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Reactive Nitrogen in the Troposphere

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Reactive nitrogen in the troposphere Christopher Paul Reed PhD University of York Chemistry May 2017 Abstract The remote marine boundary layer at the tropics represents a baseline condition for the Earth's atmosphere. Air in these regions has typically not encountered emissions for days to weeks and thus has been subject only to processing; that is degradation and reaction of species in the air. Understanding the most basic chemistry occurring in the remote marine boundary layer is fundamental to our overall understanding of the atmosphere as a whole. The source and manifestation of nitrogen oxides, NOx, in the remote marine boundary layer at the Cape Verde observatory and elsewhere has been intriguing. NOx observations could often not be reconciled easily with known chemistry. This, when coupled to the difficulty in measuring the very low concentrations present has led to various speculative theories which are thus-far unsubstantiated. By carefully characterising the NO2 photolysis{NO chemiluminescence technique for arte- facts, interferences, and uncertainties the applicability and limitations of the technique were established. It was found that peroxyacyl nitrates cause an interference of ∼ 5 % which can lead to significant errors in NO2 determination, especially in cold or high al- titude environments but are insignificant in the remote marine boundary layer at Cape Verde. Using two years of data collected at Cape Verde and a 0-D model of tropospheric chem- istry the budget of NOx was estimated. Evidence that nitric acid deposited on aerosol, previously thought to be the terminating step in the cycle of NOx, is able to be released back to the atmosphere as NO2 and HONO has been shown. This `renoxification’ pro- cess likely represents the dominant source of NOx in the tropical marine boundary layer, counter to previous theory of long range transport of reactive nitrogen species. The implication of these findings changes our fundamental understanding of NOx sources and sinks in the remote troposphere. Moreover, by understanding the limitations of NO2 measurement techniques allows for easier interpretation of sometimes puzzling data with- out the need for unprovable mechanisms. Additionally, a new technique for quantifying atmospheric HONO was developed by exploiting the limitations of photolytic NO2 mea- surement and photolysing differentially at two different wavelengths. 3 All measurements are wrong, some measurements are useful! Contents Abstract3 Contents5 List of figures9 List of tables 11 Declaration 13 1 Introduction 15 1.1 The atmosphere................................. 17 1.2 Tropospheric chemistry............................. 18 1.2.1 NOx chemistry.............................. 19 1.2.1.1 Renoxification......................... 20 1.2.1.2 NOy chemistry........................ 21 1.2.2 O3 production.............................. 22 1.2.3 OH production.............................. 22 1.2.4 The nitrate radical and night-time oxidation chemistry....... 23 1.2.5 Halogen chemistry............................ 24 1.3 Summary..................................... 25 1.4 Techniques.................................... 25 1.4.1 Nitric Oxide, NO............................ 26 1.4.2 Nitrogen Dioxide, NO2 ......................... 27 1.4.3 Nitrous Acid, HONO.......................... 28 1.5 Aim........................................ 30 1.6 Thesis outline.................................. 30 2 Interferences in photolytic NO2 measurements: explanation for an ap- parent missing oxidant? 31 2.1 Abstract...................................... 32 2.2 Introduction.................................... 33 2.3 Experimental details............................... 37 5 Contents 2.3.1 Instrumentation............................. 37 2.3.1.1 Laboratory NOx analyser................... 38 2.3.1.2 Aircraft NOx analyser..................... 39 2.3.2 NO2 converters.............................. 39 2.3.2.1 Standard BLC......................... 40 2.3.2.2 High-powered BLC...................... 41 2.3.3 TD-LIF analyser............................. 41 2.3.4 PAN preparation............................. 42 2.3.5 Zero air.................................. 44 2.3.6 Experimental procedure......................... 45 2.3.7 Residence time.............................. 47 2.4 Impact of PAN on NO2 measurements..................... 48 2.4.1 Standard BLC and laboratory NOx analyser in constant mode... 48 2.4.2 Standard BLC and laboratory NOx analyser in switching mode... 49 2.4.3 Inlet residence time effects........................ 50 2.4.4 High-powered and actively cooled photolytic NO2 converter..... 51 2.5 Results....................................... 52 2.5.1 Possible photolytic interferences of BLCs............... 52 2.5.2 Possible thermal interferences in BLCs................. 56 2.6 Atmospheric implications............................ 59 2.7 Conclusions.................................... 63 3 HONO measurement by differential photolysis 65 3.1 Abstract...................................... 66 3.2 Introduction.................................... 67 3.3 Experimental details............................... 68 3.3.1 Differential photolysis instrument.................... 68 3.3.2 NO2{HONO photolytic converter.................... 69 3.3.3 Characterisation............................. 69 3.3.4 HONO and NO2 conversion efficiencies................. 71 3.3.5 Measurement precision and limit of detection............. 75 3.3.6 Measurement artefacts, interferences, and uncertainties....... 76 3.4 Results and discussion.............................. 78 3.4.1 Chamber measurements......................... 79 6 Contents 3.4.2 Field measurements........................... 82 3.5 Conclusions.................................... 86 4 Evidence for renoxification in the tropical marine boundary layer 87 4.1 Abstract...................................... 88 4.2 Introduction.................................... 89 4.3 Methodology................................... 90 4.3.1 NO and NO2 observations........................ 90 4.3.2 HONO observations........................... 92 4.3.3 Box model................................ 92 4.4 Results and discussion.............................. 94 4.4.1 Diurnal cycles in NOx and HONO................... 94 4.4.2 Box modelling of NOx sources..................... 96 4.4.3 NOx sinks................................. 100 4.4.4 HOI/HOBr-NOx chemistry....................... 103 4.5 Conclusions.................................... 107 5 Concluding remarks 109 Appendix 115 A Tables of chemical reaction rates and reactive uptake coefficients used in the DSMACC model 115 Abbreviations 125 References 129 7 List of Figures 1.1 The vertical structure of the atmosphere.................... 18 1.2 A simplified representation of tropospheric NOx chemistry.......... 25 2.1 The average raw counts recorded by a LIF instrument when sampling var- ious mixing ratios of PAN............................. 47 2.2 The measured NO2 artefact signal for three BLC units operating in constant mode........................................ 49 2.3 The measured NO2 artefact signal for three BLC units operating in switch- ing mode...................................... 50 2.4 The measured NO2 artefact signal for the cooled and un-cooled high-powered BLC......................................... 51 2.5 Spectral output of various UV-LED BLC lamps................ 53 2.6 Absorption cross sections of various reactive atmospheric nitrogen compounds. 55 2.7 Model of thermal decomposition of PAN to NO2................ 58 2.8 Thermal decomposition profiles of various thermally labile atmospheric ni- trate compounds.................................. 60 2.9 GEOS-Chem model output showing the monthly maximum percentage over- reporting of NO2 determined by BLC...................... 61 2.10 Leighton ratio calculated for each surface model grid-box for each daylight hour for March by the GEOS-Chem model................... 62 3.1 The measured spectral output of two UV-LED and the HONO absorption spectrum...................................... 70 3.2 Difference in HONO conversion between 385 and 395 nm UV-LEDs over a range of dilutions................................. 75 3.3 FT-IR spectra of dominant HONO absorbance lines.............. 80 3.4 HONO determined by FT-IR versus HONO measured by the differential photolysis instrument............................... 81 3.5 Simulated conversion and difference in conversion for photolytic converters with different j................................... 82 9 List of Figures 3.6 HONO time series during July 2015 at the Weybourne Atmospheric Obser- vatory........................................ 84 3.7 Correlation between HONO measured by LOPAP and pHONO instrument. 86 4.1 The observed seasonal diurnal cycles in NO, NO2, NOx, and O3 at the CVO GAW station during 2014 and 2015....................... 95 4.2 The observed average HONO diurnal measured at CVO during 24 November{ 3 December 2015.................................. 96 4.3 Measured and modelled NOx and HONO diurnal behaviour at the CVO GAW station ................................... 97 4.4 The difference between measured and modelled NOx at CVO during sum- mer months.................................... 99 4.5 The modelled diurnal profile of NOx at CVO during summer months when photolysis of nitrate and a tropospheric PAN source are considered..... 100 4.6 Total production and loss analysis for NOx of the combined model of par- ticulate nitrate photolysis and PAN decomposition over 24 h......... 101 4.7 Sensitivity analysis of the effect
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