=
wind speed and =
concentration (cm-3):
No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other University, or any other institution of learning
Abstract
Novel measurements of aspects of surface atmosphere exchange of aerosol are presented. These were gathered using a custom designed and built eddy covariance system based around a modified commercially available particle counter.
Aerosol flux measurements have been made above a managed grass canopy of variable height. The analysis shows that accumulation mode (0.1 m < Dp < 0.2 m) aerosol deposition velocities are 1.13 mm s-1 and 0.87 mm s-1 to long and short grass canopies respectively. The corresponding fine mode deposition velocities are 0.275 mm s-1 and 0.066 mm s-1, where the long canopy height is 0.65 m – 1.0 m with a roughness length of 0.063 m and the short canopy height 0.07 m – 0.14 m with a roughness length of 0.022 m. These fine mode fluxes are shown to be representative of aerosol in the diameter range 11nm < Dp < 100 nm.
Fertilisation of the short grassland with 100 kg N ha-1 is shown to cause aerosol growth, the postulated cause being simultaneous condensation of ammonia and nitric acid onto the surface of existing particles. Diameter growth rates at 11 nm were calculated, ranging between zero and around 3490 nm hr-1. This large systematic growth is shown to persist for around five days following fertilisation of the canopy.
Measurements have been
made above the City of Edinburgh (UK). Aerosol concentration, number
flux and emission velocity (11 nm < Dp < 3 m;
D3) are presented, and the results shown to be
representative of the sub-100 nm size interval. Aerosol (D3)
concentrations are found typically to range between 3,000 cm-3
and 20,000 cm-3. The mean D3 aerosol flux is
found to be 42,500 cm-2 s-1, with the mode
being 30,000 cm-2 s-1 and a typical range of
9,000 cm-2 s-1 to 90,000 cm-2 s-1.
The mean and mode emission velocities are 45 mm s-1 and 35
mm s-1 respectively, typically ranging from 20 mm s-1
to 75 mm s-1. It is shown that urban aerosol concentration
can be adequately modelled by the following, where
is the stability parameter, A = 13,000, b = 1.6,
=
wind speed and =
concentration (cm-3):
The energy balance for an urban area is examined. It is demonstrated that the non-radiative heating is equivalent to the addition of 44 W m-2 to the system. Of this additional energy, which is assumed to be anthropogenic in origin, it is estimated that around 30% is derived from fossil fuel combustion within the city.
Contents
1.1 Aerosol Exchange 2
1.1.1 Deposition 2
1.1.2 Micrometeorological Deposition Measurements 3
1.2 Ammonia – Aerosol Interactions 3
1.3 Urban Aerosol 4
2 Micrometeorology 5
2.1 The Boundary Layer 5
2.1.1 Turbulence structure 5
2.1.2 Boundary Layer Wind Speed Profile 6
2.2 Atmospheric Stability 8
2.2.1 Introduction 8
2.2.2 Virtual Potential Temperature 9
2.2.3 Obukhov Length and the Stability Parameter 10
2.2.4 Monin – Obukhov Similarity Theory 11
2.2.5 Townsend’s Hypothesis and Similarity 12
2.2.6 Stability Correction 12
2.3 Gradient Flux Measurements 13
2.3.1 Introduction 13
2.3.2 Gradient Method Theory 14
2.4 Eddy Covariance 16
2.4.1 Introduction 16
2.4.2 Eddy Covariance Definition 17
2.4.3 Sensible Heat Flux 18
2.4.4 Latent Heat Flux 18
2.4.5 Aerosol Number Flux 19
2.4.6 Vertical Sensor Alignment 20
2.4.7 Sensor Requirements 21
2.4.8 Eddy Covariance Limitations 23
2.4.9 Stationarity 23
2.4.10 Fetch and Honegeneity 24
2.5 Interpretation of Fluxes 25
2.5.1 Deposition Velocity 25
2.5.2 Resistance Analogy 27
2.6 Fourier Analysis 30
2.6.1 Power Spectrum 30
2.6.2 Properties of the Atmospheric Power Spectrum 31
2.6.3 Co-spectral Analysis 34
2.7 Probability Density Analysis 35
3 Aerosol Flux Measurement System 37
3.1 Introduction 37
3.1.1 Necessity for Measuring Fluxes Using a CPC 37
3.2 Principles of the CPC 38
3.3 Flux Measurement Using a CPC 40
3.4 CPC Flux System Software 42
3.4.1 Logging Software 42
3.4.2 Flux Calculation Software 46
3.4.3 Fourier Analysis Tools 48
3.4.4 Data Visualisation 48
4 Aerosol Deposition to Grassland 49
4.1 Introduction 49
4.1.1 Aims of the Integrated Experiment 49
4.1.2 The Site 50
4.1.3 Measurements 53
4.1.4 Management Practices 54
4.2 Accumulation Mode Results 55
4.2.1 Surface Deposition Velocity 55
4.2.2 Deposition Velocity Above a Managed Grass Canopy 56
4.3 Performance of the CPC System 58
4.3.1 Power Spectral Analyses 59
4.3.2 Vertical Wind Speed Power Spectra 60
4.3.3 Aerosol Concentration Power Spectra 61
4.3.4 Aerosol Flux Co-spectrum 63
4.3.5 Interpretation of Diameter Range of Aerosol Measurements 66
4.4 Fine Mode Results 67
4.5 Aerosol Deposition Model 68
4.6 Fine Mode Deposition Velocity Trends 70
4.7 Summary of Results 71
5 Direct Aerosol – Ammonia Interactions 73
5.1 Introduction 73
5.1.1 Measurements 75
5.2 Apparent Emissions Following Fertilisation 75
5.2.1 Mechanism of Aerosol Growth 76
5.3 Aerosol Growth Model 77
5.4 Availability of Charge Balancing Ions 81
5.5 Aerosol Growth Rate Calculation 83
5.6 Aerosol Growth Results 87
5.7 Location of Aerosol Growth 91
5.8 Summary of Results 95
6 Urban Aerosol Fluxes 97
6.1 Introduction 97
6.1.1 Measurement and Interpretation 99
6.1.2 Scope of the Analysis 100
6.2 Measurement Site and Methods 101
6.2.1 Instrumentation 103
6.3 Results 106
6.3.1 Typical Urban Aerosol Fluxes and Concentrations 106
6.3.2 Dependence of Fluxes on Traffic Activity 110
6.3.3 Aerosol Emission Velocity 112
6.3.4 Bi-directionality of Aerosol Fluxes 116
6.4 Aerosol Concentration Parameterisation 120
6.5 Concluding Discussion 122
6.6 Further Work 123
7 Urban Micrometeorology 124
7.1 Introduction 124
7.1.1 Backgrounnd 124
7.1.2 Investigations of the Urban Heat Balance 124
7.2 Urban Heat Balance 125
7.3 Power Spectral Behaviour 128
7.3.1 Vertical Wind Speed 129
7.3.2 Air Temperature 130
7.3.3 Fine Aerosol Concentration 131
7.4 Co-spectral Behaviour 131
7.4.1 Sensible Heat Flux 131
7.4.2 Fine Aerosol Flux 133
7.5 Probability Density Analysis 135
7.5.1 Temperature 135
7.5.2 Aerosol Concentration 138
7.6 Summary 142
8 Conclusions and Further Work 143
8.1 Summary 143
8.1.1 Aerosol Deposition to Grassland 144
8.1.2 Ammonia – Aerosol Interactions 144
8.1.3 Urban Aerosol Production 145
8.1.4 Urban Micrometeorology 146
8.2 Further Work 147
8.2.1 Aerosol Fluxes Over Semi-natural Surfaces 147
8.2.2 Urban Aerosol 148
A – Aerosol Deposition Model 150
Model Equations 150
Summary 153
B – A Note on Notation 155
C – List of Symbols 156
Roman Alphabet 156
Greek Alphabet 160