Oceanic alkyl nitrate (RONO₂) production

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Stable isotopes as a tracer of reactive nitrogen emissions and aerosol

formation in the Southern Ocean

Dr. Katye Altieri

Department of Oceanography, University of Cape Town, [email protected]

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Biogeochemistry of the Southern Ocean is a strong control on atmospheric chemistry in the marine boundary layer

Climate models struggle to simulate complex ocean-ice- atmosphere system in the Southern Ocean.

Satellites are restricted by cloud cover.

Hostile conditions limit

seasonally resolved and long- term observations.

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How do we identify and trace oceanic sources of reactive nitrogen gases to the atmosphere?

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Atmospheric nitrate is the ultimate sink for NO_x

• Aerosol nitrate

• Acts as ccn, influences radiative forcing

• Contributes to reactive N deposition and acidification

• NO_x = NO + NO₂

• Cycling results in chemical production of ozone

• Controls oxidizing capacity of the troposphere

• Comes from anthropogenic and natural sources

Brown et al., 2004

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Stratosphere (19‰)

Snowpack photolysis (-40 to -60‰)

Different NO_x sources have different δ¹⁵N signatures

When NO_x is converted to nitrate, the N is conserved  δ¹⁵N-NO_x ~ ^δ¹⁵^N-NO₃^-

Elliott et al., 2019, Savarino et al., 2007, Morin et al., 2012

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Early Summer Dec 2018

Late Summer Feb-Mar 2019

Seasonally resolved ship-board observations from Cape Town to the ice

Winter Jul-Aug 2019 Spring Oct-Nov 2019

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Filter based size-segregated aerosol collections every 24h

High volume air sampler 5-stage cascade impactor

Sector controlled to avoid ship emissions Aqueous extraction

[NO₃^-], [NH₄⁺] IC

δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- denitrifier-IRMS δ¹⁵N-NH₄⁺ hypobromite-azide-IRMS

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Air mass back trajectories show influence of sea ice, surface ocean, and continent on each sample

Red circles = position of ship during aerosol sampling

Grey lines = HYSPLIT 72-hour air mass back trajectories for each sample hour White color = satellite-derived sea ice concentration data from AMSR2

Late Summer

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Spring (Southbound)

Spring (ice edge)

Spring (Northbound)

Samples were collected with varying degrees of ice, ocean, and ice+ocean influence

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Different NO_x sources contribute to aerosol NO₃^- in low-, mid-, and high-latitudes

Ice edge

Burger et al., ACP 2022

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Different NO_x sources contribute to aerosol NO₃^- in low-, mid-, and high-latitudes

Ice edge

Snow photolysis NO_x = -45‰

Lightning NO_x = 0‰

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Spring and summer have similar NO_x sources across latitudinal transect

Ice edge

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Spring and summer have similar NO_x sources across latitudinal transect

Ice edge

Mid-latitude – not a mixture!

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Proposed mid-latitude NO_x source is surface ocean emissions of alkyl nitrates (RONO₂)

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• Supersaturation of surface ocean

RONO₂ drives a net flux from ocean to atmosphere

Oceanic alkyl nitrate (RONO₂) production

RO₂ + NO → RONO₂

Photolysis of

DOM Nitrite

photolysis

Sur fac e o ce an At mo sp he re

RONO₂ + → RO + NO₂

NO₂ + OH → HNO₃

• RONO₂ formation requires NO & only accumulates in

regions of non- zero nitrite

concentration.

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NO_x from alkyl nitrate emissions has distinct isotopic signature δ¹⁵N-RONO₂ = -22 ± 7‰

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Interestingly…study in equatorial pacific used slightly different

approach and determined δ¹⁵N-RONO₂ = -23 ± 19‰.

Joyce et al., in review GRL

NO_x from alkyl nitrate emissions has distinct isotopic signature δ¹⁵N-RONO₂ = -22 ± 7‰

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Spring (Southbound) Spring (ice edge)

Springtime low δ¹⁵N must come from snow photolysis on sea ice – and not continental Antarctic snow

Burger et al., in prep

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Winter NO_x sources do not vary with latitude

Ice edge

No evidence of snow photolysis.

No evidence of alkyl nitrate emissions.

Very low

concentrations.

One sample with stratospheric NO_x

source – confirmed by δ¹⁸O and Δ¹⁷O data.

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Winter NO_x sources do not vary with latitude

Ice edge

Stratosphere NO_x = 19‰

No evidence of snow photolysis.

No evidence of alkyl nitrate emissions.

Very low

concentrations.

One sample with stratospheric NO_x

source – confirmed by δ¹⁸O and Δ¹⁷O data.

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Sea ice cover

Snow pack emissions Stratospheric input Alkyl nitrate emissions Light availability

Summer Winter Spring

NO_xSources

Summary of seasonal NO_x sources and drivers

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Summary

• Aerosol δ¹⁵N-NO₃^- observations can be used to quantify NO_x sources

• NO_x sources are similar in spring and summer across a latitudinal transect

• Surface ocean, snow on Antarctica, and snow on sea ice are all big sources of NO_x across the remote Southern Ocean

• Winter characterized by low aerosol concentrations, no snowpack or surface ocean NO_x sources (no light!)

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Biogeochemistry of the Southern Ocean is a strong control on atmospheric chemistry in the marine boundary layer

Seasonality in photolysis is critical for controlling

emissions of reactive N gases.

The ocean is not a passive recipient of atmospheric N deposition.

Surface ocean nitrite and DOM concentrations matter for air- sea fluxes of N.

Photochemistry of snow on sea ice emits reactive N gases that lead to aerosol formation (and clouds).

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Acknowledgements

Research support:

Captain and crew of the R/V SA Agulhas II

Team NAtm and NOce on the research voyages Hastings lab at Brown University

Granger lab at UCONN

Collaborators: Jessica Burger, Kurt Spence, Shantelle Smith, Emily Joyce

Funding support:

South African National Research Foundation South African National Antarctic Programme UCT Vice Chancellor’s Future Leaders Fund

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Summer

Dec 2018 & Mar 2019 Winter

July to Aug 2019 Spring

Oct to Nov 2019 Atmospheric nitrate concentration ([NO₃⁻]) with latitude (°S)