Chemistry and Climate interaCtions: development and evaluation of a Coupled Chemistry-aerosol-Climate model

Chemistry and Climate Interactions:
Development and Evaluation of a Coupled
    Chemistry-Aerosol-Climate Model

                          THÈSE NO 3763 (2007)
                           PRÉSENTÉE le 30 mars 2007
  à la faculté DE L'ENVIRONNEMENT NATUREL, ARCHITECTURAL ET CONSTRUIT
               Laboratoire de modélisation de la chimie atmosphérique
                 PROGRAMME DOCTORAL EN ENVIRONNEMENT

     ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE

          POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES

                                          PAR

                               Luca Pozzoli

          Laurea in scienze ambientali, Università degli studi di Milano, Italie
                              et de nationalité italienne

                           acceptée sur proposition du jury:

                          acceptée sur proposition du jury :
                            Prof. I. Bey, directrice de thèse
                              Dr F. Dentener, rapporteur
                               Dr M. Schultz, rapporteur
                          Prof. H. van den Bergh, rapporteur

                                         Suisse
                                          2007

Abstract

Atmospheric trace gases and aerosols and climate interact in many ways. A quantitative
assessment of the influence of trace gases and aerosols on climate can only be achieved
if the interactions and feedbacks among these three major components are accounted
for. The goal of this thesis was to develop, evaluate and apply the ECHAM5-HAMMOZ
chemistry-aerosol-climate model. The model includes fully interactive simulations of
the NOx-Ox-hydrocarbons chemistry and of the aerosol chemistry and microphysics,
the two simulations being embedded into the well established ECHAM5 climate model.
In particular, on-line calculation of the photolysis rates (that accounts for aerosols and
clouds) and heterogeneous reactions of trace gases on aerosols are accounted for in
the model. In addition, the aerosol simulation provides a prognostic representation of
the mixing state of the aerosol components, a feature that is particularly important to
examine the effect of the SO2 uptake and subsequent sulfate formation on aerosols.
    A thorough model evaluation of the model was performed using observations from
the Transport and Chemical Evolution over the Pacific (TRACE-P) aircraft campaign.
We also used ground based measurements from the European network EMEP (SO2
and sulfate), from the north American network IMPROVE (sulfate, black carbon and
organic carbon), and from AERONET (aerosol optical depth). The coupled model is
able to reproduce fairly well many of the observations over the TRACE-P region, even
though some improvements are still needed. For example, we find a mean absolute
bias of 20 ppbv (30%) and 40 ppbv (13%) for the simulated O3 and CO, respectively.
Sulfate concentrations are represented fairly well, for all the regions considered, while
SO2 is overestimated by a factor of 2 in general. Black carbon concentrations are
underestimated over the TRACE-P region (mean absolute bias of 80%), most probably
because of too low emissions, but well reproduced over north America. The aerosol
optical depths compare well with observations at many sites in general, both in terms
of annual means and seasonal variations.
    We show the results from a series of sensitivity simulations which goals are to assess
the impact of heterogeneous reactions, photolysis reactions and sulfur chemistry on the
regional and global trace gas distribution, and aerosol distribution, composition and
optical properties. We found that heterogeneous reactions result in a reduction of 7%
in the global O3 burden, and by up to 15% in surface O3 over regions rich in mineral
dust. OH burden decreases by 10%, NO and NO2 by 20% and 29%, respectively while

ii                                                                  Chapter 0. Abstract

CO burden increases by 7%. Our numbers fall in general within the range of previous
studies. We find that the effect of aerosols through the modifications of photolysis
rates do not affect significantly the trace gas distributions and global burdens, while
previous studies suggested larger effect.
     Heterogeneous reactions reduce the global mean SO2 surface concentration by 14%,
while the global burden remains unchanged. This reflects the effect of competing
processes. In particular, the depletion in SO2 by direct uptake on sea salt and dust is
counterbalanced by the decrease in SO2 oxidation that is associated with the decrease
in OH. The balance between the two competing effects differs depending on the regions
(SO2 increases by more than +10% over north Africa and Indian Ocean and decreases
by up to -20% at high latitudes). These processes, in turn, affect the sulfate formation.
We find in general an increase (by up to 4%) in sulfate burden over the oceans because
of sea salt uptake of SO2 , and a decrease by up to 6% over Sahara and India, where
SO2 increases. Our global burden of SO2 and sulfate amount to 0.77 and 0.87 (Tg(S)),
respectively, which fall in the range of previous estimates. We find that SO2 uptake on
sea salt contributes to the production of 3.69 Tg(S) yr−1 of sulfate (5% of total sulfate
production) in good agreement with recent observation-based estimates. Uptake of SO2
on dust only contributes to the production of 0.55 Tg(S) yr−1 (< 1%) of sulfate, but
it is an important process as it is responsible for the coating of mineral dust particles.
A total of 300 Tg yr−1 of dust are transferred from the insoluble to the soluble mixed
modes (56% of total emissions) because of this later process. The total burden of
black carbon does not change significantly because of the heterogeneous reactions, but
it is redistributed between insoluble/soluble modes. The changes in the mixing state of
aerosols produce significant regional variations in aerosol optical depth and absorption.
The aerosol optical depth and aerosol absorption increase by 10 to 30% and by 15%,
respectively, during winter in the main polluted regions of the north hemisphere because
of enhancing absorption efficiency of black carbon mixed with sulfate. On the contrary
aerosol absorption decreases by up to 20% over the Atlantic ocean because of enhancing
black carbon removal by wet scavenging.
    ECHAM5-HAMMOZ is a fully coupled model which includes many of the trace gas-
aerosol and chemistry-climate interactions, and is thus a tool that allows an integrated
assessment of the impact of the major short-life species that contribute to climate
change and air quality.
   Keywords: Climate, ozone, aerosol, trace gas-aerosol interactions, tropospheric
chemistry, global modeling.

Riassunto

Gli elementi che costituiscono l’atmosfera, quali gas, aerosol e nubi, sono strettamente
collegati attraverso processi chimici, fenomeni dinamici dell’atmosfera e dalla radiazione
solare. C’è pertanto il bisogno di sviluppare modelli climatici ”fully coupled” in cui le
componenti chimica, aerosol e clima siano completamente integrate fra loro e possano
essere esaminate sia separatamente che contemporaneamente, al fine di comprendere i
meccanismi che regolano il clima e prevederne i cambiamenti futuri.
    Durante questo lavoro di tesi, il modello accoppiato chimica-aerosol-clima
ECHAM5-HAMMOZ è stato sviluppato allo scopo di esaminare l’influenza delle in-
terazioni gas-aerosol sulla distribuzione di gas e aerosol e sulle proprietà ottiche degli
aerosol sia a livello regionale che a livello globale. Il modello comprende simulazioni to-
talmente interattive della chimica NOx-OX-Idrocarburi, della chimica e della microfisica
degli aerosol, comprendente il calcolo on-line delle costanti di fotolisi (tenendo in con-
siderazione l’effetto di aerosol e nuvole), e delle reazioni eterogenee dei gas sugli aerosol.
Una accurata valutazione del modello è basata sulla campagna di misura aerea Trans-
port and Chemical Evolution over the Pacific (TRACE-P). Le masse d’aria provenienti
dal continente asiatico che vengono trasportate sull’oceano Pacifico sono caratteriz-
zate da un carico elevato di aerosol e inquinanti di varia origine, pertanto si tratta di
un’area particolarmente interessante per questo studio. Inoltre sono state utilizzate le
misure provenienti dalla rete di rilevamento europea EMEP (SO2 e solfato), dalla rete
nord-americana IMPROVE (solfato e particolato carbonioso), e dalla rete AERONET
(profondità ottica degli aerosol).
    Il modello accoppiato è in grado di riprodurre in modo soddisfacente molte delle
osservazioni sull’area indagata da TRACE-P, sebbene siano ancora necessari alcuni
miglioramenti. Ad esempio, è stato riscontrato un bias assoluto di 20 ppbv (30%)
e di 40 ppbv (13%) per le concentrazioni simulate, rispettivamente, di O3 e CO. Le
concentrazioni di solfato sono ben riprodotte per tutte le regioni considerate, mentre
quelle di SO2 risultano sovrastimate di un fattore 2. Le concentrazioni di particolato
carbonioso sono sottostimate nella regione TRACE-P (bias assoluto medio pari a 80%),
molto probabilmente a causa di bassi livelli di emissione, ma risultano simulate corret-
tamente nell’area nord-americana. La profondità ottica degli aerosol assume valori ben
confrontabili con le osservazioni in molte stazioni AERONET, sia in termini di medie
annuali che di variazioni stagionali. In questo lavoro vengono inoltre presentati i risultati

iv                                                                   Chapter 0. Riassunto

di una serie di analisi di sensitività volte a valutare l’impatto delle reazioni eterogenee,
delle reazioni di fotolisi e della chimica dello zolfo sulla distribuzione globale dei gas
e sulla distribuzione, composizione e proprietà ottiche degli aerosol. È stato rilevato
che l’impatto delle reazioni eterogenee consiste in una riduzione del bilancio globale di
O3 del 7%, che raggiunge il 15% a livello della superficie terrestre nelle regioni in cui
c’è abbondanza di polvere minerale. Il bilancio di OH diminuisce del 10%, quello di
NO e NO2 del 20% e del 29%, rispettivamente. I risultati ottenuti ricadono, in gene-
rale, nell’intervallo definito da studi precedenti. Si riscontra che l’effetto degli aerosol
attraverso le modifiche delle costanti di fotolisi non influenza in modo significativo le
distribuzioni e i bilanci globali dei gas, mentre studi precedenti hanno riscontrato un ef-
fetto maggiore. Le reazioni eterogenee riducono la concentrazione media globale di SO2
alla superficie del 14%, mentre il bilancio rimane inalterato. La riduzione in SO2 per la
reazione su sale marino e polvere è controbilanciata dalla contemporanea diminuzione
di OH. Il bilancio tra i diversi effetti delle reazioni eterogenee sui processi sopra citati
può variare in diverse parti del globo (più di +10% sul Nord Africa e Oceano Indiano,
e fino a -20% a latitudini elevate); la concentrazione di SO2 è importante anche per
determinare la formazione di solfato. In generale, si rileva un aumento (fino al 4%) nel
budget di solfato sopra gli oceani a causa della reazione di SO2 su sale marino, e una
diminuzione del 6% sul Sahara e sull’India, dove la concentrazione di SO2 aumenta.
Si riscontra un bilancio globale di 0.77 Tg(S) e 0.87 Tg(S) per SO2 e solfato, rispet-
tivamente, tali valori ricadono nell’intervallo di stime fornite da studi precedenti. Il
risultato ottenuto per il solfato formato sulle particelle di sale marino è pari a 3.69 Tg
(S) anno−1 (5% della produzione totale di solfato), in buon accordo con studi recenti.
Il solfato su polvere minerale contribuisce soltanto con 0.55 Tg(S) anno−1 (

Contents

Abstract                                                                                  i

Riassunto                                                                               iii

List of Figures                                                                         ix

List of Tables                                                                         xiii

1 Introduction                                                                           1
   1.1   Climate change: a changing atmosphere . . . . . . . . . . . . . . . .           1
   1.2   Interactions between climate and trace gases . . . . . . . . . . . . . .        5
   1.3   Interactions between climate and aerosols . . . . . . . . . . . . . . . .       8
   1.4   Interactions between trace gases and aerosols . . . . . . . . . . . . . .     10
   1.5   Objectives and Outline of this work . . . . . . . . . . . . . . . . . . .     11

2 ECHAM5-HAMMOZ: a chemistry-aerosol-climate coupled model                             15
   2.1   The ECHAM5 General Circulation Model . . . . . . . . . . . . . . . .          15
   2.2   The tropospheric chemistry module MOZECH . . . . . . . . . . . . .            16
   2.3   The aerosol module HAM . . . . . . . . . . . . . . . . . . . . . . . .        18
   2.4   Coupling between aerosol and tropospheric chemistry simulation . . . .        21
         2.4.1    Sulfur chemistry . . . . . . . . . . . . . . . . . . . . . . . . .   22
         2.4.2    Photolytic reactions . . . . . . . . . . . . . . . . . . . . . . .   23
         2.4.3    Heterogeneous chemistry . . . . . . . . . . . . . . . . . . . .      24
   2.5   ECHAM5-HAMMOZ computing performances . . . . . . . . . . . . .                28
   2.6   APPENDIX I: Chemical Mechanism . . . . . . . . . . . . . . . . . . .          30
   2.7   APPENDIX II: Fast-J.2 implementation . . . . . . . . . . . . . . . . .        35

vi                                                                                Contents

           2.7.1   Description . . . . . . . . . . . . . . . . . . . . . . . . . . . .   35
           2.7.2   Vectorization . . . . . . . . . . . . . . . . . . . . . . . . . . .   37

3 Trace gas-aerosol interactions: insights from the spring 2001 TRACE-P
  experiment and regional and global effects on trace gases distributions 39
     3.1   Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    40
     3.2   Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   44
     3.3   Trace gas-aerosol interactions in the framework of the TRACE-P campaign 44
           3.3.1   General features of trace gas and aerosol distributions over the
                   TRACE-P region . . . . . . . . . . . . . . . . . . . . . . . . .      47
           3.3.2   Flight DC8-06: Black carbon continental outflow . . . . . . . .       54
           3.3.3   Flight DC8-13: Mineral dust episode . . . . . . . . . . . . . .       60
           3.3.4   Regional impact of the tropospheric chemistry-aerosol coupling        63
     3.4   Impact of the trace gas-aerosol interactions on global burden of O3 and
           O3 -related species . . . . . . . . . . . . . . . . . . . . . . . . . . . .   65
     3.5   Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    68
     3.6   Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . .       69
     3.7   APPENDIX I: TRACE-P P3B flights . . . . . . . . . . . . . . . . . .           72
     3.8   APPENDIX II: O3 and CO global evaluation . . . . . . . . . . . . . .          77

4 Impact of heterogeneous chemistry on the global aerosol distributions
  and optical properties                                                81
     4.1   Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .    82
     4.2   Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   83
     4.3   ECHAM5-HAMMOZ evaluation: global distribution of aerosol burdens
           and optical properties . . . . . . . . . . . . . . . . . . . . . . . . . .    84
           4.3.1   Aerosol mass concentrations . . . . . . . . . . . . . . . . . . .     87
           4.3.2   Aerosol optical properties . . . . . . . . . . . . . . . . . . . .    95
           4.3.3   Differences between ECHAM5-HAMMOZ and ECHAM5-HAM .                    96
     4.4   Influence of trace gas-aerosol interactions on sulfur species and aerosols    98
           4.4.1   SO2 and Sulfate distributions . . . . . . . . . . . . . . . . . .     98
           4.4.2   Mineral dust and black carbon distributions . . . . . . . . . . . 102
           4.4.3   Aerosol optical properties . . . . . . . . . . . . . . . . . . . . 104
     4.5   Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
     4.6   Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . 108

Contents                     vii

5 Conclusions and Outlook   111

Acknowledgements            129

Curriculum Vitae            131