This paper summarizes a 15-month long study (September 1996-November 1997)
on water soluble components (Na+, NH4+, K+, Mg2+, Ca2+, Cl-, NO3-, and SO42
-) in atmospheric aerosols conducted at a remote Himalayan site and a rural
Middle-Mountain site in Nepal. Most of the species sampled behave similarl
y in terms of seasonal and short-term variations. The aerosol concentration
s were low during the second half of the monsoon and post-monsoon seasons a
nd gradually increased during the winter season. Aerosol concentrations in
some samples were up to 20 times higher during the pre-monsoon and early-mo
nsoon seasons than they were in post-monsoon. The seasonal variations are c
learly related to variations in atmospheric circulation. The high concentra
tions during the pre-monsoon season are attributed to regional scale valley
wind systems and the maximum concentrations at both sites coincide with ma
xima in the local wind speed record. Occasional shifts in the large-scale c
irculation resulted in dramatic fluctuations in aerosol concentrations duri
ng the pre-monsoon and early monsoon seasons. In the middle of May the larg
e-scale circulation changed from westerly to south-easterly monsoon in the
Himalaya. The summer monsoon circulation brought considerable amounts of po
llution to the sites from southeast. Precipitation played an important role
in short-term (few samples, e.g., during the late winter at the Himalayan
site) to longer-term (several samples, e.g., after mid-June at both sites)
reduction in the aerosol concentrations, while the lack of precipitation in
the pre-monsoon and early monsoon supported a gradual build-up of pollutan
ts in the atmosphere. During the pre-monsoon and early monsoon, when concen
trations were elevated at both sites, they were often higher at the remote
Himalayan site than at the middle mountain site. A pollution layer in the m
onsoon circulation due to dry convective rise of pollutants at distant sour
ces and horizontal upper air transport by monsoon flow could have caused th
ese elevated concentrations. (C) 2000 Elsevier Science Ltd. All rights rese
rved.