Κατανομή του DMS και των προϊόντων οξείδωσης του στην ατμόσφαιρα με έμφαση στο ρόλο των ετερογενών αντιδράσεων

Abstract

Oceanic dimethylsulfide (DMS) emission is proposed to play a key role in climate regulation, through its oxidation products, especially in the remote marine atmosphere. Once emitted into the atmosphere DMS can be oxidized by OH, NO3 and possibly other radical species as BrO radicals forming a variety of products such as SO2, MSA and DMSO. The proposed climatic impact of DMS is decisively determined from the formation of new hydroscopic sulfate particles that either scattersolar radiation or act as cloud condensation nuclei (CCN) influencing the earth’s albedo and climate. The complete mechanism of DMS oxidation is still under investigation although considerable effort has been made during the last decades to clarify its oxidation scheme via field measurements, kinetic experiments and model results. In the present study, kinetics and product distribution of the reaction of DMSO and MSI- with OH radicals and H2O2 in the aqueous phase had been investigated. DMSO was found to be very stable in pure water or in solutions containing H2O2. Onthe other hand, the reaction with OH radicals was found to be very rapid (k = 4.5 x 109 M-1 s -1). MSI- has been identified as the main intermediate product, while MS- and sulfate were the final products due to the very fast reaction of MSI- with OH radicals (k = 1.2 x 1010 M-1 s -1). MS- has been identified as the major end product (yield higher than 95%) of both DMSO and MSI- reactions by OH radicals in aqueous solutions. Reactions of both DMSO and MSI- with OH radicals seem to be very important for the atmosphere, since the calculated lifetime range from few minutes to almost 1 h. Especially for DMSO, the liquid phase reaction with OH radicals seems to be more important compared to the gaseous phase reaction with the same radical. In the second part of this study, an effort was made to confirm the results from the kinetic lab experiment in the filed. Campaigns had been conducted in three locations of the planet: at Finokalia, Greece (35.24 0 N, 25.60 0 E), Kuujjuarapik, Canada (55.22 0 N, 77.44 0 E) and Kerguelen, South Indian Ocean (49.50 0 S, 50.00 0 E). Field measurements involved concurrent data collection both on the oxidants involved in the DMS oxidation and including at least the major oxidation products of DMS in gaseous and particulate phase. i) Especially during the campaign conducted in Crete in July–August 2001 intensive measurements of gaseous DMS, DMSO, SO2, H2SO4 and MSA and particulate SO42- and MS- have been performed in conjunction with parallel measurements of OH and NO3 radicals. DMS variability was mainly governed by NO3 radicals and air mass origin. An MSA yield of the order of 1.9×10−3 was estimated presenting the lowest values reported so far in the literature. With the exception of the periods with low RH, gas to particle conversion of MSA could not account for the observed levels of MS−. The above results indicate that under humidity conditions typical of the marine boundary layer (RH higher than 40%), multiphase reactions involving DMSO accounts for the observed MS− levels. ii) At Kuujjuarapik DMS, DMSO and MS- variations were mainly governed from air mass transport of northwestern origin. BrO radicals were found to enhance DMSO production. DMS diurnal variation was governed mainly from BrO and OH radicals. iii) For the area of South Indian Ocean, the simultaneous measurements of DMS and its main oxidation products had been performed for the first time during a short term period for all three islands. For Amsterdam DMS highest levels correlated with northern winds. DMS/DMSO ratio was significantly higher when compared to theoretical estimations. This differentiation confirms the active role of heterogeneous chemistry involved in the DMSO removal. Especially at Crozet the lowest DMSOlevels had been observed while MS- levels were the highest. MS-/DMSO ratio for Crozet was 17.9 during northern winds while the analogous values for Amsterdam and Kerguelen where 1.9 and 1.8. In this case also, MS- levels are connected to DMSO removal from multiphase reactions.

Οι ωκεάνιες εκπομπές του DMS προτείνονται να κατέχουν ρόλο κλειδί, μέσω των προϊόντων οξείδωσης της ένωσης, ιδιαίτερα στην απομακρυσμένη θαλάσσια ατμόσφαιρα. Στο πρώτο μέρος της παρούσας μελέτης, πραγματοποιήθηκαν η μελέτη της κινητικής και της κατανομής των προϊόντων των αντιδράσεων των DMSO και MSI- με τις ρίζες OH και παρουσία H2O2 σε υδατικά διαλύματα. Όσον αφορά στο DMSO, η αντίδραση με τις ρίζες OH στην υδατική κατάσταση γίνεται πιο σημαντική σε σχέση με την αντίστοιχη αντίδραση στην αέρια κατάσταση. Στο δεύτερο μέρος της παρούσας μελέτης συνδυάστηκαν τα αποτελέσματα των εργαστηριακών κινητικών πειραμάτων με τις μετρήσεις πεδίου με έμφαση στις ετερογενείς αντιδράσεις απομάκρυνσης του DMSO στην ατμόσφαιρα.