D. J. Creasey

On the photochemical production of new particles in the coastal boundary layer

Concurrent measurements of ultra-fine (r<5 nm) particle (UFP) formation, OH and SO2 concentrations in the coastal environment are examined to further elucidate the processes leading to tidal-related homogeneous heteromolecular nucleation. During almost daily nucleation events, UFP concentration approached ≈300,000 cm−3 under conditions of solar radiation and low tide. Simultaneous measurements of OH illustrate that, as well as occurring during low tide, these events occur during conditions of peak OH concentration, suggesting that at least one of the nucleating species is photochemically produced. Derived H2SO4 production also exhibited remarkable coherence, although phase-lagged, with UFP formation, thus suggesting its involvement, although binary nucleation of H2SO4 and H2O can be ruled out as a plausible mechanism. Ternary nucleation involving NH3 seems most likely as a trigger mechanism, however, at least a fourth condensable species, X, is required for growth to detectable sizes. Since UFP are only observed during low tide events, it is thought that species X, or its parent, is emitted from the shore biota - without which, no nucleation is detected. Species X remains to be identified. Model simulations indicate that, in order to reproduce the observations, a nucleation rate of 107 cm−3 s−1, and a condensable vapour concentration of 5 × 107 cm−3, are required.

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.

Production of peroxy radicals at night via reactions of ozone and the nitrate radical in the marine boundary layer

In this paper, a substantial set of simultaneous measurements of the sum of peroxy radicals, [HO2 + RO2], NO3, hydrocarbons (HCs), and ozone, taken at Mace Head on the Atlantic coast of Ireland in spring 1997, is presented. Conditions encountered during the experiment ranged from semipolluted air masses advected from Britain and continental Europe to clean air masses off the North and mid-Atlantic, where mixing ratios of pollution tracers approached Northern Hemispheric background mixing ratios. Average mixing ratios of peroxy radicals varied from 2.5 to 5.5 parts per trillion by volume (pptv) at night depending on wind sector, and were observed to decay only very slowly from late afternoon to early morning (0.1–0.5 pptv h−1). Measurements of OH and HO2 on two nights using the Fluorescence Assay by Gas Expansion (FAGE) technique give an upper limit for [OH] of 2.5×105 molecules cm−3 and an average upper limit [HO2]/[HO2 + RO2] ratio of 0.27. A modeling study of the 1/e lifetimes of the peroxy radicals, assuming no radical production at night, yielded mean lifetimes of between ∼8–23 min for HO2 and 3–18 min for CH3O2. Given these lifetimes, it may be concluded that the peroxy-radical mixing ratios observed could not be maintained without substantial production at night. No significant correlation is observed between measured [HO2 + RO2] and [NO3] under any conditions. Calculation of the reaction rates for ozone and NO3 with hydrocarbons (HCs) shows that the ozone-initiated oxidation routes of HCs outweighed those of NO3 in the NE, SE and NW wind sectors. In the SW sector, however, the two mechanisms operated at similar rates on average, and oxidation by NO3 was the dominant route in the westerly sector. The oxidation of alkenes at night by ozone was greater by a factor of 4 than that by NO3 over the whole data set. The HC degradation rates from the three “westerly” sectors, where tracer mixing ratios were relatively low, may be representative of the nighttime oxidative capacity of the marine boundary layer throughout the background Northern Hemisphere. Further calculations using literature values for OH yields and inferred RO2 yields from the ozone-alkene reactions show that peroxy radicals derived from the ozone reactions were likely to make up the major part of the peroxy-radical signal at night (mean value 66%). However, the NO3 source was of similar magnitude in the middle of the night, when [NO3] was generally at its maximum. The estimated total rates of formation of peroxy radicals are much higher under semipolluted conditions (mean 8.0×104 molecules cm−3 s−1 in the SE wind sector) than under cleaner conditions (mean 2.4×104 molecules cm−3 s−1 in the westerly wind sector). A model study using a campaign-tailored box model (CTBM) for semipolluted conditions shows that the major primary sources of OH, HO2, and CH3O2 (the most abundant organic peroxy radical) were the Criegee biradical intermediates formed in the reactions of ozone with alkenes.

Implementation and initial deployment of a fieldinstrument for measurement of OH and HO2 in thetroposphere by laser-induced fluorescence

An instrument to detect atmospheric concentrations of the hydroxyl (OH) and hydroperoxyl (HO 2 ) radicals has been developed using the FAGE (fluorescence assay by gas expansion) technique. The instrument is housed in a mobile laboratory and monitors the OH radical via on-resonance laser-induced fluorescence (LIF) spectroscopy of the A 2 Σ + (v′=0)–X 2 Π i (v ″ =0) transition at ca. 308 nm. Ambient air is expanded through a 1 mm nozzle to low pressure where it is irradiated by the laser pulse at a repetition rate of 7 kHz, with the resultant fluorescence being detected by gated photon counting. HO 2 is monitored by chemical conversion to OH by the addition of NO, with subsequent detection using LIF. Following laboratory and field calibrations to characterise the instrument sensitivity, detection limits of 1.8×10 6 and 2.1×10 7 molecule cm -3 were determined for OH and HO 2 respectively, for a signal-to-noise ratio, S/N, of 1 with 150 s signal integration time. The instrument was deployed for the first time during the ACSOE field campaign at Mace Head, Eire, for which illustrative results are given.

Absorption cross-section measurements of water vapour and oxygen at 185 nm. Implications for the calibration of field instruments to measure OH, HO2 and RO2 radicals

Absorption cross sections for oxygen and water vapour were measured using a low-pressure mercury lamp optically filtered to isolate the emission close to 184.9 nm. The cross-sections were determined for conditions typically used in the calibration of OH, HO2 and RO2 field instruments that employ photolysis of water vapour as a radical source, and O2 absorption as the actinometer for lamp flux. For water vapour, an absorption cross-section of 7.22 (± 0.22) × 10−20 cm² molecule−1 was determined at 25°C, in excellent agreement with recent studies, but ∼30% higher than the recommended value. The absorption cross-section for O2 was found to vary with the O2 column and the choice and operating current of the lamp, reinforcing the requirement that practitioners of OH, HO2 and RO2 field measurements who calibrate by this method should measure effective O2 cross-sections at frequent intervals under their particular field conditions.

An analysis of rapid increases in condensation nuclei concentrations at a remote coastal site in western Ireland.

Massive “bursts” in condensation nuclei (CN) concentration were recorded at a remote site on the west Irish coast during campaigns in summer 1996 and spring/summer 1997. Number concentrations of 3–7 nm diameter CN were observed to rise daily from 102–103 up to ∼105 /cm3 for 1–3 hours. Data were collected as part of the Atmospheric Chemistry Studies in the Oceanic Environment program. In a previous paper the burst phenomenon was linked to the movement of the tide, and it was suggested that enhanced biogenic emissions occurred near low tide with concomitant rapid homogeneous gas phase CN formation. In this paper possible chemical mechanisms for the burst phenomenon are investigated. Two approaches are adopted. First, by assuming a 20:80 sulfate:water molar composition and calculating the number distribution using data from condensation particle counters, the total mass of CN formed during a burst is evaluated. This is compared with that mass of sulfate produced by OH-initiated dimethyl sulfide (DMS) oxidation. The procedure is termed “mass balance.” Second, a variety of chemical species are coplotted with tidal height. DMS oxidation is not believed to play a major role in CN formation at this site because (1) the mass balance calculations imply ambient DMS concentrations higher than those observed, and (2) gas phase HCl, HNO3, SO2, and NH3 did not exhibit any discernible correlation with tidal height. Further, none of the suite of observed nonmethane hydrocarbons or DMS showed a tidal relation. No mechanism has to date been convincingly identified for the burst phenomenon.

Hydroxyl radical and ozone measurements in England during the solar eclipse of 11 August 1999

The solar eclipse on 11 August 1999 provided a rare opportunity to observe the remarkably dynamic character of atmospheric photochemistry. OH formation is driven by sunlight, and the rapid changes in light intensity associated with a solar eclipse provide a unique, yet natural perturbation experiment to study the response of OH and the ensuing chemistry. Highly time-resolved measurements of OH and its rate of primary production were made at ground level during a 97% solar eclipse at Silwood Park, Ascot (51°25′N, 0°41′W) on 11 August 1999. The solar ultraviolet flux fell almost to nighttime levels, and the OH concentration decreased dramatically to below the detection limit of the instrument (2.1×105 molecule cm−3), before increasing again. The OH concentration is well correlated (r=0.88) to its rate of primary production from ozone photolysis. Shortly after maximum eclipse the concentration of ozone fell to 60% of its value at first contact. The study provides a striking demonstration of the dynamics of photochemical processes in the planetary boundary layer.

Eastern Atlantic Spring Experiment 1997 (EASE97) 1. Measurements of OH and HO2 concentrations at Mace Head, Ireland

[i] We report measurements of the hydroxyl (OH) and hydroperoxy (HO 2 ) radicals, taken over 20 days, in the remote marine boundary layer at Mace Head, Ireland, during April and May 1997. OH was monitored directly by laser-induced fluorescence (LIF) spectroscopy at 308 nm, and HO 2 was measured by chemical conversion to OH upon the addition of NO, with subsequent detection by LIF. The detection limit of the instrument at midday for OH was 6.0 x 10 5 molecule cm -3 (0.0024 parts per trillion by volume (pptv)) and for HO 2 was 3.0 x 10 6 molecule cm -3 (0.12 pptv), as defined for a signal integration period of 2.5 min and a signal-to-noise ratio of 1. Midday OH and HO 2 concentrations were between 2.0-6.0 x 10 6 molecule cm -3 (0.08-0.24 pptv) and 0.5-3.5 x 10 8 molecule cm -3 (2.5-14 pptv), respectively. OH concentrations correlated well with the rate of OH production from ozone photolysis for clean air from the Arctic containing low concentrations of both NO x and nonmethane hydrocarbons. A lower correlation was observed in more polluted air that originated from the United Kingdom and continental Europe. Measurements of OH and HO 2 were made throughout two nights, and although no evidence was seen for OH above the detection limit, up to 2 pptv of HO 2 was observed. The measured HO 2 /OH ratio was in good agreement with the predictions of a steady state expression for NO in the range 75-400 pptv.

Fast photomultiplier tube gating system for photon counting applications

A normally “on” linear-focused 14-stage end-window photomultiplier tube (PMT) (Electron Tubes Limited 9893Q/100B), designed for fast photon counting, has been gated through control of the voltage applied to the first dynode. The gating circuit reduces the gain of the PMT during a laser pulse, in order to discriminate against the detection of scattered light, and then increases the gain promptly to observe extremely low levels of laser-induced fluorescence (LIF). An extinction factor for the laser scattered photons of >105 was observed, and has enabled count rates for photons due to LIF as low as 1 Hz to be measured for a laser pulse-repetition frequency of 7 kHz. The rise of the PMT gain is monitored directly by observation of the fluorescence using time-resolved photon counting, and the PMT turn-on time is 30 ns. No significant distortion of the temporal profile of the fluorescence was observed during PMT turn-on. The system, which can also be operated in ungated mode, is rugged and reliable, and has bee...