TROPOSPHERIC CHEMISTY · Tropospheric Chemistry, I. El Haddad & D. Brunner, 2020 2 Approximate...

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Tropospheric NO2 from SCIAMACHY© KNMI, The Netherlands

TROPOSPHERIC CHEMISTYLecture 701-1234-00LETH Zurich, SwitzerlandSpring Semester 2020

LecturersImad El Haddad, PSIDominik Brunner, Empa

2Tropospheric Chemistry, I. El Haddad & D. Brunner, 2020

Approximate Schedule and ContentDate Topic Lecturer21.02. Introduction, basics and issues, biogeochem cycles D. Brunner28.02. Gas phase chemistry (I) I. El Haddad06.03. Gas phase chemistry (II) I. El Haddad13.03. Emissions, local to global D. Brunner20.03. Aerosols: size, sources and chemistry I. El Haddad27.03. Laboratory and field measurements I. El Haddad03.04. Global scale modelling, 3D chemical transport models D. Brunner17.04. Modelling exercise D. Brunner24.04. Chemical mechanisms I. El Haddad08.05. Urban air pollution and megacities D. Brunner

15.05. Student presentationsD. Brunner/I. El Haddad

22.05. Satellite remote sensing D. Brunner29.05. Receptor modelling and impacts, summary I. El Haddad

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Grade and additional learning tasksThe final grade is determined by: Oral exam of 30 min (2/3) and presentation (1/3, compulsory continuous performance assessment). Additionally, the final grade can be improved by up to 0.25 grade points by two learning tasks.

Compulsory continuous performance assessment:One student presentation. For the presentation the students (groups of 2 or 3) select a topic from a list, prepare a 20 minutes presentation (based on a literature search).

Voluntary Learning tasks:‐ One modelling exercise on photochemical limitation regimes‐ 2‐3 paper discussions. The students will have one month time to collectively

comment on a paper using tools for online collaboration and feedbacks.The students’ comments will be finally discussed during the lecture.


Recommended books• Atmospheric Chemistry, Ann M. Holloway and

Richard P. Wayne, Royal Society of Chemistry (RSC) Publishing, 2010

• Introduction to Atmospheric Chemistry, Daniel D. Jacob, Princeton University Press, 2000, Preprint as PDF: http://acmg.seas.harvard.edu/people/faculty/djj/book/index.html

• Atmospheric Chemistry and Physics, from air pollution to Climate Change, J. H. Seinfeld and S. N. Pandis, John Wiley Publishing, 3rd edition, 2016

• Fundamentals of Atmospheric Modelling, M. Z. Jacobson, Cambridge University Press, 2nd edition, 2005

• Chemistry of the upper and lower atmosphere,B. Finlayson-Pitts and J Pitts, Elsevier Science Publishing Co Inc, 1999

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Laboratory for Air Pollution & Environmental Technology at Empa

• Operation of Swiss National Air Pollution Monitoring Network NABEL

• World-calibration center in GAW networkof World Meteor. Organization

• Development and application ofadvanced trace gas measurementtechnology: Laser spectroscopy, GC/MS

• Atmospheric modelling and remotesensing (my group)- Model development- Air quality- Inverse emission estimation- Urban climate- NO2 remote sensing (satellites, aircraft)


Goals of today• Refresh some basics

• Units, composition of the atmosphere

• Vertical profile of the atmosphere

• Differences between tropospheric and stratospheric photochemistry

• Lifetimes-timescales

• Global biogeochemical cycles of S, N and C

Requirements: what you should already know• Bachelor course in atmospheric chemistry (lecture 701-0471-01) or equivalent

• Basics in physical chemistry

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Unit prefixes and abbreviations

Prefixes:μ 10-6 microm 10-3 millik 103 kiloM 106 MegaG 109 GigaT 1012 TeraP 1015 Peta

Molar (mol/mol) or mass (kg/kg) mixing ratios:Percentage %Per mille ‰Parts per million ppm; 10-6

Parts per billion ppb; 10-9

Parts per trillion ppt; 10-12

E.g. global emissions of CH4 per year (avg. 2003‐2012): 540 – 568 Tg/yr

E.g. global anthropogenic emissions of CO2 in 2016: 36 Pg/yrSource: Global Carbon Atlas 2018

E.g. Avg. CO2 mole fraction in 2018 ~407 ppm

E.g. Avg. CH4 mole fraction in 2018 ~1850 ppbE.g. Avg. CFC mole fraction in 2018 ~230 ppt


Measures of atmospheric composition

number density

quantity symbol typical units

ns = [s] e.g.: nO3 = [O3] cm‐3

typical application

chemical kinetics

mass density(concentration)

ms e.g.: mO3 µg m‐3 air quality

mole fraction (orvolume mixing ratio) air

ss n

n e.g.: χO3

nmol/molppb (10‐9) atmospheric transport

mass mixing ratioair

ss m

mµ e.g.: µO3 µg / kg atmospheric transport

partial pressure ss

p

p Pa𝑝𝑠 𝑛𝑠 𝑘 𝑇 heterogeneous chemistry

column density 𝐶𝑠 𝑛𝑠 𝑑𝑧 cm‐2 satellite observations

molar concentration cs s aq, e.g. cSO2 SO2 aqmol l‐1

= M (Molar)liquid phase chemistry

particle numberconcentration

cm‐3nP aerosol processes

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ExerciseConvert 1 ppb of O3 to μg m-3 at standard temperature and pressure (STP)

Inputs:

STP conditions: p=1000 hPa; T=298.15 (STP)

Molar mass of ozone: 48 g/mol

Universal gas constant: R = 8.314 J mol-1 K-1

Solution:

See python code on following jupyter notebook:

https://nbviewer.jupyter.org/github/gredvis/tropchem/blob/master/unit-conversion-notebook/unit_conv.ipynb

https://github.com/gredvis/tropchem/blob/master/unit-conversion-notebook/unit_conv.ipynb

For an interactive version see

https://mybinder.org/v2/gh/gredvis/tropchem/master/


HomeworkWhat is the number density in molec cm-3 of 10 ppt of HO2at 800 hPa and T = 273.15 K?

Send by email to [email protected]

Check the units!

Avogadro’s number NAV = 6.022 1023 molecules/mol

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Composition of the atmosphere

Nitrogen

Oxygen

H2OArgon

20%

78%

1%

N2O 327

H2

CO

Ozone

500

100

30

ppb

CO2

CH4 (1.8)

ppm

400

Ne

18He (5)

HCHO 300

Ethane

SO2

NOx

500

200100

ppt

NH3 400

CH3OH 700

H2O2 500

HNO3 300

other


Layers in the atmosphere• Troposphere extends from surface to the

tropopause at about 10 km, higher in tropics (~16 km), lower at poles (~8 km)

• Troposphere contains about 80% of themass of the atmosphere. Question: what is the approx. pressure at tropopause?

• Troposphere is weakly stably stratified: US Standard Atmosphere: -6.5 K/km Dry adiabatic lapse rate: -9.8 K/km

• Vertical mixing is, therefore, quite fast in the troposphere (weeks), much faster than in stratosphere (years)

• Well-mixed gases like O2 or N2 haveconstant mole fraction throughout troposphere (even up to about 80 km)

Ionisation of N2/O2

UV abs: O3 →O2+OO2+O→O3 (exothermal)

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Why do we care about tropospheric chemistry?• The troposphere is the place where we live

• We typically take 12-18 breaths per minute, each time inhaling a volume of500 mL, i.e. about 6-9 L/min or about 10.8 m3/day ≈ 13 kg air/day(compared to about 2 kg food and 1 kg of water intake per day)

• Atmospheric composition is strongly influenced by human activities

• These activities have important implications for:• Human health (air pollution; toxic components)• Climate change (radiative forcing, temp, precipitation, ice melt, etc.)• Ecosystems (eutrophication, acidification)

• Knowing emissions is not enough to understand the impacts, but we need to consider the full lifecycle of gases and aerosols

• We would like to develop a basic understanding of the role of the atmosphere in the ‘Earth System’ and Earth’s climate

• We would like to understand the roles of natural processes versus human influences on atmosphere and biosphere


The troposphere as a chemical reactor• The troposphere is mainly an oxidizing environment, i.e. chemical processing tends to

increase oxidation state of gases

• The troposphere is limited by the Earth’s surface (at the bottom) and the tropopause (at the top). It is usually further divided into planetary boundary layer (PBL) and free troposphere.

• The PBL plays a key role since air pollutants are mainly emitted into the PBL

• The troposphere is (compared to stratosphere) relatively well mixed

• However, depending on life time many chemical compounds show strong spatial gradients

• Long-lived compounds (life times of the order of several months to years) can be transported into the stratosphere and vice versa

• Concentrations at a given receptor site are determined by emissions, transport and chemistry

• Dry and wet deposition leads to ultimate removal of compounds (often in oxidized form) at the Earth‘s surface

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Two contrasting faces of ozone

In the stratosphere:

• Ozone layer with maximum at about 20-25 km

• Protects against harmful solar UV radiation

• UV absorption by O3 acts as local heat source

In the troposphere:

• In middle/upper troposphere: greenhouse gas

• In lower/middle troposphere: precursor of OH radicals, main oxidant of the atmopsphere

• At the surface: toxic to humans and vegetation

Question: How would O3-profile in figure look like in units of mass density? As mole fraction?

Source: Scientific Assessment of Ozone Depletion: 2010, World Meteorological Organization Global Ozone Research and Monitoring Project ‐ Report No. 52

90% of total

10% of total


Why are tropospheric and stratospheric O3 chemistry so different?

Chapman cycle of stratospheric O3 production (1930)

O2 + hν→ O + O j1 λ < 242 nmO + O2 + M → O3 + M k2O3 + hν→ O + O2 j3 λ < 336 nmO + O3→ 2 O2 k4

Fast cycling withinodd oxygen family

Hei

gh

t [k

m]

photolysis rate profilesNot available in the troposphere

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Differences with respect to NOx, CO, CH4, VOCs

Stratosphere Troposphere

Source of NOx: (R1) N2O+ O(1D) →2 NO

Propagation(R2) NO + O3 → NO2 + O2

(R3) NO2 + hv → NO + O (R4) O+ O2 + M → O3

Net: zero reaction

Small fraction:(R5) NO2 + O → NO + O2

Net of R2 and R5: O + O3 → 2O2

“Net Ozone destruction”

Source of NOx: EmissionsFossil fuel burning at high T, p

Coupling of NOx and HOx cycles:(R1) O3 + hv + H2O → 2 OH + O2

(R2) CO, NMVOC + OH → HO2

If NOx sufficiently high:

(R3) NO + HO2 → NO2 + OH(R4) NO2 + hv → NO + O(R5) O+ O2 + M → O3

“Net Ozone production”

Much more details ontropospheric O3 chemistrywill be provided later


Role of atmospheric transportAtmospheric transport and mixing critically determines distribution and concentrations of tracegases and aerosols through:

• Advection: transport of pollutant by mean wind, e.g. from source to regions downwind

• Turbulent mixing: emissions are mixed into ambient air. Stronger mixing spreads emittedspecies over a larger volume of air within a given period of time.

Question: How does concentration of an emitted species depend on wind speed?

Examples for atmospheric transport and mixing on following slides:

• Stratosphere-troposphere exchange including transport of stratospheric O3 into troposphere

• Dynamics of the PBL

• Convective ventilation of the PBL

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Exchange between troposphere and stratosphere• Brewer-Dobson circulation is a slow (~2 years)

diabatic circulation exchanging air betweentroposphere and stratosphere

• Circulation is driven by wave breaking, the«extratropical pump»

• Tropospheric air enters stratosphere intropics, and slowly ascends diabaticallythrough isolines of potential temperature

• Diabatic descent occurs at mid- to highlatitudes

• Ozone-rich stratospheric air mixes intotroposphere mainly at mid-latitudes intropopause folding events

• Brewer-Dobson circulation is about twiceas strong in northern hemisphere thanin southern hemisphere

Source: Holton et al. 1995


Exchange between troposphere and stratosphere

Source: Meul et al., Atmos. Chem. Phys 2018., https://www.atmos‐chem‐phys.net/18/7721/2018/

• Model simulation with a «tagged» stratopheric ozone tracer (O3s):

- In stratosphere, O3s is identical tothe chemically active (normal) O3

- In troposphere, O3s has nosources but is depleted inthe same way as normal O3

• Arrows show main pathway ofstratospheric O3 entering thetroposphere

• Maximum of O3s in thetroposphere is found in thesubtropics, minimum in tropics

Loss rate LO3s

More O3s in SH in June becauseB‐D circulationstronger in winter

June

Question: Why is loss rate largestin troposphere in June around 30°N?

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Planetary boundary layer (PBL) and its importancefor regulating air pollution levels Radiatively driven PBL

(typical of sunny summer day)

Day: • Heating of surface by shortwave

solar radiation• transfer of heat to atmosphere

by sensible and latent heat fluxes• Approx. neutral stratification,

unstable near the surface

Night:• Cooling of surface by longwave

(thermal) radiation• Cooling of atmosphere above

forms shallow nocturnal inversion• Stable layer and residual layer

above

z (km)

1

2

Tsurface

solarirradiation

z (km)

1

2

T

thermalradiation fromearth surface

nighttimeinversion

Day Night

night morning

surfaceradiativelyheated

surfaceradiativelycooled

pollutantsresidual layer

entrainment zone

capping inversion capping inversion

pollutants

stable layer

unstable layer


Diurnal evolution of the PBL

Source: Collaud‐Coen et al. 2014

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Convective ventilation of the PBL

• frontal uplift of air in «warm conveyor belt»

• cleans the PBL• transports pollutants to

upper troposphere


Convective ventilation of the PBL

Source: Brunner et al. 1998

Nitrogen oxide plume observed in the uppertroposphere from a Swissair B-747

Question: What might have been the source of theobserved elevated NOx?

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Timescales of atmospheric processes

1 week synoptic weather, high and low pressure systems

1 year ENSO (El Niño Southern Oszillation)

1 month CO lifetime , O3 lifetime in troposphere

100 years N2O lifetime

1/10 s Turbulent fluctuations of wind

1 s OH radical chemistry

1 min NOx chemistry, photostationary state

1 h ozone formation downwind of a city

10 years CH4 lifetime

time scale processes

> 100 years ecosystem changes, deep water ocean circulation

1 d Chemical depletion of formaldehye CH2O


Relation between chemicallifetime and spatial scales

• A trace gas is only susceptible to a giventransport process, if its chemical lifetime islonger than the timescale of thetransport process

• Temporal (lifetime) and spatial scales oftrace gases are therefore tightly linkedas expressed by a «Junge diagram»

• Example:

The lifetime of CH4 is about 9 years.CH4 therefore mixes efficiently between thenorthern and southern hemisphere, becausethe time scale of interhemispheric mixingis only about 1.7 years.

NOx emitted in the NH, on the other hand,is not transported into the SH.

Source: Seinfeld and Pandis

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Relation between temporal (lifetime) and spatial scales

TROPOMI satellite observations of NO2 TROPOMI satellite observations of CH4

Schneising et al., 2019

https://www.esa.int/Applications/Observing_the_Earth/Copernicus/Sentinel‐5P/Nitrogen_dioxide_pollution_mapped


Atmosphere as a reservoir of trace elements Notes

• Of all elements in the earth system only nitrogen is dominant in the atmosphere

• Carbon mainly partitions into ocean and soil/rocks

• Phosphorous: mainly rocks

• Sulfur: ocean and rocks

• O: mainly in rocks as silicates (SiO2)

• Between mass of earth and atmosphere is a factor of 1 million

Mass of the Earth: 5.97 x 1027 gMass of atmosphere: 5.15 x 1021 g

Geochemical cycles

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Global geochemical cyclesof S, N and C• Geochemical cycling refers to the flow

of elements through Earth’s reservoirs

• Earth as a whole is considered a closed system

Jacob, Introduction to Atmospheric Chemistry


Geochemical cycle of SMajor reservoirs and burdens of S in Tg(S)

(Charlson et al. 1992)

Evolution of SO2 emissions since 1850 (in 106 t)

SO4‐2 in ice core from Summit, Greenland (Legrand et al. 1997)

Tambora1815

Katmai1912

Laki1783

1990 1770

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Geochemical cycle of SObserved mixing ratios of atmospheric S gases (Berresheim et al. 1995)

Present‐day tropospheric sulphur cycle(after Stevenson et al. 2003)

(DMS)

S(‐2)

S(+4) S(+6)

2‐

Question: What do we know about carbonyl sulfide (COS)?(See Montzka et al., JGR 2004)


Geochemical cycle of S

Question:What is atmospheric lifetime of SO2 considering the processes presented on the previous slide?

𝜏𝑄

𝐿 𝐿 𝐿 , 𝐿 , 𝐿 ,

Burden of SO2 in Tg(S)

Lx = Losses of SO2 in Tg(S)/yr

𝜏0.29

9.2 30 6.3 32 17 = 1.1 days

And what would be the lifetime of sulfur in the system SO2 + sulfate?(See Seinfeld and Pandis book for solution)

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Geochemical cycle of NProcesses in the atmospheric cycle of N compounds

(Seinfeld and Pandis)

NH4+ → NO2

- → NO3-

BNF = biological nitrogen fixation

(Haber‐Bosch)Global nitrogen fixation in 21st century (Fowler et al. 2013)

Relative trends in anthropogenic reactive (fixed) nitrogen sources since 1900(Battye et al. 2013)

RN = organic nitrogen


Biogeochemical cycle of C

By far most abundant atmospheric C species is CO2

The Carbon Cycle. Numbers in boxes represent reservoir mass, also called carbon stocks, inventories, or storage in PgC (1 Pg = 1015g = 1 Gt). 2.1 PgC = 1 ppm atmospheric CO2. Numbers next to arrows indicate fluxes in PgC/yr. Black numbers and arrows represent estimates of the natural (pre‐industrial) carbon cycle. Red numbers and arrows indicate estimates of anthropogenic effects for 2000‐2009. (IPCC 2013)

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Biogeochemical cycle of CC species more relevant for atmospheric chemistry:• Methane (CH4): ~2 ppm• Carbon monoxide (CO): ~100 ppb• Volatile organic compounds (VOC)

individually ~1 ppt to 1 ppb

Evolution of fossil fuel CO2 emissions and uptake byoceans and biosphere since 1900

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