ALBERT

Description: We developed a two-stage model called the random-forest–generalised additive model (RF–GAM), based on satellite data, meteorological factors, and other geographical covariates, to predict the surface 8 h O3 concentrations across the remote Tibetan Plateau. The 10-fold cross-validation result suggested that RF–GAM showed excellent performance, with the highest R2 value (0.76) and lowest root-mean-square error (RMSE) (14.41 µg m−3), compared with other seven machine-learning models. The predictive performance of RF–GAM showed significant seasonal discrepancy, with the highest R2 value observed in summer (0.74), followed by winter (0.69) and autumn (0.67), and the lowest one in spring (0.64). Additionally, the unlearning ground-observed O3 data collected from open-access websites were applied to test the transferring ability of the novel model and confirmed that the model was robust in predicting the surface 8 h O3 concentration during other periods (R2=0.67, RMSE = 25.68 µg m−3). RF–GAM was then used to predict the daily 8 h O3 level over the Tibetan Plateau during 2005–2018 for the first time. It was found that the estimated O3 concentration displayed a slow increase, from 64.74±8.30 µg m−3 to 66.45±8.67 µg m−3 from 2005 to 2015, whereas it decreased from the peak to 65.87±8.52 µg m−3 during 2015–2018. Besides this, the estimated 8 h O3 concentrations exhibited notable spatial variation, with the highest values in some cities of the northern Tibetan Plateau, such as Huangnan (73.48±4.53 µg m−3) and Hainan (72.24±5.34 µg m−3), followed by the cities in the central region, including Lhasa (65.99±7.24 µg m−3) and Shigatse (65.15±6.14 µg m−3), and the lowest O3 concentration occurred in a city of the southeastern Tibetan Plateau called Aba (55.17±12.77 µg m−3). Based on the 8 h O3 critical value (100 µg m−3) provided by the World Health Organization (WHO), we further estimated the annual mean nonattainment days over the Tibetan Plateau. It should be noted that most of the cities on the Tibetan Plateau had excellent air quality, while several cities (e.g. Huangnan, Haidong, and Guoluo) still suffered from more than 40 nonattainment days each year, which should be given more attention in order to alleviate local O3 pollution. The results shown herein confirm that the novel hybrid model improves the prediction accuracy and can be applied to assess the potential health risk, particularly in remote regions with few monitoring sites.

Description: Most previous modeling studies about black carbon (BC) transport and its impact over the Tibetan Plateau (TP) conducted simulations with horizontal resolutions coarser than 20 km that may not be able to resolve the complex topography of the Himalayas well. In this study, the two experiments covering all of the Himalayas with the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) at the horizontal resolution of 4 km but with two different topography datasets (4 km complex topography and 20 km smooth topography) are conducted for pre-monsoon season (April 2016) to investigate the impacts of topography on modeling the transport and distribution of BC over the TP. Both experiments show the evident accumulation of aerosols near the southern Himalayas during the pre-monsoon season, consistent with the satellite retrievals. The observed episode of high surface BC concentration at the station near Mt. Everest due to heavy biomass burning near the southern Himalayas is well captured by the simulations. The simulations indicate that the prevailing upflow across the Himalayas driven by the large-scale westerly and small-scale southerly circulations during the daytime is the dominant transport mechanism of southern Asian BC into the TP, and it is much stronger than that during the nighttime. The simulation with the 4 km topography resolves more valleys and mountain ridges and shows that the BC transport across the Himalayas can overcome the majority of mountain ridges, but the valley transport is more efficient. The complex topography results in stronger overall cross-Himalayan transport during the simulation period primarily due to the strengthened efficiency of near-surface meridional transport towards the TP, enhanced wind speed at some valleys and deeper valley channels associated with larger transported BC mass volume. This results in 50 % higher transport flux of BC across the Himalayas and 30 %–50 % stronger BC radiative heating in the atmosphere up to 10 km over the TP from the simulation with the 4 km complex topography than that with the 20 km smoother topography. The different topography also leads to different distributions of snow cover and BC forcing in snow. This study implies that the relatively smooth topography used by the models with resolutions coarser than 20 km may introduce significant negative biases in estimating light-absorbing aerosol radiative forcing over the TP during the pre-monsoon season. Highlights. The black carbon (BC) transport across the Himalayas can overcome the majority of mountain ridges, but the valley transport is much more efficient during the pre-monsoon season. The complex topography results in stronger overall cross-Himalayan transport during the study period primarily due to the strengthened efficiency of near-surface meridional transport towards the TP, enhanced wind speed at some valleys and deeper valley channels associated with larger transported BC mass volume. The complex topography generates 50 % higher transport flux of BC across the Himalayas and 30 %–50 % stronger BC radiative heating in the atmosphere up to 10 km over the Tibetan Plateau (TP) than the smoother topography, which implies that the smooth topography used by the models with relatively coarse resolution may introduce significant negative biases in estimating BC radiative forcing over the TP during the pre-monsoon season. The different topography also leads to different distributions of snow cover and BC forcing in snow over the TP.

Description: Identification of various emission sources and quantification of their contributions comprise an essential step in formulating scientifically sound pollution control strategies. Most previous studies have been based on traditional offline filter analysis of aerosol major components (usually inorganic ions, elemental carbon – EC, organic carbon – OC, and elements). In this study, source apportionment of PM2.5 using a positive matrix factorization (PMF) model was conducted for urban Shanghai in the Yangtze River Delta region, China, utilizing a large suite of molecular and elemental tracers, together with water-soluble inorganic ions, OC, and EC from measurements conducted at two sites from 9 November to 3 December 2018. The PMF analysis with inclusion of molecular makers (i.e., MM-PMF) identified 11 pollution sources, including 3 secondary-source factors (i.e., secondary sulfate; secondary nitrate; and secondary organic aerosol, SOA, factors) and 8 primary sources (i.e., vehicle exhaust, industrial emission and tire wear, industrial emission II, residual oil combustion, dust, coal combustion, biomass burning, and cooking). The secondary sources contributed 62.5 % of the campaign-average PM2.5 mass, with the secondary nitrate factor being the leading contributor. Cooking was a minor contributor (2.8 %) to PM2.5 mass while a significant contributor (11.4 %) to the OC mass. Traditional PMF analysis relying on major components alone (PMFt) was unable to resolve three organics-dominated sources (i.e., biomass burning, cooking, and SOA source factors). Utilizing organic tracers, the MM-PMF analysis determined that these three sources combined accounted for 24.4 % of the total PM2.5 mass. In PMFt, this significant portion of PM mass was apportioned to other sources and thereby was notably biasing the source apportionment outcome. Backward trajectory and episodic analysis were performed on the MM-PMF-resolved source factors to examine the variations in source origins and composition. It was shown that under all episodes, secondary nitrate and the SOA factor were two major source contributors to the PM2.5 pollution. Our work has demonstrated that comprehensive hourly data of molecular markers and other source tracers, coupled with MM-PMF, enables examination of detailed pollution source characteristics, especially organics-dominated sources, at a timescale suitable for monitoring episodic evolution and with finer source breakdown.

Description: Elucidating the relationship between characteristics of aerosol particles and optical absorption is important to deepen the understanding of atmospheric chemistry. Aerosol particles play significant roles in climate forcing via their optical absorption properties. However, the relationship between characteristics of aerosol particles and optical absorption remains poorly understood. Aerosol optical properties and morphologies were measured by a transmission electron microscope (TEM), cavity ring-down spectrometer (CRDS), a nephelometer and an Aethalometer in a urban site of Beijing from 24 May to 22 June. Five episodes were categorized according to the meteorological conditions and composition. The results showed that the clear episode (EP-2 and EP-4) featured as the low aerosol optical depth (AOD  =  0.72) and fewer pollutants compared with haze (1.14) and fog (2.92) episodes and the particles are mostly externally mixed. The high Ångström exponent (〉 2.0) suggests that coarse particles were scarcely observed in EP-2 due to the washout of a previous heavy rain, whereas they were widespread in EP-4 (Ångström exponent  =  0.04), which had some mineral particles introduced from the north. In contrast, industry-induced haze (EP-1) and biomass-burning-induced haze (EP-5) were both affected by the south air mass. Compared with the EP-2 and EP-4, the AOD values and the size distribution of particles during EP-1 and EP-5 were much greater because of relatively high particle concentrations. All of the particles were classified into nine categories, i.e. S-rich, N-rich, mineral, K-rich, soot, tar ball, organic, metal and fly ash, on the basis of TEM analysis. In contrast to the EP-1, a large fraction of soot, which sticks to KCl, sulfate or nitrate particles, was detected during EP-5. Additionally, evident enhancement of light absorption was observed during the EP-5, which was mainly ascribed to both black carbon (BC) acceleration and other absorbing substances. However, soot was found mostly internally mixed with sulfate and nitrate during a soot fog episode (EP-3), resulting in evident enhancement of light absorption. The larger size distribution was likely to be caused by both hygroscopic growth and collision between particles during the aging. About 28 % of particles were internally mixed during the foggy days, which favoured the light absorption. The comparison of all the episodes provides a deeper insight into how mixing states influence the aerosol extinction properties and also a clue as to how to control air pollution in the crop burning seasons.

Description: A spatial-temporal analysis has been conducted using satellite observed distributions of rain frequency, NO2 concentration and aerosol, with focus on the spring season. As revealed by measurements from 1998–2009 over Shanghai, China, the suppression of rain is mainly attributed to the reduction of rain occurrence rather than changes in rain intensity. The overall features emerge from the region-by-region analyses that there is an inverse relationship between the rain frequency and the pollution and associated aerosols at continental scale in spring. The enhancement of pollution-produced CCN in addition to mineral dust from long-term transport suppresses the rain frequency, as favored by topography, wind, and other meteorological conditions.

Description: The acid deposition has been considered to be a severe environmental issue in China. The pH, electrical conductivity (EC), and the concentrations of the water soluble ions (NO3−, Cl−, Ca2+, K+, F−, NH4+, Mg2+, SO42−, and Na+) in the precipitation samples collected from the 320 cities during 2011–2016 across the whole China were measured. The mean concentrations of F−, NO3− and SO42− were in the order of winter (6.10, 19.44 and 45.74 μeq/L) 〉 spring (3.45, 13.83, and 42.61 μeq/L) 〉 autumn (2.67, 9.73, and 28.85 μeq/L) 〉 summer (2.04, 7.66, and 19.26 μeq/L). The secondary ions (SO42−, NO3− and NH4+), and F− peaked in Yangtze River Delta (YRD) and Sichuan basin (SB). The crustal ions (i.e., Ca2+, Mg2+), Na+, and Cl− showed the highest concentrations in the semi-arid regions and the coastal cities, respectively. The statistical methods confirmed that the mean anthropogenic contribution ratios to SO42−, F−, NO3−, and NH4+ at a national scale were 46.12 %, 71.02 %, 79.10 %, and 82.40 %, respectively. However, Mg2+ (70.51 %), K+ (77.44 %), and Ca2+ (82.17 %) were mostly originated from the crustal source. Both Na+ (70.54 %) and Cl− (60.42 %) were closely linked to the sea-salt aerosols. On the basis of the stepwise regression (SR) analysis, it was proposed that most of the secondary ions and F− were closely related to gross industrial production (GIP), total energy consumption (TEC), vehicle ownership, and N fertilizer use, but the crustal ions (Ca2+ and K+) were mainly controlled by the dust events. The influence of dust days, air temperature, and wind speed on ions increased from Southeast China (SEC) to Central China, and then to Northwest China (NWC), whereas the influence of socioeconomic factors on acid ions (SO42− and NO3−) displayed the higher value in East China.

Description: Characteristics of aerosol optical properties, morphologies and their relationship were studied in urban Beijing during the clear, haze and fog episodes, sampled from 24th May to 22nd Jun, 2012. Transmission Electron Microscope (TEM), a Cavity Ring Down Spectrometer (CRDS), a nephelometer and an aethalometer were employed to investigate the corresponding changes of the aerosol properties. Five episodes were categorised according to the meteorological conditions, composition and optical variation. Results show the clear episode (EP-2 and EP-4) featured as the low light extinction with less pollutants, which are mostly externally mixed. Coarse particles were scarcely observed in EP-2 due to the washout of a previous heavy rain. Thus the size distribution in EP-2 was smaller than EP-4, which had some mineral particles introduced from the north. In contrast, industry-induced haze (EP-1) and biomass burning-induced haze (EP-5) were both impacted by the south air mass. Higher AOD (Aerosol Optical Depth) values illustrated heavy loading particle concentrations. Due to the collision, size of most particles was larger with the diameter of 1 μm, resulting in a higher scattering coefficient. However, as the influence of severe crop residue combustion, a large fraction of soot was detected, which sticks to the KCl transformed sulphate or nitrate particles. The light absorption enhancement was contributed by both Black Carbon (BC) acceleration and other light absorbing substances. Comparatively, soot fog period detected in EP-3 was mostly internally mixed with sulphates and nitrates, which revealed themselves after electron exposure. The larger size distribution was likely to be caused by both hygroscopic growth and collision. More internally mixed particles were observed, which favored the light absorption. The comparison of all the episodes provides a deeper insight of how mixing states influence the aerosol extinction properties and also a clue to the air pollution control in the crop burning seasons.

Description: The acid deposition has been considered to be a severe environmental issue in China. The pH, electrical conductivity (EC), and concentrations of water soluble ions (NO3-, Cl−, Ca2+, K+, F−, NH4+, Mg2+, SO42-, and Na+) in the precipitation samples collected from 320 cities during 2011–2016 across China were measured. The mean concentrations of F−, NO3-, and SO42- were in the order of winter (6.10, 19.44, and 45.74 µeq L−1) 〉 spring (3.45, 13.83, and 42.61 µeq L−1) 〉 autumn (2.67, 9.73, and 28.85 µeq L−1) 〉 summer (2.04, 7.66, and 19.26 µeq L−1). Secondary ions (SO42-, NO3-, and NH4+) and F− peaked in the Yangtze River Delta (YRD) and Sichuan basin (SB). Crustal ions (i.e. Ca2+, Mg2+), Na+, and Cl− showed the highest concentrations in the semi-arid regions and the coastal cities. The statistical methods confirmed that the mean anthropogenic contribution ratios to SO42-, F−, NO3-, and NH4+ at a national scale were 46.12 %, 71.02 %, 79.10 %, and 82.40 %, respectively. However, Mg2+ (70.51 %), K+ (77.44 %), and Ca2+ (82.17 %) mostly originated from the crustal source. Both Na+ (70.54 %) and Cl− (60.42 %) were closely linked to sea salt aerosols. On the basis of the stepwise regression (SR) analysis, it was proposed that most of the secondary ions and F− were closely related to gross industrial production (GIP), total energy consumption (TEC), vehicle ownership, and N fertilizer use, but the crustal ions (Ca2+ and K+) were mainly controlled by the dust events. The influence of dust days, air temperature, and wind speed on ions increased from southeast China (SEC) to central China, and then to northwest China (NWC), whereas the influence of socioeconomic factors on acid ions (SO42- and NO3-) displayed the higher value in east China.

Description: We describe the results from online measurements of nitrated phenols using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) with acetate as reagent ion in an oil and gas production region in January and February of 2014. Strong diurnal profiles were observed for nitrated phenols, with concentration maxima at night. Based on known markers (CH4, NOx, CO2), primary emissions of nitrated phenols were not important in this study. A box model was used to simulate secondary formation of phenol, nitrophenol (NP), and dinitrophenols (DNP). The box model results indicate that oxidation of aromatics in the gas phase can explain the observed concentrations of NP and DNP in this study. Photolysis was the most efficient loss pathway for NP in the gas phase. We show that aqueous-phase reactions and heterogeneous reactions were minor sources of nitrated phenols in our study. This study demonstrates that the emergence of new ToF-CIMS (including PTR-TOF) techniques allows for the measurement of intermediate oxygenates at low levels and these measurements improve our understanding on the evolution of primary VOCs in the atmosphere.

Description: Field studies in polluted areas over the last decade have observed large formation of secondary organic aerosol (SOA) that is often poorly captured by models. The study of SOA formation using ambient data is often confounded by the effects of advection, vertical mixing, emissions, and variable degrees of photochemical aging. An oxidation flow reactor (OFR) was deployed to study SOA formation in real-time during the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign in Pasadena, CA, in 2010. A high-resolution aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) alternated sampling ambient and reactor-aged air. The reactor produced OH concentrations up to 4 orders of magnitude higher than in ambient air. OH radical concentration was continuously stepped, achieving equivalent atmospheric aging of 0.8 days–6.4 weeks in 3 min of processing every 2 h. Enhancement of organic aerosol (OA) from aging showed a maximum net SOA production between 0.8–6 days of aging with net OA mass loss beyond 2 weeks. Reactor SOA mass peaked at night, in the absence of ambient photochemistry and correlated with trimethylbenzene concentrations. Reactor SOA formation was inversely correlated with ambient SOA and Ox, which along with the short-lived volatile organic compound correlation, indicates the importance of very reactive (τOH  ∼  0.3 day) SOA precursors (most likely semivolatile and intermediate volatility species, S/IVOCs) in the Greater Los Angeles Area. Evolution of the elemental composition in the reactor was similar to trends observed in the atmosphere (O : C vs. H : C slope  ∼  −0.65). Oxidation state of carbon (OSc) in reactor SOA increased steeply with age and remained elevated (OSC  ∼  2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (ΔCO, ambient CO above regional background) vs. photochemical age is similar to previous studies at low to moderate ages and also extends to higher ages where OA loss dominates. The mass added at low-to-intermediate ages is due primarily to condensation of oxidized species, not heterogeneous oxidation. The OA decrease at high photochemical ages is dominated by heterogeneous oxidation followed by fragmentation/evaporation. A comparison of urban SOA formation in this study with a similar study of vehicle SOA in a tunnel suggests the importance of vehicle emissions for urban SOA. Pre-2007 SOA models underpredict SOA formation by an order of magnitude, while a more recent model performs better but overpredicts at higher ages. These results demonstrate the value of the reactor as a tool for in situ evaluation of the SOA formation potential and OA evolution from ambient air.