Effect of Variable Flux Footprint on Measurement of Air/Sea DMS Transfer Velocity A Southern Ocean Case Study Thomas Bell Presented by Mingxi Yang with contributions from: Warren De Bruyn, Christa Marandino, Scott Miller, Cliff Law, Murray Smith, Brian Ward, Kai Christensen and Eric Saltzman

Modified from Wanninkhof et al Sol. Sc No. Flux ΔC ΔC K Wave Controlling Factors on K

How well do we know K over the ocean? Numerous Field Measurements of waterside controlled gases, though few in high winds Fair agreement in low to moderate winds Large divergence among different gases/methods in high winds & rough seas K DMS Lower than K of less soluble gases above U ~ 8 m/s Solubility DMS > CO 2 > 3 He

Why Measure Air/Sea DMS Exchange? Environmental importance: –Large biogenic sulphur source to atmosphere –Clouds, albedo and climate? Useful tracer for K: –Grossly supersaturated in surface ocean (strong flux signal) –Highly sensitive detector (CIMS) available for eddy covariance –A proxy for interfacial (i.e. tangential) gas exchange Relevant to other gases: e.g. CO 2, N 2 O, CH 4, CO, O 2, acetone, etc

Wave influence? k DMS measurements from Knorr_11 cruise in N. Atlantic Bell et al. ACP (2013)

Waves = 20% reduction in k DMS Rhee et al. (2007) Wave influence? Wind-wave tank measurements

DOGEE Cruise Surfactants? k DMS from DOGEE cruise in N. Atlantic Salter et al. (2011) U 10n (m/s)

Southern Ocean Aerosol Production (SOAP) Cruise Feb/March 2012 High productivity waters Natural surfactants and K? Waves and K?

Micrometeorological technique: Eddy Covariance Covariation between vertical wind velocity (w) and gas concentration in air (c) Timescale = 10 minutes ~ 1 hour Useful for assessing processes affecting gas transfer (e.g. waves, surfactants) BUT 1) Turbulence is stochastic – requires averaging 2) Spatial separation between ΔC and Flux Gas Flux U 10 Flux Footprint

SOAP Setup 3-D Winds (Sonic Anemometer) Atm. Inlet (90 L/min) Motion Sensor Internal Standard Seawater DMS Ship’s Inlet Atmospheric flux mast Atmospheric instruments in container lab

10 min average data

Bin Average - Fairly good agreement on the mean with previous DMS studies up to 14 m/s

- Scattering not just random - Positive and negative biases in 10 min K relative to COARE model Short timescales may contain information about physical processes that become lost during bin-averaging

Spearman’s ρ = 0.57, p<0.01, n= min average Scatter: Random noise + systematic bias Other processes? Waves? Surfactants? 10-m Wind speed (m/s)

Wave influence? No clear relationship with significant wave height

Surfactant influence? No obvious relationship with chlorophyll as a proxy for surfactant What else can be causing the scatter?

Flux footprint analysis 18 hour transect Consistent conditions Into bloom Into wind

Distance from Bloom (km) Neutral-stable atmosphere

DMS sw vs Flux/U 10 lag analysis - F DMS /U 10 peaks earlier than DMS sw - 8±2 min lag = max. correlation between DMS sw and F DMS /U 10 SOAP raw SOAP LagCorr COARE

~30% reduction in scatter Wind speed (m/s) No Lag Shift Seawater DMS Shifted by 8 minutes

Footprint Size Distance from sensor (m) Proportion of flux signal (%) SOAP Cruise 8 min lag (ship speed = 5.1 m/s) suggests footprint = 2.5 km (peak) Footprint model (Kormann and Meixner, 2001) predicts peak flux at 0.8 km (range = 0.3 – 1.9 km, depending on stability) Peak flux

Conclusions Scatter in SOAP K DMS –Random + systematic –Masks potential impacts of other processes e.g. surfactants, wave properties Accounting for lag between flux and seawater concentration improves gas transfer estimates –Reduces scatter in K Peak flux distance estimates: –Flux footprint model < lag-based estimate

Extra slides

Seawater DMS (UCI miniCIMS) PTFE porous membrane counter- flow equilibrator residual gas analyzer (SRS) liquid d3-DMS standard DL ~0.1 20°C (2 min avg)

DMS m/z 63 d3-DMS m/z Hz data acq. ~100 cps/ppt (Hz/ppt) Atmospheric DMS (UCI mesoCIMS)

Ho et al. (2006) Liss and Merlivat (1984) Nightingale et al. (2000) Global excess 14 C Budget techniques: Sparingly soluble gases U 10 (Horizontal Wind Speed) Gas Transfer Velocity (K) cm/hr calm (buoyancy) moderate wind (shear stress) rough (waves, bubbles) Dual tracer ( 3 He / SF 6 ) Timescale = hours-days