A study of peroxy radicals and ozone photochemistry at coastal sites in the northern and southern hemispheres

Peroxy radicals and other important species relevant to ozone photochemistry, including ozone, its photolysis rate coefficient jO(1D), NOx (NO + NO2), and peroxides, were measured at the coastal sites of Cape Grim, Tasmania, in January/February 1995 during the Southern Ocean Atmospheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ireland, in May 1995 during the Atlantic Atmospheric Photochemistry Experiment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2 + RO2) concentrations and jO(1D) switched from a square root dependence in clean oceanic or “baseline” air to a first-order dependence in more polluted air. Simple algorithms derived from a photochemical reaction scheme indicate that this switch-over occurs when atmospheric NO levels are sufficient for peroxy radical reaction with NO to compete with radical recombination reactions. At this crucial point, net tropospheric ozone production is expected to occur and was observed in the ozone diurnal cycles when the peroxy radical/jO(1D) dependencies became first order. The peroxy radical/jO(1D) relationships imply that ozone production exceeds destruction at NO levels of 55±30 parts per trillion by volume (pptv) at Mace Head during late spring and 23±20 pptv at Cape Grim during summer, suggesting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere.

Photochemical trajectory modeling studies of the North Atlantic region during August 1993

A Lagrangian photochemical trajectory model has been used to assess the factors affecting O-3 production during transport of polluted air masses across the North Atlantic Ocean. Sensitivity studies have been performed along idealized trajectories, and it is found that the potential impact of North American emission sources is maximized by transport of air at high altitudes, in drier conditions and in conditions where mixing of the air with background air masses is relatively limited. Measurements taken from the NCAR King Air aircraft as part of the North Atlantic Regional Experiment (NARE) August 1993 intensive have been used to initialize forward trajectories, calculated using European Centre for Medium-Range Weather Forecasting analyzed wind fields, from eastern North America to assess O-3 production over the Atlantic during this period. The effects of dilution of a polluted air parcel with air from the upper troposphere have also been studied, and the contribution of photochemical O-3 production to the air mass composition is found to be smaller than that of dilution, particularly for long trajectories and for conditions where dilution is relatively rapid or involves air from the stratosphere. Measurements taken from the Meteorological Research Flight Hercules aircraft over the eastern Atlantic as part of the Oxidizing Capacity of the Tropospheric Atmosphere campaign have been examined in the light of these studies. A backward trajectory analysis has been performed from one of the vertical profiles taken off the coast of Portugal on August 31, 1993, to assess the origin of the different air masses intercepted. While the lower levels are characteristic of air from the European boundary layer advected over the ocean, the upper levels show strong evidence for anthropogenic influence from North American sources, with elevated levels of O-3, NOy, CO, and aerosol. Although it cannot be concluded that this air mass definitely originated from over North America, the measured concentrations are shown to be consistent with those for an air mass from this source region experiencing some mixing with air masses in the upper troposphere.

The Annual Cycle of Peroxides and Ozone in Marine Air at Cape Grim, Tasmania

The concentration of gas-phase peroxides has been measured almost continuously at the Cape Grim baseline station (41° S) over a period of 393 days (7702 h of on-line measurements) between February 1991 and March 1992. In unpolluted marine air a distinct seasonal cycle in concentration was evident, from a monthly mean value of>1.4 ppbv in summer (December) to <0.2 ppbv in winter (July). In the summer months a distinct diurnal cycle in peroxides was also observed in clean marine air, with a daytime build-up in concentration and decay overnight. Both the seasonal and diurnal cycles of peroxides concentration were anticorrelated with ozone concentration, and were largely explicable using a simple photochemical box model of the marine boundary layer in which the central processes were daytime photolytic destruction of ozone, transfer of reactive oxygen into the peroxides under the low-NOx ambient conditions that favour self-reaction between peroxy radicals, and continuous heterogeneous removal of peroxides at the ocean surface. Additional factors affecting peroxides concentrations at intermediate timescales (days to a week) were a dependence on air mass origin, with air masses arriving at Cape Grim from higher latitudes having lower peroxides concentrations, a dependence on local wind speed, with higher peroxides concentrations at lower wind speeds, and a systematic decrease in peroxides concentration during periods of rainfall. Possible physical mechanisms for these synoptic scale dependencies are discussed.

An overview of the Lagrangian experiments undertaken during the North Atlantic regional Aerosol Characterisation Experiment (ACE‐2)

One of the primary aims of the North Atlantic regional Aerosol Characterisation Experiment (ACE-2) was to quantify the physical and chemical processes affecting the evolution of the major aerosol types over the North Atlantic. The best, practical way of

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.

Comparison of calculated and measured peroxide data collected in marine air to investigate prominent features of the annual cycle of ozone in the troposphere

Large amounts of data on the concentration of peroxides have been collected in vertical profiles over the North Atlantic Ocean by a Hercules aircraft. The measured peroxide concentrations have been compared with concentrations calculated by a simple algorithm derived assuming that the standing peroxide concentration is in equilibrium with its production and loss processes. In clean air where the peroxide and ozone concentrations are anticorrelated throughout the profile measured and calculated peroxide concentrations coincide, whereas in layers of polluted air within the profile, as determined from positive ozone peroxide correlations, calculated peroxide concentrations are greatly in excess of measured values. Using the degree of correlation between measured and calculated peroxide concentrations as a diagnostic, it is possible to show that many aspects of the seasonal cycle of ozone are caused by in situ tropospheric chemistry. Thus the summer minimum in the seasonal cycle of ozone, observed at clean marine ground-based sites such as Mace Head, is due to photochemical destruction, and elevated levels of ozone are associated with the transport of polluted air, on occasion over thousands of kilometers. Of particular interest if our analysis is correct, the broad maximum of ozone occurring between March and May at ground-based sites has a large contribution from ozone formed by tropospheric as well as stratospheric chemistry.

A seasonal comparison of the ozone photochemistry in clean and polluted air masses at Mace Head, Ireland

Measurements of the sum of peroxy radicals [HO2 + RO2],NOx (NO + NO2) and NOy (the sum of oxidisednitrogen species) made at Mace Head, on the Atlantic coast of Ireland in summer 1996 and spring 1997 are presented. Together with a suite of ancillary measurements, including the photolysis frequencies of O3 → O(1D)(j(O1D)) and NO2 (j(NO2)), the measured peroxy radicals are used to calculate meandailyozone tendency (defined as the difference of the in-situphotochemical ozone production and loss rates); these values are compared with values derived from the photochemical stationary state (PSS) expression. Although the correlation between the two sets of values is good, the PSS values are found to be significantly larger than those derived from the peroxy radical measurements, on average, in line with previous published work. Possible sources of error in these calculations are discussed in detail. The data are further divided up into five wind sectors, according to the instantaneous wind direction measured at the research station. Calculation of mean ozone tendencies by wind sector shows that ozone productivity was higher during spring (April–May) 1997 than during summer (July–August) 1996across all airmasses, suggesting that tropospheric photochemistry plays an important role in the widely-reported spring ozone maximum in the Northern Hemisphere. Ozone tendencies were close to zero for the relatively unpolluted south-west, west and north-west wind sectors in the summer campaign, whereas ozone productivity was greatest in the polluted south-east sector for both campaigns. Daytime weighted average ozone tendencies were +(0.3± 0.1) ppbv h−1 for summer 1996 and +(1.0± 0.5) ppbvh−1 for spring 1997. These figures reflect the higher mixing ratios of ozone precursors in spring overall, as well as the higher proportion of polluted air masses from the south-east arriving at the site during the spring campaign. The ozone compensation point, where photochemical ozone destruction and production processes are in balance, is calculated to be ca. 14 pptv NO for both campaigns.

Comparison of Measured Ozone Production Efficiencies in the Marine Boundary Layer at Two European Coastal Sites under Different Pollution Regimes

Ozone production efficiencies (EN), which can be defined as the netnumber of ozone molecules produced per molecule of NOxoxidised, have been calculated from measurements taken during three intensive field campaigns (one in the spring, EASE 96, and two in the summer, EASE 97 and TIGER 95), at two European coastal sites (Mace Head, Ireland (EASE) and Weybourne, Norfolk (TIGER)) impacted by polluted air masses originating from both the U.K. and continental Europe, as well as relatively clean oceanic air masses from the Arctic and Atlantic. From a detailed wind sector analysis of the EASE 96 and 97 data it is clear that two general types of pollution regime were encountered at Mace Head. The calculated ozone production efficiency in clean oceanic air masses was approximately 65, which contrasted to more polluted air, from the U.K. and the continental European plume, where the efficiency decreased to between 4 and 6. The latter values of ENagree well with literature measurements conducted downwind of various urban centres in the U.S. and Europe, which are summarised in a wide-ranging review table. The EN value calculated for clean oceanic air is effectivelyan upper limit, owing to the relatively rapid deposition of HNO3 tothe ocean. Consideration of the variation of EN with NOx forthe three campaigns suggests that ozone production efficiency is relatively insensitive to both geographical location and season. The measuredEN values are also compared with values derived from steady-state expressions. An observed anti-correlation between EN and measured ozone tendencyis briefly discussed.

Measurements of the entrainment of hydrogen peroxide into cloud systems

Abstract Detailed measurements from an aircraft of the vertical structure of hydrogen peroxide with detailed ground-based measurements of the chemistry of an aistream entering a hill cap cloud are presented together with measurements of the microphysics and chemistry of the cloud. Two case studies have been selected in which the cloud is being affected by the entrainment of air from outside its boundaries. The major findings are: (1) when the boundary layer is polluted the concentration of gas-phase hydrogen peroxide was observed to increase markedly with altitude in the lower troposphere; (2) the maximum concentrations of hydrogen peroxide were found close to the surfaces of clouds and were up to 10 times greater than concentrations in the boundary layer on the same day and (3) the entrainment of this hydrogen peroxide into the cap cloud had a profound effect on the chemistry of the cloud.