Indoor air quality is of major concern for human health and well-being. Nitrous acid (HONO) is an emerging indoor pollutant, and its indoor mixing ratios are usually higher than outdoor levels, ...ranging from a few to tens of parts per billion (ppb). HONO exhibits adverse effects to human health due to its respiratory toxicity and mutagenicity. Additionally, HONO can easily undergo photodissociation by ultraviolet light to produce hydroxyl radicals (OH•), which in turn trigger a series of further photochemical oxidation reactions of primary or secondary pollutants. The accumulation of indoor HONO can be attributed to both direct emissions from combustion sources, such as cooking, and secondary formation resulting from enhanced heterogeneous reactions of NOx on indoor surfaces. During the day, the primary sink of indoor HONO is photolysis to OH• and NO. Moreover, adsorption and/or reaction on indoor surfaces, and diffusion to the outside atmosphere contribute to HONO loss both during the day and at night. The level of indoor HONO is also affected by human occupancy, which can influence household factors such as temperature, humidity, light irradiation, and indoor surfaces. This comprehensive review article summarized the research progress on indoor HONO pollution based on indoor air measurements, laboratory studies, and model simulations. The environmental and health effects were highlighted, measurement techniques were summarized, pollution levels, sources and sinks, and household influencing factors were discussed, and the prospects in the future were proposed.
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•Indoor HONO measurements from initial to recent studies were reviewed.•Heterogeneous reactions are accelerated indoors due to higher surface-to-volume ratio.•More source and sink mechanisms are needed to improve indoor air chemistry model.•The impact of indoor HONO on outdoor atmosphere needs further evaluation.
China's “Blue Sky Action Plan” aimed at tremendous improvements in atmospheric visibility. While stringent emission control policies have substantially brought down PM2.5 mass concentrations, ...visibility improved much less than expected due to non-linear responses of visibility changes to PM2.5 reductions. In this study, we used long-term continuous humidified nephelometer system measurements of multi-wavelength aerosol scattering coefficients in both dry state and controlled relative humidity conditions in the North China Plain during spring and summer to attempt disentanlge the non-linear relationsips between visibility and PM2.5 mass.Aerosol scattering efficiency, optical hygroscopicity and air relative humidity are key factors for relating PM2.5 mass to visibility. It was found that aerosol volume scattering efficiencies (VSEs) were highly correlated (r > 0.8) with aerosol scattering coefficients. The continuous decrease of aerosol scattering Ångström exponent during pollution episodes revealed dominant contributions of secondary aerosol formation to aerosol size growth and mass accumulation, explaining aerosol VSE increases. Moreover, the optical hygroscopicity parameter κsca that describes the aerosol light scattering enhancement abilities through water uptake increased jointly with VSE and aggravated the visibility degradation during middle to final stages of pollution episodes. Thus, low visibility events (<3 km) only occurred when VSE and κsca were at their highest levels. The contribution of aerosol water to visibility degradation increased as visibility decreased, and contributed dominantly to visibility degradation under extremely low visibility conditions (<1 km). However, under hazy visibility conditions (3–10 km), which occurred most frequently, both aerosol water and scattering efficiency enhancement played significant roles. For setting up more efficient emission control strategies targeting on visibility improvement, our results highly encourage more future research on the linkages between secondary aerosol formation mechanisms and co-variations of aerosol scattering efficiency and aerosol hygroscopicity on the NCP.
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•Factors affecting the nonlinear relationship between visibility improvement and aerosol mass reduction is investigated.•Aerosol scattering efficiencies were highly correlated (r>0.8) with aerosol scattering coefficients•Joint increase of aerosol scattering efficiency and aerosol hygroscopicity aggravate visibility reduction
We investigated the seasonal and spatial ozone variations in China by using three-year surface ozone observation data from the six Chinese Global Atmosphere Watch (GAW) stations and tropospheric ...column ozone data from satellite retrieval over the period 2010–2012. It is shown that the seasonal ozone variations at these GAW stations are rather different, particularly between the western and eastern locations. Compared with western China, eastern China has lower background ozone levels. However, the Asian summer monsoon (ASM) can transport photochemical pollutants from the southern to the northern areas in eastern China, leading to a northward gradual enhancement of background ozone levels at the eastern GAW stations. Over China, the tropospheric column ozone densities peak during spring and summer in the areas that are directly and/or indirectly affected by the ASM, and the peak time lags from the south to the north in eastern China. We also investigated the regional representativeness of seasonal variations of ozone at the six Chinese GAW stations using the yearly maximum tropospheric column month as indicator. The results show that the seasonal variation characteristics of ozone revealed by the Chinese GAW stations are typical, with each station having a considerable large surrounding area with the ozone maximum occurring at the same month. Ozone variations at the GAW stations are influenced by many complex factors and their regional representativeness needs to be investigated further in a broader sense.
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Atmospheric peroxyacetyl nitrate (PAN) and ozone (O3) are two typical indicators for photochemical pollution that have adverse effects on the ecosystem and human health. Observation networks for ...these pollutants have been expanding in developed regions of China, such as North China Plain (NCP) and Pearl River Delta (PRD), but are sparse in Yangtze River Delta (YRD), meaning their concentration and influencing factors remain poorly understood. Here, we performed a one-year measurement of atmospheric PAN, O3, particulate matter with aerodynamic diameter smaller than 2.5 μm (PM2.5), nitrogen oxides (NOx), carbon monoxide (CO), and meteorological parameters from December 2016 to November 2017 in Shanghai. Overall, high hourly maximum PAN and O3 were found to be 7.0 and 185 ppbv in summer, 6.2 and 146 ppbv in autumn, 5.8 and 137 ppbv in spring, and 6.0 and 76.7 ppbv in winter, respectively. Continental air masses probably carried atmospheric pollutants to the sampling site, while frequent maritime winds brought in less polluted air masses. Furthermore, positive correlations (R: 0.72–0.85) between PAN and O3 were found in summer, indicating a predominant role of photochemistry in their formation. Unlike in summer, weak or no correlations between PAN and O3 were featured during the other seasons, especially in winter, due to their different loss pathways. Unexpectedly, positive correlations between PAN and PM2.5 were found in all seasons. During summer, moderate correlation could be attributed to the strong photochemistry acting as a common driver in the formation of secondary aerosols and PAN. During winter, high PM2.5 might promote PAN production through HONO production, hence resulting in a good positive correlation. Additionally, the loss of PAN by thermal decomposition (TPAN) only accounted for a small fraction (ca. 1%) of the total (PAN + TPAN) during a typical winter episode, while it significantly reached 14.4 ppbv (71.1% of the total) in summer.
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•Hourly maximum PAN and O3 were measured as 7.0 and 185 ppbv in summer, respectively.•Continent air masses carried high loadings of atmospheric pollutants to sample site.•Large loss of PAN by thermal decomposition was found in summer.
Atmospheric ammonia (NH3) as the most important alkaline gas in the atmosphere has attracted much attention in recent years due to its critical role in haze formation, especially on the North China ...Plain (NCP). Comprehensive studies are needed for investigating diurnal variations of NH3 and underlying mechanisms in different seasons and their potential impacts on atmospheric chemistry. In this study, continuous long-term observation (Mar. 2016 to May 2017) of NH3 at a rural site in the NCP was used to characterize the diurnal variation of NH3 in different seasons and to unveil its causes and potential impacts on atmospheric chemistry. NH3 concentrations displayed rapid increases during the morning, reaching very prominent peaks mostly between 8:00 to 11:00 LT. Such frequent (55%) morning peaks were mainly caused by the evaporation of dew and guttation water droplets. Average dew and guttation water volume concentrations of 750 mL m−2 was estimated for spring, which resulted in approximate NH3 emissions of 800 ng m−2 s− 1. Such high emission fluxes from dew and guttation water evaporation have never been reported before, suggesting dew and guttation droplets to be significant night-time reservoirs and strong morning sources for NH3. In light of recent studies putting forward that NH3 can promote the heterogeneous formation of HONO and nitrate under high humidity conditions, we investigated the differences in HONO and aerosol chemical composition diurnal variations between days with and without NH3 morning spikes during November. HONO, nitrate and sulfate concentrations were significantly higher for days with NH3 morning spikes, with HONO displaying a morning peak near that of NH3. These results demonstrate that the prevailing NH3 morning spikes on the NCP have significant influences on aerosol formation and atmospheric chemistry. NH3 emission mitigation strategies and regulations are urgently needed.
•Highly frequent morning peaks of NH3 caused by dew and guttation water.•Dew and guttation water exhibit very high NH3 storage and emission capacity.•NH3 diurnal variation has potential impact on rapid haze formation.
Dew and guttation water exhibit very high NH3 storage and emission capacity, causing highly frequent NH3 morning peaks, inducing rapid haze formation in the North China Plain.
Ozone pollution is still considered a severe environmental problem in China despite the fact that great efforts have been devoted to monitoring and alleviating its impact by the Chinese government ...including the establishment of numerous observational networks. One of the issues most relevant to the design of emission reduction policies is to distinguish the O3 chemical regime. Here a method of quantifying the fraction of the radical loss versus NOx chemistry was applied to identify the O3 chemical regime inferred from the weekly pattern of atmospheric O3, CO, NOx, and PM10, which were monitored by Ministry of Ecology and Environment of China (MEEC). During spring and autumn, O3 and the total odd oxygen (Ox, Ox = O3 + NO2) weekend afternoon concentrations are both higher than the weekday values during 2015–2019 except in 2016, while CO and NOx weekend morning concentrations were generally both smaller than weekday values except 2017. Results from the calculated values of fraction of the radical loss by NOx chemistry relative to total radical loss (Ln/Q) suggested a volatile organic compound (VOC)-limited regime at this site in the spring of 2015–2019, as expected from the decreasing trend in NOx concentration and essentially constant CO after 2017. With respect to autumn, a shift from a transition regime during 2015–2017 to a VOC-limited regime in 2018 was found, which rapidly took place to a NOx-limited regime in 2019. No significant differences were detected in the Ln/Q values under different assumptions on photolysis frequencies both in spring and autumn mostly from 2015 to 2019, giving the same conclusion of determining the O3 sensitivity regime. This study develops a new method in determining the O3 sensitivity regime in the typical season in China and provides insight into efficient O3 control strategies in different seasons.
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•High O3 and Ox were found on weekend but low NOx and CO on weekdays.•A VOC-limited regime mostly dominated O3 formation in the spring of 2015–2019.•A shift of O3 chemical regime was elucidated in the autumn of 2017.•No differences were found under different assumptions on J value and O3.
Peroxyacetyl nitrate (PAN) is an important indicator for photochemical pollution, formed similar to ozone in the photochemistry of certain volatile organic compounds (VOCs) in the presence of ...nitrogen oxides, and has displayed surprisingly high concentrations during wintertime that were better correlated to particulate rather than ozone concentrations, for which the reasons remained unknown. In this study, wintertime observations of PAN, VOCs, PM2.5, HONO, and various trace gases were investigated to find the relationship between aerosols and wintertime PAN formation. Wintertime photochemical pollution was affirmed by the high PAN concentrations (average: 1.2 ± 1.1 ppb, maximum: 7.1 ppb), despite low ozone concentrations. PAN concentrations were determined by its oxygenated VOC (OVOC) precursor concentrations and the NO/NO2 ratios and can be well parameterized based on the understanding of their chemical relationship. Data analysis and box modeling results suggest that PAN formation was mostly contributed by VOC aging processes involving OH oxidation or photolysis rather than ozonolysis pathways. Heterogeneous reactions on aerosols have supplied key photochemical oxidants such as HONO, which produced OH radicals upon photolysis, promoting OVOC formation and thereby enhancing PAN production, explaining the observed PM2.5–OVOC–PAN intercorrelation. In turn, parts of these OVOCs might participate in the formation of secondary organic aerosol, further aggravating haze pollution as a feedback. Low wintertime temperatures enable the long-range transport of PAN to downwind regions, and how that will impact their oxidation capacity and photochemical pollution requires further assessment in future studies.
Significant upward trends in surface ozone (O3) have been widely reported in China during recent years, especially during warm seasons in the North China Plain (NCP), exerting adverse environmental ...effects on human health and agriculture. Quantifying long-term O3 variations and their attributions helps to understand the causes of regional O3 pollution and to formulate according control strategy. In this study, we present long-term trends of O3 in the warm seasons (April–September) during 2006–2019 at an agricultural site in the NCP and investigate the relative contributions of meteorological and anthropogenic factors. Overall, the maximum daily 8-h average (MDA8) O3 exhibited a weak decreasing trend with large interannual variability. < 6 % of the observed trend could be explained by changes in meteorological conditions, while the remaining 94 % was attributed to anthropogenic impacts. However, the interannual variability of warm season MDA8 O3 was driven by both meteorology (36 ± 28 %) and anthropogenic factors (64 ± 27 %). Daily maximum temperature was the most essential factor affecting O3 variations, followed by ultraviolet radiation b (UVB) and boundary layer height (BLH), with rising temperature trends inducing O3 inclines throughout April to August, while UVB mainly influenced O3 during summer months. Under changes in emissions and air quality, warm season O3 production regime gradually shifted from dominantly VOCs-limited during 2006–2015 to NOx-limited afterwards. Relatively steady HCHO and remarkably rising NOx levels resulted in the fast decreasing MDA8 O3 (−2.87 ppb yr−1) during 2006–2012. Rapidly decreasing NOx, flat or slightly increasing HCHO promoted O3 increases during 2012–2015 (9.76 ppb yr−1). While afterwards, slow increases in HCHO and downwards fluctuating NOx led to decreases in MDA8 O3 (−4.97 ppb yr−1). Additionally, continuous warming trends might promote natural emissions of O3 precursors and magnify their impacts on agricultural O3 by inducing high variability, which would require even more anthropogenic reduction to compensate for.
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•Weak O3 decrease in agricultural region contrasting to reported increases in NCP.•Rural O3 trend and variability are determined by anthropogenic processes.•Meteorology affects O3 formation by magnifying anthropogenic impacts.
Secondary organic aerosol (SOA) constitutes a large fraction of organic aerosol worldwide, however, the formation mechanisms in polluted environments remain poorly understood. Here we observed fast ...daytime growth of oxygenated organic aerosol (OOA) (with formation rates up to 10 μg m–3 h–1) during low relative humidity (RH, daytime average 38 ± 19%), high RH (53 ± 19%), and fog periods (77 ± 13%, fog occurring during nighttime with RH reaching 100%). Evidence showed that photochemical aqueous-phase SOA (aqSOA) formation dominantly contributed to daytime OOA formation during the periods with nighttime fog, while both photochemical aqSOA and gas-phase SOA (gasSOA) formation were important during other periods with the former contributing more under high RH and the latter under low RH conditions, respectively. Compared to daytime photochemical aqSOA production, dark aqSOA formation was only observed during the fog period and contributed negligibly to the increase in OOA concentrations due to fog scavenging processes. The rapid daytime aging, as indicated by the rapid decrease in m,p-xylene/ethylbenzene ratios, promoted the daytime formation of precursors for aqSOA formation, e.g., carbonyls such as methylglyoxal. Photooxidants related to aqSOA formation such as OH radical and H2O2 also bear fast daytime growth features even under low solar radiative conditions. The simultaneous increases in ultraviolet radiation, photooxidant, and aqSOA precursor levels worked together to promote the daytime photochemical aqSOA formation. We also found that biomass burning emissions can promote photochemical aqSOA formation by adding to the levels of aqueous-phase photooxidants and aqSOA precursors. Therefore, future mitigation of air pollution in a polluted environment would benefit from stricter control on biomass burning especially under high RH conditions.
The characteristics of benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations, their temporal and spatial variations, and their source origins from September–December 2017 at an urban and a ...background site in Beijing, China were investigated. The averaged (±σ) total mixing ratios of benzene, toluene, ethylbenzene, m, p-xylenes, and o-xylene were 0.40 ± 0.39 ppbv, 0.31 ± 0.34 ppbv, 0.08 ± 0.07 ppbv, 0.08 ± 0.08 ppbv, and 0.05 ± 0.05 ppbv at the SDZ site, which were 63%, 79%, 83%, 85%, and 89% lower than those at the Chinese Academy of Meteorological Sciences site (CMA). It is worth noting that the average mixing ratios of BTEX at SDZ and CMA were 0.86 ± 1.03 ppbv and 3.38 ± 2.80 ppbv during the heating period (HP), which were 2.3% and 21.9% lower than those before the HP, a decrease that was mainly related to the frequent occurrence of strong northerly and northwesterly winds and low relative humidity (RH) during the HP. Obvious differences were also observed between the BTEX composition proportions at the SDZ and CMA sites. On average, benzene comprised 44% of the total BTEX at SDZ, whereas toluene was the largest contributor to the total BTEX at CMA, accounting for 37%. In addition, the contributions of C8 aromatics (the sum of ethylbenzene, m, p-xylenes, and o-xylene) at CMA (36%) were also higher than those at SDZ (21%), reflecting the different emission sources of the two sites. In addition, the BTEX species showed similar and pronounced diurnal profiles at SDZ and CMA, all characterized by much higher values at night than during the day. Diagnostic ratios and source implications suggested that SDZ was affected mainly by biomass/biofuel/coal burning, with substantially elevated benzene levels during the winter HP, whereas CMA was affected both by traffic-related emissions and biomass/biofuel/coal burning emissions. These findings suggest the necessity of regionally-tailored control strategies both to reduce BTEX levels and to mitigate their environmental impact. Further analysis of the backward trajectories revealed that the BTEX compounds varied greatly in terms of air mass origins, but generally exhibited high values for slow air masses passing over areas south of Beijing, with dominant contributions from benzene, toluene, and m, p-xylenes.
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•The characteristics and temporal variations of BTEX were investigated at the two sites in Beijing.•Benzene and toluene were the largest contributor to BTEX at background and urban sites.•The emission sources of BTEX were resolved during the whole period at the two sites.
