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Abstract: The soot redox reaction with an uptake coefficient (γ) of 10−4 at night and the soot heterogeneous photochemistry with the γ formula of J NO2/10 in the daytime, where J NO2 denotes the NO2 photolysis frequency, were inserted into the WRF-Chem model to evaluate the effects of γ changes of several orders of magnitude on the concentrations of HONO, HO x , O3, NO3 −, and NH4 + in the Beijing–Tianjin–Hebei (BTH) region. Results demonstrated that the simulations in the BTH region support the redox mechanism at night and the heterogeneous photochemistry mechanism in the daytime for HONO formation, and suggested that the order of magnitude of γ is likely to be 10−4. Minor increases in HONO, HO x , O3, NO3 −, and NH4 + were produced when γ was decreased to ≤10−5, whereas the concentrations of HONO and NO3 − were substantially overestimated in most cases when γ was increased to 10−3. The soot surface area per unit volume of air showed a sharp decrease from 3×103 μm2 cm−3 near the ground to 6×102 μm2 cm−3 at ∼200m at night. The surface area fractions of soot, nitrate, and sulfate showed minor variations between day and night, although the surface areas of the major aerosol components at night were approximately three times those in the daytime. [Copyright &y& Elsevier]

Abstract: In the summers of 2002–2003, acidic gases, ammonia and water-soluble ions in PM10 were measured in Beijing. The mean concentrations of HCl, HONO, HNO3, SO2 and NH3 are 0.6, 3.6, 1.9, 14.1 and 16.6μgm−3, respectively, and 2.2, 14.6, 19.3 and 8.9μgm−3 for in PM10. The concentrations of secondary ions in PM10 are found to have strong dependence on the pathway of trajectories. The most frequent southerly air flow is connected with high concentrations of secondary water-soluble ions during summertime. Other trajectories with northwest and north direction lead to lower concentrations of secondary ions. Hebei and Shandong Provinces and the Tianjin Municipality are the main source areas for sulfate as identified by Potential Source Contribution Function. This result emphasizes that the non-Beijing sources play an important role in the sulfate mass concentration in the urban atmosphere of Beijing and validates conclusions based on model calculations for the region. [Copyright &y& Elsevier]

Abstract: Much rain and strong winds caused by a cold front occurred in Beijing during the period of Sep. 27 to Oct. 4, 2004 and led to sharp drops in maximum and mean concentrations of HONO, HCHO, O3, and NO2, i.e., the maximum concentrations were reduced by 5.9, 21.3, 45.6, and 44.4 ppb, respectively, and the mean concentrations were decreased by 4.0, 5.5, 30.3, and 32.3 ppb, respectively. For daily HO x production rates HONO photolysis was the largest contributor and over 90% contributions were from photolysis of HONO and HCHO. Large number and area percentages of soot aggregate from PM10, and high correlations between concentrations of PM10 and chemical formation of HONO suggested that heterogeneous reactions of NO2 on surfaces of soot aggregate could be a key source of HONO in the heavy traffic areas of Beijing during the night and should be considered in air quality simulations for such areas. [Copyright &y& Elsevier]

Measurements of nitrous acid (HONO) and nitrogen dioxide (NO_2) in Beijing City have been performed by means of a developed differential optical absorption spectroscopy (DOAS) system based on photodiode array (PDA), during the autumn of 2004. HONO and NO_2 were simultaneously identified by their characteristic absorption bands in the spectral region between 337 nm and 372 nm with high sensibility and time resolution. The concentrations of HONO exhibit obviously diurnal variation with a nocturnal maximum and a daytime minimum. The highest HONO value up to 11.8 μg/m^3 was observed during the night of 2/3 September. Possible sources of the observed HONO were discussed. Good correlation to NO_2 indicates that NO2 is a main source component. The measurement also shows direct emission of HONO is an important source in strongly polluted urban area. [ABSTRACT FROM AUTHOR]

In this study, continuous measurements of HONO and related parameters were carried out at an urban site in Beijing during intense summer and winter campaigns. Highly variable HONO levels were observed ranging from 0.53 ppb to 2.41 ppb in summer and from 0.02 ppb to 3.18 ppb in winter. An additional unknown source with HONO production rates of 2.10 ppb h−1 and 0.87 ppb h−1 were derived, accounting for 86.4% and 70.4% of the total HONO daytime sources in summer and winter, respectively. In order to explore the potential sources and formation pathways of HONO, a zero-dimensional box model (F0AM) based on Master Chemical Mechanism (MCM3.3.1) coupled with six updated HONO sources was used to reveal the HONO chemistry and its impact on atmospheric oxidation capacity. With the additional sources, the simulation results reproduced the observations well, and suggested the HONO concentrations during the summer daytime (7:00–17:00) mainly came from the homogeneous OH + NO reaction (22.0%), photosensitized heterogeneous conversion on aerosol surfaces (19.2%) and direct emission (18.5%). Whereas, photosensitized heterogeneous conversion on aerosol surfaces (29.7%), photosensitized heterogeneous conversion on ground surfaces (24.9%) and direct emission (20.4%) became the three major sources during winter daytime. The surprisingly high OH oxidation rates simulated in the winter campaign, which was significantly underestimated in current MCM, highlighting the urgent need for full consideration of additional HONO sources in the numerical model, especially in the cases of winter haze in megacities of North China. Image 1 [ABSTRACT FROM AUTHOR]