This study reveals aerosol liquid water content (ALWC) in PM 2.5 ranged from 2% up to 74%, and the associated secondary inorganic fraction rose from 24% to 55%, while ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM 2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry's law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining ALWC. Coincident with a significant reduction in SO 2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving ALWC. Thus, the abundance of ALWC and its effects on the aerosol chemistry and climate should be reconsidered.

The molecular composition of humic-like substances (HULIS) in different aerosol samples was analyzed using an ultrahigh-resolution mass spectrometer to investigate the influence of biomass burning on ambient aerosol composition. HULIS in background aerosols were characterized with numerous molecular formulas similar to biogenic secondary organic aerosols. The abundance of nitrogen-containing organic compounds (NOC), including nitrogen-containing bases (N-bases) and nitroaromatics, increased dramatically in ambient aerosols affected by crop residue burning in the farm field. The molecular distribution of N-bases in these samples exhibited similar patterns to those observed in smoke particles freshly emitted from lab-controlled burning of straw residues but were significantly different with those observed from wood burning. Signal intensity of the major N-bases correlated well with the atmospheric concentrations of potassium and levoglucosan. These N-bases can serve as molecular markers distinguishing HULIS from crop residue burning with from wood burning. More nitroaromatics were detected in ambient aerosols affected by straw burning than in fresh smoke aerosols, indicating that many of them are formed in secondary oxidation processes as smoke plumes evolve in the atmosphere. This study highlights the significant contribution of crop residue burning to atmospheric NOC. Further study is warranted to evaluate the roles of NOC on climate and human health.

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.