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

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.

Atmospheric Measurements at the Foot and the Summit of Mt. Tai – Part I: HONO Formation and Its Role in the Oxidizing Capacity of the Upper Boundary Layer

“…For the same sampling site at the summit of Mt. Tai, as listed in Table 2, the observed mean HONO mixing ratios in summer is similar to those observed at the same site in winter (150 pptv, December 2017) and spring (130 pptv, March -April 2018) reported by Jiang et al (2020), but the variation of HONO mixing ratios in summer was within a much narrower range (1 -880 pptv) than in winter (0 -1140 pptv) and spring (0.5 -3230 pptv).…”

“…For instance, J(HONO) = J(HONO)model * J(NO2)measured / J(NO2)model. Water-soluble ions, including particulate nitrate (pNO3) of PM2.5, were collected by filter method and analyzed by an ion chromatograph (Liu et al, 2020) (Jiang et al, 2020). In this study, 10-min averaged data were used for the following analysis.…”

“…to be much less important compared to O3 photolysis (Ye et al, 2018;Zhang et al, 2009). However, in mountainous regions, mountain winds, including mountain breeze (downslope) and valley breeze (upslope) can accelerate the air mass exchange between the mountain top and the ground levels, which may affect HONO levels and the atmospheric oxidizing capacity at the summit level (Jiang et al, 2020;Schmid et al, 2020;Ye et al, 1987).…”

Winter ClNO<sub>2</sub> formation in the region of fresh anthropogenic emissions: seasonal variability and insights into daytime peaks in northern China

“…The downward trans- port of ClNO 2 at Wangdu in summer has been illustrated by Tham et al (2016), and the upward transport to the top of Mt. Tai has also been implicated by the increasing daytime concentrations of O 3 and other pollutants (e.g., Gao et al, 2005;Zhou et al, 2009;Jiang et al, 2020). Measurements in the residual layers are needed to further investigate the transport of ClNO 2 within the entire boundary layer.…”

Atmospheric oxidation capacity and ozone pollution mechanism in a coastal city of Southeast China: Analysis of a typical photochemical episode by Observation-Based Model

“…The HONO photolysis was a very important primary ROx source at the high-altitude or background sites. (Acker et al, 2001;Jiang et al, 2020).…”