Molecular characteristics and diurnal variations of organic aerosols at a rural site in the North China Plain with implications for the influence of regional biomass burning
Authors: Li Jianjun , Wang Gehui , Zhang Qi , Li Jin , Wu Can , Jiang Wenqing , Zhu Tong , Zeng Limin 
Published: August 2019

Abstract. Field burning of crop residue in early summer releases a
large amount of pollutants into the atmosphere with significant impacts on the air quality and
aerosol properties in the North China Plain (NCP). In order to investigate
the influence of this regional anthropogenic activity on molecular
characteristics of organic aerosols, PM2.5 filter samples were
collected with a 3 h interval at a rural site of NCP from 10 to
25 June 2013 and analyzed for more than 100 organic tracer compounds,
including both primary (n-alkanes, fatty acids/alcohols, sugar compounds,
polycyclic aromatic hydrocarbons, hopanes, and phthalate esters) and
secondary organic aerosol (SOA) tracers (phthalic acids, isoprene-, α-/β-pinene, β-caryophyllene, and toluene-derived products),
as well as organic carbon (OC), elemental carbon (EC), and water-soluble
organic carbon (WSOC). Total concentrations of the measured organics ranged
from 177 to 6248 ng m−3 (mean 1806±1308 ng m−3) during the
study period, most of which were contributed by sugar compounds, followed by
fatty acids and fatty alcohols. Levoglucosan (240±288 ng m−3)
was the most abundant single compound and strongly correlated with OC and
WSOC, suggesting that biomass burning (BB) is an important source of
summertime organic aerosols in this rural region. Based on the analysis of
fire spots and backward trajectories of air masses, two representative
periods were classified, which are (1) Period 1 (P1), 13 June
21:00–16 June at 15:00 CST (China Standard Time),
when air masses were uniformly distributed from the southeast
part of NCP, where intensive open-field biomass burning occurred; and (2) Period 2 (P2),
22 June at 12:00 to 24 June at 06:00 CST, which is representative
of local emission. Nearly all the measured PM components showed much higher
concentrations in P1 than in P2. Although n-alkanes, fatty acids, and fatty
alcohols presented similar temporal–diurnal variations as those of
levoglucosan throughout the entire period, their molecular distributions
were more dominated by high molecular weight (HMW) compounds in P1,
demonstrating an enhanced contribution from BB emissions. In contrast,
intensive BB emission in P1 seems to have limited influence on the
concentrations of polycyclic aromatic hydrocarbons (PAHs), hopanes, and
phthalate esters. Both 3-hydroxyglutaric acid and β-caryophyllinic
acid showed strong linearly correlations with levoglucosan (R2=0.72
and 0.80, respectively), indicating that BB is also an important source for
terpene-derived SOA formation. A tracer-based method was used to estimate
the distributions of biomass-burning OC, fungal-spore OC, and secondary
organic carbon (SOC) derived from isoprene, α-/β-pinene,
β-caryophyllene, and toluene in the different periods. The results
showed that the contribution of biomass-burning OC to total OC in P1
(27.6 %) was 1.7 times that in P2 (17.1 %). However, the contribution
of SOC from oxidation of the four kinds of volatile organic compounds (VOCs)
increased slightly from 16.3 % in P1 to 21.1 % in P2.