This reaction was incorporated into the CMAQ model by assuming deposited HNO 3 and nitrate on surfaces to equal the accumulation of dry deposition since the last precipitation event, referring to the method of Sarwar et al. The photolysis rate of deposited HNO 3 and nitrate was estimated as the following equation:.
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Eight simulations were conducted considering different HONO sources, including the following. The same instrument was employed by our group in several previous field campaigns Zha et al. The reader referred to these sources e. Other instruments whose data are used in the present paper have been summarized in Yun et al. Briefly, PM 2. Sulfate, nitrate, and ammonium in PM 2. Additionally, hourly PM 2. It should be noted that NO 2 concentrations in the national network were measured using the catalytic conversion method, which overestimates NO 2 , especially during the period with active photochemistry and at the location away from the primary emission sources Xu et al.
The NO 2 observation data were adjusted based on the method of Zhang et al. As shown in Fig. S1, the episode average PM 2. The hourly PM 2. It can be seen that the whole western PRD region suffered from severe pollution. The simulated HONO levels in the afternoon were lower than 0.
The simulated average daytime HONO mixing ratios increased from 0. The revised model reproduced the HONO diurnal variation. One possible reason was that some HONO sources were not considered in this study. For example, previous studies have proposed that deposited HONO can be reserved in dew water on the ground or vegetation during the night and re-released when the dew water evaporates in the morning He et al. The total HONO dry deposition was 1. This suggests that dew evaporation may be a considerable source, but other sources may also exist. The inclusion of the four additional HONO sources also changed the model performance for other air pollutants.
S1, the O 3 prediction by the default CMAQ model was not satisfactory, especially for the peak value. In contrast, the revised model was better to reproduce the magnitude. Therefore, it is crucial to include these additional HONO sources into air quality models. For NO 2 , the revised model could reproduce its temporal variation in general, although underestimation was seen for some peak NO 2 values.
This underprediction could also partially explain the underestimation of HONO peak values. For PM 2. For the sulfate, nitrate and ammonium components in PM 2. S3, the revised model performed well in reproducing their temporal variations, with correlation coefficients R values of 0. The possible reason was underestimation of the emissions of NH 3 or other alkaline species e. We have conducted a sensitivity test to compare the simulations with those considering more recent emission by linearly adjusting the emissions based on the ratio of emissions to emissions for China reported in a very recent paper Zheng et al.
Despite the uncertainty, we think that the current emission setting is acceptable for simulations of the present case. It could be used to analyze the formation and impact of HONO in this episode.
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These findings on the dominant contribution from heterogeneous generation were similar to those in the previous modeling studies Sarwar et al. High relative humidity and solar radiation were two important driving factors for the formation of the high HONO concentrations in this pollution episode.
In addition to enhancing the heterogeneous generation, the existence of light also increased HONO values through the photolysis of particle nitrate in the atmosphere and total nitrate deposited on the surface. With the increasing of O 3 concentrations from sunrise to afternoon, the contribution of the O 3 photolysis began to increase and reached a peak of Compared to the reported results for summer in the previous studies Czader et al. For example, Xue et al. Tham et al. A similar high contribution can be found in other winter studies with high HONO concentrations, e.
The O 3 enhancement was much higher than the previous simulation results for the summer cases Sarwar et al. For example, Zou et al. Therefore, the enhancement of O 3 concentrations was more significant Li et al. Particle nitrate can be generated through the partition of HNO 3 to particle phase.
Therefore, HONO chemistry can accelerate the formation of nitrate through the enhancement of atmospheric oxidation capacity HO x and O 3. Considering the uncertainty of the partition of gaseous HNO 3 to particle nitrate see Sect. The absolute enhancement was more significant in the areas with higher aerosol concentrations. Meanwhile, the HONO chemistry also accelerated the formation of other secondary inorganic aerosols.
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Particle sulfate can be generated through the partition of H 2 SO 4 to the particle phase. The above results indicated that the HONO chemistry also aggravated the particulate pollution during this episode. This study has identified the major contributors to the observed high HONO levels during a severe winter pollution episode and highlighted the importance of HONO chemistry in the combined photochemical and haze pollution in a subtropical region.
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The model simulations suggested a predominant contribution from NO 2 heterogeneous reactions enhanced by humidity and solar radiation. The high HONO concentration significantly increased the atmospheric oxidation capacity and the levels of ozone and secondary aerosols, especially total nitrate. This study highlights the key role of HONO chemistry in the formation of winter haze in a subtropical environment and indicates the critical need to include and update HONO sources in regional air quality models in order to predict ozone and other secondary pollutants during heavy pollution events in southern China and similar regions.
Additional efforts are needed to improve the current representation of HONO sources, such as evaporation of dew and a more accurate simulation of deposited nitrate. Model codes and input data are available from Xiao Fu. The measurement data used in this study are available from Tao Wang cetwang polyu. All authors contributed to discussion and commented on the paper. It is not associated with a conference. Edited by: Dwayne Heard Reviewed by: two anonymous referees.
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Alicke, B. Appel, K. Model Dev. Baergen, A. Bernard, F. Cao, J.
Plant Physiol. Elstner, E.