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. Nitrous acid (HONO) is a significant precursor of atmospheric“detergent” OH radicals and plays a vital role in tropospheric chemistry.The current knowledge about daytime HONO sources is incomplete, and itsimpact on the tropospheric radical chemistry has not been fully quantified.Existing observational studies of HONO were mostly conducted at the surface,with few efforts focusing on the high-elevation atmosphere. In order tobetter understand the characteristics and sources of HONO in the upperboundary layer and lower free troposphere, two intensive field observationswere carried out at the summit of Mt. Tai (1534 m a.s.l.), the peak of theNorth China Plain (NCP), in winter 2017 and spring 2018. HONO showed moderateconcentration levels (average ± standard deviation: 0.15±0.15and 0.13±0.15 ppbv), with maximum values of 1.14 and 3.23 ppbv inwinter and spring, respectively. Diurnal variation patterns with broadnoontime maxima and lower nighttime concentrations were observed during bothcampaigns, which is distinct from most of the previous studies at the groundlevel. The Lagrangian particle dispersion model (LPDM, WRF-FLEXPART v3.3) simulations indicated the combined effects of theplanetary boundary layer evolution and valley breeze on the daytime HONOpeak. A photostationary state (PSS) analysis suggested a strong unknowndaytime HONO source with production rates of 0.45±0.25 ppb h−1 inwinter and 0.64±0.49 ppb h−1 in spring. Correlation analysis supportedthe important role of photo-enhanced heterogeneous conversion of NO2 toHONO on the aerosol surface at this high-elevation site. HONO photolysis isthe predominant primary source of OH radical and plays a major role in theradical chemistry at Mt. Tai. The model only considering a homogenous HONOsource predicted much lower levels of the HOx radicals and atmosphericoxidation capacity than the model constrained with measured HONOdata. This study sheds light on the characteristics, sources, chemistry, andimpacts of HONO in the upper boundary layer and lower free troposphere inthe NCP region.

atmospheric chemistry and physics

Sources of nitrous acid (HONO) in the upper boundary layer and lower free troposphere of the North China Plain: insights from the Mount Tai Observatory