Abstract
Abstract. 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 its impact 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 to better understand the characteristics and sources of HONO in the upper boundary layer and lower free troposphere, two intensive field observations were carried out at the summit of Mt. Tai (1534 m a.s.l.), the peak of the North China Plain (NCP), in winter 2017 and spring 2018. HONO showed moderate concentration levels (average ± standard deviation: 0.15±0.15 and 0.13±0.15 ppbv), with maximum values of 1.14 and 3.23 ppbv in winter and spring, respectively. Diurnal variation patterns with broad noontime maxima and lower nighttime concentrations were observed during both campaigns, which is distinct from most of the previous studies at the ground level. The Lagrangian particle dispersion model (LPDM, WRF-FLEXPART v3.3) simulations indicated the combined effects of the planetary boundary layer evolution and valley breeze on the daytime HONO peak. A photostationary state (PSS) analysis suggested a strong unknown daytime HONO source with production rates of 0.45±0.25 ppb h−1 in winter and 0.64±0.49 ppb h−1 in spring. Correlation analysis supported the important role of photo-enhanced heterogeneous conversion of NO2 to HONO on the aerosol surface at this high-elevation site. HONO photolysis is the predominant primary source of OH radical and plays a major role in the radical chemistry at Mt. Tai. The model only considering a homogenous HONO source predicted much lower levels of the HOx radicals and atmospheric oxidation capacity than the model constrained with measured HONO data. This study sheds light on the characteristics, sources, chemistry, and impacts of HONO in the upper boundary layer and lower free troposphere in the NCP region.
Highlights
Nitrous acid (HONO) is an important reactive trace gas in the Earth’s atmosphere and plays a pivotal role in tropospheric oxidation chemistry
This paper describes the measurement results of HONO and related parameters, which are the first report of HONO chemistry in the upper planetary boundary layer (PBL) and lower free troposphere (FT) in the North China Plain (NCP) region
Tai were dominated by cold and dry weather (especially in winter with average (± standard deviation; SD) temperature and relative humidity (RH) of −5.2 ± 3.8 ◦C and 48 ± 20 %, respectively) as well as relatively lower concentrations of air pollutants. This was due to the high elevation of the station, which is primarily located in the lower FT and/or upper PBL during the measurement periods
Summary
Nitrous acid (HONO) is an important reactive trace gas in the Earth’s atmosphere and plays a pivotal role in tropospheric oxidation chemistry. Photolysis of HONO by sunlight releases the hydroxyl radical (OH), which initiates the oxidation reaction cycles, resulting in the degradation of various primary contaminants and formation of secondary pollutants such as ozone (O3) and secondary aerosols (Alicke et al, 2003). HONO is a key player in biosphere–atmosphere interactions and the biogeochemi-. Y. Jiang et al.: Sources of nitrous acid (HONO) in the upper boundary layer cal cycle via dry and wet depositions as well as soil microbial processes (Sörgel et al, 2015). It is of high significance to better understand the characteristics, sources, sinks, and environmental consequences of tropospheric HONO
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