Second Progress Report
Subgrid Scale Investigations of Factors Determining the Occurrence of Ozone and Fine Particles
Ian Colbeck and Charlotte Bryant
University of Essex, Colchester, Essex
WORK PACKAGE 1-FIELD MEASUREMENT
2. LAND CAMPAIGN METHODOLOGY 3
2.1 Aerosol Scattering Coefficients 3
2.2 Black Carbon 4
2.3 Aerosol Number Concentration (LASX) 4
2.4 Chemical Analysis 4
2.5 Additional Parameters 4
3. BOAT CAMPAIGN METHODOLOGY 5
3.1 Aerosol Number Concentration (SMPS) 5
3.2 Ozone Analysis 6
3.3 Chemical Analysis 6
3.4 Additional Parameters 6
WORK PACKAGE 2-ANALYSIS OF
THE FIELD MEASUREMENTS
4. DATA ANALYSIS 7
4.1 Aerosol Scattering Coefficients 7
4.2 Aerosol Absorption Coefficients 7
4.3 Aerosol Extinction Coefficients 8
5. LAND CAMPAIGN RESULTS 9
5.1 Aerosol Number Concentration (LASX) 9
5.2 Black Carbon 10
5.3 Scattering Coefficient, Absorption and Extinction 11 &12
5.4 Wind Rose Diagrams 13
5.5 Correlation Coefficients 14
5.6 Chemical Analysis 14
6. BOAT CAMPAIGN RESULTS
6.1 SMPS 15
6.2 Ozone 15
6.3 Correlation Coefficients 16
6.4 Chemical Analysis 17
7. DETAILED STUDY 14 JULY
BC, LASX and Nephelometer 18&19
7.2 Correlation Coefficients 19
WORK PACKAGE 1-FIELD MEASUREMENT PROGRAM
The aim of the SUB-AERO research project is the understanding of the formation, accumulation, fate and effects of ozone (O3), other photochemical oxidants and fine particulate matter in subgrid scale in the Medditerean area. This report describes the contribution of the University of Essex to Work Packages 1 and 2, in which an intensive measurement program on land and at sea was conducted to access physical and chemical processes which are factors causing increases in ozone concentrations and fine aerosol particles.
2. LAND CAMPAIGN METHODOLOGY
This report outlines an intensive field campaign conducted over 4 weeks during July 2000 at a remote coastal site on the Greek Island of Crete. The sampling station at Finokalia (35 19’ N, 25 40’ E) is situated 70 km eastwards of Heraklion at the top of a hilly elevation (130m AOD) facing seawards. The measurements conducted during the land campaign and the corresponding instruments used are described in the following sections.
2.1. Aerosol Scattering Coefficients
erosol scattering coefficients were measured with a three-wavelength integrating nephelometer (TSI model 3563). Anderson et al., (1996) describes in detail the performance characteristics and all the main features of this model. The instrument shown in Fig. 1 measures both the total particle scattering coefficient (sp) and the hemispherical backscattering coefficient (bsp) at three wavelengths: 450nm, 550nm and 700nm. The TSI 3563 also has sensors that measure other relevant parameters such as the temperature, pressure, and relative humidity of the sampled air. This additional data was measured concurrently with the scattering coefficients. During the SUB-AERO summer land campaign the nephelometer was set to record all measured data at 5-minute intervals.
Fig 1. Integrating Nephelometer
2.2. Black (or Elemental) Carbon (contribution by Demokritos)
An Anderson Instrumentation from the Demokritos Institute (Athens) was used to determine black carbon (BC) concentrations. Fig. 2 illustrates the aethelometer.
The principle of operation involves measuring the optical attenuation of aerosol samples deposited on a filter and converting it to the equivalent BC concentration through the application of a calibrated factor. Sampling was conducted at 5-minute intervals.
Fig 2 Aethelometer
2.3. Aerosol Number Concentration (LASX)
The laser aerosol spectrometer (LASX, Particle Measuring Systems) is an optical particle counter that uses light scattering principles to measure particle diameter and aerosol concentration in 16 nominal size bins of supermicron range from 0.1-3m. Measurements were made at 3-min time intervals throughout the summer Finokalia campaign.
2.4. Chemical Analysis
During the summer campaign in Finokalia between 9 and 12 July 2000, integrated gas and fine particulate species concentration measurements were made by means of denuder/filterpack systems as described in WP 1 of the 1st progress report (Lazaridis, 2000). These denuder/filterpack systems sampled at 12-hour time intervals concurrently with cascade impactors and the horizontal type denuders run by ECPL to provide comparative data.
Ion Chromatography is currently being used to determine concentrations of HCl, HNO3, HONO, NO2, SO2, NO3-, SO42-, Cl-, NH4+ and H+. The final results should be available by April 2001.
2.5. Additional Parameters
A number of micro-meteorological parameters were measured by a Vaisala portable weather station at Finokalia. The parameters included temperature, wind speed and humidity.
3. BOAT CAMPAIGN METHODOLOGY
A five-day cruise took place between 25 and 30 July 2000 . This coincided with the land-based SUB-AERO field campaign in Finokalia. The research vessel, ‘Aigaion’, was subcontracted from the National Centre for Marine Research (NCMR), and was equipped with a chemistry laboratory and other essential consumables. The boat cruised in the Aegean Sea along selected tracks defined by forward and back trajectory modelling, defined in WP2, as calculated with the sampling site in Crete as the end point. The following section briefly describes sampling and instrumentation.
3.1 Aerosol Number Concentration (SMPS)
Scanning Mobility Particle Sizer (SMPS) measured submicrometer aerosols in the range 3 to 1000 nm in diameter (Fig 3). The SMPS employed an Electrostatic Classifier to determine particle size, and a Condensation Particle Counter (CPC) that determined particle concentrations. This instrument was set to record measurements at 3-minute time intervals. SMPS data for the boat campaign is currently being analysed in collaboration with ICPF, Czech Republic.
Fig 3. SMPS on-board the research vessel ‘Aigaion’.
3.2. Ozone Analysis
A calibrated O3 analyser (Analysis Automation) was used to sample O3 and was set to sample at 1-minute time intervals. The method is based on the photometric assay of O3 concentrations in a dynamic flow system. The concentration of O3 is determined in an absorption cell from the measurement of the amount of light absorbed at the wavelength of 254nm.
3.3. Chemical Analysis
Denuder/filterpack systems, as shown in Fig. 4 and described in WP 1, sampled at 6-hour time intervals at a 10 L min-1-flow rate.
Ion Chromatography is currently being conducted at ECPL to determine concentrations of HCl, HNO3, HONO, NO2, SO2, NO3-, SO42-, Cl-, NH4+ and H+. The final results should be available by April 2001.
Fig. 4. On-board denuder/filterpack set up.
3.4 Additional Parameters
On-board meteorological measurements were conducted using the ship weather station.
WORK PACKAGE 2-ANALYSIS OF THE FIELD MEASUREMENTS
All data was first inspected to ensure that it was free of gross errors and/or contamination. Certain measurements were then used to derive other essential parameters, as discussed below. Time averaging was applied to data sets recorded at high frequencies e.g. O3, nephelometer and meteorological measurements.
4.1 Aerosol Scattering Coefficients
Light scattering is one of two attenuating effects of aerosols on solar radiation (the other being absorption) and can be quantified directly by means of scattering coefficients sp and bsp. These were simultaneously measured during the SUB-AERO field campaign by a multiwave nephelometer (TSI 3563).
Back/total scattering ratios
From the aerosol scattering coefficients, the aerosol backscattered fraction or back/total scattering ratios R=bsp/sp were derived for the three nephelometer wavelengths (450nm, 550nm and 700nm). These ratios gave information about the angular dependence of scattering and are necessary for the estimation of aerosol scattered diffuse radiation reaching the ground (Iqbal, 1983). R is therefore very useful for describing the cooling effect of aerosol on climate, as it is a measure of the fraction of the scattered radiation that is returned to space (Sagen and Pollack, 1967).
Asymmetric factor g
It was possible to derive the asymmetric factor g from R using the following relationship given by Kokhanovsky and Zege (1996): R = (1-g)/3. This factor is a useful optical property required to estimate aerosol forcing of climate and is used in most radiative transfer calculations.
4.2 Aerosol Absorption Coefficients
Absorption is the other main radiative property of aerosols, and is strongly related to the aerosol BC content. In fact, BC is considered to be almost exclusively responsible for all light absorption by aerosols (Horvath, 1993), although absorption by large dust particles (diameter up to ~16 m) is also significant. Absorption by BC occurs over a broad range of wavelengths, and within the visible region, it is only slightly dependent on wavelength.
Using the procedure given by Bodhaine, (1995) and Hansen et al., (1984), the aerosol BC content was converted to AP using a specific absorption coefficient of 10 m2 g-1.
The absorption/backscattering ratio (AP/SP) was also calculated. This represents the approximate amount of trapped thermal energy, relative to back scatter into space, due to atmospheric aerosol particles (Nyeki, 2000).
4.3 Aerosol Extinction Coefficients
The aerosol extinction coefficients act as important parameters related to visibility as shown in Kneizys et al., (1990). The formulae EXT = SP + AP (Ichoku, 1999) was used to obtain hourly geometric means of EXT during SUB-AERO campaign July 2000.
Single-scattering albedo ()
Single-scattering albedo () is also a useful term used for studying the radiative effects of aerosols, and is defined as = SP/EXT.
5. LAND CAMPAIGN RESULTS
5. 1 Aerosol Number Concentration (LASX)
Measurements were conducted in the 0.1um – 3 m range. Highest concentrations of up to 9500 particles cm–3 were recorded at the 0.1 m level, although concentrations of 1500 – 4000 particles cm-1 were more typical for the land based campaign.
There was a strong diurnal variation evident shown in fig. 5 at the 0.1m and 0.15m levels with peak concentrations occurring at about 15:00 and minimum concentrations at about 06:00. The same variation occurs to a lesser degree for the courser size fractions. It is theorised that this variation resulted from a combination of solar influenced convective conditions and photochemical aerosol production. Chemical analysis of denuder/filterpack samples will aid in deriving the dominant processes responsible for this observed diurnal variation.
Fig. 5. Nephelometer and LASX Total Hourly Averages, 7 – 31 July 2000, Sub-Aero Campaign
5.2 Black Carbon
Figures 6 and 7 illustrate diurnal and 3-hourly average BC. Values typically ranged between 200 and 1000 ng m-3. A prolonged peak in BC concentration on the 14 July was selected as a case study episode and investigated in greater detail (refer to Section 7). Generally, elevated BC concentrations were observed during afternoons, corresponding with fine particle and scattering measurements. However, the diurnal variation was not as pronounced as for other aerosol measurements.
ig. 6. BC Three-Hourly Average Concentration, 7- 31 July 2000, Sub-Aero Campaign
Fig. 7. BC Diurnal Average Concentrations, 7 – 31 July 2000, Sub – Aero Campaign
5.3 Scattering Coefficient, Absorption and Extinction
Total scatter coefficient (550nm) ranged from 2x10-5 to 1.3x10-4 for the study period (mean = 4.35x 10-5, SD = 1.74 x 10-5. A strong diurnal variation, consistent with LASX observations was noted for the three measured wavelengths (450nm, 550nm and 700nm) as illustrated in Fig. 5.
The values of R measured at Finokalia are illustrated in Fig. 8. Average R values over campaign: 450 nm = 0.13 (SD = 0.02, range = 0.09 – 0.16), 550 nm = 0.15 (SD = 0.02, range = 0.11 – 0.19), 700 nm = 0.18 (SD = 0.02, range = 0.14 – 0.23). The range of 0.11 – 0.19 at 550 nm compared well with other remote sites such as NOAA Sable Island monitoring site where R ranged from 0.14-0.16 (CMDL, 1996).
ig. 8 Aerosol Backscattered Fraction R for Sub-Aero Campaign, Finokalia, July 2000
Asymmetry factor (g)
The average g values over campaign: 450 nm = 0.61 (SD = 0.05, range = 0.51 – 0.72), 550 nm = 0.56 (SD = 0.05, range = 0.44 – 0.67), 700 nm = 0.47 (SD = 0.06, range = 0.32 – 0.58). These values are consistent with contributions from both soluble aerosols (g = 0.6) and soot aerosols (g = 0.3).
Absorption Coefficient AP
The average AP at 550nm over campaign: 6.26 x 10-6 (SD = 2.77 x 10-5, range = 1.30x10-6-2.00 x 10-5). These values were used to calculate extinction (EXT) and single-scattering albedo () coefficients for the campaign.
Extinction Coefficient EXT
The average EXT at 550nm during the campaign was 5.05x10-5 (SD = 1.95x10-5, range = 1.61 x 10-5– 1.3 x 10-4). EXT is an important parameter related to visibility.
Single-Scattering Albedo Coefficient
The average at 550nm during the campaign was 0.87 (SD = 0.04, range = 0.65 – 0.97). Purely scattering aerosols (e.g. sulphuric acid) exhibit values of = 1, whilst very strong absorbers (e.g. elemental carbon) have values of 0.3.
5.4. Meteorological Results /Wind Rose Diagrams
ig. 9 and 10 illustrate the wind direction in relation to particle size and scatter measured with the LASX and nephelometer, respectively. Both the size concentration and scattering coefficients indicated dominant transport of aerosols from the western and north-western sectors, with no evidence of preferential size sorting related to wind direction.
Fig. 9. Percentage Total Scatter in Relation to Wind Direction at 550 nm.
ig. 10. Percentage Total Size Concentration in Relation to Wind Direction for size class 0.1 (size classes 0.15 – 3m exhibit the same trend).
The fine particle fraction (0.1 m to 0.25 m) exhibited significant correlations with scattering coefficients for the whole campaign (approximately 0.50). Correlations increased to around 0.9 when calculated on a diurnal basis.
5.6 Chemical Analysis
BOAT CAMPAIGN RESULTS
Currently being analysed in collaboration with ICPF, Czech Republic.
Fig. 11 illustrates the diurnal variation of ozone. The maximum value was observed at mid-day, resulting from warm, sunny anticyclonic conditions, which as well as bringing elevated ozone levels during the day, are also associated with the formation of night time inversions caused by radiative cooling at the surface. This situation establishes a shallow stable layer near the ground and vertical mixing of air is severely restricted (Kalabokas et al., 2000). Vertical ozone measurements have shown that above the inversion ozone is not destroyed, while below the inversion ozone is removed by dry deposition enhanced by chemical destruction from emitted substances. Following the disappearance of the inversion in the morning, the down ward mixing of the air from the ozone reservoir in the upper layers results in a rapid increase in ground level ozone (Colbeck and McKenzie,1994).
Fig. 11. Total Hourly Average Ozone Concentrations
Boat Campaign Correlation Coefficients 24-29 July, 2000
As with the land campaign the fine particle fraction exhibited significant correlations with scattering coefficients for the whole campaign. Increased correlations were derived for diurnal measurements. Ozone also showed moderate diurnal correlation with scattering.
.4 Chemical Analysis
Awaiting results from ECPL.
DETAILED STUDY, 14 JULY 2000
7.1 BC, LASX and Nephelometer
The 14 July was selected for a detailed study. Fig12, 13 and 14 show LASX nephelometer, BC and LASX, and BC and nephelometer values. All show significant correlations over this period. This pollution episode needs further investigation before any firm conclusions can be made. However possible sources include Heraklion power station emissions and the predominance of forest fires occurring during this time.
ig.12 5 Minute LASX and Nephelometer Values
ig 13 5-Minute Nephelometer and BC Values
Fig.14 5 Minute LASX and BC Values
7.2 Correlations 14 July 2000
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