Characteristics and sources of peroxyacetyl nitrate(PAN)in the rural North China Plain:Results from1-year continuous observations

Peroxyacetyl nitrate(PAN)is an important photochemical pollutant in the troposphere,whereas long-term measurements are scarce in rural areas in North China Plain(NCP),resulting in unclear seasonal variations and sources of PAN in rural NCP.In this study,we conducted a 1-year observation of PAN during 2021-2022 at the rural NCP site.The average concentrations of PAN were 1.10,0.75,0.65,and 0.88 ppbv in spring,summer,autumn,and winter,respectively,with a 1-year average of 0.81±0.60 ppbv.Calculations indicate that the loss of PAN through thermal decomposition in summer accounts for 43.2% of the total formed PAN,which is an important reason for the low concentration of PAN in summer.We speculate that since the correlation between PAN and O_(3) in winter is significantly lower than that in other seasons,the observed regional transport of PAN cannot be ignored in winter.Through budget analysis,regional transport accounted for 12.8% and 55.9% of the observed PAN on the spring and winter pollution days,respectively,which showed that regional transport played key roles during the photochemical pollution of the rural NCP in winter.The potential source contribution function revealed that the transported PAN mainly comes from southern Hebei in spring.In winter,the transported PAN was mainly from Langfang,Hengshui,and southern Beijing.Our findings may aid in understanding PAN variations in different seasons in rural areas and highlight the impact of regional transport on the PAN budget.

The levels, sources and reactivity of volatile organic compounds in a typical urban area of Northeast China

Air concentrations of volatile organic compounds(VOCs) were continually measured at a monitoring site in Shenyang from 20 August to 16 September 2017. The average concentrations of alkanes, alkenes, aromatics and carbonyls were 28.54, 6.30, 5.59 and9.78 ppbv, respectively. Seven sources were identified by the Positive Matrix Factorization model based on the measurement data of VOCs and CO. Vehicle exhaust contributed the most(36.15%) to the total propene-equivalent concentration of the measured VOCs,followed by combustion emission(16.92%), vegetation emission and secondary formation(14.33%), solvent usage(10.59%), petrochemical industry emission(9.89%), petrol evaporation(6.28%), and liquefied petroleum gas(LPG) usage(5.84%). Vehicle exhaust, solvent usage and combustion emission were found to be the top three VOC sources for O_3 formation potential, accounting for 34.52%, 16.55% and 11.94%, respectively. The diurnal variation of the total VOCs from each source could be well explained by their emission characteristics,e.g., the two peaks of VOC concentrations from LPG usage were in line with the cooking times for breakfast and lunch. Wind rose plots of the VOCs from each source could reveal the possible distribution of the sources around the monitoring site. The O_3 pollution episodes during the measurement period were found to be coincident with the elevation of VOCs, which was mainly due to the air parcel from the southeast direction where petrochemical industry emission was found to be dominant, suggesting that the petrochemical industry emission from the southeast was probably a significant cause of O_3 pollution in Shenyang.

Pollution levels, composition characteristics and sources of atmospheric PM2.5 in a rural area of the North China Plain during winter

The pollution levels,composition characteristics and sources of atmospheric PM2.5 were investigated based on field measurement at a rural site in the North China Plain(NCP) from pre-heating period to heating period in winter of 2017.The hourly average concentrations of PM2.5 frequently exceeded 150 μg/m3 and even achieved 400 μg/m3,indicating that the PM2.5pollution was still very serious despite the implementation of stricter control measures in the rural area.Compared with the pre-heating period,the mean concentrations of organic carbon(OG),element carbon(EC) and chlorine ion(Cl-) during the heating period increased by 20.8%,36.6% and 38.8%,accompanying with increments of their proportions in PM2.5from 37.5%,9.8% and 5.5% to 42.9%,12.7% and 7.2%,respectively.The significant increase of both their concentrations and proportions during the heating period was mainly ascribed to the residential coal combustion.The proportions of sulfate,nitrate and ammonium respectively increased from 9.9%,10.9% and 9.0% in nighttime to 13.8%,16.2% and 11.1% in daytime,implying that the daytime photochemical reactions made remarkable contributions to the secondary inorganic aerosols.The simulation results from WRF-Chem revealed that the emission of residential coal combustion in the rural area was underestimated by the current emission inventory.Six sources identified by positive matrix factorization(PMF) based on the measurement were residential coal combustion,secondary formation of inorganic aerosols,biomass burning,vehicle emission and raising dust,contributing to atmospheric PM2.5 of 40.5%,21.2%,16.4%,10.8%,8.6% and 2.5%,respectively.

Rural vehicle emission as an important driver for the variations of summertime tropospheric ozone in the Beijing-Tianjin-Hebei region during 2014-2019

Tropospheric ozone(O_(3))pollution is increasing in the Beijing-Tianjin-Hebei(BTH)region despite a significant decline in atmospheric fine aerosol particles(PM_(2.5))in recent years.However,the intrinsic reason for the elevation of the regional O_(3)is still unclear.In this study,we analyzed the spatio-temporal variations of tropospheric O_(3)and relevant pollutants(PM_(2.5),NO_(2),and CO)in the BTH region based on monitoring data from the China Ministry of Ecology and Environment during the period of 2014-2019.The results showed that summertime O_(3)concentrations were constant in Beijing(BJ,0.06μg/(m^(3)·year))but increased significantly in Tianjin(TJ,9.09μg/(m^(3)·year))and Hebei(HB,6.06μg/(m3·year)).Distinct O_(3)trends between Beijing and other cities in BTH could not be attributed to the significant decrease in PM_(2.5)(from-5.08 to-6.32μg/(m3·year))and CO(from-0.053 to-0.090 mg/(m^(3)·year))because their decreasing rates were approximately the same in all the cities.The relatively stable O_(3)concentrations during the investigating period in BJ may be attributed to a faster decreasing rate of NO_(2)(BJ:-2.55μg/(m^(3)·year);TJ:-1.16μg/(m^(3)·year);HB:-1.34μg/(m3·year)),indicating that the continued reduction of NOx will be an effective mitigation strategy for reducing regional O_(3)pollution.Significant positive correlations were found between daily maximum8 hr average(MDA8)O_(3)concentrations and vehicle population and highway freight transportation in HB.Therefore,we speculate that the increase in rural NO_(x)emissions due to the increase in vehicle emissions in the vast rural areas around HB greatly accelerates regional O_(3)formation,accounting for the significant increasing trends of O_(3)in HB.

Activity maintenance of the excised branches and a case study of NO_2 exchange between the atmosphere and P.nigra branches

The efficient maintenance of the activity of excised branches is the powerful guarantee to accurately determine gas exchange flux between the detached branches of tall trees and the atmosphere. In this study, the net photosynthetic rate(NPR) of the excised branches and branches in situ were measured simultaneously by using two photosynthetic instruments to characterize the activity of the excised branches of Phyllostachys nigra. The ratio of normalized NPR of excised branches to NPR in situ was used to assess the photosynthetic activity of detached branches. Based on photosynthetic activity, an optimal hydroponics protocol for maintaining activity of excised P. nigra branches was presented:1/8 times the concentration of Gamborg B5 vitamin mixture with p H = 6. Under the best cultivation protocol, photosynthetic activity of excised P. nigra branches could be maintained more than 90% within 6 hr in the light intensity range of 200–2000 μmol/(m2·sec) and temperature range of 13.4–28.7°C. The nitrogen dioxide(NO2) flux differences between in situ and in vitro branches and the atmosphere were compared using double dynamic chambers.Based on the maintenance method of excised branches, the NO2 exchange flux between the excised P. nigra branches and the atmosphere(from-1.01 to-2.72 nmol/(m2·sec) was basically consistent with between the branches in situ and the atmosphere(from-1.12 to-3.16 nmol/(m2 sec)) within 6 hr. Therefore, this study provided a feasible protocol for in vitro measurement of gas exchange between tall trees and the atmosphere for a period of time.

Serious BTEX pollution in rural area of the North China Plain during winter season

Atmospheric BTEX compounds（benzene, toluene, ethylbenzene and xylenes） in a rural site of the North China Plain（NCP） were preliminarily investigated in winter, and the outdoor concentrations（25.8–236.0 μg/m^3） were found to be much higher than those reported in urban regions. The pollution of BTEX inside a farmer＇s house was even more serious, with combined concentrations of 254.5–1552.9 μg/m^3. Based on the ratio of benzene to toluene（1.17 ± 0.34） measured, the serious BTEX pollution in the rural site was mainly ascribed to domestic coal combustion for heating during the winter season. With the enhancement of farmers＇ incomes in recent years, coal consumption by farmers in the NCP is rapidly increasing to keep their houses warm, and hence the serious air pollution in rural areas of the NCP during winter, including BTEX, should be paid great attention.

Characteristics and influence factors of NO2 exchange flux between the atmosphere and P. nigra

Nitrogen dioxide(NO2)is an important substance in atmospheric photochemical processes and can also be absorbed by plants.N02 fluxes between the atmosphere and P.nigrc seedlings were investigated by a double dynamic chambers method in Beijing from June 15to September 3,2017.The range of N02 exchange fluxes between P.nigra seedlings and the atmosphere was from-14.6 to 0.8 nmol/(m2.sec)(the positive data represent N02 emissior from trees,while the negative values indicate absorption).Under ambient concentrations the mean NO2 flux during the fast-growing stage(Jun.15-Aug.4)was-3.0 nmol/(m2·sec)greater than the flux of-1.5 nmol/(m2-sec)during the later growth stage(Aug.8-Sept.3)The daily exchange fluxes of NO2 obviously fluctuated.The fluxes were largest in the morning and decreased gradually over time.Additionally,the N02 fluxes were larger undei high light intensities than under low light intensities during the whole growth period.The effects of temperature on N02 fluxes were different under two growth periods.The NO2exchange fluxes were larger in a range of temperatures close to 44℃in the fast-growing stage,whereas there were no evident differences in N02 exchange fluxes under widel differing temperatures in the later growth stage.Under polluted conditions,the uptake ability of N02 was weakened.Additionally,the compensation point of N02 was 5.6 ppb ir the fast-growing stage,whereas it was 1.4 ppb in the later growth stage.The depositior velocities of NO2 were between 0.3 and 2.4 mm/sec.

Evaluation of offline sampling for atmospheric C3-C11 non-methane hydrocarbons

The concentration variation of C3-C11 non-methane hydrocarbons(NMHCs)collected in several types of commercial flexible bags and adsorption tubes was systematically inves-tigated using a gas chromatography-flame ionization detector(GC-FID)system.The per-centage loss of each NMHC in the polyvinyl fluoride(PVF)bags was less than 5%during a 7-hr storage period;significant NMHCs loss was detected in aluminum foil composite film and fluorinated ethylene propylene bags.The thermal desorption efficiency of NMHCs for adsorption tubes filled Carbopack B and Carboxen1000 sorbents was greater than 95%at 300℃,and the loss of NMHCs in the adsorption tubes during 20-days storage at 4℃was less than 8%.The thermal desorption efficiency for C11 NMHCs in the adsorption tube filled with Carbograph 1 and Carbosieve SⅢabsorbents was less than 40%at 300℃,and pyrolysis of the absorbents at 330℃interfered significantly with the measurements of some alkenes.The loss of alkenes was significant when NMHCs were sampled by cryo-enrichment at-90℃in the presence of O3 for the online NMHC measurements,and negligible for enrichment us-ing adsorption tubes at 25℃.Although O_(3)scrubbers have been widely used to eliminate the influence of O_(3)on NMHC measurements,the loss of NMHCs with carbon numbers greater than 8 was more than 10%.Therefore,PVF bags and adsorption tubes filled Carbopack B and Carboxen1000 sorbents were recommended for the sampling of atmospheric NMHCs.

Photochemical production of carbonyl sulfide,carbon disulfide and dimethyl sulfide in a lake water

Photochemical production of carbonyl sulfide（COS）,carbon disulfide（CS_2） and dimethyl sulfide（DMS） was intensively studied in the water from the Aohai Lake of Beijing city.The lake water was found to be highly supersaturated with COS,CS_2 and DMS,with their initial concentrations of 0.91 ± 0.073 nmol/L,0.55 ± 0.071 nmol/L and 0.37 ± 0.062 nmol/L,respectively.The evident photochemical production of COS and CS_2 in the lake water under irradiation of 365 nm and 302 nm indicated that photochemical production of them might be the reason for their supersaturation.The similar dependence of wavelength and oxygen for photochemical production of COS,CS_2 and DMS implied that they might be from the same precursors.The water cage effect was found to favor COS production but inhibit CS_2and DMS formation,indicating that COS photochemical production was mainly from direct degradation of the precursors and the formation of CS_2 and DMS needed intermediates via combination of carbon-centered radicals and sulfur-centered radicals.The above assumptions were further confirmed by simulation experiments with addition of carbonyls and amino acids（cysteine and methionine）,and the photochemical formation mechanisms for COS,CS_2 and DMS in water were derived from the investigations.