 |
| Titel |
Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season |
| VerfasserIn |
P. Shrestha, A. P. Barros, A. Khlystov |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 10, no. 23 ; Nr. 10, no. 23 (2010-12-08), S.11605-11621 |
| Datensatznummer |
250008943
|
| Publikation (Nr.) |
 copernicus.org/acp-10-11605-2010.pdf |
|
|
|
|
|
| Zusammenfassung |
| Aerosol particle number size distribution and chemical composition were
measured at two low altitude sites, one urban and one relatively pristine
valley, in Central Nepal during the 2009 pre-monsoon season (May–June). This
is the first time that aerosol size distribution and chemical composition
were measured simultaneously at lower elevations in the middle Himalayan
region in Nepal. The aerosol size distribution was measured using a Scanning
Mobility Particle Sizer (SMPS, 14–340 nm), and the chemical
composition of the filter samples collected during the field campaign was
analyzed in the laboratory. Teflon membrane filters were used for ion
chromatography (IC) and water-soluble organic carbon and nitrogen analysis.
Quartz fiber filters were used for organic carbon and elemental carbon
analysis. Multi-lognormal fits to the measured aerosol size distribution
indicated a consistent larger mode around 100 nm which is usually the oldest,
most processed background aerosol. The smaller mode was located around 20 nm,
which is indicative of fresh but not necessarily local aerosol. The diurnal
cycle of the aerosol number concentration showed the presence of two peaks
(early morning and evening), during the transitional periods of boundary
layer growth and collapse. The increase in number concentration during the
peak periods was observed for the entire size distribution. Although the
possible contribution of local emissions in size ranges similar to the
larger mode cannot be completely ruled out, another plausible explanation is
the mixing of aged elevated aerosol in the residual layer during the morning
period as suggested by previous studies. Similarly, the evening time
concentration peaks when the boundary layer becomes shallow concurrent with
increase in local activity. A decrease in aerosol number concentration was
observed during the nighttime with the development of cold (downslope)
mountain winds that force the low level warmer air in the valley to rise.
The mountain valley wind mechanisms induced by the topography along with the
valley geometry appear to have a strong control in the diurnal cycle of the
aerosol size distribution. During the sampling period, the chemical
composition of PM2.5 was dominated by organic matter at both sites. Organic
carbon (OC) comprised the major fraction (64–68%) of the aerosol
concentration followed by ionic species (24–26%, mainly SO42−
and NH4+). Elemental Carbon (EC) compromised 7–10% of the
total composition and 27% of OC was found to be water soluble at both
sites. The day-to-day variability observed in the time series of aerosol
composition could be explained by the synoptic scale haze that extended to
the sampling region from the Indian Gangetic Plain (IGP), and rainfall
occurrence. In the presence of regional scale haze during dry periods, the
mean volume aerosol concentration was found to increase and so did the
aerosol mass concentrations. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|