Observations of nitrous acid and its relative humidity dependence in Shanghai

Significant roles of aged dust aerosols on rapid nitrate formation under dry conditions in a semi-arid city

Mineral dust can accelerate secondary aerosol formation under humid conditions. However, it is unclear whether it can promote secondary aerosol formation under dry conditions. To investigate this issue, two years of comprehensive observations was conducted at a semi-arid site, near the dust source regions. Three types of episodes were selected: dust, anthropogenic-dominated, and mixed (mixed with dust and anthropogenic aerosols). Compared to anthropogenic-dominated episodes under humid conditions, rapid nitrate formation was still observed in mixed episodes under dry conditions, suggesting that active metallic oxides in dust, such as titanium dioxide, could promote photochemical reactions of nitrogen dioxide. The detailed evolutionary processes are further illustrated by a typical dust-to-mixed episode. After the arrival of the dust, titanium sharply increased ten-fold and rapid nitrate formation was observed, together with a rapid increase in the two most important photochemical pollutants, ozone and peroxyacetyl nitrate. The increased secondary organic carbon further illustrated that the suspended dust particles accelerated the atmospheric oxidative capacity, thereby enhancing secondary aerosol formation and eventually leading to haze pollution. These results differ from those in humid regions and therefore expand the scientific understanding of the impact of dust aerosols on haze pollution under dry conditions.

Fractal analysis of impact of PM2.5 on surface O3 sensitivity regime based on field observations

Properties of PM_2.5 that can change aerosol chemistry and photolysis rates have great impacts on O₃ sensitivity regime, further affecting the production rate of surface O₃. However, responses of O₃ sensitivity regime to changes in PM_2.5 levels are difficult to be accurately determined, due to the complexity and nonlinearity of atmospheric chemistry. Here, based on long-term time series (2016-2020) of air quality variables in north and south Taiwan, fractal analysis along with Pearson correlation analysis are used to directly reveal the impacts of PM_2.5 on O₃ sensitivity regime in real atmosphere, by capturing the nonlinear dynamic relations among air pollutants. Great regional and seasonal difference in impacts of PM_2.5 on O₃ sensitivity regime may be ascribed to meteorological factors, PM_2.5 components and levels of SO₂, NO, NO₂, etc. For north Taiwan, increased PM_2.5 level can enhance the sensitivity of O₃ formation to VOC in spring and summer, whereas the opposite effect can be observed in winter. But for south Taiwan, the influence of PM_2.5 on O₃ sensitivity regime is not statistically significant, excluding spring. Furthermore, feasibility and availability of fractal analysis is tested by simulations with Empirical Kinetics Modeling Approach (EKMA). The results demonstrate the capability of fractal analysis to identify the impacts of PM_2.5 on O₃ sensitivity regime in real atmosphere, which can provide suggestions for PM_2.5-O₃ coordinated control strategies in regions suffering combined air pollution.

Observations by Ground-Based MAX-DOAS of the Vertical Characters of Winter Pollution and the Influencing Factors of HONO Generation in Shanghai, China

Analyzing vertical distribution characters of air pollutants is conducive to study the mechanisms under polluted atmospheric conditions. Nitrous acid (HONO) is a kind of crucial species in photochemical cycles. Exploring the influence and sources of HONO in air pollution at different altitudes offers some insights into the research of tropospheric oxidation chemistry processes. Ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS) measurements were conducted in Shanghai, China, from December 2017 to March 2018 to investigate vertical distributions and diurnal variations of trace gases (NO2, HONO, HCHO, SO2, and water vapor) and aerosol extinction coefficient in the boundary layer. Aerosol and NO2 showed decreasing profile exponentially, SO2 and HCHO concentrations were observed relatively high values in the middle layer. SO2 was caused by industrial emissions, while HCHO was from secondary sources. As for HONO, below 0.82 km, the heterogeneous reactions of NO2 impacted on forming HONO, while in the upper layers, vertical diffusion might be the dominant source. The contribution of OH production from HONO photolysis at different altitudes was mainly controlled by the concentration of HONO. MAX-DOAS measurements characterize the vertical structure of air pollutants in Shanghai and provide further understanding for HONO formation, which can help deploy advanced measurement platforms of regional air pollution over eastern China.

Insight into the formation of secondary inorganic aerosol based on high-time-resolution data during haze episodes and snowfall periods in Zhengzhou, China

Episodic haze is frequently observed in Zhengzhou, China. Such haze typically contains secondary inorganic aerosols. In this paper, we explore the formation mechanisms of sulfate, nitrate, and ammonium (SNA) in Zhengzhou from January 3 to 25, 2018 based on the results of a series of online instruments and a size-segregated filter sampler. Our results document the remarkable contributions of SNA to winter haze episodes in Zhengzhou, where they account for about 50% of PM_2.5 mass concentration. SNA were mainly concentrated in droplet-mode particles, which increased remarkably with the aggravation of the haze episode. In addition, KNO₃ and NaNO₃ were formed in droplet-mode particles and coarse-mode particles respectively with increasing PM_2.5 concentration. The atmosphere during the observation period was ammonia-rich, and the aerosol was acidic under high PM_2.5 concentration. Homogeneous reactions dominated the formation of nitrate. HONO photolysis played a more important role in the origin of OH radicals when O₃ decreased during haze episode. Under high relative humidity (RH), nitrate formation was influenced by heterogeneous hydrolysis reactions of N₂O₅. Sulfates were mainly formed through aqueous-phase reactions, especially when the RH was higher than 60%. Under these conditions, there were amounts of liquid water content existed in aerosols. Finally, we observed enhanced conversion of SO₂ and NO₂ during snowfall periods. This effect may be attributable to the higher RH and O₃ levels despite the unfavorable effects of wet deposition and low concentrations of gaseous precursors.

An observational study of nitrous acid (HONO) in Shanghai, China: The aerosol impact on HONO formation during the haze episodes

Continuous HONO measurement was conducted to study the formation features of HONO during the haze episodes at Shanghai, China. The HONO concentration ranged from 0.26 to 5.84ppb and averaged at 2.31ppb during the measurement period. The HONO concentration during the haze episode (P1), the haze-fog episode (P2) and the clean period (P3) were 2.80, 2.35 and 1.78ppb, respectively. Heterogeneous conversion of NO₂ was the dominate pathway for nocturnal HONO formation, and the heterogeneous conversion efficiency of NO₂ to HONO was closely associated with the PM_2.5 concentration. The averaged heterogeneous conversion rate of NO₂-to-HONO (C_HONO) during the pollution periods (P1+P2) was 1.58×10^-2h^-1, higher than that during the clean period (P3) (0.93×10^-2h^-1), suggesting the higher conversion potential of NO₂ to HONO during the pollution episodes. The daytime unknown HONO production rate (P_unknown) in the pollution period was 2.98ppb/h, higher than 1.78ppb/h in the clean period. Further, aerosol played a role in P_unknown during the transformation of the clean period to the pollution period. At a single particle scale, transmission electron microscopy (TEM) images revealed that most of the particles during P1 and P2 were agglomerated, whereas the particles collected from P3 were uniformly distributed and showed simple morphologies. The number percentage of the S/N-bearing particles during P1 (34%) and P2 (27%) were higher than that during P3 (20%). In addition, particles contained more internally mixed nitrates during P1 and P2 than those during P3, suggesting more intense heterogeneous conversion of NO₂ to HONO on particle surfaces during the pollution episodes. In the present study, the averaged HONO/NO_x ratio (5.60%), especially during P1 (7.80%) and P2 (7.50%) was much higher than that assumed global averaged value of 2.0%, suggesting a potentially important role for the HONO chemistry in Shanghai. This study provides new insights into the HONO formation mechanism in the atmosphere characterized by high fine particle level.

Exploring the nitrous acid (HONO) formation mechanism in winter Beijing: direct emissions and heterogeneous production in urban and suburban areas

Continuous measurements of nitrous acid (HONO) were performed from December 12 to December 22, 2015 in both urban and suburban areas of Beijing to study the formation mechanism of HONO. The measurement campaign in both sites included a clean-haze-clean transformation process. HONO concentrations showed similar variations in the two sites, while they were always higher in the urban area. Moreover, correlations of HONO with NOx, NO2, NO, PM2.5 and relative humidity (RH) were studied to explore possible HONO formation pathways, and the contributions of direct emissions, heterogeneous reactions, and homogeneous reactions were also calculated. This showed that HONO in urban and suburban areas underwent totally different formation procedures, which were affected by meteorological conditions, PM2.5 concentrations, direct emissions, homogeneous reactions and heterogeneous reactions. PM2.5 concentrations and RH would influence the NO2 conversion efficiency. Heterogeneous reactions of NO2 were more efficient in suburban areas and in clean periods while direct emissions and homogeneous reactions contributed more in urban areas and in polluted periods when the concentrations of NOx and NO were at a high level.

Concentration and sources of atmospheric nitrous acid (HONO) at an urban site in Western China

Highly time-resolved measurements of nitrous acid (HONO) were carried out with a highly sensitive long path absorption photometer (LOPAP) at an urban site of Xi'an in Western China from 24 July to 6 August 2015 to investigate the atmospheric variations, sources, and formation pathways of HONO. The concentrations of HONO vary from 0.02 to 4.3ppbv with an average of 1.12ppbv for the entire measurement period. The variation trends of HONO and NO₂ are very similar and positively correlated which, together with the similar diurnal profiles of HONO/NO₂ ratio and HONO, suggest the importance of heterogeneous conversion of HONO from NO₂. The nocturnal HONO level is governed by heterogeneous formation from NO₂, followed by homogeneous formation of NO with OH and then by direct emissions. Further, it is found that the heterogeneous formation of HONO is largely affected by relative humidity and aerosol surface. Daytime HONO budget analysis indicates that an additional unknown source with HONO production rate of 0.75ppbvh^-1 is required to explain the observed HONO concentration, which contributes 60.8% of the observed daytime HONO.

Contributions of vehicular emissions and secondary formation to nitrous acid concentrations in ambient urban air in Tokyo in the winter

Nitrous acid (HONO) plays an important role in the formation of OH radicals, which are involved in photochemical oxidation. HONO concentrations in ambient air at urban sites have previously been measured, but very few studies have been performed in central Tokyo. In this study, HONO concentrations in ambient air in southeast central Tokyo (near Tokyo Bay) in winter were determined by incoherent cavity enhanced absorption spectroscopy. The O₃, NO, NO₂, and SO₂ concentrations were simultaneously determined. The NO concentrations were used to classify the parts of the study period into types I (high pollution), II (medium pollution), and III (low pollution). The maximum HONO concentrations in the type I, II, and III periods were 7.1, 4.5, and 3.0ppbv, respectively. These concentrations were comparable to concentrations previously found in other Asian megacities. The mean HONO concentration varied diurnally, and HONO was depleted between 00:00 and 03:00 each day. The sampling site is surrounded by roads with high traffic loads, but vehicular emissions were estimated to contribute <10% of the HONO concentrations. Two positive and negative relative humidity dependences of the HONO to NO₂ ratio were confirmed, implying the existence of the two different secondary formation process of HONO. The NO₂ to HONO conversion rates at night in the type I, II, and III periods were 6.3×10^-3, 7.6×10^-3, and 4.2×10^-3h^-1, respectively.

Measurements of nitrous acid (HONO) in urban area of Shanghai, China

Nitrous acid (HONO), as a precursor of the hydroxyl radical (OH), plays an important role in the photochemistry of the troposphere, especially in the polluted urban atmosphere. A field campaign was conducted to measure atmospheric HONO concentration and that of other pollutants (such as NO2 and particle mass concentration) in the autumn of 2009 at Shanghai urban areas. HONO mixing ratios were simultaneously measured by three different techniques: long path absorption photometer (LOPAP), differential optical absorption spectroscopy (DOAS) and chemical ionization mass spectrometer (CIMS). The measurements showed that the mixing ratios of HONO were highly variable and depended strongly on meteorological parameters. The HONO levels ranged from 0.5 to 7 ppb with maximum values during early morning and minimum levels during late afternoon. The three instruments reproduced consistent diurnal pattern of HONO concentrations with higher concentration during the night compared to the daylight hours. Comparison of HONOLOPAP/HONOCIMS ratios during daytime and nighttime periods exhibited a non-systematic disagreement of 0.93 and 1.16, respectively. This would indicate different chemical compositions of sampled air for the LOPAP and the CIMS instruments during daytime and nighttime periods, which have possibly affected measurements. Mean HONO concentration reported by LOPAP was 33 % higher than by DOAS on the whole period with no significant difference between daytime and nighttime periods. This revealed a systematic deviation from both instruments. The present data provides complementary information of HONO ambient levels in the atmosphere of Shanghai urban areas.

Investigation of Ground-Level Ozone and High-Pollution Episodes in a Megacity of Eastern China

Differential Optical Absorption Spectroscopy (DOAS) was used for the long-term observation of ground-level ozone (O₃) from March 2010 to March 2013 over Shanghai, China. The 1-hour average concentration of O₃ was 27.2 ± 17.0 ppbv. O₃ level increased during spring, reached the peak in late spring and early summer, and then decreased in autumn and finally dropped to the bottom in winter. The highest monthly average O3 concentration in June (41.1 ppbv) was nearly three times as high as the lowest level recorded in December (15.2 ppbv). In terms of pollution episodes, 56 hourly samples (on 14 separate days) in 2010 exceeded the 1-hour ozone limit of 200 μg/m³ specified by the Grade II of the Chinese Ambient Air Quality Standards (CAAQS, revised GB 3095-2012). Utilizing the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the primary contribution to high ozone days (HODs) was identified as the regional transportation of volatile organic compounds (VOC) and high concentrations of O₃ from the chemical industrial zone in the Jinshan district of Shanghai. HODs showed higher concentrations of HONO and NO₂ than non-episode conditions, implying that HONO at high concentration during HODs was capable of increasing the O₃ concentration. The photolysis rate of HONO was estimated, suggesting that the larger number of OH radicals resulting from high concentrations of HONO have a considerable impact on ozone concentrations.

Occurrence of atmospheric nitrous acid in the urban area of Beijing (China)

The atmospheric concentrations of nitrous acid (HONO) have been measured during two field campaigns in the winter and summer of 2007 at Beijing (China). The results were discussed from the standpoint of temporal and diurnal variations and meteorological effects. The daily average HONO concentrations were in the range of 0.03-2.91ppb and didn't show temporal variation between the winter and summer periods. The temporal trends seemed to be largely affected by meteorological conditions. HONO concentrations showed very typical diurnal variations during intensive winter and summer periods. Nitrogen oxides were key precursors of HONO formation and the HONO/NO2 values were higher than those reported for direct emission (<1%), indicating the prevalence of secondary chemical HONO formation on direct emission during both periods. We used a pseudo steady state approach (PSS), which included homogeneous and heterogeneous reactions and direct emission, explaining on average about 83% and 48% of the observed HONO levels during the intensive winter and summer periods, respectively. The daytime unknown HONO production was on average 2.58ppbh(-1) during the summer period. The HNO3 and fine particulate NO3(-) photolysis contributed weakly as HONO source. Including these sources in the PSS calculation, we explained about 53% of the observed HONO levels. The results showed that heterogeneous JNO2 dependent processes on aerosol and ground surfaces, involving NO2 as HONO precursor, were HONO sources during the summer measurements.

Interactions of NO2 with Zr-based MOF: effects of the size of organic linkers on NO2 adsorption at ambient conditions

Zirconium-based metal organic framework (Zr-MOF), UiO-66 and UiO-67, were synthesized and used as adsorbents of NO(2) at ambient temperatures in either dry or moist conditions. The samples were characterized before and after exposure to NO(2) by X-ray diffraction, scanning electron microscopy, N(2)-adsorption at 77 K, thermal analysis, and infrared spectroscopy. The results indicate the important effect of a ligand size on the adsorption of NO(2) on Zr-MOF materials. While the large size of the 4,4-benzenebiphenyldicarboxylic acid (BDPC) ligand has a positive impact on the adsorption of NO(2) on UiO-67 in moist conditions, the opposite effect is found in dry conditions. The large pore volume of UiO-67 enhances the adsorption of moisture and formation of nitric and nitrous acids. The small pore sizes of UiO-66 favor the NO(2) removal in dry conditions via dispersive forces. Upon interaction of NO(2) molecules with the Zr-MOF in dry conditions, the bond between the organic linker and metallic oxide center is broken, leading to the formation of nitrate and nitrite species. Moreover, organic ligands also contribute to the NO(2) reactive adsorption via nitration reaction.