Enhancement in Particulate Organic Nitrogen and Light Absorption of Humic-Like Substances over Tibetan Plateau Due to Long-Range Transported Biomass Burning Emissions

To elucidate the influence of long-range transported
biomass burning
organic aerosols (BBOA) on the Tibetan Plateau, the molecular compositions
and light absorption of HUmic-Like Substances (HULIS), major fractions
of brown carbon, were characterized during the premonsoon season.
Under the significant influence of biomass burning, HULIS concentrations
increased to as high as 26 times of the background levels, accounting
for 54% of water-soluble organic carbon (WSOC) and 50% of organic
carbon (OC). The light absorption of HULIS also enhanced up to 42
times of the background levels, contributing 61% of the WSOC absorption
and 50% of OC absorption. Meanwhile, elevated nitrogen-containing
compounds (NOCs) among HULIS were observed. The NOCs from fresh and
aged BBOA were unambiguously identified on the molecular level, through
comparing with the molecular compositions of NOCs from lab-controlled
and field burning experiments. N-Heterocyclic bases represent major
fractions in the reduced nitrogen compounds from fresh BBOA, and nitroaromatic
compounds are important groups among the oxidized nitrogen compounds
from aged BBOA. The nitrogen-containing compounds, including nitroaromatics
and N-heterocyclic compounds, were also important chromophores, which
contributed to the enhanced light absorption of extracted HULIS during
biomass burning-influenced periods.