Molecular Characterization of Nitrogen-Containing Organic Compounds in Humic-like Substances Emitted from Straw Residue Burning

Abstract

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.

Wet and Dry Nitrogen Depositions in the Pearl River Delta, South China: Observations at Three Typical Sites With an Emphasis on Water‐Soluble Organic Nitrogen

“…the atmosphere is expected to be higher when biomass burning occurs (Wang et al, 2017). We found that in wet deposition K + /WSOC ratio showed higher level during fall in both years, even at the urban GIG site (Figure 4), indicating an enhanced biomass burning during the period .…”

“…K + is normally regarded as a tracer of biomass burning (Andreae, ; Chen et al, ; Mardi et al, ; Shi et al, ; Wang et al, ), and water‐soluble organic carbon (WSOC) is mostly derived from biomass burning and secondary formation (Ding et al, ; Weber et al, ). Therefore, the ratio of K + /WSOC in the atmosphere is expected to be higher when biomass burning occurs (Wang et al, ). We found that in wet deposition K + /WSOC ratio showed higher level during fall in both years, even at the urban GIG site (Figure ), indicating an enhanced biomass burning during the period (He et al, ).…”

“…Considering that biomass burning could be substantially enhanced during dry season in the PRD (Ding et al, 2012;Yu et al, 2016;Yu et al, 2017) and that biomass burning has been recognized as an important source of WSON species (Mace, Artaxo, & Duce, 2003;Zamora et al, 2011;Yu et al, 2017), we propose that the enhanced concentration and proportion of WSON in wet deposition during fall were attributed to biomass burning (discussed below). K + is normally regarded as a tracer of biomass burning (Andreae, 1983;Chen et al, 2010;Mardi et al, 2018;Shi et al, 2010;Wang et al, 2017), and water-soluble organic carbon (WSOC) is mostly derived from biomass burning and secondary formation (Ding et al, 2008;Weber et al, 2007). Therefore, the ratio of K + /WSOC in…”

Responses to Referee 1

“…These are just some examples with higher relative intensity (RI). The RI may not accurately represent their relative concentration levels in each sample, as the MS responses of different OSs are also influenced by different carbon chain structures (Wang et al, 2017d). The OS species of low MW and short carbon chain structures (with fewer than six carbon atoms in the molecule) are less retained on the SPE cartridges due to their highly water-soluble and more hydrophilic properties Lin et al, 2012Lin et al, , 2010.…”

“…However, one noted limitation is a lack of commercially available authentic standards. As a result, surrogate standards are often used for quantification Riva et al, 2015;, which adds uncertainty to the concentrations (Wang et al, 2017d). Recently, a few research groups quantified some OS species using synthetic authentic standards (e.g., hydroxyacetone sulfate, glycolic acid sulfate, lactic acid sulfate, methyltetrol sulfate, aromatic OSs, αand β-pinene OS, limonene OS, and limonaketone OS; Hettiyadura et al, 2017Hettiyadura et al, , 2015Olson et al, 2011;Wang et al, 2017d;Ma et al, 2014;Staudt et al, 2014), which was very important for understanding the variation and formation of OSs in ambient aerosols.…”