The researchers at Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science conducted high-resolution vertical profile measurements of HONO and NO2 to investigate the nocturnal sources of HONO at different pollution levels. And their work was published in Atmospheric Chemistry and Physics.
Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry. Due to its importance, in spite of decades of research, how HONO is formed remains controversial.
In this study, the study team conducted high-resolution vertical gradient measurements based on the Beijing 325m meteorological tower combined incoherent broadband cavity enhanced absorption spectrometer (or IBBCEAS).
Through the work, they obtained high-resolution vertical profiles which revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and they also observed a shallow inversion layer affected the vertical distribution of HONO. Direct HONO emissions from traffic contributed 29.3% ± 12.4% to the ambient HONO concentrations at night.
The ground surface dominated HONO production by heterogeneous conversion of NO2 during clean episodes. However, the production of HONO on aerosol surfaces explained the HONO observations in the residual layer during haze episodes, suggesting that the aerosol production was an important HONO source.
In addition, average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h-1 were identified during the clean and haze episodes, respectively, implying that significant amounts of HONO were deposited to the ground surface at night.
This work was supported by the National Natural Science Foundation of China (grant nos. 91544104, 41571130023, and 41875154).
Link to the paper: High-resolution vertical distribution and sources of HONO and NO2 in the nocturnal boundary layer in urban Beijing, China