Carbonaceous aerosol composition in air masses influenced by large-scale biomass burning: a case study in northwestern Vietnam

Abstract

Abstract. We investigated concentrations of organic carbon (OC), elemental\ncarbon (EC), and a wide range of particle-bound organic compounds in daily\nsampled PM 2.5 at the remote Pha Din (PDI) – Global Atmosphere Watch\n(GAW) monitoring station in northwestern Vietnam during an intense 3-week\nsampling campaign from 23\xa0March to 12\xa0April 2015. The site is\nknown to receive trans-regional air masses during large-scale biomass\nburning (BB) episodes. BB is a globally widespread phenomenon and BB\nemission characterization is of high scientific and societal relevance.\nEmissions composition is influenced by multiple factors (e.g., fuel and\nthereby vegetation type, fuel moisture, fire temperature, available oxygen).\nDue to regional variations in these parameters, studies in different world\nregions are needed. OC composition provides valuable information regarding\nthe health- and climate-relevant properties of PM 2.5 . Yet, OC composition\nstudies from PDI are missing in the scientific literature to date.\nTherefore, we quantified 51 organic compounds simultaneously by in situ\nderivatization thermal desorption gas chromatography and time-of-flight mass\nspectrometry (IDTD-GC-TOFMS). Anhydrosugars, methoxyphenols, n -alkanes,\nfatty acids, polycyclic aromatic hydrocarbons, oxygenated polycyclic\naromatic hydrocarbons, nitrophenols, and OC were used in a\nhierarchical cluster analysis highlighting distinctive patterns for periods\nunder low, medium, and high BB influence. The highest particle phase\nconcentration of the typical primary organic aerosol (POA) and possible\nsecondary organic aerosol (SOA) constituents, especially nitrophenols, were\nfound on 5\xa0and 6\xa0April. We linked the trace gas mixing ratios\nof methane (CH 4 ), carbon dioxide (CO 2 ), carbon monoxide (CO), and\nozone (O 3 ) to the statistical classification of BB events based on OA\ncomposition and found increased CO and O 3 levels during medium and high\nBB influence. Likewise, a backward trajectory analysis indicates different\nsource regions for the identified periods based on the OA clusters, with\ncleaner air masses arriving from the northeast, i.e., mainland China and the Yellow\nSea. The more polluted periods are characterized by trajectories from the\nsouthwest, with more continental recirculation of the medium cluster and\nmore westerly advection for the high cluster. These findings highlight that\nBB activities in northern Southeast Asia significantly enhance the regional\norganic aerosol loading and also affect the carbonaceous PM 2.5 \nconstituents and the trace gases in northwestern Vietnam. The presented analysis adds valuable data on the carbonaceous and chemical composition of PM 2.5 , 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.