Severe events of wintertime particulate air pollution in Beijing (winter haze) are associated with high relative humidity (RH) and fast production of particulate sulfate from the oxidation of sulfur dioxide (SO2) emitted by coal combustion. There has been considerable debate regarding the mechanism for SO2 oxidation. Here we show evidence from field observations of a haze event that rapid oxidation of SO2 by nitrogen dioxide (NO2) and nitrous acid (HONO) takes place, the latter producing nitrous oxide (N2O). Sulfate shifts to larger particle sizes during the event, indicative of fog/cloud processing. Fog and cloud readily form under winter haze conditions, leading to high liquid water contents with high pH (>5.5) from elevated ammonia. Such conditions enable fast aqueous-phase oxidation of SO2 by NO2, producing HONO which can in turn oxidize SO2 to yield N2O.This mechanism could provide an explanation for sulfate formation under some winter haze conditions.

/pdf/fast-sulfate-formation-from-oxidation-of-so-2-by-no-2-and-39srzjjcad.pdf

Fast sulfate formation from oxidation of SO 2 by NO 2 and HONO observed in Beijing haze

The photolysis of nitrous acid (HONO) in the early morning hours is an important source of OH radicals, the most important daytime oxidizing species. Although the importance of this mechanism has been recognized for many years, no accurate quantification of this OH source is available, and the role of HONO photolysis is often underestimated. We present measurements of HONO and its precursor NO 2 by Differential Optical Absorption Spectroscopy (DOAS) during the Berliner Ozonexperiment (BERLIOZ) field campaign in July/August 1998 at Pabstthum near Berlin, Germany. HONO concentrations, formation rates, and simultaneously measured HONO photolysis frequencies are used to calculate the total amount of OH formed by HONO photolysis during a full diurnal cycle. A comparison with the OH formation by photolysis of O 3 and HCHO and by the reaction of alkenes with ozone shows that HONO photolysis contributed up to 20% of the total OH formed in a 24 hour period during this campaign. In the morning hours, HONO photolysis was by far the most important OH source during BERLIOZ.

OH formation by HONO photolysis during the BERLIOZ experiment : Photochemistry experiment in BERLIOZ (PHOEBE)

Ozone and SOA formation potential based on photochemical loss of VOCs during the Beijing summer.

Abstract. Co-occurrences of high concentrations of PM2.5 and ozone (O3) have
been frequently observed in haze-aggravating processes in the North China
Plain (NCP) over the past few years. Higher O3 concentrations on hazy
days were hypothesized to be related to nitrous acid (HONO), but the key sources
of HONO enhancing O3 during haze-aggravating processes remain unclear.
We added six potential HONO sources, i.e., four ground-based (traffic, soil,
and indoor emissions, and the NO2 heterogeneous reaction on ground
surface (Hetground)) sources, and two aerosol-related (the NO2
heterogeneous reaction on aerosol surfaces (Hetaerosol) and nitrate
photolysis (Photnitrate)) sources into the WRF-Chem model and designed
23 simulation scenarios to explore the unclear key sources. The results
indicate that ground-based HONO sources producing HONO enhancements showed a
rapid decrease with height, while the NO + OH reaction and aerosol-related
HONO sources decreased slowly with height. Photnitrate contributions to
HONO concentrations were enhanced with aggravated pollution levels. The enhancement of
HONO due to Photnitrate on hazy days was about 10 times greater than on
clean days and Photnitrate dominated daytime HONO sources
(∼ 30 %–70 % when the ratio of the photolysis frequency of
nitrate (Jnitrate) to gas nitric acid (JHNO3) equals 30) at higher
layers (>800 m). Compared with that on clean days, the
Photnitrate contribution to the enhanced daily maximum 8 h averaged
(DMA8) O3 was increased by over 1 magnitude during the haze-aggravating process. Photnitrate contributed only ∼ 5 %
of the surface HONO in the daytime with a Jnitrate/JHNO3 ratio of 30
but contributed ∼ 30 %–50 % of the enhanced O3 near the
surface in NCP on hazy days. Surface O3 was dominated by volatile
organic compound-sensitive chemistry, while O3 at higher altitudes
(>800 m) was dominated by NOx-sensitive chemistry.
Photnitrate had a limited impact on nitrate concentrations (<15 %) even with a Jnitrate/JHNO3 ratio of 120. These results
suggest the potential but significant impact of Photnitrate on O3
formation, and that more comprehensive studies on Photnitrate in the
atmosphere are still needed.

Amplified role of potential HONO sources in O3 formation in North China Plain during autumn haze aggravating processes

PM2.5 pollution is a complex process mainly affected by emission sources and meteorological conditions. However, it is hard to accurately assess the effects of emission sources and meteorological conditions on the variation of PM2.5 concentrations in the complex atmospheric environment. In this study, the Random Forest model with Shapley Additive exPlanations (RF-SHAP) and Partial Dependence Plot (RF-PDP) was combined with Positive Matrix Factorization (PMF) to evaluate the impacts of various factors on PM2.5 pollution. The results show that anthropogenic emissions and meteorological conditions contributed about 67% (40.5 μg/m3) and 33% (19.7 μg/m3) to variation in PM2.5 concentrations, respectively. Specifically, secondary nitrate (SN) had the greatest impact among all sources (about 45%). Hence, we further explore the impacts of the primary sources and meteorological conditions on SN formation. Coal combustion and vehicle emissions significantly contribute to the formation of SN by providing a large number of precursor NOX. Additionally, the RF-PDP method was further employed to estimate the synergistic effects of primary sources and meteorological conditions on SN formation. The results help reveal strategies to simultaneously reduce SN by controlling primary emissions under suitable meteorological conditions. This work also suggests that the machine learning model can utilize online datasets well and provide a reliable approach for analyzing the causes of PM2.5 pollution. 

Machine learning combined with the PMF model reveal the synergistic effects of sources and meteorological factors on PM2.5 pollution.

. Secondary aerosol is a major component of PM2.5, yet its formation mechanism in the ambient atmosphere is still an open question. Based on field measurements in downtown Beijing, we show that the photolysis of nitrous acid (HONO) could promote the formation of organic and nitrate aerosol in wintertime Beijing as evidenced by the growth of the mass concentration of organic and nitrate aerosols linearly increasing as a function of consumed HONO from early morning to noon. The increased nitrate also lead to the formation of particulate matter ammonium by enhancing the neutralization of nitric acid by ammonia. We further illustrate that over 50 % of the ambient HONO during pollution events in wintertime Beijing might be related to traffic-related emission including direct emission and formation via the reaction between OH and vehicle-emitted NO. Overall, our results highlight that the traffic-related HONO plays an important role in the oxidative capacity and in turn, contribute to the haze formation in winter Beijing. Mitigation of HONO and NOx emission from the vehicles might be an effective way to reduce secondary aerosol mass formation and severe haze events in wintertime Beijing.

/pdf/the-promotion-effect-of-nitrous-acid-on-aerosol-formation-in-2njyr440ly.pdf

The promotion effect of nitrous acid on aerosol formation in wintertime in Beijing : the possible contribution of traffic-related emissions

The invention discloses a generating system of standard gas-state nitrous acid. The system is based on the reaction of gas-state hydrochloric acid and solid-state nitrite at a certain temperature and humidity to produce the gas-state nitrous acid. The specific concentration of gas-state nitrous acid, i.e., the standard gas-state nitrous acid can be accurately, simply and conveniently provided. The system has the advantages that the gas-state nitrous acid with the required concentration can be conveniently and accurately produced. The concentration range of the gas-state nitrous acid produced by the generating system is 1ppt to 10ppm; the gas-state nitrous acid with the concentration can be used for laboratory simulation study and can also be used as a standard source to be used for calibration of an HONO analysis instrument; the important effect is achieved on improving the accuracy of the instrument for precisely measuring the HONO. The invention also discloses a preparation method of the standard gas-state nitrous acid. According to the preparation method, the generating system is used; the preparation method of the standard gas-state nitrous acid can be used for preparing the gas-state nitrous acid with the specific concentration, i.e., the standard gas-state nitrous acid.

Preparation method and generating system of standard gas-state nitrous acid

The invention relates to a long-optical-path liquid flow cell and a defoaming detection method, belongs to the technical field of detection equipment, and solves the problems that in the prior art, liquid to be tested needs to be subjected to suspended particle filtering in advance, bubbles are easily attached to the inner wall of a liquid core waveguide, and the like. The long-optical-path liquidflow cell comprises a liquid core waveguide tube (7), a vacuum container (8), a vacuum air pump (9), a first connector (3) and a second connector (4), wherein the liquid core waveguide tube (7) is arranged in the vacuum container (8); the vacuum air pump (9) is used for forming a vacuum environment in the vacuum container (8); and the first connector (3) and the second connector (4) are both of tee structures. The adsorption state bubbles in the liquid core waveguide tube can be removed, so that the detection efficiency and accuracy are improved.

Long-optical-path liquid flow cell and defoaming detection method

As an important indoor pollutant, nitrous acid (HONO) can contribute to the concentration of indoor OH radicals by photolysis via sunlight penetrating into indoor environments, thus affecting the indoor oxidizing capability. In order to investigate the concentration of indoor HONO and its impact factors, three different indoor environments and two different locations in urban and suburban areas were selected to monitor indoor and outdoor pollutants simultaneously, including HONO, NO, NO2, nitrogen oxides (NOx), O3, and particle mass concentration. In general, the concentration of indoor HONO was higher than that outdoors. In the urban area, indoor HONO with high average concentration (7.10 ppbV) was well-correlated with the temperature. In the suburban area, the concentration of indoor HONO was only about 1-2 ppbV, and had a good correlation with indoor relative humidity. It was mainly attributed to the heterogeneous reaction of NO2 on indoor surfaces. The sunlight penetrating into indoor environments from outside had a great influence on the concentration of indoor HONO, leading to a concentration of indoor HONO close to that outdoors.

Evaluation and impact factors of indoor and outdoor gas-phase nitrous acid under different environmental conditions.

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Papers

The promotion effect of nitrous acid on aerosol formation in wintertime in Beijing : the possible contribution of traffic-related emissions

Preparation method and generating system of standard gas-state nitrous acid

Long-optical-path liquid flow cell and defoaming detection method

Evaluation and impact factors of indoor and outdoor gas-phase nitrous acid under different environmental conditions.