Pacific Atmospheric Sulfur Experiment PASE dyna

Authors: Alan Bandy Ian C Faloona Byron W Blomquist Barry J Huebert Antony D Clarke Steven G Howell R L Mauldin Christopher A Cantrell James G Hudson Brian G Heikes John T Merrill Yuhang Wang Daniel W O’Sullivan Wolfgang Nadler Douglas D Davis

Publish Date: 2012/02/15

Volume: 68, Issue: 1, Pages: 5-25

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Abstract

The Pacific Atmospheric Sulfur Experiment PASE was a comprehensive airborne study of the chemistry and dynamics of the tropical trade wind regime TWR east of the island of Kiritibati Christmas Island 157º 20′ W 2º 52′ N Christmas Island is located due south of Hawaii Geographically it is in the northern hemisphere yet it is 6–12º south of the intertropical convergence zone ITCZ which places it in the southern hemisphere meteorologically Christmas Island trade winds in August and September are from east south east at 3–15 ms−1 Clouds if present are fair weather cumulus located in the middle layer of the TWR which is frequently labeled the buffer layer BuL PASE provided clear support for the idea that small particles 80 nm were subsiding into the tropical trade wind regime TWR where sulfur chemistry transformed them to larger particles Sulfur chemistry promoted the growth of some of these particles until they were large enough to activate to cloud drops This process promoted by sulfur chemistry can produce a cooling effect due to the increase in cloud droplet density and changes in cloud droplet size These increases in particle size observed in PASE promote additional cooling due to direct scattering from the aerosol These potential impacts on the radiation balance in the TWR are enhanced by the high solar irradiance and ocean albedo of the TWR Finally because of the large area involved there is a large factional impact on earth’s radiation budget The TWR region near Christmas Island appears to be similar to the TWR that persists in August and September from southwest of the Galapagos to at least Christmas Island Transport in the TWR between the Galapagos and Christmas involves very little precipitation which could have removed the aerosol thus explaining at least in part the high concentrations of CCN ≈300 at 05 supersaturation observed in PASE As expected the chemistry of sulfur in the trade winds was found to be initiated by the emission of DMS into the convective boundary layer BL the lowest of three layers However the efficiency with which this DMS is converted to SO2 has been brought into further question by this study This unusual result has come about as result of our using two totally different approaches for addressing this long standing question In the first approach based on accepted kinetic rate constants and detailed steps for the oxidation of DMS reflecting detailed laboratory studies a DMS to SO2 conversion efficiency of 60–73 was determined This range of values lies well within the uncertainties of previous studies However using a completely different approach involving a budget analysis a conversion value of 100 was estimated The latter value to be consistent with all other sulfur studies requires the existence of a completely independent sulfur source which would emit into the atmosphere at a source strength approximately half that measured for DMS under tropical Pacific conditions At this time however there is no credible scientific observation that identifies what this source might be Thus the current study has opened for future scientific investigation the major question is there yet another major tropical marine source of sulfur Of equal importance then is the related question is our global sulfur budget significantly in error due to the existence of an unknown marine source of sulfur Pivotal to both questions may be gaining greater insight about the intermediate DMS oxidation species DMSO for which rather unusual measurements have been reported in previous marine sulfur studies The 3 pptv bromine deficit observed in PASE must be lost over the lifetime of the aerosol which is a few days This observation suggests that the primary BrO production rate is very small However considering the uncertainties in these observations and the possible importance of secondary production of bromine radicals through aerosol surface reactions to completely rule out the importance of bromine chemistry under tropical conditions at this time cannot be justified This point has been brought into focus from prior work that even at levels of 1 pptv the effect of BrO oxidation on DMS can still be quite significant Thus as in the case of DMS conversion to SO2 future studies will be needed In the latter case there will need to be a specific focus on halogen chemistry Such studies clearly must involve specific measurements of radical species such as BrOThe Pacific Atmospheric Sulfur Experiment PASE was a comprehensive study of the chemistry of a trade wind regime TWR of the tropical Pacific near 157º 20′ W 2º 52′ N Christmas Island Kiritibati flown in August and September of 2007 on the NSF/NCAR C130 To reduce the complexity of the field observations PASE focused on the chemistry microphysics and dynamics of the cloud free convective boundary layer BLA major goal of PASE was to study the chemistry occurring in the TWR that may influence cloud droplet chemistry and aerosol concentrations and composition Aerosol size number chemistry and surface areas are key characteristics that influence their role in boundary layer processes associated with nucleation gas to particle conversion and their activation as cloud condensation nuclei CCN Aerosol sources include direct injection of seasalt from the ocean surface entrainment from the free troposphere and insitu nucleation The process of aerosol growth via the accretion of gases such as water vapor and sulfuric acid H2SO4 can add mass while conserving number Incloud collisioncoalescence and sedimentation can alter number chemical properties mass and shape of the size distributions These influences may in turn modulate overall cloud albedo Cloud convection and scavenging can also introduce gases aerosols and water vapor into the free troposphere FT over the intertropical convergence zone ITCZ Clarke et al 1999 but is thought to be negligible in the TWR due to strong subsidenceSince the discovery that dimethyl sulfide DMS is emitted in large quantities from the ocean Barnard et al 1982 speculation has abounded that DMS plays an important role in climate Charlson et al 1987 Atmospheric DMS oxidation is initiated mainly by OH in the remote marine environment to produce SO2 which in turn is oxidized both homogeneously and heterogeneously to H2SO4 Chen et al 2000 Davis et al 1999 Aerosol can accrete H2SO4 NH3 and H2O to form larger particles that scatter sunlight or act as cloud condensation nuclei CCN that affect cloud physical and chemical characteristics Finally under favorable conditions with low aerosol surface area there is evidence that DMS chemistry can lead to the formation of new particles Clarke et al 1998Previous studies involving more limited suites of instruments have produced valuable but fragmented information on TWR chemistry One of these a 1994 study on Christmas Island Bandy et al 1996 produced SO2 and DMS time series that unequivocally showed that DMS was oxidized to SO2 Nonsea salt sulfate NSS and methane sulfonate MS also had diurnal variations Huebert et al 1996 Pacific Exploration Missions Tropics A and B and Aerosol Characterization Experiment 1 were the first airborne studies containing a large suite of instruments including H2SO4 OH methane sulfonic acid MSA NO O3 SO2 and DMS In addition to further confirming that DMS oxidation produces SO2 these programs showed that OH was one of the main oxidants and that H2SO4 was one of the principal products They left unanswered the role of the intermediate DMSOPrevious experiments demonstrated the importance of nucleation from sulfuric acid in cloud outflow from deep convection as a source of new particles to the free troposphere They also included a rare example of nucleation at the top of the boundary layer linked to sulfur chemistry after deep convection and precipitation had removed most aerosol surface area in the MBL Clarke et al 1999

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