Clean Episodes Research Articles

Budget of atmospheric nitrous acid (HONO) during the haze and clean periods in Shanghai: Importance of heterogeneous reactions

Nitrous acid (HONO) plays a significant role in radical cycling and atmospheric oxidative chemistry. While the source and evolution of HONO in the Yangtze River Delta (YRD) region of China after 2018 remains largely unknown, this work monitored HONO and other air pollutants throughout 2019 at an urban site (Pudong, PD) and a suburban site (Qingpu, QP) in Shanghai. Episodes with high HONO mixing ratios but different PM2.5 levels, namely haze and clean episodes, were chosen for HONO budget analysis. Using an observation-based photochemical box model, relative importance of different sources and sinks of HONO were evaluated. Gas-phase reaction of NO with OH was found to be one of the most important daytime HONO formation sources, especially during the QPhaze period (accounting for 40.3 % of daytime HONO formation). In particular, heterogeneous conversion of NO2 on ground and aerosol surface was found to be the dominant source for nocturnal HONO. Photo-enhanced NO2 conversion on ground surface plays an important role in daytime HONO production (19.4 % in PDhaze vs. 27.6 % in PDclean, and 19.8 % in QPhaze vs. 25.9 % in QPclean). In addition, photo-enhanced NO2 conversion at the aerosol surface during haze episodes made more significant contributions to HONO formation compared to the clean periods (20.9 % in PDhaze vs. 17.1 % in PDclean, and 19.7 % in QPhaze vs. 11.2 % in QPclean). The role of multiphase reactions was found to be increasingly important in HONO generation with enhanced relative humidity (RH) during daytime. Significant unknown HONO source was further analyzed and found to be positively related with photolytic as well as multiphase pathways. Overall, our study sheds light on the budget of HONO in one of the biggest megacities in east China, which would help developing future mitigation strategies for urban HONO and atmospheric oxidation capacity.

Chemical composition, sources and evolution of wintertime inorganic and organic aerosols in urban Shanghai, China

China experienced severe haze pollution in the past decades. However, systematic characterization of atmospheric fine particles with advanced measurement techniques has been very scarce in Shanghai, which is the largest megacity in China. Herein, we present the characterization of non-refractory submicron aerosol (NR-PM1) in urban Shanghai during winter 2017 by applying an Aerosol Chemical Speciation Monitor (ACSM, Aerodyne Research Inc.). The NR-PM1 is predominated by organics (43%), followed by nitrate (29%), sulfate (18%), ammonium (7%), and chloride (3%). Regarding the organic aerosol (OA) factors, three organic aerosol factors were identified and classified as hydrocarbon-like organic aerosol (HOA), less-oxygenated organic aerosol (LO-OOA), and more-oxygenated organic aerosol (MO-OOA), which contributed 32%, 22%, and 46% to total organic aerosol respectively. The NR-PM1 composition exhibited an increase in secondary inorganic aerosol (SIA) (sulfate, nitrate and ammonium) contribution from clean episodes (49%) to pollution episodes (59%), while organic aerosol contribution decreased accordingly from 48% to 39%. The strong increase of sulfate in high-relative humidity (RH) pollution episodes indicated that aqueous-phase oxidation of SO2 could be an important formation process for sulfate during particulate air pollution period. The contribution of nitrate was elevated from 25% during clean episodes to 32% during polluted episodes, likely owing to the increase of relative humidity which facilitates the hydrolysis of N2O5 and the gas-to-particle partitioning of hydrophilic NH4NO3 and. Further analysis of atmospheric formation relevance suggested that less-oxygenated organic aerosol formation was mainly driven by aqueous-phase chemistry reactions, whereas photochemical oxidation became an important process for more-oxygenated organic aerosol formation. Meanwhile, less-oxygenated organic aerosol formation may also be influenced by atmospheric oxidative tracer (i.e., Ox), as less-oxygenated organic aerosol exhibited a distinct peak at noon under high-relative humidity condition.

Investigation on the vertical distribution and transportation of PM2.5 in the Beijing-Tianjin-Hebei region based on stereoscopic observation network

In recent years, the regional PM2.5 pollution occurred frequently, especially in the Beijing-Tianjin-Hebei (BTH) region, China. Understanding the characteristics of PM2.5 pollution and its traceability within the boundary layer is necessary to better implement effective emission control strategies. In this study, we used the observed data from stereoscopic observation network, the simulated concentrations and regional transport of PM2.5 from CMAQ-ISAM model, combined with the PM2.5 transport flux, to investigate a comprehensive analysis of the PM2.5 pollution over the BTH region during January 18–31, 2018. The results showed that there were two pollution episodes (E1 and E3) and two clean episodes (E2 and E4) during the research period. The PM2.5 pollutants at the surface layer in Beijing were dominated by the contributions from local source, accounting for 55.63% in E1, 62.20% in E2, 58.83% in E3, and 65.94% in E4, respectively. The pollutant transport from the southern and southwestern pathways played the important role in controlling the PM2.5 pollution levels in Beijing city, the height of the pollutant transport was mainly concentrated from 0.5 to 2.0 km, and the maximum transport flux reached 972 μg/(m2·s) during the pollution episodes. The transport from eastern and southeastern pathways were significant within 1.0 km under the prevailing wind, the maximum transport flux reached 629 μg/(m2·s). During the cleaning episodes, the pollution transport of PM2.5 within the boundary layer mainly came from northern and southwestern pathways, with a maximum transport flux of 482 μg/(m2·s). Overall, the pollution process of Beijing is not only from local emissions, but also affected by regional transport during the research period. Therefore, it is necessary to control local emissions and strengthen regional-scale collaborations to reduce regional pollution events.

Conjoint impacts of continental outflows and marine sources on brown carbon in the East China sea: Abundances, optical properties, and formation processes

Brown carbon (BrC), as a short-lived climate forcer, is still poorly understood due to its insufficient quantification of chemical compositions, ambiguous optical properties, and complex formation mechanisms. This study firstly investigated the characteristics of BrC over a remote island of the East China Sea during the winter of 2019. Driven by the continental outflows, BrC increased 3–8 times than the clean episodes. Different from most urban studies, high water-soluble organic carbon to organic carbon (WSOC/OC) ratios (66% ± 18%) and low MS-BrC/WS-BrC ratios (methanol extracts vs. water extracts, 1.1) were observed, which resulted from the aging processes via the long-range transport over the ocean. When air masses originated from northern China with long travelling history over the ocean, aqueous phase processing promoted the formation of BrC under rich ammonium and nitrate. In contrast, air masses from eastern China travelled over less oceanic areas and were characterized of higher oxidation capacity, suggesting a dominant role of gas-phase processing in the formation of BrC. Biomass burning, fossil fuels combustion, and marine sources were apportioned as the major contributors to carbonaceous aerosols, of which marine sources accounted for more than 30%. The fractional solar absorption by BrC relative to elemental carbon in the ultraviolet range (300–400 nm) was simulated as 17.3 ± 7.51% for WS-BrC and 21.18 ± 7.97% for MS-BrC, indicating a non-negligible role of BrC in perturbating the radiative forcing in the marine atmosphere.

Retrieval of Boundary Layer Height and Its Influence on PM 2.5 Concentration Based on Lidar Observation over Guangzhou

Wavelet analysis was applied to lidar observations to retrieve the planetary boundary layer height (PBLH) over Guangzhou from September 2013 to November 2014 over Guangzhou. Impact of the boundary effect and the wavelet scale factor on the accuracy of the retrieved PBLH has been explored thoroughly. In addition, the PBLH diurnal variations and the relationship between PM_2.5 concentration and PBLH during polluted and clean episodes were studied. Results indicate that the most steady retrieved PBLH can be obtained when scale factor is chosen between 300-390 m. The retrieved maximum and minimum PBLH in the annual mean diurnal cycle were ~1100 m and ~650 m, respectively. The PBLH was significantly lower in the dry season than in the wet season, with the average highest PBLH in the dry season and the wet season being ~1050 m and ~1200 m respectively. Compared to the wet season, the development of PBLH in the dry season was delayed by at least one hour due to the seasonal cycle of solar radiation. Episode analysis indicated that the PBLH was ~50% higher during clean episodes than during haze episodes. The average highest PBLH in the haze episodes and clean episodes were ~800 m and ~1300 m, respectively. A significant negative correlation between PBLH and PM_2.5 concentration(<i>r</i>=-0.55^**) is discovered.According to China "Ambient Air Quality Standard", the PBLH values in good and slightly polluted conditions were 1/6-1/3 lower than that in excellent conditions, while the corresponding PM_2.5 concentration were ~2-2.5 times higher.

Characteristics and Sources of Organic Aerosol Markers in PM2.5

ABSTRACT To investigate the molecular compositions, sources, and evolution processes of organic aerosols (OAs), PM2.5 samples in wintertime were collected in Jinan, a typically polluted city in the North China Plain (NCP). The concentrations of PM2.5, carbonaceous species, and organic components (e.g., sugars, fatty acids, polycyclic aromatic hydrocarbons (PAHs) and oxygenated-PAHs (OPAHs)) in haze episodes were 1.8–2.7 times higher than those in clean episodes. Levoglucosan was the most abundant saccharide, which exhibited significant correlations with carbonaceous species during the whole sampling period, demonstrating that biomass burning has an important effect on the concentrations of carbonaceous species in PM2.5 of Jinan during wintertime. The higher ratios of both secondary OC/OC (SOC/OC) and C18:1/C18:0 was observed during the clean days than those during haze days, implying that OAs during clean periods were more aged. The higher ratio of OPAHs/PAHs in the daytime reflected the more photochemical formation of OPAHs in the daytime. Positive matrix factorization (PMF) model proved that biomass burning was the major source of OAs during the whole sampling period. Potential source contribution function (PSCF) and concentration-weight trajectory (CWT) results indicating that Shandong Province and Beijing-Tianjing-Hebei (BTH) region made important contribution to the levels of PM2.5 in Jinan during the winter. Moreover, the CWT results showed that OAs during haze periods were largely originated from local and surrounding regions. These results confirmed that biomass burning can significantly influence the concentrations and chemical compositions of OAs, thereby they can affect human health and atmospheric chemistry.

Effect of Pollution Level on Size Distributions and Mixing State of Ambient Black Carbon Particles in an Urban Area during Wintertime

We deployed a Single Particle Soot Photometer (SP2) and Single Particle Aerosol Mass Spectrometer (SPAMS) to investigate the size distribution and mixing state of ambient black carbon (BC) particles in Shanghai during the winter of 2017. The mixing state of the particles changed drastically under different meteorological conditions, and higher concentrations often occurred during haze episodes. The BC particles existed in two size modes: the condensation mode, which mainly consisted of fresh traffic-emitted particles, and the droplet mode, which encompassed heavily aged traffic-emitted particles, biomass burning particles and heavy-duty diesel engine emission particles. Whereas the heavily aged traffic-emitted particles exhibited relatively small BC cores (60–80 nm in diameter) surrounded by a thick coating (100–160 nm), the biomass burning particles displayed slightly larger cores (80–130 nm) covered by an even thicker layer (160–300 nm). However, we observed a large number of particles with large cores (150–200 nm) but a relatively thin coating (40–80 nm) on clean days. Trajectory analyses of the air masses showed that they usually originated in local areas or on the North China Plain during haze episodes but mostly migrated from the East China Sea during clean episodes. Interestingly, the BC particles in the clean air masses contained V more frequently and generated intense mass spectrum signals for Ca+, suggesting that these particles probably arose from heavy machinery emissions near ports. Our real-time single particle data indicates that the sources of BC aerosol can be easily identified by measuring the size distribution and mixing state of the particles.

Vertical Gradient Variations in Radiation Budget and Heat Fluxes in the Urban Boundary Layer: A Comparison Study Between Polluted and Clean Air Episodes in Beijing During Winter

AbstractPrevious studies have reported air pollution‐radiation interactions in the urban boundary layer (UBL), but vertical gradient variations in the radiation budget and heat flux under air pollution conditions are relatively sparse. In this study, based on gradient observations from the Beijing 325 m meteorological tower in December 2015, the characteristics of near‐surface radiation balance and energy budget at three levels under different pollution conditions were comparatively investigated. Relative to clean days, both downward and upward shortwave radiation (DSR and USR) dropped during daytime, while downward and upward longwave radiation (DLR and ULR) enhanced during nighttime on heavily polluted days, showing that with evaluated height, the drop magnitudes of DSR and USR decreased, while the enhancement magnitude of DLR (ULR) decreased (increased). The combined effects of four radiation components significantly induced the reduction in net radiation (Rn) on polluted days, leading to the near‐surface energy budget change. In addition, the monthly averaged anthropogenic heat flux (Qf) was estimated to quantitatively calculate the heat storage (G) term in the surface energy budget. During daytime, compared to the clean episodes, the sensible heat flux (H) was reduced more than Rn in the whole near‐surface UBL during heavy polluted episodes, resulting in smaller H/(Rn + Qf) and larger G/(Rn + Qf). Finally, we revealed that weak thermal forcing effects caused by insufficient availability of net radiation energy at the surface and weak dynamic motion associated with weak winds were both responsible for the larger reduction (increase) in H (G) during pollution episodes in the whole near‐surface UBL.

High-resolution vertical distribution and sources of HONO and NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in the nocturnal boundary layer in urban Beijing, China

Abstract. Nitrous acid (HONO), an important precursor of the hydroxyl radical (OH), plays a key role in atmospheric chemistry, but its sources are still debated. The production of HONO on aerosol surfaces or on ground surfaces in nocturnal atmospheres remains controversial. The vertical profile provides vertical information on HONO and NO2 to understand the nocturnal HONO production and loss. In this study, we report the first high-resolution (&lt;2.5 m) nocturnal vertical profiles of HONO and NO2 measured from in situ instruments on a movable container that was lifted on the side wiring of a 325 m meteorological tower in Beijing, China. High-resolution vertical profiles revealed the negative gradients of HONO and NO2 in nocturnal boundary layers, and a shallow inversion layer affected the vertical distribution of HONO. The vertical distribution of HONO was consistent with stratification and layering in the nocturnal urban atmosphere below 250 m. The increase in the HONO ∕ NO2 ratio was observed throughout the column from the clean episode to the haze episode, and relatively constant HONO∕NO2 ratios in the residual layer were observed during the haze episode. Direct HONO emissions from traffic contributed 29.3 % ± 12.4 % to the ambient HONO concentrations at night. The ground surface dominates HONO production by heterogeneous uptake of NO2 during clean episodes. In contrast, the HONO production on aerosol surfaces (30–300 ppt) explained the observed HONO increases (15–368 ppt) in the residual layer, suggesting that the aerosol surface dominates HONO production aloft during haze episodes, while the surface production of HONO and direct emissions into the overlying air are minor contributors. Average dry deposition rates of 0.74±0.31 and 1.55±0.32 ppb h−1 were estimated during the clean and haze episodes, respectively, implying that significant quantities of HONO could be deposited to the ground surface at night. Our results highlight the ever-changing contributions of aerosol and ground surfaces in nocturnal HONO production at different pollution levels and encourage more vertical gradient observations to evaluate the contributions from varied HONO sources.

Filling the gaps of in situ hourly PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; concentration data with the aid of empirical orthogonal function analysis constrained by diurnal cycles

Abstract. Data gaps in surface air quality measurements significantly impair the data quality and the exploration of these valuable data sources. In this study, a novel yet practical method called diurnal-cycle-constrained empirical orthogonal function (DCCEOF) was developed to fill in data gaps present in data records with evident temporal variability. The hourly PM2.5 concentration data retrieved from the national ambient air quality monitoring network in China were used as a demonstration. The DCCEOF method aims to reconstruct the diurnal cycle of PM2.5 concentration from its discrete neighborhood field in space and time firstly and then predict the missing values by calibrating the reconstructed diurnal cycle to the level of valid PM2.5 concentrations observed at adjacent times. The statistical results indicate a high frequency of data gaps in our retrieved hourly PM2.5 concentration record, with PM2.5 concentration measured on about 40 % of the days suffering from data gaps. Further sensitivity analysis results reveal that data gaps in the hourly PM2.5 concentration record may introduce significant bias to its daily averages, especially during clean episodes at which PM2.5 daily averages are observed to be subject to larger uncertainties compared to the polluted days (even in the presence of the same amount of missingness). The cross-validation results indicate that our suggested DCCEOF method has a good prediction accuracy, particularly in predicting daily peaks and/or minima that cannot be restored by conventional interpolation approaches, thus confirming the effectiveness of the consideration of the local diurnal variation pattern in gap filling. By applying the DCCEOF method to the hourly PM2.5 concentration record measured in China from 2014 to 2019, the data completeness ratio was substantially improved while the frequency of days with gapped PM2.5 records reduced from 42.6 % to 5.7 %. In general, our DCCEOF method provides a practical yet effective approach to handle data gaps in time series of geophysical parameters with significant diurnal variability, and this method is also transferable to other data sets with similar barriers because of its self-consistent capability.

High-resolution simulation of wintertime fossil fuel CO2 in Beijing, China: Characteristics, sources, and regional transport

In this study, a high-resolution simulation of fossil fuel CO2 in Beijing and surrounding areas, China, during January 2014 is performed to investigate the characteristics and sources of Beijing fossil fuel CO2 (FFCO2) mixing ratios and the impact of regional transport. The model reasonably reproduces the observed meteorological fields in the study domain, including temperature, relative humidity, wind speed, and wind direction. The simulated CO2 and CO mixing ratios in Beijing are in good agreement with the measurements. Elevated FFCO2 levels are produced by the model in megacities, such as Beijing, Tianjin, Shijiazhuang, and Baoding. The model result shows that FFCO2 mixing ratios are significantly correlated with observed total CO2, CO, and PM2.5 concentrations in Beijing. Sensitivity experiments show that Beijing FFCO2 is mainly from industry and residential emissions (35% for each), followed by power plant (21%) and transportation (9%) emissions in January 2014. Spatially, the largest contributor for Beijing FFCO2 is the local source, and regional transport also plays an important role in winter. The impact of regional transport is associated with wind direction, in which south/east (north/west) wind tends to accumulate (dilute) Beijing FFCO2. In addition, the roles of regional transport during haze and clean episodes are significantly distinct in winter, and more contribution is found in haze episodes.

Particle number size distribution and new particle formation under the influence of biomass burning at a high altitude background site at Mt. Yulong (3410 m), China

Abstract. Biomass burning (BB) activities have a great impact on the particle number size distribution (PNSD) in the upper troposphere of the Tibetan Plateau, which could affect regional and global climate. An intensive campaign focused on the measurement of the PNSD, gaseous pollutants, and meteorological parameters was conducted at Mt. Yulong, a high-altitude site (3410 m a.s.l.) on the southeastern Tibetan Plateau during the pre-monsoon season (22 March to 15 April). During this period, intensive BB activities in southern Asia were detected by fire maps. The long-range transport of BB pollutants can increase the accumulation mode particles in the background atmosphere at Mt. Yulong. As a consequence, the cloud condensation nuclei (CCN) concentration was found to be 2–8 times higher during BB periods than during clean periods. Apart from BB, variations of the planet boundary layer (PBL) and new particle formation (NPF) were other factors that influenced the PNSD. However, only three NPF events (with a frequency of 14 %) were observed at Mt. Yulong. The occurrence of NPF events during clean episodes corresponded to an elevated PBL or transported BB pollutants. Due to the lack of condensable vapors including sulfuric acid and organic compounds, the newly formed particles were not able to grow to CCN size. Our study emphasizes the influences of BB on the aerosol and CCN concentration in the atmosphere of the Tibetan Plateau. These results also have the potential to improve our understanding of the variation of the particle concentration in the upper troposphere, and provide information for regional and global climate models.

Impacts of Synoptic Weather Patterns and their Persistency on Free Tropospheric Carbon Monoxide Concentrations and Outflow in Eastern China

AbstractThe relationship between synoptic weather patterns (SWPs) and variability of air pollution has not been fully understood. In this study, we assess the sensitivity of carbon monoxide (CO) concentrations and outflow in the free troposphere over eastern China to SWPs and their persistency, using daily CO data from the spaceborne Atmospheric Infrared Sounder (AIRS) instrument from 2003 to 2015. The SWPs over eastern China are classified into eight patterns following the Kirchhofer approach. On a regional average over the 13 years, distinct differences are observed in CO distributions and outflows among the eight SWPs. CO concentrations tend to increase under three SWPs that are characterized with anticyclonic circulations and downdrafts in all or part of eastern China. In contrast, three SWPs related to the East Asian summer monsoon (EASM) tend to reduce CO concentrations. The remaining two SWPs, dominant in winter and autumn, respectively, tend to bring more clean episodes. CO outflow from eastern China is sensitive to the influence of the SWPs on zonal winds. CO outflow is enhanced under two SWPs that prevail in winter and spring. Under the other SWPs, CO outflow decreases or changes only slightly. CO concentrations either increase or decrease by 5–15% under the persistent control of a SWP for two to seven days. In most cases, the anomalies of CO concentrations and CO outflow tend to be amplified if a SWP persists for several days. This study statistically illustrates the synoptic influences on the regional distribution and transport of air pollutants.

Observations of ozone vertical profiles and corresponding precursors in the low troposphere in Beijing, China

A comprehensive investigation of ozone, its precursors as well as aerosols in the low troposphere was conducted over the University of Chinese Academy of Sciences site (abbreviated as UCAS site, a northern suburban site) in Beijing during the period of October 26 to November 6, 2014, with a focus on the comparison between polluted and clean conditions. The vertical profiles of ozone and aerosol extinction coefficients in the low troposphere were measured by differential absorption lidar, and the tropospheric vertical column densities (VCDs) as well as vertical profiles of HCHO and NO2 were measured by ground-based Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS). O3 in the boundary layer (0.3–1.0 km) had a positive gradient with height and presented a stratified structure at night on haze days, in contrast to a uniform distribution during clean episodes. The polluted conditions were characterized by higher levels of atmospheric pollutants (O3, aerosol, HCHO and NO2) and more dynamic variations with respect to clean periods. During lightly to moderately polluted periods, elevated concentrations of O3 within the boundary layer were observed with a maximum exceeding 60 ppbv. In haze days, the air masses from urban Beijing and southern Hebei industrial and urbanized areas could bring large amounts of precursors, enhancing O3 formation over the lidar site. The photochemical production of O3 within the boundary layer was controlled by a VOC-limited regime in haze days and turned into a mixed-VOC-NOx-limited regime on clean days. According to sensitivity analysis, when air masses came from southern polluted regions, the sensitivity of ozone production was dominated by VOC-limited regime. The reduction of VOC emissions from urban Beijing and Hebei industrial and urbanized areas would favor the mitigation of O3 pollution at UCAS site.

Chemical Components, Variation, and Source Identification of PM1 during the Heavy Air Pollution Episodes in Beijing in December 2016

Air pollution is a current global concern. The heavy air pollution episodes (HPEs) in Beijing in December 2016 severely influenced visibility and public health. This study aims to survey the chemical compositions, sources, and formation processes of the HPEs. An aerodyne quadruple aerosol mass spectrometer (Q-AMS) was utilized to measure the non-refractory PM1 (NR-PM1) mass concentration and size distributions of the main chemical components including organics, sulfate, nitrate, ammonium, and chloride in situ during 15–23 December 2016. The NR-PM1 mass concentration was found to increase from 6 to 188 μg m–3 within 5 days. During the most serious polluted episode, the PM1 mass concentration was about 2.6 times that during the first pollution stage and even 40 times that of the clean days. The formation rates of PM2.5 in the five pollution stages were 26, 22, 22, 32, and 67 μg m–3 h–1, respectively. Organics and nitrate occupied the largest proportion in the polluted episodes, whereas organics and sulfate dominated the submicron aerosol during the clean days. The size distribution of organics is always broader than those of other species, especially in the clean episodes. The peak sizes of the interested species grew gradually during different HPEs. Aqueous reaction might be important in forming sulfate and chloride, and nitrate was formed via oxidization and condensation processes. PMF (positive matrix factorization) analysis on AMS mass spectra was employed to separate the organics into different subtypes. Two types of secondary organic aerosol with different degrees of oxidation consisted of 43% of total organics. By contrast, primary organics from cooking, coal combustion, and traffic emissions comprised 57% of the organic aerosols during the HPEs.

Effect of aerosols on different regime of clouds and their precipitation over Tehran city

In this study we investigate the impact of aerosols on different cloud regimes and amount of their precipitation over Tehran for the period of 2003-2014, utilizing data from Moderate-Resolution Imaging Spectroradiometer (MODIS), Tropical Rainfall Measuring Mission (TRMM) and surface data from local synoptic stations. The regimes were determined using a k-means clustering method on retrieved cloud properties. The results indicate that in mixed clouds, increase in aerosols has led to increase in the mean cloud effective radius (CER), cloud height as well as lighting and precipitation amounts of weak and moderate convective cells which may be attributed to more freezing of cloud droplets above the 0 °C isotherm and its associated latent heat releases. Vice versa, warm clouds intensity in polluted air condition are weeker than clean air condition. The results also showed that, nimbostratus clouds thickness and their precipitation increase under high aerosol index (AI). Both TRMM and rain gauges data, however, showed that the average amount of precipitation has decreased during polluted episodes in comparison with clean episodes during the period of study. Elaborating on the observed overall changes in the amount of precipitation, it may be concluded that over Seeding was a critical factor in reducing the mean precipitation over Tehran. Also, observed differences on clouds microphysics over different parts of the city may be related to local pollutions, type of ambient air aerosols and topography conditions.

Sources and Characteristics of Regional Background PM1 in North China During the Autumn and Winter Polluted Period

The campaign of investigating the chemical compositions and particle size distributions of NR-PM1(non-refractory PM1) was conducted by using a High Resolution Time of Flight Aerosol Mass Spectrometer(HR-ToF-AMS) at the Shangdianzi(SDZ) regional atmospheric background site(117.07°E, 40.39°N), northeast of Beijing, from October 17th 2015 to January 27th 2016. The results showed that organics was the main component of PM1, and the proportion of nitrate was higher than that of sulfate in autumn and winter. The mean mass-resolved size distributions for the main components displayed accumulation mode. The wider organic peak shape and larger nitrate peak size indicated that the organics contributed to both small and large particles at the beginning of the particle formation, growth and aging processes, while most of nitrate particles preferred to grow into large particles during the aging process. The ratios of elements between OM and OC, O/C and H/C were calculated as 1.91, 0.58 and 1.58 respectively. The slope of Van Krevelen diagram of organic aerosols during polluted episode was -0.21, whose oxidation state was higher than those of other city sites. Nitrate was the major contributor of NR-PM1 during the polluted period, while organics was significantly higher than that during clean period. The results of back trajectories analysis demonstrated that the air masses were complex during the pollution episode. The northwest wind from central Inner Mongolia and Siberia dominated the clean episodes, which was conducive to the spread of pollutants.

Evaluating the Effects of Springtime Dust Storms over Beijing and the Associated Characteristics of Sub-Micron Aerosol

ABSTRACTIn order to understand the characteristics, sources and processes of non-refractory submicron particles (NR-PM1), an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed to acquire observational data during the spring (April 1 to 30) in Beijing, China, in 2012. Based on PM10, PM2.5 and NR-PM1 mass concentrations observation, satellite images and the back trajectory analysis, one haze and dust storm episodes were recorded during the campaign, in addition, one clean episodes was also added to the comparison as a reference. The NR-PM1 mass concentration was 97 µg m–3 during the haze episodes, while it was approximately 12 times and 1.7 times that on the clean and dust episodes, respectively. In addition, the secondary inorganic aerosol (sulfate, nitrate and ammonium) contributed the largest fraction of NR-PM1 (69%) during the haze episodes. The dust storms originated from the northwestern caused the PM10 peaking at 826 µg m–3, with an average of 364 ± 186 µg m–3 and higher than the haze episodes (241 µg m–3). In addition, compared to the clean episodes (the NR-PM1 mass was 8 µg m–3), the dust storms caused the average NR-PM1 mass reaching 56 µg m–3, corresponding to the secondary components significantly increased, including sulfate (9.5 µg m–3), nitrate (8 µg m–3), ammonium (6 µg m–3) and OOA (6 µg m–3). The backward trajectory clustering analysis indicated the air mass from the southeast (at a frequency more than 30%) contained the higher NR-PM1 concentration (more than 80 µg m–3) corresponding to the higher sulfate, nitrate and ammonium contributions.

Impact of atmospheric circulations on aerosol distributions in autumn over eastern China: observational evidence

Abstract. Regional heavy pollution events in eastern China (110–122° E, 28–40° N) are causing serious environmental problems. In this study, the relationship between the degree of regional pollution and the patterns of large-scale atmospheric circulation over eastern China in October is investigated using 10-year (2001–2010) Terra/MODIS aerosol optical depth and NCEP reanalysis data by both case study and composite analysis. Eighteen polluted and 10 clean episodes are selected and categorised into six polluted types and three clean types respectively. Generally speaking, weather patterns such as a uniform surface pressure field in eastern China or a steady straight westerly in the middle troposphere, particularly when being at the rear of the anticyclone at 850 hPa, are typically responsible for heavy pollution events. Meanwhile, clean episodes occur when strong southeastward cold air advection prevails below the middle troposphere or air masses are transported from sea to land. Uniform descending motion prevails over the study region, trapping pollutants in the lower atmosphere. Therefore, the value of vertical velocity averaged from 1000 to 100 hPa and divergence of wind field in the lower troposphere are used in this study to quantify the diffusion conditions in each circulation type. The results reveal that it is often a clean episode when both the mean downward motion (larger than 2.56 × 10−2 Pa s−1) and the divergence of low-level winds (larger than 1.79 × 10−2 s−1) are strong. Otherwise, it is more likely to be a polluted episode.

Long-term measurements of aerosol and carbon monoxide at the ZOTTO tall tower to characterize polluted and pristine air in the Siberian taiga

Abstract. Siberia is one of few continental regions in the Northern Hemisphere where the atmosphere may sometimes approach pristine background conditions. We present the time series of aerosol and carbon monoxide (CO) measurements between September 2006 and December 2011 at the Zotino Tall Tower Observatory (ZOTTO) in Central Siberia (61° N; 89° E). We investigate the seasonal, weekly and diurnal variations of aerosol properties (including absorption and scattering coefficients and derived parameters, such as equivalent black carbon (BCe), Ångström exponent, single scattering albedo, and backscattering ratio) and the CO mixing ratios. Criteria were established to distinguish polluted from near-pristine air masses, providing quantitative characteristics for each type. Depending on the season, 23–36% of the sampling time at ZOTTO was found to be representative of a clean atmosphere. The summer pristine data indicate that primary biogenic and secondary organic aerosol formation are quite strong particle sources in the Siberian taiga. The summer seasons 2007–2008 were dominated by an Aitken mode around 80 nm size, whereas the summer 2009 with prevailing easterly winds produced particles in the accumulation mode around 200 nm size. We found these differences to be mainly related to air temperature, through its effect on the production rates of biogenic volatile organic compounds (VOC) precursor gases. In winter, the particle size distribution peaked at 160 nm, and the footprint of clean background air was characteristic for aged particles from anthropogenic sources at great distances from ZOTTO and diluted biofuel burning emissions from domestic heating. The wintertime polluted air originates mainly from large cities south and southwest of the site; these particles have a dominant mode around 100 nm, and the ΔBCe / ΔCO ratio of 7–11 ng m−3 ppb−1 suggests dominant contributions from coal and biofuel burning for heating. During summer, anthropogenic emissions are the dominant contributor to the pollution particles at ZOTTO, while only 12% of the polluted events are classified as biomass-burning-dominated, but then often associated with extremely high CO concentrations and aerosol absorption coefficients. Two biomass-burning case studies revealed different ΔBCe / ΔCO ratios from different fire types, with the agricultural fires in April~2008 yielding a very high ratio of 21 ng m−3 ppb−1. Overall, we find that anthropogenic sources dominate the aerosol population at ZOTTO most of the time, even during nominally clean episodes in winter, and that near-pristine conditions are encountered only in the growing season and then only episodically.