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)

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.

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

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

The relative importance of common activities on indoor nitrous acid (HONO) mixing ratios was explored during high time resolution, month-long measurements by chemical ionization mass spectrometry in a previously unoccupied house. Indoor HONO varied from 0.2 to 84.0 ppb (mean: 5.5 ppb; median 3.8 ppb), an order of magnitude higher than simultaneously measured outdoor values, indicating important indoor sources. They agree well with simultaneous measurements of HONO by Laser-Photofragmentation/Laser-Induced Fluorescence. Before any combustion activities, the mixing ratio of 3.0 ± 0.3 ppb is indicative of secondary sources such as multiphase formation from NO2. Cooking (with propane gas), especially the use of an oven, led to significant enhancements up to 84 ppb, with elevated mixing ratios persisting for a few days due to slow desorption from indoor surface reservoirs. Floor bleach cleaning led to prolonged, substantial decreases of up to 71-90% due to reactive processes. Air conditioning modulated HONO mixing ratios driven by condensation to wet surfaces in the AC unit. Enhanced ventilation also significantly lowered mixing ratios. Other conditions including human occupancy, ozone addition, and cleaning with terpene, natural product, and vinegar cleaners had a much smaller influence on HONO background levels measured following these activities.

Cooking, Bleach Cleaning, and Air Conditioning Strongly Impact Levels of HONO in a House.

. Although the anthropogenic emissions of SO 2 have decreased
significantly in China, the decrease in SO 4 2 - in PM 2.5 is
much smaller than that of SO 2 . This implies an enhanced formation rate
of SO 4 2 - in the ambient air, and the mechanism is still under
debate. This work investigated the formation mechanism of particulate
sulfate based on statistical analysis of long-term observations in
Shijiazhuang and Beijing supported with flow tube experiments. Our main
finding was that the sulfur oxidation ratio (SOR) was exponentially
correlated with ambient RH in Shijiazhuang (SOR  = 0.15 + 0.0032 × exp⁡ ( RH / 16.2 ) ) and Beijing (SOR  = - 0.045 + 0.12 × exp⁡ ( RH / 37.8 ) ). In
Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water
content (AWC) in PM 2.5 (SOR  = 0.15 + 0.40 × AWC / PM 2.5 ).
Our results suggest that uptake of SO 2 instead of oxidation of S(IV) in
the particle phase is the rate-determining step for sulfate formation.
NH 4 NO 3 plays an important role in the AWC and the change of
particle state, which is a crucial factor determining the uptake kinetics of
SO 2 and the enhanced SOR during haze days. Our results show that
NH 3 significantly promoted the uptake of SO 2 and subsequently the
SOR, while NO 2 had little influence on SO 2 uptake and SOR in the
presence of NH 3 .

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Ammonium nitrate promotes sulfate formation through uptake kinetic regime

. 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.

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

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.

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.

Wang Xuefei

Papers

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

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

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

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