Summertime Urban Ammonia Emissions May Be Substantially Underestimated in Beijing, China.

Ammonia (NH3) is critical to the nitrogen cycle and PM2.5 formation, yet a great deal of uncertainty exists in its urban emission quantifications. Model-underestimated NH3 concentrations have been reported for cities, yet few studies have provided an explanation. Here, we explore reasons for severe WRF-Chem model underestimations of NH3 concentrations in Beijing in August 2018, including simulated gas-particle partitioning, meteorology, regional transport, and emissions, using spatially refined (3 km resolution) NH3 emission estimates in the agricultural sector for Beijing-Tianjin-Hebei and in the traffic sector for Beijing. We find that simulated NH3 concentrations are significantly lower than ground-based and satellite observations during August in Beijing, while wintertime underestimations are much more moderate. Further analyses and sensitivity experiments show that such discrepancies cannot be attributed to factors other than biases in NH3 emissions. Using site measurements as constraints, we estimate that both agricultural and non-agricultural NH3 emission totals in Beijing shall increase by ∼5 times to match the observations. Future research should be performed to allocate underestimations to urban fertilizer, power, traffic, or residential sources. Dense and regular urban NH3 observations are necessary to constrain and validate bottom-up inventories and NHx simulation.

On-road mobile mapping of spatial variations and source contributions of ammonia in Beijing, China.

Ammonia (NH3) measurements were performed with a mobile platform deploying a cavity ring-down spectroscopy NH3 analyzer in Beijing. The transect and loop sampling strategy revealed that the Beijing urban area is more strongly affected by NH3 emissions than surrounding areas. Although average enhancements of on-road NH3 were small compared to background levels, traffic emissions clearly dominated city enhancements of NH3, carbon dioxide (CO2), acetaldehyde and acetone. Increments of on-road NH3 ranged between 5.1 ppb and 11.4 ppb in urban areas, representing an enhancement of 20.6 % to 47.9 % over the urban background. The vehicle NH3:CO2 emission ratio was 0.26 ppb/ppm, about a factor of 1.5 higher than the value derived from the available emission inventory. The obtained NH3 emission factor was approximately 306.9 mg/kg. If the annual gasoline consumption in Beijing is accurate, annual NH3 emissions from vehicles are estimated at 1.5 Gg. The influx and outflux of NH3 in Beijing during monitoring periods fluctuated due to variations of wind direction (WD), wind speed (WS), and planetary boundary layer height (PBLH). Net fluxes at the 4th Ring Road were larger than zero, suggesting that local emissions were important in urban Beijing. Negative net fluxes at the 6th Ring Road reveal a large amount of NH3 transported from agricultural regions south of Beijing lost during transport across the city, for example by deposition or particle formation in the city. Our analyses have important implications for regional NH3 emission estimates and for improving vehicular NH3 emission inventory allocations.

[Characteristics of Two Pollution Episodes Before and After City Heating in Beijing from February to March of 2019].

The characteristics of meteorological conditions and pollutant concentrations were analyzed based on two pollution episodes before and after city heating in Beijing during February to March of 2019. The backward trajectory and WRF-CAMx models were used to analyze the evolution of pollutants before and after city heating, and the influences of meteorological conditions, regional transport, and secondary transformation on the episodes were discussed. There was little difference in the average ρ(PM2.5) between February 21-24 (episode 1) and March 18-20 (episode 2), with concentrations of 100.1 µg·m-3 and 97.2 µg·m-3, respectively. However, compared with that of episode 2, in episode 1 the average peak value was higher with two peak stages, the diurnal variation was clearer, and the process developed much more rapidly. Moreover, episode 1 was regional pollution, while episode 2 was more related to local pollution in Beijing. The SO2 concentrations in both episodes were not higher than 16 µg·m-3, thereby indicating the effectiveness of coal-burning treatment and other measures. In addition, two peaks occurred in the diurnal fluctuation of SO2 in episode 1, whereas only one peak occurred for episode 2. In episode 1, the CO concentration was high and the ratio of ρ(CO)/ρ(SO2) increased around February 22-23 (phase 1); moreover, the pollutant concentrations in the central and southern areas of the Beijing-Tianjin-Hebei region and those in the background sites located in the southern part of the Beijing plain were higher than those in the urban area, thereby indicating that the diffusion conditions of episode 1 were unfavorable and the first PM2.5 peak was mainly affected by regional transport. A high ratio of ρ(PM2.5)/ρ(CO) in episode 2 suggested a slightly larger proportion of secondary generation for PM2.5, whereas higher ratios of ρ(NO2)/ρ(CO), ρ(SO2)/ρ(CO), and ρ(SO42-)/ρ(PM2.5) in episode 2 and the similar SOR value to that of episode 1 demonstrated that episode 1 was more advantageous for gas phase transformation and episode 2 was more affected by the coal industry. Phased analysis of episode 1 showed that the indicators of second generation for PM2.5 in phase 2 (around February 23-24) of episode 1 and episode 2 were similar, and both were higher than that in phase 1 of episode 1, which implied that the second PM2.5 peaks of episode 1 and episode 2 were mainly related to local emissions and chemical conversion. Both WRF-CAMx with and without assimilation experiments could better reproduce the temporal variation in pollutants, and the correlation between the simulation and observations increased but with lower values after assimilation. The model performance for the PM2.5 trend simulation significantly increased with data assimilation, and the simulated lower NO2 in February and higher NO2 in March as well as the overestimated SO2 were also improved. In addition, the pollutant concentration simulation in Beijing was more sensitive to that of Hebei in episode 1, which suggested that episode 1 was more affected by regional transport. The simulation ability for the rapid growth of pollutants needs to be promoted, and the response of pollutant types to emission reduction and the feedback related to the atmospheric oxidant and aerosol properties may be important for the simulation effect, which all require further study.

[Characteristics of BTEX and Health Risk Assessment During Typical Pollution Episodes in Summer and Winter in Tianjin Urban Area].

Real-time BTEX(including benzene, toluene, ethylbenzene, m-, p-, and o-xylenes) were measured continuously in Tianjin urban site in July 2019 and January 2020 using a Syntech Spectras GC955 analyzer. The BTEX concentration levels, composition, and evolutionary mechanisms during typical pollution episodes were investigated. The potential sources of BTEX were analyzed qualitatively using the diagnostic ratios method. Finally, the BTEX health risk was evaluated by using the human exposure analysis and evaluation method according to US EPA. The averaged total mixing ratio of BTEX were 1.32×10-9 and 4.83×10-9 during ozone pollution and haze episodes, respectively. Benzene was the most abundant species, followed by toluene. The mixing ratio of BTEX was largely affected by short southwestern distance transportation in January, while local emissions in July. In addition, the BTEX mixing ratio depended on the influence of temperature and relative humidity(RH) in July, while the concentration was more sensitive to changes in RH when the temperature was low in January. Diagnostic ratios and source implications suggested that the BTEX was affected mainly by biomass/biofuel/coal burning during haze episodes. The traffic related emissions also had an impact except for the influence of biomass/biofuel/coal burning in July. The averaged hazard quotient(HQ) values were 0.072 and 0.29 during ozone pollution and haze episodes, respectively, which were in the upper safety range limit recommended by the US EPA. The carcinogenic risk posed by benzene in both cleaning and pollution processes was higher than the safety threshold set by the US EPA, which should be monitored carefully.

Regional transport and urban emissions are important ammonia contributors in Beijing, China.

Measuring ammonia (NH3) is important for understanding the role of NH3 in secondary aerosol formation and the atmospheric deposition of reactive N. In this study, NH3 was measured in an urban area, a background region, and a tunnel in Beijing. The average NH3 concentrations between September 2017 and August 2018 were 24.8 ± 14.8 ppb and 11.6 ± 10.3 ppb in the urban area and background region, respectively. Higher NH3 concentrations at both the urban and background sites, relative to some earlier measurements indicated a likely increase in the NH3 concentrations in these regions. The urban NH3 level in Beijing was much higher than that typically observed at urban and industrial sites in other domestic and foreign cities, suggesting that the Beijing urban area was affected by greater NH3 emissions than other regions. Based on the relationship among NH3, wind direction, and wind speed, the urban area was affected by both local emissions and air transported from North China Plain (NCP). Potential source contribution function analyses suggested that regional transport from the NCP could greatly affect local concentrations of NH3 in both urban and background areas in spring and autumn; however, in addition to the NCP, urban emissions could also affect NH3 levels in the background region in summer and winter. The average NH3 concentration at the Fenshuiling Tunnel was 8.5 ± 7.7 ppb from December 2017 to February 2018. The NH3:CO emission ratio measured in the tunnel test was 0.022 ± 0.038 ppb/ppb, which was lower than values in the USA and South Korea. The contribution of traffic to NH3 in Beijing did not agree well with the available emission inventories, suggesting that vehicular emissions were underestimated and further evaluation is necessary.

Potential source regions of air pollutants at a regional background station in Northern China.

Understanding the potential source regions of air pollutants and their relative contribution from surrounding areas are of great importance for air pollution control strategies in Northern China. Six years of measurement of air pollutants was conducted from 2005 to 2010 in Shangdianzi (SDZ) regional background station. During the study period, the annual average concentrations of sulfur dioxide (SO2), carbon monoxide (CO), nitrogen dioxide and ozone (Ox), and fine particle matter less than or equal to 2.5 µm (PM2.5) range from 15.7 to 20.0 µg/m3, 577.7 to 856.0 µg/m3, 90.4 to 101.8 µg/m3, and 39.8 to 62.4 µg/m3, respectively. In this work, Potential Source Contribution Function (PSCF) and Trajectory Sector Analysis (TSA) methods are applied to identify locations of sources and their relatively contribution of air pollutants at SDZ. PSCF analysis shows that central Inner Mongolia, north Shanxi, west and south Hebei, and west Liaoning are all potential sources of SO2. The North China Plain (NCP) region, especially south Hebei and north Shandong, are major potential source regions for CO, Ox, and PM2.5. Therefore, reducing anthropogenic emissions from the coal industry, biomass burning, agricultural activities, and vehicles in these areas could be an effective way of controlling air pollution at SDZ. Based on the TSA results, the contributions of SO2, CO, Ox, and PM2.5 from long-distance transport are 5.5 µg/m3, 301.4 µg/m3, 14.8 µg/m3, and 25.8 µg/m3, accounting for approximately 22.6%, 32.3%, 13.1%, and 37.5% of the respective air pollutant concentrations at SDZ.

The Shape of the Concentration-Response Association between Fine Particulate Matter Pollution and Human Mortality in Beijing, China, and Its Implications for Health Impact Assessment.

BACKGROUND: Studies found approximately linear short-term associations between particulate matter (PM) and mortality in Western communities. However, in China, where the urban PM levels are typically considerably higher than in Western communities, some studies suggest nonlinearity in this association. Health impact assessments (HIA) of PM in China have generally not incorporated nonlinearity in the concentration-response (C-R) association, which could result in large discrepancies in estimates of excess deaths if the true association is nonlinear. OBJECTIVES: We investigated nonlinearity in the C-R associations between with PM with aerodynamic diameter [Formula: see text] ([Formula: see text]) and mortality in Beijing, China, and the sensitivity of HIA to linearity assumptions. METHODS: We modeled the C-R association between [Formula: see text] and cause-specific mortality in Beijing, China (2009-2012), using generalized linear models (GLM). [Formula: see text] was included through either linear, piecewise-linear, or spline functions to investigate evidence of nonlinearity. To determine the sensitivity of HIA to linearity assumptions, we estimated [Formula: see text]-attributable deaths using both linear- and nonlinear-based C-R associations between [Formula: see text] and mortality. RESULTS: We found some evidence that, for nonaccidental and circulatory mortality, the shape of the C-R association was relatively flat at lower concentrations of [Formula: see text], but then had a positive slope at higher concentrations, indicating nonlinearity. Conversely, the shape for respiratory mortality was positive and linear at lower concentrations of [Formula: see text], but then leveled off at the higher concentrations. Estimates of excess deaths attributable to short-term [Formula: see text] exposure were, in some cases, very sensitive to the linearity assumption in the association, but in other cases robust to this assumption. CONCLUSIONS: Our results demonstrate some evidence of nonlinearity in [Formula: see text]-mortality associations and that an assumption of linearity in this association can influence HIAs, highlighting the importance of understanding potential nonlinearity in the [Formula: see text]-mortality association at the high concentrations of [Formula: see text] in developing megacities like Beijing. https://doi.org/10.1289/EHP4464.

Long-term trend of chemical composition of atmospheric precipitation at a regional background station in Northern China.

Understanding the trend of chemical composition of precipitation is of great importance for air pollution control strategies in Northern China. A comprehensive study on the long-term chemical compositions of precipitation was carried out from 2003 to 2014 at the Shangdianzi (SDZ) regional background station in northern China. All samples were analyzed for pH, electrical conductivity and major ions (F-, Cl-, NO3-, SO42-, NH4+, Mg2+, Ca2+, K+ and Na+). The average pH during this period was 4.53±0.35, which is considerably lower than those reported in other background stations in China (Linan, Waliguan and Longfengshan). NH4+, SO42-, Ca2+ and NO3- were the dominant ions in precipitation, with concentrations (volume-weighted mean) of 212.99µeqL-1, 200.20µeqL-1, 116.88µeqL-1 and 98.56µeqL-1, respectively. The ion concentrations at SDZ were much higher than those of other background stations and megacities in China. A significantly increasing trend was observed for NO3- (7.26%year-1), and a decreasing trend was observed for SO42-/NO3-, suggesting that the precipitation of SDZ evolved from a sulfuric acid type to a mixed type dominated by both sulfuric and nitric acid. The source identification indicated that SO42-, NO3-, NH4+ and F- were dominated by secondary sources, Mg2+, Ca2+ and Na+ mostly originated from natural sources, and K+ and Cl- probably associated with anthropogenic sources. Long-range transport of air masses could influence the acidity, electrical conductivity and ion concentrations of precipitation at SDZ. The higher acidity and ion concentrations mainly occurred in the southerly and westerly trajectory pathways and partially in northwest pathways. Anthropogenic pollutants and crustal sources along these pathways were significant contributors to the chemical composition of precipitation in SDZ.

[Meteorological mechanism for the formation of a serious pollution case in Beijing in the background of northerly flow at upper levels].

There is a kind of serious air pollution case occurring in the situation of northerly flow at the levels above 850 hPa in Beijing area. Meteorological data, NCEP reanalysis data and PM2.5 concentration survey were used to analyze the roles of meteorological mechanism in this kind of air pollution case. The results showed that the stable stratification did not exist all the time during the gradual increase of PM2.5 concentration. The favorable meteorological conditions in PM2.5 accumulation came from the layered structure of vertical speed and convergence in vertical direction. From the surface to the middle level of troposphere, the vertical velocity showed an updraft-downdraft-updraft distribution. And the divergence displayed a convergence-divergence-convergence structure. The convergence at lower levels led to the gathering of the polluted air from the surrounding and the ascending current brought it upwards. However, the polluted air could not go further up due to the prolonged downdraft layer above. As the result, PM2.5 near the surface was concentrated gradually. The vertical speed in layer structure was attributed to the high level northerly flow without invading into the boundary layer. In such case, the wind was still in southerly direction or kept calm at the surface. The weak cold air or no cold air at all was the main reason that the northerly flow could not hit the surface. The developed downdraft layer was relative to the convergence above which was resulted from the wind speed pulses in the northerly flow at upper levels. In conclusion, the air dynamical effect played a key role in the pollution type of northerly flow at upper levels. Therefore, it will be beneficial to enhance our capability in the analysis and forecasting of this kind of pollution case by focusing on 24 hours temperature variation at the middle and lower troposphere as well as the vertical distribution of vertical velocity and divergence.

[Impact of atmospheric pollutants transport pathways on aerosol optical properties at Shangdianzi background station].

The impact of transport pathways on aerosol properties at Shangdianzi (SDZ) background station was analyzed using the back trajectory analysis method with the observation of PM2.5 concentration and aerosol scattering coefficients ( σ(sp)) during 2005-2010. The results showed that the pollutant level at SDZ was influenced by both the direction and motion state of air masses. The southerly transport pathways of air masses had higher PM2.5 concentration and σ(sp) than those of northerly pathways. The air masses with lower moving speed and altitude generally corresponded to higher PM2.5 concentration and σ(sp). Air masses originated from the North China Plain in spring, summer and autumn, and air masses from the north of North China region in winter had a significant contribution to PM2.5 concentration and σ (sp) at SDZ. Dust aerosol appeared frequently in spring, and the mean scattering efficiency (α(sp)) was 0.78 m2 x g(-1). The mean α(sp) of anthropogenic aerosol of four seasons was 4.00 m2 x g(-1) with the highest value in winter and the lowest in spring. The west-northwest, southerly pathways, and the northerly pathways with low moving speed in spring, summer and autumn correspond to higher α(sp) ( > 4.0 m2 x g(-1)). This indicated that these pathways were affected by the emissions form human activities. However, the α(sp) in winter kept at a high level among all pathways, which suggested that the emission effect was relatively uniform in surrounding regions in winter. The other northerly pathways in spring, summer and autumn were affected by anthropogenic pollutants mixed with dust.

[PM2.5 pollution and aerosol optical properties in fog and haze days during autumn and winter in Beijing area].

A study on the PM2.5 pollution and aerosol optical properties in haze-fog days was carried out from Sep. 1st to Dec. 7th, 2011 in Beijing area by using PM2.5 concentration, aerosol scattering coefficient (sigma sca) and absorption coefficient (sigma abs) measured under urban and rural environment. The effect of weather condition on the PM25 pollution and aerosol optical properties was discussed as well. The results showed that the PM2.5 concentration, sigma sca and sigma abs, were evidently higher in haze-fog days than those in non-haze-fog days. The average PM2.5 concentrations in haze-fog days with values of 97.6 microg m-3 and 64.4 microg.m-3 were as 3.3 and 4.8 times as those in non-haze-fog days at urban and rural stations, respectively. The higher PM2.5 concentration in urban area resulted in the more frequent fog and haze phenomena than that in rural area. The PM25 concentration, sigma sca, and sigma abs were significantly higher in urban area than that in rural area in mist days, while relatively close in mist-haze days. This difference suggested that the effect of regional transport of pollution was relatively evident in mist-haze days but weak in mist day. In fog days the sigma sca showed no evident difference between urban and rural area, and was the highest in all types of fog and haze weather. The scattering property of aerosol was the strongest in fog days. The different weather conditions resulted in various characteristics of spatial distribution of PM2.5 concentration, sigma sca and sigma abs, as well as the strength of PM2,5 pollution and aerosol extinction. The pollutants transported by the strong southwest wind above the boundary layer and subsided in the boundary layer companying with the local accumulation of pollutants due to the weak diffusion resulted in the most serious haze-fog episode with the strongest PM2.5 pollution and aerosol extinction.

[Effect of weather condition on the aerosol scattering property at Shangdianzi].

A study on the effect of weather condition on the aerosol scattering property has been carried out using one year measurement data sets of aerosol scattering coefficient (ASC) and meteorological parameters at Shangdianzi (SDZ). The results showed that the ASC was highest in haze-fog day with 608.4 Mm(-1) and higher in fog day with 500.6 Mm(-1) and haze day with 423.7 Mm (-1) those were 6.4-9.2 times higher than the ASC in normal day. The ASC was highest in summer in all kinds of weather conditions. The lower ASC in fog day and haze-fog day was observed in autumn and winter, respectively. There was no evident difference of the ASC between other three seasons in haze day and normal day. Pronounced seasonal variation of the mass scattering efficiency (MSE) of PM2.5 was observed in fog day with the highest value in summer. Significant diurnal variations in ASC were observed in haze-fog day and normal day with a unimodal pattern and a bimodal pattern, respectively. The wind was the most important factor for the ASC at SDZ. The transport of aerosol particles by the strong southwest wind should be responsible for the higher level of ASC in SDZ area and regional scale in low visibility weather conditions. The northeast wind was favourable to the reduction of ASC, especially in normal day.