Studies of the oxidation efficiency in a coastal environment
Adam, Max Gerrit
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The hydroxyl radical, OH, is the key oxidant in the atmosphere as it reacts with most trace gas species, often being the first and rate limiting step. The oxidation leads to products that are more easily removed from the atmosphere, e.g. via wet deposition. OH thus controls the removal and thereby the concentrations and lifetimes of many trace gases, including greenhouse gases, e.g. methane, CH4, and pollutants, e.g. carbon monoxide, CO. Moreover, in the marine atmosphere the OH initiated oxidation of dimethyl sulfide, CH3SCH3 (DMS), via the intermediates sulfur dioxide, SO2, or sulfur trioxide, SO3, leads to the production of sulfuric acid, H2SO4, a key component in new particle formation and growth of aerosols. Another product of DMS oxidation is methane sulfonic acid, CH3S(O2)OH (denoted MSA(g) to specify the gaseous compound), which on a lesser scale than H2SO4 contributes to aerosol growth as well. In the present work, measurements of OH, H2SO4, and MSA(g) made with a selected-ion chemical ionization mass spectrometer (SI-CIMS) at Mace Head have been analysed in conjunction with support measurements of sulfur dioxide, SO2, nitrogen oxides, NO and NO2, ozone, O3, photolysis frequencies of ozone, J(O1D), and nitrogen dioxide, J(NO2), aerosol size distribution and composition, and meteorological parameters. Measurements have been made between May 2010 -- September 2013 with the analysis focusing on the period of August 2010 -- August 2012. Clear diurnal and seasonal cycles related to photochemical production have been observed for OH and H2SO4 with maxima for both compounds during summer with 2.2 x 10^6 molecules cm-3 and 1.3 x 10^7 molecules cm-3, respectively. OH shows a robust relationship with J(O1D) with a correlation coefficient of R = 0.75 and a slope of 1.06 (+-0.05) x 10^11 cm-3 s in the marine sector (190 degrees - 300 degrees). OH levels showed no systematic relation with tidal cycles. For the land sector (< 190 degrees and > 300 degrees) with NO < 50 pptv, R = 0.81 and the slope 1.13 (+-0.03) x 10^11 cm-3 s. For levels of NO > 50 pptv, R = 0.88 and the slope significantly increases to 2.43 (+-0.12) x 10^11 cm-3 s. A sulfuric acid balance calculation, taking into account measurements of SO2 during the summer of 2011, reveals a significant discrepancy between measured and calculated H2SO4 with a mean ratio of 4.7 +- 2.4. This result points to the presence of one or more missing oxidants of SO2 yielding H2SO4 in addition to the SO2 + OH reaction which is corroborated by measurements of the background signal in OH mode having a similar diurnal cycle as OH, indicating the presence of another oxidant(s) X which oxidises SO2 to H2SO4. Possible candidates for X may be simple short-chain Criegee radicals, such as CH2OO, being formed in the atmospheric photochemical processing of biogenic iodine compounds emitted from marine macroalgae (seaweed). Halogen oxides (e.g. IO, ClO, BrO), however, appear to be unlikely candidates. Alternative to the SO2 + OH production pathway, the oxidation of DMS by OH (and/or X) may also yield SO3 instead of SO2 as intermediate product which rapidly reacts with water vapour to produce H2SO4. Also, additional DMS oxidation by X may change the product yield ratio of MSA(g) / H2SO4 as compared to OH oxidation alone. Analysis of air mass back-trajectories in conjunction with satellite remote sensing data indicates that DMS chemistry significantly contributes towards H2SO4 levels in marine air at Mace Head. MSA(g) also has a seasonal cycle with maximum values of ca. 1 x 10^8 molecules cm-3 in summer indicative of emissions from its biogenic precursor dimethyl sulfide. However, the diel cycle has been found to be very sensitive to changes in relative humidity, particularly below 80%. Frequent peaks of MSA(g) at nighttime have been observed during the summer period which are also anti-correlated with relative humidity. It could be shown for individual case studies that vertical entrainment of drier air from the free troposphere causes re-volatolization from aerosol MSA to the gas-phase.
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