COVID-19 Impact on the Concentration and Composition of Submicron Particulate Matter in a Typical City of Northwest China.

In this study, we evaluated the variations of air quality in Lanzhou, a typical city in Northwestern China impacted by the COVID-19 lockdown. The mass concentration and chemical composition of non-refractory submicron particulate matter (NR-PM1) were determined by a high-resolution aerosol mass spectrometer during January-March 2020. The concentration of NR-PM1 dropped by 50% from before to during control period. The five aerosol components (sulfate, nitrate, ammonium, chloride, and organic aerosol [OA]) all decreased during the control period with the biggest decrease observed for secondary inorganic species (70% of the total reduction). Though the mass concentration of OA decreased during the control period, its source emissions varied differently. OA from coal and biomass burning remained stable from before to during control period, while traffic and cooking related emissions were reduced by 25% and 50%, respectively. The low concentration during the control period was attributed to the lower production rate for secondary aerosols.

Seasonal characterization of aerosol composition and sources in a polluted city in Central China.

We characterized the aerosol composition and sources of particulate matter (PM) in Sanmenxia, a polluted city located in the Fen-Wei Plain region of Central China. The PM2.5 concentration decreased by 18% from 72 µg m-3 in 2014 to 59 µg m-3 in 2019. All chemical species presented pronounced seasonal variations, with their highest concentrations in winter due to enhanced emissions and the frequent stagnant meteorological conditions. Nitrate was the major fraction of PM2.5 during all seasons (35-41%) except summer (25%), while sulfate was a dominant species in summer (29%) compared to other seasons (16-18%) from July 2018 to June 2019. The detailed analysis of a wintertime severe haze episode that lasted for approximately half a month demonstrated that secondary aerosols, including secondary organic aerosol, sulfate, nitrate, and ammonium, contributed 89% to non-refractory PM1 (NR-PM1), indicating the remarkable role of secondary aerosol formation in air pollution in Sanmenxia. Positive matrix factorization analysis further showed considerably enhanced low-volatility oxygenated organic aerosol (OA) and hydrocarbon-like OA during severe haze episodes, while significant contributions in semi-volatile oxygenated OA and coal combustion OA during clean periods. Severe pollution events in the city were generally associated with air masses from the southwest, and we also found that aerosol species, especially secondary aerosol species, showed distinct forenoon increases that were caused by the subsidence of air pollutants aloft. Our results highlight that future air quality improvement would benefit substantially from a more efficient control of gaseous precursors, particularly the NOx emissions from industry and vehicle emissions.

[Pollution Characteristics and Regional Transport of Atmospheric Particulate Matter in Beijing from October to November, 2016].

In this study, the Aerosol Chemical Speciation Monitor (ACSM) was used to conduct real-time and continuous comprehensive observation of chemical components in non-refractory submicron aerosols (NR-PM1) from October 15 to November 15, 2016. In addition to that, the evolution characteristics of NR-PM1 chemical components were discussed. The potential source contribution function (PSCF) method and a meteorology-air quality coupling model system (WRF-CAMx) were applied to identify the potential PM2.5 emission sources and transport path in Beijing, and the vertical distribution characteristics of PM2.5 net transport flux. The results indicate that the monthly average mass concentrations of NR-PM1 and PM2.5 were (59.16±57.05) µg·m-3 and (89.82±66.66) µg·m-3, respectively. On average, NR-PM1 accounted for (70.31±22.28)% of PM2.5. During the whole study period, Org, NO3-, SO42-, NH4+, and Chl represented (42.75±11.35)%, (21.27±7.72)%, (19.11±7.08)%, (12.19±2.64)%, and (4.68±3.24)% of NR-PM1, respectively. The diurnal variation characteristics of different chemical components were disparate. The high potential source areas were mainly located in southern Hebei, northeastern Henan, and western Shandong provinces during the whole study period. During the haze episode, the potential regions of higher contribution were concentrated in Baoding, southern Beijing, and Langfang. The simulation results of WRF-CAMx showed that the vertical distribution characteristics of PM2.5 net flux varied with different altitudes. The adjacent cities mainly export PM2.5 to Beijing, and the PM2.5 net fluxes mainly occurred at 600-800 m during the whole study period. PM2.5 in Beijing from external sources mainly occurred in high altitudes during the early stage of the heavy pollution episode. Then it turned to near-ground transport until November 5, when the pollution was the most severe. This indicated that high-altitude and near-ground transport both played an essential role in the formation of heavy PM2.5 pollution in Beijing during the autumn. Moreover, two important transport pathways were identified:the southwest-northeast pathway (Baoding→Beijing→Chengde) and the northwest-southeast pathway (Zhangjiakou→Beijing→Langfang-south→Tianjin).

Chemical characterization of submicron aerosol particles during wintertime in a northwest city of China using an Aerodyne aerosol mass spectrometry.

An Aerodyne quadrupole aerosol mass spectrometry (Q-AMS) was utilized to measure the size-resolved chemical composition of non-refractory submicron particles (NR-PM1) from October 27 to December 3, 2014 at an urban site in Lanzhou, northwest China. The average NR-PM1 mass concentration was 37.3 µg m-3 (ranging from 2.9 to 128.2 µg m-3) under an AMS collection efficiency of unity and was composed of organics (48.4%), sulfate (17.8%), nitrate (14.6%), ammonium (13.7%), and chloride (5.7%). Positive matrix factorization (PMF) with the multi-linear engine (ME-2) solver identified six organic aerosol (OA) factors, including hydrocarbon-like OA (HOA), coal combustion OA (CCOA), cooking-related OA (COA), biomass burning OA (BBOA) and two oxygenated OA (OOA1 and OOA2), which accounted for 8.5%, 20.2%, 18.6%, 12.4%, 17.8% and 22.5% of the total organics mass on average, respectively. Primary emissions were the major sources of fine particulate matter (PM) and played an important role in causing high chemically resolved PM pollution during wintertime in Lanzhou. Back trajectory analysis indicated that the long-range regional transport air mass from the westerly was the key factor that led to severe submicron aerosol pollution during wintertime in Lanzhou.