Measurement and modelling of pollutant emissions from Hong Kong

Environmental Modelling & Software 17 (2002) 87–94 www.elsevier.com/locate/envsoft

Measurement and modelling of pollutant emissions from Hong Kong J.N. Carras *, M. Cope, W. Lilley, D.J. Williams CSIRO Energy Technology, PO Box 136, N Ryde, NSW, 1670, Australia

Abstract During November 1997 a detailed airborne investigation of air pollution in the Hong Kong region was undertaken. The airborne investigation formed part of a larger study funded by the Hong Kong Environmental Protection Department (EPD) and included the development of a state of the art numerical air quality modelling system to simulate air pollution in the Hong Kong region. The system consisted of a numerical weather prediction module, a prognostic air–chemistry/transport model, an emissions inventory system and a Graphical User Interface for display of results and preparation of simulations. The purpose of the airborne investigations was to provide data on the fluxes of selected pollutants arising from or entering the Hong Kong airshed as a check on the inventory. In addition the aircraft was to provide data on other pollutants of interest particularly with respect to the formation of photochemical smog. This paper describes the inventory data obtained from the aircraft and makes comparisons between the predictions of the model and the aircraft data for one of the days when the aircraft was able to be used to estimate the total fluxes of NMHC and NOx from the study area.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Aircraft plume data; Air quality modelling; Inventory validation; Urban air pollution; Plume tracking

1. Introduction In 1996 the Hong Kong Environment Protection Department (EPD) commissioned ERM Hong Kong to develop a multi scale, multi species, state of the art modelling system, to be used for investigating the impact of different development and control scenarios in the Special Administrative Region of Hong Kong (SAR). The development of the modelling system (know as PATH — Pollutants in the Atmosphere and their Transport over Hong Kong) was one component of the Territory-Wide Air Quality Modelling Study. Another component was the generation of modelling data sets for eleven case-study days. These case-study days covered a range of meteorological conditions including those conducive to the generation of photochemical smog and aerosols. Daily-average PM10 concentrations from seven of the case-study days were used to generate an estimate of annual average PM10. The case-study days were also used to validate components of PATH, using available

* Corresponding author. E-mail address: [email protected] (J.N. Carras).

near-surface and upper air observations of air quality and meteorology. In order to provide another level of data in addition to that normally available, EPD also funded an airborne investigation of air pollution within the SAR, the Airborne Inventory Validation Study (AIVS). The AIVS was carried out by CSIRO Energy Technology and its objectives were as follows: 앫 to measure the fluxes of carbon dioxide (CO2), nitrogen oxides (NOx), methane (CH4), non-methane hydrocarbons (NMHC), sulfur dioxide (SO2), particles and volatile organic compounds (VOCs) emitted into the atmosphere of Hong Kong; 앫 to provide windfield data and measurements of selected atmospheric pollutants for the Hong Kong region for comparison with model predictions; and 앫 to provide measurements and speciation (by grab sampling and subsequent chemical analysis) of VOCs in the Hong Kong region. In the current paper we describe briefly the numerical model developed, the aircraft data obtained and provide a comparison between the aircraft measurements and the

1364-8152/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 4 - 8 1 5 2 ( 0 1 ) 0 0 0 5 5 - X

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model predictions for a day when the aircraft data allowed estimates of the total fluxes of NMHC and NOx from the study area.

2. Model description The major role in developing the modelling system was undertaken by CSIRO Atmospheric Research who acted as sub-contractor to ERM. The main modelling approach has been described by Cope et al. (2000). The development of the PATH (Pollutants in the Atmosphere and their Transport over Hong Kong) system included the adaption, coupling and operation of three substantial numerical modules: a numerical weather prediction system (MM5, Dudhia, 1993), an emissions inventory module system (EMS-95, Wilkinson et al., 1994), and an air quality module chemical/transport model (SAQM, Chang et al., 1997). Fig. 1 (adapted from Cope et al., 2000) shows the overall structure of the modelling system illustrating the manner in which the modules are combined. SAQM, the chemical/transport model, may be considered a derivative of the Regional Acid Deposition Model (RADM; Chang et al., 1987). Major advances that were incorporated into SAQM include: implementation of a transport equation compatible with meteorological data from the non-hydrostatic version of MM5; implementation of a

Fig. 1. Schematic diagram of the PATH modelling system showing the principal computational modules (adapted from Cope et al., 2000).

more accurate numerical algorithm for calculating advection (in the horizontal and vertical); the option for using three alternative gas-phase chemical mechanisms (note that the CB-IV mechanism, — Gery et al., 1989 was used exclusively during the PATH study); improved description of dry deposition; and one-way or two-way nesting algorithms. SAQM also has the capability of performing plume-in-grid calculations. A number of additional changes were undertaken during the course of the current study to adapt SAQM to the requirements of PATH. These changes included: incorporation of modules for modelling the gas-phase production of sulfate and nitrate aerosol (Saxena et al., 1986), and for performing Process Analysis (Wang and Jeffries, 1997); incorporation of a module for modelling the transport and deposition of 18 (size- and sourcefractionated) primary aerosol species; incorporation of a deformation-based horizontal diffusion algorithm, and revisions to the vertical dispersion algorithm. SAQM was configured with a five-level series of oneway nested horizontal grids (grid spacings of 40.5 km, 13.5 km, 4.5 km, 1.5 km and 0.5 km). Each grid was assigned 49×49 points in the horizontal. As prescribed, the outer grids enable explicit modelling of sources in southern China, and the inner grids enable sources within the SAR to be accurately resolved. Thus the system is designed to model the evolution of the Hong Kong urban plume within a large-scale southern-China air mass. SAQM was configured with 15 levels in the vertical, with one-for-one matching of the MM5 levels within the lowest 1000 m of the atmosphere. The lowest level of the model was centred approximately 10 m above ground level and the top level was located at 20,970 m. Initial and boundary concentrations for the chemical transport modelling were developed with reference to outcomes reported in the PEM-West B study (Talbot et al., 1997). Air pollutant fields on the 40.5 km grid were pre-conditioned by integrating SAQM for up to a 48hour “spin-up” period prior to modelling each case study. The spin-up fields were then used as the initial conditions for all grid resolutions. The PATH modelling system has been developed as a single, cohesive software package which can be operated from a graphical user interface (GUI). The GUI resides on a personal computer and may be used to set up and/or modify the input data sets for the numerical modelling components of PATH. The GUI is able to upload data sets onto a Silicon Graphics workstation and instigate the execution of the models. The GUI also provides the capability of seamlessly interrogating output data sets from the chemical/transport model via a graphical information system. The software system is extensively documented, including a comprehensive online help system at the level of the GUI.

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3. Airborne experimental methods 3.1. Measurement of fluxes A number of workers have used airborne techniques to estimate total fluxes of emissions from distributed sources, such as cities. The basic concept is that by measuring the cross wind concentration distribution in both the vertical and lateral directions the flux of material passing through a plane at right angles to the wind direction can be calculated. The technique requires the concentration and wind fields to be mapped simultaneously by aircraft sensors in sufficient detail to reduce the uncertainty associated with the flux calculation. Carras et al. (1991) used this technique to evaluate the flux of methane from Sydney, Australia while Williams et al. (1995) used the same technique to estimate the total flux of non methane hydrocarbons (NMHC) from Perth, Australia. Lind and Kok (1999) described their use of the technique to estimate the fluxes of NOy and CO from the Hong Kong region. These latter authors reported a flux of NOy of 5.4×1025 molecules s⫺1 emanating from the Hong Kong region, or 4.13 kg s⫺1 expressing NOy as NO2. 3.2. Aircraft description and instrumentation In the current study the aircraft used was a twin engine Cessna 404. The aircraft was flown to Macau International Airport (from its home base in Perth, Western Australia) where it was based for the duration of the airborne campaign. Navigational data were obtained from a Garmin global positioning system (GPS). Wind data was obtained from the on-board aircraft instrumentation (Shadin) which accepted data from the GPS system and an in-built gyrocompass. Outputs included latitude, longitude, track, heading, ground speed, air speed, pitch, yaw and roll, as well as wind speed and direction. Data from the Shadin were logged at the rate of 1 Hz on the logger hard disk and backed up to floppy disk at the conclusion of each flight. Temperature measurements for lapse rate determination were made with an on-board Vaissala resistance thermometer and the aircraft’s outside air temperature indicator. For the chemical measurements a stainless steel gas sampling inlet (2.5 cm in diameter) was fitted on the starboard side of the fuselage through a false window panel, behind the plane of the propellers but ahead of the engine exhaust. Sampled air was ducted from this inlet via a polyethylene sample tube to all of the chemical sensors, except for the ozone instrument. The ozone sample line was made entirely of Teflon and was inserted through the polyethylene tube so that it was flush with the outside air, ensuring that no ozone was lost to the main sample line.

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Real-time chemical analysers mounted in the aircraft and used during the campaign are described in Table 1. Calibrations of the chemical sensors were carried out before and after the field programme for all the instruments, and also during the measurement programme for the ozone, carbon dioxide and methane/hydrocarbon detectors. The CH4/NMHC, CO2 and CO monitors used in the aircraft are altitude-sensitive. In the concentrations quoted in this paper the effects of pressure, temperature, time response and zero offsets have been taken into account as far as is possible.

4. Results and discussion 4.1. Aircraft data Total fluxes of material entering or leaving the airshed can be estimated from the aircraft measurements by the expression: Q⫽

冕冕

uc(x,y,z)dydz

(1)

where Q is the flux of material, u the wind speed at right angles to flight path, c the concentration of pollutant and x, y and z the downwind, crosswind and vertical dimensions. From a knowledge of the plume concentrations and wind speed, the above expression can be approximated from the aircraft data and estimates of the fluxes of pollutants calculated. In practice the traverses must be done sequentially, which may take 40 to 60 minutes. The calculated fluxes therefore refer to an averaging time corresponding to the duration of the flux measurements. It has further been assumed that the mixing height did not changed significantly over this period, which in a maritime situation is well justified. The usefulness of flights for inventory validation depends heavily on the occurrence of favourable winds during the field campaign. Ideally winds should be about 5 m s⫺1 in strength and with a coherent direction. In Hong Kong, further constraints were imposed by air traffic control, which led to some restrictions of the flight paths. For the flights along the western perimeter of the SAR this required flying close to the coast along Lantau Island (see Fig. 2). This meant that the effects of local and terrain influenced winds could not be avoided. One day, 17 November, was suitable for inventory calculations to be made. This was a day of relatively strong northerly winds. However, the strong winds meant that the concentrations were less than ideal. Nevertheless enough traverses were carried out to allow an estimate of the total flux of material leaving the southern boundary of China and the SAR to be made. The overall flight path of the aircraft is depicted in Fig. 3.

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Table 1 Chemical detectors carried in the a2ircraft Instrument

Species

Approximate response time

Detection technique

Monitor labs NOx (Dual Channel) Bendix ozone analyser Horiba methane and total hydrocarbon BINOS CO/CO2 Particle PAH detector TSI Laser particle counter

NO O3 CH4 total HC CO/CO2 PAH Optical diameter

2s 2s 30–40 s 1s 5s 1s

Gas phase chemiluminescence with ozone Gas phase chemiluminescence with ethylene Flame ionisation detector Non-dispersive infra-red Photo-ionisation Light scattering

Fig. 3.

Fig. 2. SAQM computational domains as configured for the PATH study (adapted from Cope et al., 2000).

Outline of flight paths for the day 17 November 1997.

The first three traverses were to the south of Hong Kong and allowed the total flux of material leaving southern China and the SAR to be estimated. The last two traverses were along the border between China and the SAR and allowed the emission fluxes from southern China to be estimated. The SAR emission fluxes were determined as the difference between the two fluxes. The wind speed data showed a decrease in the lee of Hong Kong Island at the three altitudes flown. This was factored into the estimates of the fluxes. Fig. 4 shows the chemical data from the 17th November for the traverse beginning at 0824 hrs and proceeding from west to east at an altitude of 500’ above mean sea level. Note that some of the signals have been offset for purposes of display. From data such as those displayed in Fig. 4 and the measured wind speed and direction the emission fluxes for non methane hydrocarbons and NOx were calculated as shown in Table 2. Since it is possible that the traverses did not adequately capture the power station plumes from the

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Fig. 4. Chemical sensor data for the traverse beginning at 0824 on 17 November, south of Hong Kong, traverse from west to east at an altitude of 500’. Table 2 Results of inventory flight (17 November) Source

Traverse (start time)

NMHC (kg s⫺1)

NOxa (kg s⫺1)

Southern China

0936 0952

1.94 1.24 3.18 2.76 1.91 1.13 5.80 2.62

1.86 1.13 2.99 1.83 1.80 1.96 5.59 2.60

Total (southern China) Hong Kong plus southern China

0832 0843 0856

Total (Hong Kong plus southern China) Total (Hong Kong) a

(as NO2).

SAR, the emission fluxes determined are likely to reflect the lower level sources from the SAR. The NOy result of 2.6 kg s⫺1, obtained from the aircraft in the current study for Hong Kong, may be compared with the value of 4.13 kg s⫺1 for the Hong Kong plume reported by Lind and Kok (1999) and obtained from measurements made during October 1994. It is interesting that while the flight paths flown in the current study were essentially east–west because of the prevailing northerly winds the data obtained by Lind and Kok were for a general easterly flow.

spective and because conditions were similar to November 17. (The day on which the aircraft data were obtained was not modelled explicitly as detailed data for this day were not available at the time the calculations were performed). The modelled crosswind fluxes were derived from Eq. (2), for east–west traverses located to the north and south of Hong Kong (see Fig. 3). Qmod(x)⫽

冕冕

uH[c(x,y,z)⫺cthreshold](c(x,y,z)

(2)

⫺cthreshold)dydz

4.2. Comparison of aircraft-based and modelled pollutant fluxes Two periods were chosen for modelling predictions. These were the 22–24 November 1995 and 2–4 December 1995. These days were chosen because they had been well characterised from a meteorological per-

Note that Eq. (2) differs from Eq. (1) through the inclusion of Heaviside’s unit step function and a threshold concentration cthreshold. This has been done in order to investigate the sensitivity of the flux estimates to a range of assumed instrument baseline concentrations

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(defined as the lowest detectable concentration by the instrument). The modelled crosswind integrated fluxes were calculated using concentration fields taken from the 4.5 km SAQM grid (see Fig. 2). This was done because the emissions database for this resolution included emission estimates for the Guangdong Sheng region. The modelled crosswind integrated fluxes were calculated for each hour of a simulation period. Instrument threshold concentrations of 0, 2 and 4 ppb were considered for the NOy calculations. Threshold concentrations of 0, 10 and 20 ppb-C were assumed for the NMHC calculations. The modelled NOy concentration was calculated as the sum of: NOy⫽NO⫹NO2⫹HNO3⫹PAN⫹N2O5

(3)

in order to account for the instrument tendency to oxidize all nitrate compounds when determining the concentrations of NOy. Note that nitrate aerosol hasn’t been included because this particulate species may have been filtered from the air sample prior to measurement. Note also that organic nitrate compounds have not been included. Thus, the modelled NOy flux may be considered a lower bound estimate. The modelled NMHC concentration is given by the sum of: NMHC⫽ETH⫹OLE⫹PAR⫹TOL⫹XYL⫹ISO

(4)

where compounds on the RHS of Eq. (4) are representative of CB-IV hydrocarbon species (ETH — ethylene, OLE — higher olefins, PAR — paraffins [not including methane], TOL — toluene, XYL — xylene, ISO — isoprene),. Note that the carbon mass, speciated as nonreactive, has not been included in Eq. (4). This may amount to up to 苲10% of the total and therefore the modelled NMHC flux may be considered a lower bound estimate. The calculated fluxes were filtered to eliminate hours for which the northerly component of the wind speed was ⬍5 ms⫺1. The fluxes were also filtered to eliminate cases where the mixed-layer height was less than 500 m. The filtering stage was included in order to restrict consideration to cases in which the meteorological conditions were similar to those observed during the aircraft observation period. Table 3 shows the results of the simulations along with the estimates from the aircraft. The modelled flux estimates of NMHC for mainland China for the November 1995 simulation range between 12 and 1.9 kg s⫺1 depending on the value chosen for the instrument threshold. The corresponding values for the December 1995 event were 6.8 to 2.4 kg s⫺1. The zero threshold flux estimates for the two modelled events differ by a factor of about two (ie 6.8–12 kg s⫺1). This difference is due to the fact that the modelled mixing depths were are

larger for the November 1995 event and a large component of the modelled mass flux for this event resulted from the background NMHC concentrations, which were specified by the boundary conditions for the model. The modelled NMHC fluxes for the two events decreased in magnitude as the assumed instrument threshold concentration increased to 10 ppb-C and 20 ppb-C. For the November 1995 event, the calculated flux decreased from 12 kg s⫺1 to 7 kg s⫺1 and to 1.9 kg s⫺1. A similar pattern of decrease is also event in the case of the December 1995 event (6.8 to 4.6 to 2.4 kg s⫺1). Given that the instrument threshold most likely lies between 10 and 20 ppb-C this suggests that the modelled NMHC emission fluxes, for mainland China for both events, are are not inconsistent with the value observed by the aircraft on 17 November 1997. A similar conclusion may be drawn for the NMHC flux from Hong Kong. In this case, a value of 2.6 kg s⫺1 was determined on the basis of the aircraft data. The model estimates range between 1.4 and 3.5 kg s⫺1. The Hong Kong NMHC flux estimates are less sensitive to the assumed instrument thresholds than are the mainland China estimates. This is a result of the higher (relative to the threshold concentrations) NMHC concentrations present in the Hong Kong plume. In the case of the November 1995 event, the observed flux is bounded by the 10- and 20 ppb-C threshold results. In the case of the December 1995 event, the observed flux is underestimated for all modelled threshold cases. Nevertheless, the level of agreement between the modelled and observed flux is reasonable. Observed and estimated NOy fluxes for mainland China and for Hong Kong are also given in Table 3. The modelled NOy flux estimates for mainland China ranged from 苲0.2 to 苲5 kg s⫺1 for the event of November 1995, with a large sensitivity evident to the assumed instrument threshold concentration. For the December 1995 event the corresponding range was 0.4 to 2.6 kg s⫺1. There was an approximate factor of two difference between the emissions calculated for the two events for zero threshold. Given that the the instrument threshold is between 苲1 to 2 ppb, then the modelled estimated NOy flux may be considered to be comparable to the value obtained from the aircraft observations. The estimated NOy fluxes for Hong Kong for the November 1995 event ranged from 4.0 to 4.9 kg s⫺1 while the values for the December 1995 event ranged from 2.2 to 2.5 kg s⫺1. As was the case for the Hong Kong NMHC fluxes the larger absolute concentrations of NOy downstream of the Hong Kong plume reduced the sensitivity of the modelled fluxes to the threshold concentration. The estimated NOy fluxes for the instrument threshold of between 1 and 2 ppb bound the observed value of 2.6 kg s⫺1 and show reasonable consistency between the observations and model predictions.

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Table 3 Observed and modelled fluxes of NMHC and NOy for mainland China and Hong Kong Source

Species Observed

Mainland China Hong Kong

Mainland China

Hong Kong

Mainland China

Hong Kong

a b c

cthresholdb 0 10 ppb-C 20 ppb-C 0 10 ppb-C 20 ppb-C

cthresholdc 0 10 ppb-C 20 ppb-C 0 10 ppb-C 20 ppb-C

NMHC (kg s⫺1)

NOya (kg s⫺1)

3.18 2.62 Modelled (951122F24)

2.99 2.60

12.0 (12.0)c 7.0 (7.1) 1.9 (2.4) 3.5 (3.3) 3.0 (2.6) 2.3 (2.3) Modelled (951202F04)

6.8 4.6 2.4 1.6 1.5 1.4

(6.8) (4.5) (2.3) (2.1) (1.6) (1.3)

cthreshold 0 2 ppb 4 ppb 0 2 ppb 4 ppb

4.9 1.8 0.2 4.9 4.6 4.0

(5.0) (2.2) (0.17) (4.3) (4.1) (3.8)

cthreshold 0 2 ppb 4 ppb 0 2 ppb 4 ppb

2.6 1.3 0.4 2.5 2.4 2.2

(2.6) (1.3) (0.32) (2.3) (2.1) (1.9)

(As NO2). See Eq. (2). Arithmetic mean and (bracketed values) median values.

5. Conclusion A comparison between modelled and observed fluxes of NMHC and NOx was carried out for aircraft based measurements on 17 November 1997. Air quality modelling of two days with similar overall meteorology to 17 November showed reasonable consistency between the predicted and measured fluxes, suggesting that the NMHC and NOx inventories for Hong Kong are broadly correct.

Acknowledgements The authors wish to thank EPD Hong Kong for their support of this work and for permission to publish the results. In addition the assistance provided by ERM Hong Kong during the operational phase of the work is gratefully acknowledged. The authors would also like to thank Capt. S Lawrey of Kevron Pty Limited whose flying skills made the measurements possible.

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