Global modeling of the nitrate radical (NO3) for present and pre-industrial scenarios

AbstractIncreasing the complexity of the chemistry scheme in the global chemistry transport model STOCHEM to STOCHEM-CRI (Utembe et al., 2010) leads to an increase in NOx as well as ozone resulting in higher NO3 production over forested regions and regions impacted by anthropogenic emission. Peak NO3 is located over the continents near NOx emission sources. NO3 is formed in the main by the reaction of NO2 with O3, and the significant losses of NO3 are due to the photolysis and the reactions with NO and VOCs. Isoprene is an important biogenic VOC, and the possibility of HOx recycling via isoprene chemistry and other mechanisms such as the reaction of RO2 with HO2 has been investigated previously (Archibald et al., 2010a). The importance of including HOx recycling processes on the global budget of NO3 for present and pre-industrial scenarios has been studied using STOCHEM-CRI, and the results are compared. The large increase (up to 60% for present and up to 80% for pre-industrial) in NO3 is driven by the reduced lifetime of emitted VOCs because of the increase in the HOx concentration. The maximum concentration changes (up to 15ppt) for NO3 from pre-industrial to present day are found at the surface between 30oN and 60oN because of the increase in NOx concentrations in the present day integrations

Determination of the photolysis rate coefficient of Monochlorodimethyl Sulfide (MClDMS) in the atmosphere and its implications for the enhancement of SO2 production from the DMS + Cl2 reaction

In this work, the photolysis rate coefficient of CH3SCH2Cl (MClDMS) in the lower atmosphere has been determined and has been used in a marine boundary layer (MBL) box model to determine the enhancement of SO2 production arising from the reaction DMS + Cl2. Absorption cross sections measured in the 28000-34000 cm(-1) region have been used to determine photolysis rate coefficients of MClDMS in the troposphere at 10 solar zenith angles (SZAs). These have been used to determine the lifetimes of MClDMS in the troposphere. At 0° SZA, a photolysis lifetime of 3-4 h has been obtained. The results show that the photolysis lifetime of MClDMS is significantly smaller than the lifetimes with respect to reaction with OH (?4.6 days) and with Cl atoms (?1.2 days). It has also been shown, using experimentally derived dissociation energies with supporting quantum-chemical calculations, that the dominant photodissocation route of MClDMS is dissociation of the C-S bond to give CH3S and CH2Cl. MBL box modeling calculations show that buildup of MClDMS at night from the Cl2 + DMS reaction leads to enhanced SO2 production during the day. The extra SO2 arises from photolysis of MClDMS to give CH3S and CH2Cl, followed by subsequent oxidation of CH3S

Impacts of mechanistic changes on HOx formation and recycling in the oxidation of isoprene

Recently reported model-measurement discrepancies for the concentrations of the HOx radical species (OH and HO2) in locations characterized by high emission rates of isoprene have indicated possible deficiencies in the representation of OH recycling and formation in isoprene mechanisms currently employed in numerical models; particularly at low levels of NOx. Using version 3.1 of the Master Chemical Mechanism (MCM v3.1) as a base mechanism, the sensitivity of the system to a number of detailed mechanistic changes is examined for a wide range of NOx levels, using a simple box model. The studies consider sensitivity tests in relation to three general areas for which experimental and/or theoretical evidence has been reported in the peer-reviewed literature, as follows: (1) implementation of propagating channels for the reactions of HO2 with acyl and β-oxo peroxy radicals with HO2, with support from a number of studies; (2) implementation of the OH-catalysed conversion of isoprene-derived hydroperoxides to isomeric epoxydiols, as characterised by Paulot et al.~(2009a); and (3) implementation of a mechanism involving respective 1,5 and 1,6 H atom shift isomerisation reactions of the β-hydroxyalkenyl and cis-δ-hydroxyalkenyl peroxy radical isomers, formed from the sequential addition of OH and O2 to isoprene, based on the theoretical study of Peeters et al. (2009). All the considered mechanistic changes lead to simulated increases in the concentrations of OH, with (1) and (2) resulting in respective increases of up to about 7% and 16%, depending on the level of NOx. (3) is found to have potentially much greater impacts, with enhancements in OH concentrations of up to a factor of about 3.3, depending on the level of NOx, provided the (crucial) rapid photolysis of the hydroperoxy-methyl-butenal products of the cis-δ-hydroxyalkenyl peroxy radical isomerisation reactions is represented, as also postulated by Peeters et al.~(2009). Additional tests suggest that the mechanism with the reported parameters cannot be fully reconciled with atmospheric observations and existing laboratory data without some degree of parameter refinement and optimisation which would probably include a reduction in the peroxy radical isomerisation rates and a consequent reduction in the OH enhancement propensity. However, an order of magntitude reduction in the isomerisation rates is still found to yield notable enhancements in OH concentrations of up to a factor of about 2, with the maximum impact at the low end of the considered NOx range. A parameterized representation of the mechanistic changes is optimized and implemented into a reduced variant of the Common Representative Intermediates mechanism (CRI v2-R5), for use in the STOCHEM global chemistry-transport model. The impacts of the modified chemistry in the global model are shown to be consistent with those observed in the box model sensitivity studies, and the results are illustrated and discussed with a particular focus on the tropical forested regions of the Amazon and Borneo where unexpectedly elevated concentrations of OH have recently been reported

A study of the atmospherically relevant reaction between molecular chlorine and dimethylsulfide (DMS): establishing the reaction intermediate and measurement of absolute photoionization cross-sections

In a study using UV photoelectron spectroscopy (PES) of the atmospherically relevant reaction CH3SCH3 + Cl-2, CH3SCH2Cl +HCl bands associated with a reaction intermediate have been observed. These have been assigned to ionization of the covalently bound molecule (CH3)(2)SCl2 on the basis of the intensity of the observed bands as a function of reaction time, molecular orbital calculations of vertical ionization energies and evidence from infrared spectroscopy. A method has also been developed, with the flow-tube/PE spectrometer combination used, to measure photoionization cross-sections of the reagents and products at the photon energy utilized and this has allowed the photoionization cross-section of the intermediate to be estimated. This work augments an earlier study in which the rate constant of the reaction between CH3SCH3 (DMS) and Cl-2 has been measured at room temperature