Abstract. Gaseous nitrous acid (HONO) is an important source of OH radicals in the
troposphere. However, its source, especially that during daytime hours
remains unclear. We present an instrument for simultaneous unambiguous
measurements of HONO and NO2 with high time resolution based on
incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). To
achieve robust performance and system stability under different environment
conditions, the current IBBCEAS instrument has been developed with
significant improvements in terms of efficient sampling as well as resistance
against vibration and temperature change, and the IBBCEAS instrument also has
low power consumption and a compact design that can be easily deployed on
different platforms powered by a high-capacity lithium ion battery. The
effective cavity length of the IBBCEAS was determined using the absorption of
O2-O2 to account for the “shortening” effect caused by the
mirror purge flows. The wall loss for HONO was estimated to be 2.0 % via
a HONO standard generator. Measurement precisions (2σ) for HONO and
NO2 are about 180 and 340 ppt in 30 s, respectively. A field
inter-comparison was carried out at a rural suburban site in Wangdu, Hebei
Province, China. The concentrations of HONO and NO2 measured by
IBBCEAS were compared with a long optical path absorption photometer (LOPAP)
and a NOx analyzer (Thermo Fisher Electron Model 42i), and the
results showed very good agreement, with correlation coefficients (R2)
of HONO and NO2 being ∼0.89 and ∼0.95, respectively; in
addition, vehicle deployments were also tested to enable mobile measurements
of HONO and NO2, demonstrating the promising potential of using
IBBCEAS for in situ, sensitive, accurate and fast simultaneous measurements
of HONO and NO2 in the future.
Abstract. Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry, but its sources are still debated. The production of HONO on aerosol surfaces or on ground surfaces in nocturnal atmospheres remains controversial. The vertical profile provides vertical information on HONO and NO2 to understand the nocturnal HONO production and loss. In this study, we report the first high-resolution (<2.5 m) nocturnal vertical profiles of HONO and
NO2 measured from in situ instruments on a movable container that was lifted on the side wiring of a 325 m meteorological tower in Beijing, China. High-resolution vertical profiles revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and a shallow inversion layer affected the vertical distribution of HONO. The vertical distribution of HONO was consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. The increase in the HONO ∕ NO2 ratio was observed throughout the column from the clean episode to the haze episode, and relatively constant HONO∕NO2 ratios in the residual layer were observed during the haze episode. Direct HONO emissions from traffic contributed 29.3 % ± 12.4 % to the ambient HONO concentrations at night. The ground surface dominates HONO production by heterogeneous uptake of NO2 during clean episodes. In contrast, the HONO production on aerosol surfaces (30–300 ppt) explained the observed HONO increases (15–368 ppt) in the residual layer, suggesting that the aerosol surface dominates HONO production aloft during haze episodes, while the surface production of HONO and direct emissions into the overlying air are minor contributors. Average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h−1 were estimated during the clean and haze episodes, respectively, implying that significant quantities of HONO could be deposited to the ground surface at night. Our results highlight the ever-changing contributions of aerosol and ground surfaces in nocturnal HONO production at different pollution levels and encourage more vertical gradient observations to evaluate the contributions from varied HONO sources.
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