44 Distinguishing Between Remote and Local Air Pollution …
281
aerosol data assimilation GEOS-5 uses AOD retrievals from the Multiangle Imaging
SpectroRadiometer (MISR), the Moderate Resolution Imaging Spectroradiometer
(MODIS) on both NASA Terra and Aqua satellites; the Advanced Very High Resolution Radiometer (AVHRR) instruments over bright surfaces, and ground-based
AERONET direct measurements. Month to month changes in MERRA AOD were
analyzed over the following two areas: the open ocean area in the vicinity of Taiwan
(25.5 N–29 N; 122.5E–124.5E) and the Taiwan area (22 N–25.5 N; 120E–122E). In
addition, a spatial distribution of wind vectors in the 700–800 hPa layer was analyzed,
using 15-year monthly mean MERRA data.
44.3 Results and Discussion
Month to month changes in AERONET AOD measurements in Taiwan show the
primary maximum (AOD ≈ 0.75) in spring (March–April) and the secondary maximum (AOD ≈ 0.45) in autumn (September–October) (Fig. 44.1b). AOD from the
high-elevated site in Lulin is essentially lower than AOD from the low-elevated
AERONET monitoring sites. Therefore, air pollution in Taiwan is mainly vertically
distributed below 2800 m a.s.l., in line with Kishcha et al. [2]. As shown in Fig. 44.1b,
the standard deviation of AERONET AOD in spring is significantly lower than that
in autumn, indicating the more homogeneous AOD spatial distribution in spring
than in autumn. Therefore, aerosols originating from remote sources were more
homogeneously distributed over Taiwan than local aerosols [2]. As a measure of air
pollution homogeneity, we used the AOD standard deviation: the more homogeneous
the spatial distribution of air pollution is, the lower the AOD standard deviation is.
To quantify changes in aerosol homogeneity over Taiwan over the year, we analyzed
month to month changes in the AERONET AOD standard deviation in percentage to
the 15-year mean AOD (Fig. 44.1c). One can see that, in autumn (October), the AOD
standard deviation is essentially higher than that in spring (March–April) (Fig. 44.1c):
this fact points out that, in autumn, inhomogeneous aerosols from local sources are
predominant, while, in spring, homogeneous aerosols from remote sources dominate.
The above mentioned finding is supported by the analysis of spatial distributions of 15-year monthly mean MERRA wind vectors in the 700–800 hPa layer
[2], their Fig. 7). In particular, Kishcha et al. [2] showed that, in the spring season
(March), prevailing strong west and south-west winds blow mainly from land to sea,
causing transport of anthropogenic air pollution (including biomass burning) from
its sources in continental Asia towards Taiwan. Therefore, in spring, aerosols from
remote sources are likely to be predominant. By contrast, in autumn (October), weak
east winds are observed over Taiwan, indicating insignificant transport of continental
air pollution towards Taiwan [2]. Therefore, in autumn, aerosols from local sources
are likely to dominate.
To support our findings about the origin of air pollution responsible for the spring
and autumn AOD maxima, we conducted a comparison of month to month changes
in MERRA AOD over the open ocean area in the vicinity of Taiwan (25.5 N–29 N;
281
aerosol data assimilation GEOS-5 uses AOD retrievals from the Multiangle Imaging
SpectroRadiometer (MISR), the Moderate Resolution Imaging Spectroradiometer
(MODIS) on both NASA Terra and Aqua satellites; the Advanced Very High Resolution Radiometer (AVHRR) instruments over bright surfaces, and ground-based
AERONET direct measurements. Month to month changes in MERRA AOD were
analyzed over the following two areas: the open ocean area in the vicinity of Taiwan
(25.5 N–29 N; 122.5E–124.5E) and the Taiwan area (22 N–25.5 N; 120E–122E). In
addition, a spatial distribution of wind vectors in the 700–800 hPa layer was analyzed,
using 15-year monthly mean MERRA data.
44.3 Results and Discussion
Month to month changes in AERONET AOD measurements in Taiwan show the
primary maximum (AOD ≈ 0.75) in spring (March–April) and the secondary maximum (AOD ≈ 0.45) in autumn (September–October) (Fig. 44.1b). AOD from the
high-elevated site in Lulin is essentially lower than AOD from the low-elevated
AERONET monitoring sites. Therefore, air pollution in Taiwan is mainly vertically
distributed below 2800 m a.s.l., in line with Kishcha et al. [2]. As shown in Fig. 44.1b,
the standard deviation of AERONET AOD in spring is significantly lower than that
in autumn, indicating the more homogeneous AOD spatial distribution in spring
than in autumn. Therefore, aerosols originating from remote sources were more
homogeneously distributed over Taiwan than local aerosols [2]. As a measure of air
pollution homogeneity, we used the AOD standard deviation: the more homogeneous
the spatial distribution of air pollution is, the lower the AOD standard deviation is.
To quantify changes in aerosol homogeneity over Taiwan over the year, we analyzed
month to month changes in the AERONET AOD standard deviation in percentage to
the 15-year mean AOD (Fig. 44.1c). One can see that, in autumn (October), the AOD
standard deviation is essentially higher than that in spring (March–April) (Fig. 44.1c):
this fact points out that, in autumn, inhomogeneous aerosols from local sources are
predominant, while, in spring, homogeneous aerosols from remote sources dominate.
The above mentioned finding is supported by the analysis of spatial distributions of 15-year monthly mean MERRA wind vectors in the 700–800 hPa layer
[2], their Fig. 7). In particular, Kishcha et al. [2] showed that, in the spring season
(March), prevailing strong west and south-west winds blow mainly from land to sea,
causing transport of anthropogenic air pollution (including biomass burning) from
its sources in continental Asia towards Taiwan. Therefore, in spring, aerosols from
remote sources are likely to be predominant. By contrast, in autumn (October), weak
east winds are observed over Taiwan, indicating insignificant transport of continental
air pollution towards Taiwan [2]. Therefore, in autumn, aerosols from local sources
are likely to dominate.
To support our findings about the origin of air pollution responsible for the spring
and autumn AOD maxima, we conducted a comparison of month to month changes
in MERRA AOD over the open ocean area in the vicinity of Taiwan (25.5 N–29 N;