Radical chemistry and ozone production at a UK coastal receptor site

OH, HO2, total and partially speciated RO2, and OH reactivity (kOH′) were measured during the July 2015 ICOZA (Integrated Chemistry of OZone in the Atmosphere) project that took place at a coastal site in north Norfolk, UK. Maximum measured daily OH, HO2 and total RO2 radical concentrations were in the range 2.6–17 × 106, 0.75–4.2 × 108 and 2.3–8.0 × 108 molec. cm−3, respectively. kOH′ ranged from 1.7 to 17.6 s−1, with a median value of 4.7 s−1. ICOZA data were split by wind direction to assess differences in the radical chemistry between air that had passed over the North Sea (NW–SE sectors) and that over major urban conurbations such as London (SW sector). A box model using the Master Chemical Mechanism (MCMv3.3.1) was in reasonable agreement with the OH measurements, but it overpredicted HO2 observations in NW–SE air in the afternoon by a factor of ∼ 2–3, although slightly better agreement was found for HO2 in SW air (factor of ∼ 1.4–2.0 underprediction). The box model severely underpredicted total RO2 observations in both NW–SE and SW air by factors of ∼ 8–9 on average. Measured radical and kOH′ levels and measurement–model ratios displayed strong dependences on NO mixing ratios, with the results suggesting that peroxy radical chemistry is not well understood under high-NOx conditions. The simultaneous measurement of OH, HO2, total RO2 and kOH′ was used to derive experimental (i.e. observationally determined) budgets for all radical species as well as total ROx (i.e. OH + HO2 + RO2). In NW–SE air, the ROx budget could be closed during the daytime within experimental uncertainty, but the rate of OH destruction exceeded the rate of OH production, and the rate of HO2 production greatly exceeded the rate of HO2 destruction, while the opposite was true for RO2. In SW air, the ROx budget analysis indicated missing daytime ROx sources, but the OH budget was balanced, and the same imbalances were found with the HO2 and RO2 budgets as in NW–SE air. For HO2 and RO2, the budget imbalances were most severe at high-NO mixing ratios, and the best agreement between HO2 and RO2 rates of production and destruction rates was found when the RO2 + NO rate coefficient was reduced by a factor of 5. A photostationary-steady-state (PSS) calculation underpredicted daytime OH in NW–SE air by ∼ 35 %, whereas agreement (∼ 15 %) was found within instrumental uncertainty (∼ 26 % at 2σ) in SW air. The rate of in situ ozone production (P(Ox)) was calculated from observations of ROx, NO and NO2 and compared to that calculated from MCM-modelled radical concentrations. The MCM-calculated P(Ox) significantly underpredicted the measurement-calculated P(Ox) in the morning, and the degree of underprediction was found to scale with NO

Measurements of peroxides in the atmosphere and their relevance to the understanding of global tropospheric chemistry

Peroxides are produced as termination products in atmospheric chain reactions involving peroxy radicals, both organic and inorganic. They are the principal sink for radicals produced in the troposphere from the photolysis of ozone in the presence of water vapour and as such are excellent indicators of the extent of free radical chemistry taking place at any given location. Their measurement is relatively simple and data on the concentration of peroxides in the atmosphere with respect to time and space can be collected easily and extensively. New data on peroxide measurements collected at different parts of the atmosphere, principally by the Meteorological Office C-130 Hercules aircraft are presented. They indicate that the extent of hydroxyl radical chemistry during the summer is controlled mostly by the water vapour content of the atmosphere. Both negative and positive correlations are observed between ozone and peroxide concentrations in vertical profiles over the North Atlantic Ocean and the equatorial Pacific. The negative correlations demonstrate that the ozone concentration throughout the troposphere is determined mostly by in situ photochemistry. This is borne out by the close correlation between calculated and measured concentrations of peroxides in vertical profiles. Positive correlations over the North Atlantic allow us to make an estimate of the amount of ozone, present there in the summer, which is formed from tropospheric as opposed to stratospheric chemistry

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 products 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

Seasonal variation of peroxyacetylnitrate (PAN) in coastal Antarctica measured with a new instrument for the detection of sub-part per trillion mixing ratios of PAN

An automated gas chromatograph with sample pre-concentration for the measurement of peroxyacetylnitrate (PAN) was constructed with a minimum detection limit below 1 pptv. This instrument was deployed at the British Antarctic Survey's Halley Research Station, Antarctica (75.6 degrees S, 26.6 degrees W) as part of the CHABLIS (Chemistry of the Antarctic Boundary Layer and the Interface with Snow) campaign. Hourly measurements were carried out between July 2004 and February 2005 with observed maximum and minimum mixing ratios of 52.3 and < 0.6 pptv, respectively with a mean PAN mixing ratio for the measurement period of 9.2 pptv (standard deviation: 6.2 pptv). The changes in PAN mixing ratios typically occurred over periods of several days to a week and showed a strong similarity to the variation in alkenes. The mixing ratio of PAN at Halley has a possible seasonal cycle with a winter maximum and summer minimum, though the cycle is incomplete and the data are very variable. Calculations indicate that gross local PAN production is approximately 1 pptv d(-1) in spring and 0.6 pptv d(-1) in summer. Net loss of PAN transported to Halley in the summer is a small gas-phase source of NOx and net production of PAN in the spring is a very small NOx sink

Evolution of the aerosol, cloud and boundary-layer dynamic and thermodynamic characteristics during the 2nd Lagrangian experiment of ACE-2

We present observations from the 2nd Aerosol Characterisation Experiment where over a 29‐h period between 16–18 July 1997 a tagged column of air was followed by a fully instrumented aircraft. The Lagrangian framework this offered made it possible to measure the evolution of the aerosol size distribution, the cloud structure and microphysics, and the dynamic and thermodynamic structure of the marine boundary layer within a polluted airmass advecting off northwest Europe over the sub‐tropical North Atlantic Ocean. The salient observations are presented and analysed. Processes responsible for the evolution are suggested, but quantification of their respective rates must be taken up by future modelling studies. Stratocumulus capped the boundary layer throughout the period that produced negligible washout of aerosol. This implies that the conversion of a continental to a maritime airmass within the cloud‐capped sub‐tropical marine boundary layer is not controlled by the drizzle process but by entrainment from the free troposphere. We find evidence of processing of aerosol particles by stratocumulus cloud, in particular by aqueous‐phase reactions. The processing of the aerosol, realised by modification of the aerosol size distribution in the particle diameter range 0.1–0.5 μm, was complicated by rapid changes in boundary layer height and structure, and also by entrainment of both polluted and relatively clean aerosol from the free troposphere. The cloud microphysics was affected by these changes in the boundary layer aerosol through changes in the cloud condensation nuclei activation spectra. The cloud microphysics was also strongly affected by changes in the dynamics of the boundary layer which included variations (e.g., diurnal) in cloud thickness and an increase in vertical wind speed. Thermodynamic changes within the boundary layer included decoupling due to an increasing sea‐surface temperature and a change in the subsidence rate in the free troposphere superimposed on diurnal decoupling. Hypotheses have been devised so that future modellers can focus their efforts to either validate or invalidate potentially important processes

Observation on Great Dun Fell of the pathways by which oxides of nitrgoen are converted to nitrate.

Two field experiments to investigate the formation of nitrate as an airstream passes through a hill cap cloud have been performed at the UMIST field station on Great Dun Fell. Techniques chosen for the measurement of various nitrogen species are described. The results of the second field experiment are discussed and compared with those of the first. Evidence is found in support of the hypothesis that under the range of conditions studied, the dominant pathway for nitrate production is the solution of N2O5 formed from the reaction of NOx with O3 upwind. The effectiveness of this pathway by night and by day is observed to be a function of the NOx mixing ratio. A surface reaction rate constant of around 300 cm3 cm−2 s−1 for the hydration of N2O5 is inferred from the observations. These results are shown to be consistent with recent laboratory measurements of the rates of reaction of nitrogen species. It is suggested that pathways other than via N2O5 may be significant sources of nitrate under certain conditions that merit further investigation