Daytime Production of Airborne Pollutants Including Brown Carbon by NO2 Interaction With Surface Microlayer of Lake Water in Southwestern China

Abstract

Surface microlayer at freshwater (rivers, lakes, ponds, streams, and groundwater) and seawater is abundant with organic compounds compared to subsurface water. These organics adsorbed at the air-water interface can interact with the atmospheric oxidants and influence the exchange of organic materials between the water and the atmosphere. Here, we assess the chemical interaction between gaseous NO₂ and authentic surface microlayers collected at the lake water (Dianchi Lake) situated in China. The formation of the gas-phase product compounds was evaluated in real time using a novel secondary electrospray ionization ultrahigh-resolution quadrupole Orbitrap mass spectrometer (SESI-UHR-MS) upon exposure of surface microlayer to gaseous NO₂ (20 or 50 ppb) in dark and under simulated sunlight irradiation at two different temperatures: 5°C and 25°C. The obtained results revealed that the sampling sites of the lake impacted by human activities (municipal sewage and agricultural activities) significantly impact the number and the composition of the formed gas-phase product compounds. The formation of nitrogen (N)-containing compounds was observed as well, which contain most likely nitro or amino functional groups, or alternatively, they could be aromatic compounds. The observed N-containing compounds may contribute to the “brown carbon” which act as light-absorbing compounds, thus influencing the radiative forcing of aerosols in the atmosphere.