Chemical composition and aerosol size distribution of the middle mountain range in the Nepal Himalayas during the 2009 pre-monsoon season

Abstract

Abstract. 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 watersoluble 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 shalCorrespondence to: A. P. Barros (barros@duke.edu) low 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 PM 2.5 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 SO 2− 4 and NH + 4 ). 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.

Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon season

“…Note that a partial coarse particulate influence over Hetauda was observed earlier in the AOD vs. AE relationship thus complementing the SSA variation. Compared to Hetauda, dust/coarse particle influence is found to weaken over Dhulikhel that shows a relatively flat spectrum suggesting a mix of local pollution aerosols from upslope transport as well as long-range transport of dust (Carrico et al, 2003;Shrestha et al, 2010). Unlike over Western IGP, SSA over Hetauda and Dhulikhel is generally lower at longer wavelengths (670-1020 nm) and less sensitive to variation in wavelength.…”

“…Unlike over Western IGP, SSA over Hetauda and Dhulikhel is generally lower at longer wavelengths (670-1020 nm) and less sensitive to variation in wavelength. Chemical composition analysis carried out by Shrestha et al (2010) over Dhulikhel during the 2009 pre-monsoon period indicates organic carbon as a major fraction of the aerosol concentration with major sources of pollution in Kathmandu valley, in particular, and over the IGP, in general. Additionally, their chemical analysis found about 10 % contribution from elemental carbon in the total aerosol composition suggesting presence of significant absorbing aerosol concentration thus implying overall low spectral SSA (greater absorption) as reported here by AERONET over Dhulikhel.…”

Seasonal variations of water-soluble organic carbon, dicarboxylic acids, ketocarboxylic acids, and α-dicarbonyls in Central Himalayan aerosols

“…During the last three decades, chemical studies of aerosols and snow in the Himalayan region have become of great interest in order to evaluate the impact of anthropogenic pollution over the high elevation mountain regions. Several studies have been reported from the northern slope of the Himalayas (Mayewski et al, 1983;Wake et al, 1994;Shrestha et al, 1997;Shrestha et al, 2000;Shrestha et al, 2002) and also from lower elevations in middle Himalayan region (Shrestha et al, 2010). Wake et al (1994) P. Hegde and K. Kawamura: Water-soluble organic carbon, dicarboxylic acids, ketocarboxylic acids studied aerosol samples collected from the southern slopes of the Himalayas to the northern margin of the Tibetan Plateau and reported high concentrations of organic compounds.…”

Modeling Aerosol-Cloud-Precipitation Interactions in Mountainous Regions: Challenges in the Representation of Indirect Microphysical Effects with Impacts at Subregional Scales

“…The aerosol pool is eventually scavenged by the formation of low-level clouds and fog, and washed out by rainfall similar to subregional scale forcing in the inner region of the Southern Appalachians investigated by [21,22]. Specifically, [26] showed that, in the presence of regional scale aerosol clouds and during dry periods, the mean volume aerosol concentration increased, and so did the aerosol mass concentrations in two different valleys of Central Nepal, the Marshyangdi and the Kathmandu, followed by rain-out. In addition, the topography of the region was found to play an important role in modulating the diurnal cycle of aerosol number concentration due to the diurnal cycle of katabatic and anabatic winds.…”