| Abstract | To study the air pollution by airborne polycyclic aromatic hydrocarbons (PAHs) in
Bangkok urban air (a tropical atmosphere), two tasks were conducted: (a) the spatial
distributions of airborne P AHs was evaluated and (b) the source contribution of P AHs
airborne particulates using the factor analysis-multiple regression (FA-MR) technique was
estimated.
Task I : Phase distribution and particle size dependency of airborne P AHs in Bangkok
Urban Area
The level and distribution of airborne P AHs in Bangkok urban air were investigated.
Their characteristics were evaluated with respect to gas-particle partitioning and dependency
on particle size by using 8 stage size fractionating cascade impactor (Andersen "low volume"
Sampler) and a downstream XAD-2 adsorbent tube for sample collection. Nine P AHs,
classified as carcinogenic and co-carcinogenic compounds - Pyrene (PYR),
Benz(a)Anthracene (BaA), Benzo(e)Pyrene (BeP), Dibenz(a,c)Anthracene (DBacA),
Benzo(k)Fluoranthene (BkF), Benzo(a)Pyrene (BaP), Dibenz(a,h)Anthracene (DBahA),
Benzo(ghi)Perylene (BghiP) and Trimethylcholanthrene (3MC) - were quantified by
ultrasonic extraction and HPLC with fluorescence detection. It was found that the Andersen
cascade impactor in combination with the XAD-2 adsorbent tube constituted a suitable
alternative for obtaining the total amount of airborne PAHs (gas and particulate phases) in
the Bangkok urban atmosphere. The collection of the gas phase P AHs by the XAD-2
adsorbent tube was found to be a well-adapted tool for air sampling in tropical climates,
because even at the sampling temperatures of about 35°C (daytime), ,none of the listed PAHs
(with molecular weights of 202 and above) was traceable in the backup layer of the XAD-2
adsorbent tube.
PYR, BaA, BeP, BkF, BaP and BghiP were found prevailing in the Bangkok urban
air. The lower molecular weight P AHs such as PYR, BeP and BaA were present mainly in
the gaseous phase (80%, 40% and 24%, respectively), while the higher molecular weight
compounds were present almost totally in the particulate fraction. About 30%-60% of each
P AHs content were found on particles smaller than 0.43 µm and more than 70% on particles
smaller than 2.1 µm. The incorporation of P AHs into particulate matter was consistent with
the adsorption and condensation processes with a marked preference for the sub-micron size
particles. The particle size dependency of P AHs in Bangkok revealed the incorporation of
low molecular weight P AHs into the large fraction of aerosols. This incorporation decreased
as the molecular weight of P AHs increased.
The relationship between the particle/gas partition coefficient (Kp) and the sub-cooled
liquid vapour pressure (p0
L) was explored. The slopes of the relationship differed slightly
within the Bangkok urban sites (mr = -0.77 to -0.74), contrary to the shallowest slope
observed in rural site (IDr = -0.52). This indicated that P AHs partitioning was not at
equilibrium (mr ":/: -1 ). Given the state of air pollution and tropical climate in Bangkok, the
iii shallow slopes were most probably related to the different activity effects of the
incorporation of P Alls with the liquid aerosols and the depletion of their corresponding gas
phase by photochemical reactions in the atmosphere.
Task Il : Estimation of source contribution to P AHs airborne particulates using
Factor Analysis-Multiple Regression (FA-MR) model
Factor Analysis-Multiple Regression Model was used to estimate the source
contributions to the Bangkok urban aerosols. This multivariate technique enabled major air
pollution source categories to be identified along with the quantitative contributions of
pollutant species to each source group. Source apportionment of the atmospheric PAHs in
Bangkok urban area was based on the combined measurements of P AHs and elemental
compositions in aerosol samples. The aerosol characterisation was done by using
instrumental neutron activation analysis, high performance liquid chromatography, ion
chromatography and differential thermal analysis. The results of factor analysis revealed five
factors solution from the 42 samples and 23 variables. The cumulative percentage variance
explained by these five factors was of satisfaction (86% of the total). The multiple regression
analysis revealed soil and re-entrained road dust, automobiles and refuse incineration and
open burning as the major sources of suspended particulate matter in Bangkok at a
significance level of p = 0.01 and the regression of 0.79. The source contribution in
percentage of total SPM mass in Bangkok urban air was given as follows: the contribution of
soil and re-entrained dust was the largest at 51 %. Emissions from automobile were other
prominent source contribution at 18% of total ambient aerosol. The contributions from refuse
and biomass burning, industry and sea salt, and secondary pollutants were rather small,
ranging from 6% to 11 %. The unknown part was estimated at 10% which is ascribed to
unidentified sources. P Alls were found to be associated predominantly with emissions from
automobiles.
The relationship between absolute contribution from each factor and the prevailing
wind direction revealed the representativeness of the area. The local SPM pollution from soil
and re-entrained road dust as well as automobiles in Bangkok due to construction activities
and traffic were observed by factor 1 and 2 in which factor scores were always high in each
wind direction. The transported pollutants were also obvious in the remaining factors in
which they were consistent to the particular areas located in suburbs of Bangkok, i.e.
industrial area in the south-east wind direction, urban and agricultural areas in the west wind
direction. |