Carbonaceous aerosol composition in air masses influenced by large-scale biomass burning: a case study in northwestern Vietnam
Authors: Nguyen Dac-Loc , Czech Hendryk , Pieber Simone M. , Schnelle-Kreis Jürgen , Steinbacher Martin , Orasche Jürgen , Henne Stephan , Popovicheva Olga B. , Abbaszade Gülcin , Engling Guenter , Bukowiecki Nicolas , Nguyen Nhat-Anh , Nguyen Xuan-Anh , Zimmermann Ralf 
Published: May 2021

Abstract. We investigated concentrations of organic carbon (OC), elemental
carbon (EC), and a wide range of particle-bound organic compounds in daily
sampled PM2.5 at the remote Pha Din (PDI) – Global Atmosphere Watch
(GAW) monitoring station in northwestern Vietnam during an intense 3-week
sampling campaign from 23 March to 12 April 2015. The site is
known to receive trans-regional air masses during large-scale biomass
burning (BB) episodes. BB is a globally widespread phenomenon and BB
emission characterization is of high scientific and societal relevance.
Emissions composition is influenced by multiple factors (e.g., fuel and
thereby vegetation type, fuel moisture, fire temperature, available oxygen).
Due to regional variations in these parameters, studies in different world
regions are needed. OC composition provides valuable information regarding
the health- and climate-relevant properties of PM2.5. Yet, OC composition
studies from PDI are missing in the scientific literature to date.
Therefore, we quantified 51 organic compounds simultaneously by in situ
derivatization thermal desorption gas chromatography and time-of-flight mass
spectrometry (IDTD-GC-TOFMS). Anhydrosugars, methoxyphenols, n-alkanes,
fatty acids, polycyclic aromatic hydrocarbons, oxygenated polycyclic
aromatic hydrocarbons, nitrophenols, and OC were used in a
hierarchical cluster analysis highlighting distinctive patterns for periods
under low, medium, and high BB influence. The highest particle phase
concentration of the typical primary organic aerosol (POA) and possible
secondary organic aerosol (SOA) constituents, especially nitrophenols, were
found on 5 and 6 April. We linked the trace gas mixing ratios
of methane (CH4), carbon dioxide (CO2), carbon monoxide (CO), and
ozone (O3) to the statistical classification of BB events based on OA
composition and found increased CO and O3 levels during medium and high
BB influence. Likewise, a backward trajectory analysis indicates different
source regions for the identified periods based on the OA clusters, with
cleaner air masses arriving from the northeast, i.e., mainland China and the Yellow
Sea. The more polluted periods are characterized by trajectories from the
southwest, with more continental recirculation of the medium cluster and
more westerly advection for the high cluster. These findings highlight that
BB activities in northern Southeast Asia significantly enhance the regional
organic aerosol loading and also affect the carbonaceous PM2.5
constituents and the trace gases in northwestern Vietnam. The presented analysis adds valuable data on the carbonaceous and chemical composition of PM2.5, in particular of OC, in a region of scarce data availability, and thus offers a reference dataset from Southeast Asian large-scale BB for future studies. Such a reference dataset may be useful for the evaluation of atmospheric transport simulation models, or for comparison with other world regions and BB types, such as Australian bush fires, African savannah fires, or tropical peatland fires.