Chapter 3 Model development and evaluation
3.3 Model validation
In order to evaluate the performance of the model, the modelled ClNO2 formation has been compared with ClNO2 measured by Chemical Ionization Mass Spectrometry (CIMS) in Leicester on August 2014 during the fieldwork performed by the University of Leicester at the AURN monitoring station located in the University campus (Figure 3.9) (Sommariva et al., 2018).
The data measured in Leicester are used in this study to establish the initial experimental conditions of the box model. The mean temperature (287.5 K), mean mixing ratios of NO, NO2, O3 (5.95, 10.93, and 20.45 ppbv) as shown in Figure 3.5, and mean concentrations of chloride, and nitrate particulate aerosol (1.49 and 3.57 µgm-3) recorded over August 2014 in Leicester were used. The ten primary VOCs were integrated into the model with their initial mixing ratios taken from measurements during the TORCH campaign carried out in a rural area in Essex, UK in 2003 (Lee et al.,2006) (explained in Chapter 2.4.3). Detail of the model input data is listed in Table 3.3.
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Figure 3. 5 Diurnal profile of NO, NO2, and O3 measured in Leicester during August 2014. Blue, orange, and the grey line represent NO, NO2, and O3 respectively. No data measured for NO and NO2 during the period of 8th August at 7 am to 19th August at 11 am, and O3 from 13th August at 7am to 14th August at 10 am.
Ambient ozone and nitrogen oxides (NOx) emissions rate have been integrated into the model as this is important to sustain the key chemical species (OH, HO2, NO, NO2, O3, and ClNO2) in a steady state level. These species are assumed to be in a steady state level if the variations in their peak concentration remain < 5% for the whole model run period, which is 6 days in this study.
The amount of emissions (ENOx) that has used in this study close to the emission rate (6.94x109 molec cm-3 s-1) that was used by Bright et al. (2011) which was calculated for an urban major road (Bristol Road in Birmingham) by using the UK Road Vehicle Emission Factors, 2009. Vehicle emissions per kilometre driven were determined using vehicle speed emission factors, vehicle fleet composition data, and total activity. The number of vehicles per hour on the major road was rounded to the nearest 500 to become 1500 vehicles per hour, which represents moderate traffic flow for a 30 mile per hour speed limit (Figure 5.1).
Traffic is assumed to be the main source of NOx emissions in the model, and the primary emissions of NO and NO2 were estimated to be 90% and 10% respectively, of the total NOx emissions. The 10% represents the lowest value of the estimated range (10-15%) of NO2 primary emissions in London (Carslaw et al., 2011, Grice et al., 2009).
In urban areas, traffic volume often peaks during the weekday morning and evening due to the daily commute to and from work or school (Knibbs et al., 2011, Ragettli et al., 2013),
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resulting in a greater emission of pollutants when compared to off-peak times or in rural areas (Hitchcock and Carslaw, 2016). Figure 3.6 shows typical average hourly distribution of traffic trips in the UK on weekdays and weekends during 1997-1999. During weekdays, driver’s journeys peak in the morning and the afternoon, while travel during the weekend peak around middle of the day (Charlton and Baas, 2002).
Therefore, NOx emissions peak for two hours during the morning and evening, i.e. during
‘peak-rush hour’ scenario is included in the model (Figure 3.7), that represents a typical emission condition in the UK urban areas during weekdays, i.e. when traffic activity is high.
Figure 3. 6 Hourly distribution of traffic trips during weekdays, weekends in the UK (Charlton and Baas, 2002).
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Figure 3. 7 Modelled NOx emissions scenario represents a typical emission condition in the UK urban areas during weekdays.
The model was simulated for a period of 6 days with a one-minute time step. The first 5 days used as a spin up period to minimize the effect of initial conditions on the concentrations of the output chemical species from the model simulation, as well as to allow the short/intermediate or long lived species like O3 to reach a steady state level (Figure 3.8).
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b)
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Figure 3. 8 Shows the concentrations of short lived species [a) OH, b) HO2, c) CH3O2],
intermediate and long lived species [d) NO2, e) O3] are in steady state level on the 6th day of the model simulation
c)
d)
e)
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Table 3. 3 Initial chemical and physical data applied to the box model
The measured average ClNO2 mixing ratio for the period from 20th to 28th August 2014 which peaked at ~60 ppt is compared with the modelled ClNO2 (peaked at 112.86 pptv) (Figure 3.10A). Since data for NO, NO2, and O3 was not recorded before 20th August, thus the comparison is made for that period (20th to 28th August) only.
However, the modelled ClNO2 mixing ratio which peaked at 112.86 pptv, is comparable with the measured ClNO2 on the 10th August with the peak 114.97 pptv (Figure 3.10B).
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Figure 3. 9 Measured ClNO2 mixing ratio in Leicester on the 10th August 2014 during fieldwork performed by the University of Leicester at the AURN monitoring station on the University campus (Sommariva et al., 2018).
As shown in Figure 3.10A and B, the modelled and measured ClNO2 mixing ratios have relatively same pattern as both modelled and measured ClNO2 peaked in the early morning, followed by almost complete depletion around 3pm as ClNO2 is readily photolysis to release chlorine. The frequency distribution of the modelled and measured ClNO2 was investigated and the result revealed that the modelled ClNO2 has a same distribution (Right skewed) as the measured, with low ClNO2 mixing ratio (< 10ppt) shows a high frequency in both cases (Figure 3.11).
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Figure 3. 10 A) Comparisons between modelled with mean measured ClNO2 over 20-28th August 2014 in Leicester, and B) comparison between modelled and measured ClNO2 on 10th August 2014 in Leicester
B) A)
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Figure 3. 11 Positive-Right-skewed distribution of modelled and measured (over the August 2014) ClNO2.
The general discrepancies between modelled and measured ClNO2 mixing ratios may be related to the variation in emissions or abundance of NOx and VOC, which can affect NOx, O3, N2O5, and thus ClNO2 concentrations. The differences could also be related to meteorological factors such as rainfall or wind speed, which can considerably affect the measurements (Oikonomakis et al., 2018). For example, there is a decrease in measured ClNO2 from 4 to 5am of the local time and an increase in modelled ClNO2 from 6pm which is not observed in the measurement (Figure 3.10B). According the weather station of Met Office (https://www.metoffice.gov.uk/climate/uk/summaries/2014/august), August 2014 was wet and the coolest August since 1992, with rainfall and showers almost every day.
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Further to evaluate the model, the simulated and average measured NO, NO2 and O3 over August 2014 are compared (Figure 3.12). The model relatively well reproduced measured NO2 and O3, but not for NO as the modelled NO peaked at 3 ppb whereas the average measured NO peaked at ~13 ppb, which may be related to the difference in the amount of NO emissions between the model and measurement.
A)
B)
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Figure 3. 12 Comparison between model diurnal with mean measured A) NO, B) NO2. C) O3 for the period 20 – 28th August 2014 in Leicester
3.4 Chapter summary
The MCM box model has been developed to include parameters and reactions describe the N2O5 reaction with aerosol particles. Each single parameter that involved in the N2O5 heterogeneous reactions and ClNO2 formation have been explained in detail, highlighting the main factors that can affect the atmospheric lifetime of N2O5.
The developed model (Chapter 2 and Chapter 3) has been evaluated against measurements to examine the accuracy of the model. The model and measurements are in good agreement for predicting ClNO2 concentrations. Therefore, the model can be used to investigate N2O5 and ClNO2 chemistry and to predict the concentrations of the chemical species in the atmosphere.
The following chapters of this thesis describe the use of the developed MCM box models for the investigation of effect of temperature on ClNO2 formation and chemistry (Chapter 4), the effect of diurnal emission distribution from traffic on the concentrations of the atmospheric chemical species (Chapter 5), and finally to predict the nitrate aerosol formation from N2O5 heterogeneous reactions in the atmosphere.
C)
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