Abstract. Recent studies have revealed a significant influx of anthropogenic aerosol
from South Asia to the Himalayas and Tibetan Plateau (TP) during pre-monsoon
period. In order to characterize the chemical composition, sources, and
transport processes of aerosol in this area, we carried out a field study
during June 2015 by deploying a suite of online instruments including an
Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-AMS)
and a multi-angle absorption photometer (MAAP) at Nam Co station
(90∘57′ E, 30∘46′ N; 4730 m a.s.l.) at the
central of the TP. The measurements were made at a period when the transition
from pre-monsoon to monsoon occurred. The average ambient mass concentration
of submicron particulate matter (PM1) over the whole campaign was
∼ 2.0 µg m−3, with organics accounting for 68 %,
followed by sulfate (15 %), black carbon (8 %), ammonium (7 %), and
nitrate (2 %). Relatively higher aerosol mass concentration episodes were
observed during the pre-monsoon period, whereas persistently low aerosol
concentrations were observed during the monsoon period. However, the chemical
composition of aerosol during the higher aerosol concentration episodes in
the pre-monsoon season was on a case-by-case basis, depending on the
prevailing meteorological conditions and air mass transport routes. Most of
the chemical species exhibited significant diurnal variations with higher
values occurring during afternoon and lower values during early morning,
whereas nitrate peaked during early morning in association with higher
relative humidity and lower air temperature. Organic aerosol (OA), with an oxygen-to-carbon ratio (O ∕ C) of 0.94, was more
oxidized during the pre-monsoon period than during monsoon (average O ∕ C ratio of 0.72), and an average O ∕ C was 0.88
over the entire campaign period, suggesting overall highly oxygenated aerosol
in the central TP. Positive matrix factorization of the high-resolution mass
spectra of OA identified two oxygenated organic aerosol (OOA) factors: a less
oxidized OOA (LO-OOA) and a more oxidized OOA (MO-OOA). The MO-OOA dominated
during the pre-monsoon period, whereas LO-OOA dominated during monsoon. The
sensitivity of air mass transport during pre-monsoon with synoptic process
was also evaluated with a 3-D chemical transport model.
Chemical characteristics of submicron particles at the central Tibetan Plateau: insights from aerosol mass spectrometry