High resolution vertical distribution and sources of HONO and NO 2 in the nocturnal boundary layer in urban Beijing, China

Abstract. The production of HONO on aerosol surfaces and ground surfaces in urban atmosphere is of interests. However, ground surface measurement commonly in our society is not able to distinguish these two parts. Here, for the first time, we reported high-resolution vertical profile measurements of HONO and NO2 in urban Beijing at night using an incoherent broadband cavity enhanced absorption spectrometer (IBBCEAS) amounted on a movable container which attached to a meteorological tower of 325 m high. The mixing ratios of HONO during one haze episode (E1), the clean episode (C2) and another haze episode (E3) were 4.26 ± 2.08, 0.83 ± 0.65, and 3.54 ± 0.91 ppb, respectively. High-resolution vertical profiles revealed that the vertical distribution of HONO is consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. Direct emissions from combustion processes contributed 51.1 % to ambient HONO concentration at night. The HONO production from the heterogeneous conversion of NO2 on the aerosol surfaces cannot explain HONO vertical measurements at night, indicating that the heterogeneous reaction of NO2 on ground surfaces dominated the nocturnal HONO production. The nocturnal HONO in the boundary layer is primarily derived from the heterogeneous conversion of NO2 at ground level and direct emissions; it is then transported throughout the column by vertical convection. ϕNO2 → HONO, the HONO yield from deposited NO2, is used to evaluate HONO production from the heterogeneous conversion of NO2 at night. The derived ϕNO2 → HONO values on 9 (C2), 10 (C2) and 11 December (E3) were 0.10, 0.08, and 0.09, respectively, indicating a significant production of HONO from heterogeneous reaction of NO2 at ground level. The similar ϕNO2 → HONO values measured during clean and haze episodes suggest that the heterogeneous conversion potential of NO2 at ground level is consistent at night. Furthermore, the dry deposition loss of HONO to the ground surface and vertical mixing effects associated with convection reached a near steady state at midnight on 11–12 December, indicating that significant quantities of HONO are deposited to the ground surface at night, and the ground surface is the source and sink of HONO at night.