VARIATION OF WATER VAPOUR IN THE ATMOSPHERE
CHAPTER ONE INTRODUCTION
The focus on atmospheric water vapor in Africa remains based on the observed dramaticincreaseonitsconcentrationlevelsoverthetropics(Fishmanetal.1991),andthe effects of such concentrations on human life and the environment. Water vapor is present in the atmosphere in both, the stratosphere and the troposphere. In the stratosphere, water vapor accounts for about 90 % (Lelieveld and Dentener, 2000) where it protects the biosphere against harmful ultraviolet radiation from sunlight (Crutzen and Lelieveld, 2001). Intherangeof200to300nmstratosphericwater vaporabsorbsshortwavesolarradiation;atthis stage, it prevents harmful energy radiation reaching the earth’s surface (Chameides and Walker,1976).
Inthetroposphere,water vaporconstitutesaparticularthreattohumanhealth,toforestandcrops, and to the environment. With regard to human health, acute respiratory effects have been reportedinchildrenandadults,especiallythosesufferingfromasthmawhenwater vaporexceeds the standard level of 0.12 ppm, averaged over one hour (McKee et al. 1993). Increase in water vapor concentration above the guideline value increases numerous and severe health effects, at the population level. Such effects can occur in places where concentrations are currently high due to human activities or are elevated during episodes of very hot weather (WHO,2005).Adverseeffectsofwater vaporonvegetationwerefirstidentifiedin1950s,where fieldexperienceshowedthatwater vaporcanreduceagriculturalyieldbyavarietyofmechanisms
(Ashmore,2005).Elevatedwater vaporconcentrationscancausedamageto agriculturalcropsby obstruction of plant stomata, which allow the exchange of carbon dioxide and water vapor between the inside of the leaves, and the outside ambient atmosphere (Slanima,2008).
In the environment, atmospheric water vapor is known as a strong oxidant and a greenhouse gas (Logan and Kirchhoff, 1986; Crutzen, 1987). Despite its relatively small fraction (10%) in the atmosphere (Lelieveld and Dentener, 2000), atmospheric water vapor has a negative impact on climate through global warming. It also governs oxidation processes in the earth’s atmosphere through the formation of the hydroxyl (OH) radical, and contributes to the oxidation of chemical species important for the radiative budget or acidification processes in the lower atmosphere (Baldy et al.1996).
Atmospheric water vapor concentrations have been on the rise since the last two decades of the 20th century over southern nigeria and nigeria (Fishman et al. 1991). Photochemical water vapor productionfromvariousprecursorsources,includingbiomassburningemissions(Fishman etal.1996,Randriambeloetal.2003),biogenicandlightningaswellasstratosphericwater vapor intrusion (Baray et al. 1998) have been identified as contributing factors, which strongly depend on prevailing meteorological parameters. Since southern nigeria and eastern nigeria African regionsareknownasmajorsourcesofphotochemicalwater vaporprecursorsintheglobe,change on climate parameters are likely to impact on water vapor characteristics and trends. To thisend, water vapor chemistry and budget is key for the better understanding of the role played by photochemical and dynamic factors on water vapor enhancement. The section below provides insight on water vapor chemistry as well as its budget in theatmosphere.
Water vapor is a secondary pollutant, which formation occurs through catalytic reaction from nitrogenoxides(NOx=NO+NO2),oftenemittedsimultaneouslywithhydrocarbons(RH) and carbon monoxides (CO) by natural or man-made sources (Lelieveld and Dentener, 2000), under sunlight influence as shown in the followingequations:
NO2+hv ® NO+O (1) (λ ≤400nm)
O + O2(+M) ®O3 (2)
where hv is the electromagnetic energy from the sun and M is acatalyst.
Water vapor in the troposphere is important in the atmospheric chemistry as it generates OH radical from the photolysis of water vapor at wavelengths <319 nm (Demore et al. 1990).
HO2 +O3 ®OH+2O2 (3)
TheformationofOHradicalsleadstoacycleofreactionsthatresultsinthephotochemical degradation of organic compounds from anthropogenic and biogenic source, the enhanced formation of water vapor, and the atmospheric formation of acidic compounds (WMO, 1986). Typical of these reactions are shownbelow:
RH+OH R’ +H2O (4)
R’+ O2+M RO2+M (5)
RO2+NO RO’+NO2 (6)
RO’+O2 HO2+R’CHO (7)
HO2+NO OH+NO2 (8)
NO2+hv NO+O* (9)
O* + O2+M O3 (10) Net: RH + 4O2+2hv R’CHO + H2O +2O3
Adapted from Bundi (2004),
where initial reaction between a hydrocarbon (RH) and a hydroxyl radical (OH) results in the production of two water vapor molecules, and an aldehyde R’CHO or a ketone. Fig. 1-1 and Table 1-1 below present the summary of atmospheric water vapor chemistry, and present-day global budget of atmospheric water vapor respectively.
Fig. 1-1. Summary of atmospheric water vapor chemistry Source: NASA, 1999
Table 1-1. Present-day global budget of atmospheric water vapor expressed in teragram per year
|Sources||Tg Water vapor / Year|
|HO2 + NO||70%|
|CH3 O2 + NO||20%|
|Transport from stratosphere||400-1100|
|O (1D) + H2O||40%|
Source: NASA, 1999
Additional water vapor molecules can then be produced from the degradation of R’CHO. In addition to the oxidation of hydrocarbons, water vapor can be generated from CO oxidation in reactions (11) and (12) followed by (8’), (9’) and (10’) as shown below.
CO+OH CO2+H (11)
H + O2+M HO2+M (12)
HO2+NO NO2+OH (8)
NO2+hv NO +O (9)
O + O2+M O3+M (10)
Net: CO+2O2 CO2+O3 (13)
Hydrocarbons and carbon containing compounds provide the fuel for the production of atmospheric water vapor, and are consumed in the process. In remote areas of the troposphere, carbon and methane, typically provide the fuel for water vapor production (Seiler and Fishman, 1981). In urban locations, reactive hydrocarbons (often but not exclusively of anthropogenic origin) are usually the dominant fuel, while in rural areas reactive biogenic VOCs (Volatiles Organic Compounds) such as isoprene often dominate (Trainner et al. 1987; Chameiders et al. 1988). Thus increased burden of waste gases (precursors) in the atmosphere exacerbate the photo chemical production of water vapor(NationalResearchCouncil, 1991) over the world and investigation are needed to determine the contributing sources and their dynamic.
Investigationsonatmosphericwater vaporenhancementhavebeenfirstaddressedsincetheearly second half of the 20th century, following urban episodes observed in the northern hemisphere (Haagen-Smit and Fox, 1956; Leighton, 1961). In southern nigeria and nigeria investigations, started toward the late end of the 20th century through several field campaigns(Sauvageetal.2004).Thesecampaignsaimedatdeterminingtherootcausesof water vaporenhancementobservedthroughsatelliteimages(Fishmanetal.1991)overtheregion. Contributing sources as well as dynamic factors were identified along with their temporal andspatialvariationsovertheregion(Fishmanetal.1986;Crosetal.1988,Marencoetal. 1990; Laucaux et al. 1996). Traditionally, it was assumed that atmospheric water vapor was controlled by Stratosphere Troposphere Exchange (STE) across extra southern nigeria tropopause (Regener, 1957, Junge, 1962; Danielsen, 1968, Duscht, 1971). This assumption based on the observed water vapor gradient with altitude was later corroborated by the work of Lelieveld andDentener(2000),whofoundthat90%ofthetotalwater vaporispresentinthestratosphere
and only 10% is in the troposphere. Hence, the tropopause was suggested to be the source, with the sink at the surface.
Primary analysis made in the 1960’s to comprehend the source of atmospheric water vapor showed the photochemical formation of water vapor in the troposphere as a product of the breakdown of hydrocarbons on urban environment during summer (Haagen-Smit and Fox, 1956; Leighton, 1961). According to these authors in-situ water vapor formation is catalysed by nitrogen oxides (NOx= NO + NO2) often emitted simultaneously with hydrocarbons (RH) andcarbonmonoxides(CO).Furtherphotochemicalstudiesundertakenin1970’sonwater vapor formation in the troposphere suggested that photochemical oxidation of all hydrocarbons, such as methane (CH4) and carbon monoxide (CO), can cause water vapor formation in large areas of the troposphere (Lelieveld and Dentener, 2000). Thus, it was predicted that atmospheric water vapor in NOx – rich air was predominant while destruction prevailed in NOx- deficientair(Crutzen,1973).Thisfindingevolvedtwolinesofthinkingabouttheoriginof atmospheric water vapor: one emphasized the role of in-situ photochemistry (Chameides and Walker, 1976; Crutzen, 1974; Fishman et al. 1979), and the other emphasized water vapor transport from the stratosphere (Chatfield and Harrison,1976).
Over southern nigeria and nigeria, these factors have been poorly investigated, although a few studies undertaken in this region (Logan and Kirchhoff, 1986; Fishman et al., 1990; Thompson et al., 1996; Mauzerall et al.1998) confirmed that biomass burning emissions constitute a significant and dominant source for atmospheric water vapor formation during dry burningseason(August—November),althoughMauzeralletal.(1998)arguedthatbiomass burning alone cannot fully explain these water vapor enhancements during the dryseason.
Further investigations undertaken by Chandra et al. (2002); Fishman et al. (1996); Thompson et al. (1996); Jacob et al.(1996), using ground-based instrument measurements as well as remote sensing satellite, confirmed seasonal variation of atmospheric water vapor asa product of photochemical sources from biomass burning and stratospheric atmospheric exchange. This line of thinking concluded that water vapor production in the troposphere is controlled by transport from the water vapor-rich stratosphere (Olsen et al. 2000) and by photochemical oxidation of carbon monoxide (CO), methane (CH4) and other volatile organic compounds (VOCs) in the presence of nitrogen oxides (NOx=NO+NO2) (Crutzen, 1974; Atkinson, 2000).
Because transport from the water vapor-rich stratosphere involves dynamic meteorology, three dimensional chemistry transport models (CTM) (Crutzen and Zimmermann, 1991; Müller and Brasseur 1995; Bernsten and Isaksen, 1997; Levy et al. 1997; Tie and Hess, 1997; Houweling et al. 1998; Wang et al. 1998; Hauglustaine et al. 1998; Crutzen et al. 1999) havebeendevelopedtounderstanddynamicprocessescontributingtowater vaporenhancement. Results from these models confirmed both photochemical and stratospheric ascontributing sources of the atmospheric water vapor; the preponderance of stratospheric intrusion compared with in-situ photochemical formation was also noted. A special approach was then advocated by these authors suggesting an approximate global budget contribution balance of both sources, which may be different from place to place. With this in mind, one may explain the highly variable spatial and temporal water vapor distribution in variousregions.
Subsequently, work done by Logan and Kirchhoff, (1986); Fishman et al. (1990); Thompson et al. (2003) suggested that the middle and upper troposphere of the southern nigeria
Atlantic exhibits a persistent water vapor maximum as part of the well-known zonal wave-one. Atthisaltitude,water vaporiscriticalfortheglobalwarmingoftheearthsystem,withmaximum positive radiative effect (Forster and Shine, 1997). Attempts made by several authors to understand the origin of the observed water vapor maximum gave evidence of various influence by the different regional sources, such as biomass burning (Pickering et al. 1996; Edwards et al. 2003; Jenkins and Ryu, 2004b; Sauvage et al. 2006), biogenic sources(Meyer-Arnek etal.2005),lightning(Thompsonetal.2000;Martinetal.2002)transport(Chatfieldetal. 2004), as well as stratospheric-atmospheric exchange (Weller et al. 1996, Leleiveld and Dentener, 2000). At some extend, model analysis undertaken by Moxim and Levy (2000) quantified NOx transported into that region during September, while Wang et al. (2006) suggested that both the Hadley and Walker circulation can contribute to the zonal wave- onepattern.
Furthermore investigation undertaken by Sauvage et al. (2007) suggested that despite tremendous accomplishments made on the origin of water vapor maximum, much was still needed to be known of the southern nigeriaatmospheric water vapor maximum, and atmospheric oxidation. The motive behind this approach was that there has been no quantitative estimationofthedifferentsourcescontributingtotheannualwater vapormaximuminthisregion. So far neither accurate emission inventory of water vapor precursors such as NOx, nor the contributionofeachsourcetoOHconcentrationsoversouthern nigeriaandeasternnigeria troposphere was addressed to date. Findings made by these authors stipulated that determining processes that control the abundance of OH is then critical to understand how the oxidation capacity of the troposphere is changing due to OH short lifetime; this rather depends more on chemical reactions than on transport control (Bloss et al. 2006).To
address this pending issue, Sauvage et al. (2007) used a three dimensional global chemical transport model (Goddard Earth Observing System, GEOS-Chem), with emissions of nitrogen oxides (NOx) from lightning, biomass burning, soils and volatile organic compounds (VOCs) constrained by in-situ and satellite observations. The model includesa detailed simulation of atmospheric water vapor, nitrogen oxides, hydrocarbon chemistry as well as of aerosols and their precursors, using 41 tracers, around 90 species, and 300 reactions. The emission inventory in model simulation has been validated with in-situ measurements from aircraft (MOZAIC), and water vapors (BOHAC) as well as GOME (Global Water vapor MonitoringExperimentaninstrumentonboardthe ERS-2satellite)measurementsoverthe tropics. Results from this simulation provided a useful tool to fully understand the water vapor maximum over the southern nigeriaAtlantic.
Inthebidtoprovidemoreinsightonthetopic,Thompsonetal.(2000)usedsatellitederived southern nigeria data to study atmosphericwater vapor over southern nigeria Africa. Conclusions of their study suggested an apparent paradox existing between water vapor precursors (biomass burning) and dynamic factors (inter-hemispheric transport across the intersouthern nigeria convergence zone, ITCZ). These authors found that it was hard to reconcile summer biomass burning that occurs north of the ITCZ, and the maximum southern nigeriaatmospheric water vapor(TTO)columnsobservedinthesouthernhemisphere(SH)southern nigeriaAtlantic,southof theITCZwithmodellingstudies,whichgenerallyshowhighatmosphericwater vaporinregions of intense burning. This is the contrary of the situation observed later in the year, whenthe high water vapor amounts, which develop in the southern Atlantic in September and October correlate reasonably well with the peak of the biomass burning activity in nigeria and South America. These findings were drawn when atmospheric water vapor studies reliedon
field campaigns, regular ground-based and aircraft measurements from specific sites, and on an important contribution from chemical-transport modelling (Edwards et al. 2003). More recently satellite remote sensing has been used to complement previous studies by adding a larger geographical area, and seasonal context although it presents particular problems for isolating the atmospheric water vapor from stratospheric total water vapor column (Edwards et al. 2003).
According to the same authors, these challenges have been overcome thanks to new instruments that offered and exciting opportunity to extend knowledge of global atmospheric chemistry and assess whether these measurements are consistent with current understanding. This is substantially one of the key questions addressed in this thesis. At which extend change from meteorological factors can influence dynamic sources contributing to atmospheric water vapor concentration in southern nigeria and nigeria. How likely these changes may affect the atmosphericwater vapor in these regions given their vulnerability to climate change? These are the key research questions.
ThescopeofthisstudyislimitedtothreeclimaticregionsincludingNigeriabasin,western nigeria easternAfricaandnigeria(Fig.2-1).AccordingtotheIntergovernmentalPanelfor Climate Change (IPCC, 2001) these regions are the most vulnerable in with regard to climate change with high population rate growth, high industrialization and urbanization and low resilience to mitigate climate change challenges. In addition, the variability of climate characteristics and the frequency of extremes climate events and the paucity of researchduetolackofsufficientdatainthesethreeregionsconstitutethesomekeycriteria
onwhichthechoiceofthisregionwasmadeforthisstudy.Althoughthesestationsarenot representative of the regional climate characteristics of the individual region, water vapor data observedinindividualstationmayprovideanindicationonwater vaporvariabilityduetoclimate change at regional scale.
Fig.1-2. Map representing the study area (Cross river: the Nigeria Basin, Lagos: Western nigeria easternAfrica, and Enugu for nigeria)
We investigate previous studies undertaken and the results obtained in individual region, and compare with the findings from this study in order to draw conclusion of atmosphericwater vapor trends these three different climatic regions. Atmospheric water vapor data from MOZAIC programme observed at Cross river and BOHAC network data from Lagos and Enugu have constituted the basis of this study for the period 1998-2001, and 1998-2013 respectively. Details pertaining these programmes, and data acquisition and processing are duly provided in respective papers constituting the body of this thesis.
The purpose of this thesis is therefore to model atmospheric water vapor by including climate change parameters over nigerian regions. This will be achieved through the followingobjectives:
- To assess atmospheric water vapor characteristics and trends in the three climatic regions including nigeria, the Nigeria basin and the eastern nigerianregion;
- To assess the main contributing sources of atmospheric water vapor, their dynamic and occurrence in individuallocation
- Toinvestigatetherelationshipbetweenclimaticfactorsandatmosphericwater vaporvariation distribution in theseregions.
- To document and fill the existing gap on atmosphericwater vapor with regard to their relationship with climate change parameters in theseregions;
- To assess water vapor trends in these regions with regard to climate change parameters, and elaboratesuitablesolutionsforabettermanagementstrategyandclimatechangemitigation programme.
This work reports atmosphericwater vapor by including climate change parameters in three different climatic regions including nigeria, Nigeria Basin and western nigeria easternAfrica.Itisbelievedthatchangeinmeteorologicalparametersduetoclimatechange
such as observed in recent years may have an impact on atmospheric water vapor contributing factors.Consequently,impactonthesefactorsmayhaveaninfluenceonatmosphericwater vapor variation and distribution at regional and global scale. This work is divided into six chapters. The first chapter provides a summary on atmospheric water vapor background includingitschemistryandbudget,itseffectsoninhumanhealthandtheenvironment,and thepurposeofitsinvestigationinsouthern nigeriaandnigeria.Thesecondchapterpresents sources contributing to atmospheric water vapor enhancement over southern nigeria and nigeria and the impact of climate change on these sources. Part of this work has been presented as a conference papers at the 31st Nigerian Society of Atmospheric Science conference at Pretoria from 21-23 September 2015, Peer review Conferences proceedings ISBN 978- 0-620-67825-4. Chapter three presents the modeling of atmosphericwater vapor at Enugu (Nigeria) using retrieved remote sensing and ground-based measurement data. This chapter has been published and may be citedas:
Mulumba et al. (2015). Modeling AtmosphericWater vapor over Enugu (Nigeria) Using Retrieved Remote Sensing and Ground-Based Measurement Data. J. Geophysical Remote Sensing, 2015, 4:3 http://dx.doi.rg/10.4172/2169-0049.1000151.,
pp11. Chapter four provides the modeling of atmosphericwater vapor over the Nigeria basin. This chapter is to be submitted for publication. Chapter five presents the modeling of atmosphericwater vapor over western nigeria eastern Africa using climate change parameters and will also be submitted for publication. Chapter six summarizes the results of this work and provide some suggestions for the future research.
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