Insight into naturally-charged Highly Oxidized Molecules (HOMs) in the boreal forest
Atmospheric Chemistry and Physics Discussions
In order to investigate the role of the naturally charged highly oxidised molecules (HOMs) in the boreal forest we have performed measurements to chemically characterize the composition of negatively charged ions. Additionally, we compared this information with the chemical composition of the neutral (HOMs) detected in the ambient air during the same period. The chemical composition of the ions was retrieved using an Atmospheric Pressure interface Time-Of-Flight mass spectrometer (APi-TOF)
... eter (APi-TOF) while the gas phase neutral molecules (mainly sulphuric acid and HOMs) were characterized using the same mass spectrometer coupled to a nitrate-based chemical ionization unit (CI-APi-TOF). Overall, we divided the identified HOMs in two classes, HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or organonitrates (ONs). During the day, among the ions, in addition to the well-known sulphuric acid clusters, we found a large number of HOMs clustered with the two most common inorganic acids, nitrate (NO<sub>3</sub><sup>−</sup>) or bisulphate (HSO<sub>4</sub><sup>−</sup>), the first one being predominant. During the night, the detected ions were very similar to the neutral compounds and were mainly composed of HOMs clustered with NO<sub>3</sub><sup>−</sup>. <br><br> For the first time, we identified several clusters contain up to 40 carbon atoms clustered with NO<sub>3</sub><sup>−</sup>. At this regard, we think that these naturally charged clusters are formed by up to 4 oxidized α-pinene units. <br><br> Finally, diurnal profiles of the negative ions were consistent with the neutral compounds revealing that ONs peak during the day while non-nitrate HOMs are more abundant at night-time. However, during the day, a big fraction of the negative charge is taken up by the sulphuric acid clusters causing differences between detected neutral and ion HOM/ON species. As a result, the total signal of the ionised organic compounds was much lower during day than during the night.