Southern California air quality study: A search for methyl nitrate

Methyl nitrate, CH₃ONO₂, was measured by electron capture gas chromatography (EC-GC) under conditions which allowed resolution of methyl nitrate, PAN, and several chlorinated hydrocarbons. Calibrations involved both EC-GC andNO_x chemiluminescence and were in agreement with independent calibrations involving i.r. spectroscopy. The rate constant for photolysis of methyl nitrate in sunlight was< 2.3 × 10⁻⁶s⁻¹. Detection limits of field instruments were 0.1–0.4 ppb. Some 3000 EC-GC chromatograms grams of ambient air recorded between June and December 1987, during the Southern California Air Quality Study (SCAQS) at up to nine Southern California locations yielded only seven possible, but unlikely observations of methyl nitrate. Thus, methyl nitrate was only a minor component among nitrogenous air pollutants during SCAQS. The measured CH₃ONO₂/PAN ratios of<0.003–0.2 during SCAQS are discussed in terms of available kinetic data for PAN unimolecular decomposition (a major source of methyl nitrate), PAN thermal decomposition and CH₃ONO₂ photolysis.     

Wintertime peroxyacetyl nitrate (PAN) in the megacity Beijing: Role of photochemical and meteorological processes

Previous measurements of peroxyacetyl nitrate(PAN) in Asian megacities were scarce and mainly conducted for relative short periods in summer. Here, we present and analyze the measurements of PAN, O3, NOx, etc., made at an urban site(CMA) in Beijing from 25 January to 22 March 2010. The hourly concentration of PAN averaged 0.70 × 10 9mol/mol(0.23 × 10 9–3.51 × 10 9mol/mol) and was well correlated with that of NO2but not O3, indicating that the variations of the winter concentrations of PAN and O3in urban Beijing are decoupled with each other. Wind conditions and transport of air masses exert very significant impacts on O3, PAN, and other species. Air masses arriving at the site originated either from the boundary layer over the highly polluted N-S-W sector or from the free troposphere over the W-N sector. The descending free-tropospheric air was rich in O3, with an average PAN/O3ratio smaller than 0.031, while the boundary layer air over the polluted sector contained higher levels of PAN and primary pollutants, with an average PAN/O3ratio of 0.11. These facts related with transport conditions can well explain the observed PAN-O3decoupling. Photochemical production is important to PAN in the winter over Beijing. The concentration of the peroxyacetyl(PA) radical was estimated to be in the range of 0.0014 × 10 12–0.0042 × 10 12 mol/mol. The contributions of the formation reaction and thermal decomposition to PAN's variation were calculated and found to be significant even in the colder period in air over Beijing, with the production exceeding the decomposition.     

Laboratory tests of KI and alkaline annular denuders

Annular denuders coated with KI and with alkaline solutions have been tested for their ability to remove atmospheric pollutants including ozone, NO₂, SO₂, formaldehyde, methyl nitrate and peroxyacetyl nitrate. Tests were carried out at flow rates of 0.4–2.0 ℓ min⁻¹, using particle-free ambient air as well as purified air to study the effect of atmospheric CO₂ on alkaline denuder performance. Denuders coated with KI were efficient in removing O₃, NO₂, SO₂ (> 95%) and PAN (84±3%) but not methyl nitrate (44%) and formaldehyde (<5%). Selective removal of PAN from NO₂, and vice versa, could be obtained with annular denuders coated with NaOH, which removed 100% PAN and ⩽15% NO₂, and with alkaline guaiacol, which removed ⩾99% NO₂ and ⩽6% PAN.     

Nitrogenous air pollutants at a southern California mountain forest smog receptor site

Ambient levels of the nitrogenous pollutants NO, NO₂, nitric acid, nitrous acid, ammonia, particulate nitrate, particulate ammonium, peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) have been measured at a southern California mountain forest location severely impacted by urban photochemical smog. Air quality at the mountain forest location was characterized by high levels of nitric acid (up to 18 ppb) and the phytotoxic peroxyacyl nitrates PAN (up to 22 ppb) and PPN (up to 5 ppb). Alkyl nitrates were below our detection limits of 0.05–0.5 ppb. The (PAN + PPN)/NO₂ ratios varied substantially (range 0.03–2.27) and were generally large, with typical 24-h averages of 0.19–0.50. Diurnal variations of the (PAN/PPN)/NO₂ ratio exhibited both nighttime and daytime maxima reflecting diurnal variations in PAN (and PPN) thermal stability and photochemical production rates, respectively. Organic nitrogen-containing oxidation products (PAN + PPN) were more abundant than inorganic nitrate (HNO₃ + NO₃⁻), with an average organic/inorganic concentration ratio of 2.5 (daytime ratio 1.0; nighttime ratio 3.7). The four oxidation products PAN, PPN, HNO₃ and NO₃⁻ together accounted for 0.26 of the total reactive nitrogen. The results are discussed with respect to diurnal and seasonal variations and in terms of NO₂ atmospheric oxidation pathways. Deposition fluxes and velocities to ponderosa pine have been measured for inorganic nitrate and for ammonium and have been compared with those obtained at other mountain forest locations.     

A captive-air irradiation study of the response of nitric acid and peroxyacetyl nitrate to ozone control strategies in Los Angeles

Outdoor smog chamber experiments were used to study the sensitivity of the yields of two important nitrogen-containing pollutants, nitric acid (HNO₃) and peroxyacetyl nitrate (PAN) to changes in nonmethane hydrocarbon (HC) and nitrogen oxide (NO_x) concentrations in Los Angeles. The experiments were conducted at two sites in the Los Angeles Basin using eight chambers filled with morning Los Angeles air on 33 days. At least one chamber was unchanged and served as a control, while the initial HC and/or NO_x concentrations were changed by 25–50% in up to seven chambers to simulate O₃ control strategies and to broaden the range of HC - NO_x conditions studied. Empirical models that predict the maximum yields of HNO₃ and PAN were used to determine the response of these pollutants to three possible ozone control strategies. All three strategies (reductions in HC, NO_x or both HC and NO_x) reduced PAN while only NO_x reductions decreased HNO₃. However, reducing NO_x increased the HC reductions required to attain lower O₃ levels. Thus, there is a conflict between the O₃ and HNO₃ control strategies.     

Seasonal and diurnal variations of atmospheric peroxyacetyl nitrate, peroxypropionyl nitrate, and carbon tetrachloride in Beijing

Atmospheric peroxyacetyl nitrate(PAN), peroxypropionyl nitrate(PPN), and carbon tetrachloride(CCl4) were measured from September 2010 to August 2011 in Beijing. PAN exhibited low values from mid-autumn to early spring(October to March) with monthly average concentrations ranging from 0.28 to 0.73 ppbV, and increased from early spring to summer(March to August), ranging from 1.37–3.79 ppbV. The monthly variation of PPN was similar to PAN, with low values(below detection limit to 0.18 ppbV) from mid-autumn to early spring, and a monthly maximum in September(1.14 ppbV). The monthly variation of CCl4was tightly related to the variation of temperature, exhibiting a minimum in winter(69.3 pptV) and a maximum of 180.6 pptV in summer. Due to weak solar intensity and short duration, PAN and O3showed no distinct diurnal patterns from morning to night during winter, whereas for other seasons, they both exhibited maximal values in the late afternoon(ca. 15:00 to 16:00 local time) and minimal values during early morning and midnight. Good linear correlations between PAN and PPN were found in autumn(R = 0.91), spring(R = 0.94), and summer(R = 0.81), with slopes of 0.130, 0.222, and 0.133, respectively, suggesting that anthropogenic hydrocarbons dominated the photochemical formation of PANs in Beijing. Positive correlation between PAN and O3 in summer with the low slopes( O3 / PAN) ranging from 9.92 to 18.0 indicated serious air pollution in Beijing, and strong negative correlation in winter reflected strong O3consumption by NO titration and less thermal decompositin of PAN.     

Dynamic calibration of peroxyacetyl nitrate (PAN) analysers by annular denuder and ion-chromatographic techniques

Peroxyacetyl nitrate (PAN) at ppb-ppt levels is produced by irradiating a mixture of acetaldehyde and NO₂ dynamically generated by permeation tubes. The quantitative and selective removal of NO₂ and ozone from the reaction mixture performed by using two annular denuders allows the quantification of PAN as nitrite by ion chromatography carried out on the hydrolysed sample. The method is suitable for the field calibration of GC-based PAN-analysers.     

Sulfur dioxide,hydrogen sulfide,total reduced sulfur,chlorinated hydrocarbons and photochemical oxidants in southern California museums

Indoor and outdoor concentrations of the air pollutants ozone, NO₂, SO₂, H₂S, total reduced sulfur (TRS), peroxyacetyl nitrate (PAN), methyl chloroform and tetrachloroethylene, have been measured at three southern California museums. Indoor maxima were 175 ppb for NO₂, 77 ppb for O₃, 0.7 ppb for PAN, 1.2 ppb for C₂Cl₄, >6.3 ppb for CH₃CCl₃, 2.5 ppb for SO₂, 1.4 ppb for TRS, and 46 ppt for H₂S. Indoor levels and indoor/outdoor (I/O) ratios for the chlorinated hydrocarbons pointed out to indoor sources. Outdoor and indoor levels of SO₂ and TRS were low at all three museums, but I/O ratios for SO₂ were high and averaged 0.89. H₂S concentrations were low, 16–46 ppt at one museum and less than 6 ppt at the other two museums. I/O ratios for the air pollutants with outdoor sources (ozone, PAN and NO₂) showed substantial variations, from low values of 0.02–0.33 at locations without influx of outdoor air to high values of 0.85–0.88 at locations experiencing high influx of outdoor air. Of the 10 institutions we have surveyed in southern California to date, eight exhibit high I/O ratios, e.g. 0.60–1.00 for PAN. Of the four museums surveyed that were equipped with HVAC and chemical filtration, only two yielded the expected low I/O ratios.     

A comparison of calibration and measurement techniques for gas chromatographic determination of atmospheric peroxyacetyl nitrate (PAN)

A 24 day intercomparison study of peroxyacetyl nitrate (PAN) measurement was conducted in Egbert, Ontario, in February–March 1989. Two GC/ECD systems and one GC using a luminolchemiluminescence based detection device were involved. To separate the influence of calibration errors vs possible chromatographic interferences, a series of laboratory standards (gaseous PAN concentrations determined by either i.r. or by chemiluminescence) were exchanged. For all three GCs the results of analysis of standard samples were within ±25% of the standard concentration. For ambient air PAN measurements (concentrations ranging from 0.15 to 2 ppb), plots of each set of measurement data regressed against those of the other two techniques were obtained. Linear regression of the data from ambient air analysis for the two GC/ECD systems yielded a slope and intercept of 1.14±0.01 and −0.03±0.05, respectively (r = 0.995). The plot of the Luminol-based GC against the AES GC/ECD yielded a slope of 1.08±0.03 and an intercept of 0.08±0.24 (r = 0.864). The results demonstrated the reliability of the chemiluminescence calibration technique, and indicated that the luminol-based PAN measurement device can yield accurate and sensitive measurements of ambient PAN concentrations.     

Southern California air quality study: Peroxyacetyl nitrate

Information on the spatial and seasonal variations of ambient peroxyacetyl nitrate (PAN) has been obtained from simulataneous measurements made at five, seven or nine Southern California locations in June-September and November-December, 1987. Summertime (smog season) levels of PAN were consistent with photochemical formation during transport, and increased substantially from coastal to inland locations. Daily maxima (up to 30 ppb) coincided with those of ozone at all locations and shifted from midday at coastal sites to late afternoon inland.Elevated levels of PAN, e.g. up to 19 ppb on 3 December 1987, were observed during the fall at all coastal and central locations, where they consistently exceeded summertime levels.     

广州番禺大气成分站一次典型光化学污染过程PAN和O3分析

通过对广州番禺大气成分站（GPACS）的光化学相关污染物（O₃、PAN、VOCs、NO₂、NO）以及气象要素进行观测,分析2010~2016年期间发生在广州地区一次典型光化学污染过程.结果表明,该光化学污染过程期间,O₃和PAN总体体积分数比较高,最大O₃小时体积分数为140.6×10^-9,而最大PAN小时体积分数为4.7×10^-9.NO整体体积分数较低,对O₃的化学滴定和PAN的去除影响较小.NO₂整体体积分数较高、辐射较强和风速较低则有利于O₃和PAN的形成和积累.PAN和O₃具有一定的线性关系（R²=0.55）,而形成PAN和O₃前体物VOCs物种不完全相同影响着它们的线性关系,在生成PAN的VOCs物种中,乙烯、丙烷、异戊二烯和甲苯所占的比例较大,而对臭氧生成潜势较大的物种有异戊二烯、1,3,5-三甲苯、丙烯、间,对-二甲苯以及甲苯.对PA自由基体积分数进行估算,发现它的日均值体积分数在0.11×10^-12~0.16×10^-12范围变化,远高于其它地区,表明此次发生的光化学反应较为强烈.     

Formic acid as an alternative reducing agent for the catalytic nitrate reduction in aqueous media

Formic acid was used for the nitrate reduction as a reductant in the presence of Pd:Cu/γ-alumina catalysts. The surface characteristics of the bimetallic catalyst synthesized by wet impregnation were investigated by SEM, TEM-EDS. The metals were not distributed homogeneously on the surface of catalyst, although the total contents of both metals in particles agreed well with the theoretical values. Formic acid decomposition on the catalyst surface, its influence on solution pH and nitrate removal efficacy was investigated. The best removal of nitrate (50 ppm) was obtained under the condition of 0.75 g/L catalyst with Pd:Cu ratio (4:1) and two fold excess of formic acid. Formic acid decay patterns resembled those of nitrate removal, showing a linear relationship between kf (formic acid decay) and k (nitrate removal). Negligible amount of ammonia was detected, and no nitrite was detected, possibly due to buffering effect of bicarbonate that is in situ produced by the decomposition of formic acid, and due to the sustained release of H2 gas.     

Field tests of a passive sampler for atmospheric ozone at California mountain forest locations

Field tests of a colorant-based ozone passive sampler have been carried out during the 1990 smog season at five mountain forest locations in California. Co-located measurements of ambient ozone were made at all field sites using the passive sampler and a reference method, ultraviolet (u.v.) photometry. The sampling duration ranged from 3 to 30 days. Nitrogen dioxide, aldehydes and the phytotoxic oxidants peroxyacetyl nitrate (PAN) and peroxypropionyl nitrate (PPN) were measured at one location to assess possible interferences. The average precision of the measurements, calculated from data for 42 sets of colocated passive samplers, was 12%. Data for all field locations could be reduced to a single equation relating color change (ΔE units) to ozone dose (units: ppb-days). This polynomial equation could be reduced to a linear equation for color changes not exceeding 8 ΔE units (equivalent to sampling durations of up to 15 days), with good agreement between field data and earlier calibration results obtained in the laboratory. Both polynomial and linear equations can be used to obtain quantitative, time-integrated measurements of ambient ozone. The performance of the passive sampler showed no dependence on changes in ambient temperature and humidity. Interferences from air pollutants other than ozone contributed a total positive bias of less than 5% to the measured ozone concentrations: 3% for NO₂, 1.3% for PAN, 0.2% for PPN, and 0.% or less for formaldehyde and acetaldehyde.     

Nitrogen budget for eastern Canada

A Lagrangian model incorporating S and N chemistry has been developed by the Atmospheric Environment Service. It was used to produce an annual nitrogen budget including annual nitrogen transboundary flux estimates for 1980 for eastern Canada.The model used trajectories computed from the analyzed winds produced at the Canadian Meteorological Centre (CMC) in addition to the temperature and precipitation analyses. The model parameterized the dry and wet deposition and chemical transformation of NO₂, nitrate (including HNO₃) and PAN using monthly, time-dependent dry deposition velocities, scavenging ratios and transformation rates. The model was integrated throughout eastern Canada on a 6-h basis for 1980. The boundary point concentrations were used to compute input and output fluxes which were summed over several boundary segments to give annual transboundary flux estimates.The total annual N deposition was about 0.6 TgN with high deposition in August and December and the lowest in February. Wet NO₃⁻ deposition (0.2 TgN) was higher than the dry deposition of any single N species (NO₂, nitrate or PAN) but lower than all dry N deposition combined. The annual N input (output) flux was about 1.0 (0.5) TgN. The highest input and output fluxes occurred in December and the lowest input and output fluxes occurred in May.     

Peroxyacetyl nitrate observed in Beijing in August from 2005 to 2009

Measurements of peroxyacetyl nitrate（PAN） were made at a Beijing urban site each August from2005 to 2009. Over this 5-year period, the average PAN concentration for August in each year increased from 3（2005） to 11.7 μg/m3（2007）; however, it decreased rapidly in 2008（4.1 μg/m3）.Generally, the variation over the 5 years showed a rise in the first part of the study period,followed by a decline. We considered two categories of local and regional air masses in this study, which revealed that the PAN concentration in Beijing was affected mainly by southeastern air masses. The August PAN variation was influenced predominantly by local air masses in 2005,but by 2009 regional air masses had become more important. This study showed the level and variation of PAN in the month of August in 5 consecutive years for the first time, and proved that control measures are useful in decreasing photochemical pollution; hence, these measures are probably feasible for other megacities too. Furthermore, this method of analyzing regional and local impacts might be useful for other studies as well.     

Analysis of peroxyacetyl nitrate (PAN) in the vertical profile of a spruce stand

Continuous analyses of peroxyacetyl nitrate (PAN) were performed at a forest site in the Eggegebirge, F.R.G., where severe forest decline has occurred in the last decade. An automatically working analysis system was installed inside a spruce stand. Air samples from three elevations (1 m, 7.5 m and 12.5 m above ground) were simultaneously analyzed. The results obtained show that PAN-concentrations in the range below 0.2 ppb prevailed. Several episodes with elevated PAN concentrations, up to 4.6 ppb, were measured. High PAN-levels were more abundant in spring and summer than in winter. PAN concentrations above the canopy were usually higher than within the canopy. Near ground level the lowest concentrations were found. This concentration profile and the detailed discussion of episodes with elevated PAN concentrations, in conjunction with additional meteorological data, showed that PAN formation within or above the forest canopy is relatively low compared to the input by long-range transport. Thus it was concluded that biogenic hydrocarbons emitted by the forest trees do not significantly contribute to the formation of observed PAN peak concentrations. The measured concentrations did not indicate a phytotoxic burden by PAN and PAN homologous photooxidants at this site.     

天津夏季边界层低层大气中PAN和O3的输送特征分析

采用在线仪器监测分析2017年夏季天津气象铁塔220 m观测平台大气中过氧乙酰硝酸酯(PAN)和O_3的体积分数,并结合气象观测资料和后向轨迹分析PAN和O_3的输送特征.观测期间PAN和O_3体积分数平均值分别为(0.73±0.56)×10~(-9)和(53±25)×10~(-9),最大小时体积分数分别为3.49×10~(-9)和137×10~(-9),PAN和O_3体积分数具有相似的日变化特征,白昼PAN和O_3浓度高于夜间,且PAN和O_3浓度相关系数(R2=0.52)显著高于夜间(R2=0.21).观测期间偏南风下PAN和O_3浓度最高,偏东风下最低,风玫瑰图和后向轨迹聚类分析都表明,来源于西南方向的气流轨迹对应的污染物浓度最高,途经渤海和河北、辽宁沿海地区的偏东气流对应的PAN和O_3体积分数最低,边界层内输送对PAN和O_3的体积分数分布起到了重要作用.     

Effects of cathode potentials and nitrate concentrations on dissimilatory nitrate reductions by Pseudomonas alcaliphila in bioelectrochemical systems

The effects of cathode potentials and initial nitrate concentrations on nitrate reduction in bio- electrochemical systems （BESs） were reported. These factors could partition nitrate reduction between denitrification and dissimilatory nitrate reduction to ammonium （DNRA）. Pseudomonas alcaliphilastrain MBR utilized an electrode as the sole electron donor and nitrate as the sole electron acceptor. When the cathode potential was set from -0.3 to -I.1 V （vs. Ag/AgC1） at an initial nitrate concentration of 100 mg NO~-N/L, the DNRA electron recovery increased from （10.76 ± 1.6）% to （35.06 ± 0.99）%; the denitrification electron recovery decreased from （63.42 ± 1,32）% to （44.33 ± 1.92）%. When the initial nitrate concentration increased from （29.09 ± 0.24） to （490.97 ± 3.49） mg NO3-N/L at the same potential （-0.9 V）, denitrification electron recovery increased from （5.88 ± 1.08）% to （50.19 ±2.59）%; the DNRA electron recovery declined from （48.79 ±1.32）% to （16.02 ± 1.41）%. The prevalence of DNRA occurred at high ratios of electron donors to acceptors in the BESs and denitrification prevailed against DNRA under a lower ratio of electron donors to acceptors. These results had a potential application value of regulating the transformation of nitrate to N2 or ammonium in BESs for nitrate removal.     

Products and mechanisms of the reactions of the nitrate radical (NO3) with isoprene, 1,3-butadiene and 2,3-dimethyl-1,3-butadiene in air

Products and mechanisms of the reaction of NO₃ with isoprene have been studied under simulated atmospheric conditions with in situ FTIR spectroscopy as analytical technique. The study addressed also the reactions of NO₃ with 1,3-butadiene and 2,3-dimethyl-1,3-butadiene as well as with the deuterated species 1,1,4,4-d₄-1,3-butadiene, d₆-1,3-butadiene and 4,4-d₂-2-methyl-1,3-butadiene (d₂-isoprene). The dienes examined apparently follow very similar reaction pathways. The decay of the intermediate peroxynitrates formed after the addition of NO₃ to one of the methylene groups, leads to unsaturated ketone-nitrate, aldehyde-nitrate, alcohol-nitrate and perhaps also dinitrate species. The yield of unsubstituted carbonyl compounds was insignificant in the reactions of NO₃ with 1,3-butadiene and isoprene. In the case of isoprene, NO₃ reacts adding preferentially to the 1-position and 3-methyl-4-nitroxy-2-butenal appears to be the main product. 1,3-Butadiene predominantly reacts with NO₃ via trans-1,4-addition and 1,2-addition while the cis-1,4-addition path is of minor importance. trans-4-Nitroxy-2-butenal and 1-nitroxy-3-buten-2-one were found as main products. Contrary to the daytime OH-initiated degradation of isoprene, the nighttime oxidation of isoprene by NO₃ leads to formation of large quantities of organic nitrate compounds; this may have consequences for the tropospheric NO_y budget as briefly discussed.     

不同含量硫酸根和硝酸根作为反离子对聚二甲基二烯丙基铵盐絮凝性能的影响

在实验室烧杯实验中,通过利用光散射监控技术,以及测定絮凝后上清液的残余浊度和Zeta电位,研究了硫酸根和硝酸根聚二甲基二烯丙基铵盐聚电解质（PDADMAX）的絮凝性能．以不同浊度的高岭土悬浊液为目标物,详细地讨论了反离子中不同含量的SO4^2-或NO3^-对絮凝效能的影响．为了进一步阐明PDADMAX的絮凝机理,用原子力显微镜（AFM）和比浓粘度表征了反离子对PDADMAX溶液性质和吸附形貌的影响实验结果表明：反离子明显影响了PDADMAX的絮凝性能．含有NOr的PDADMAX具有更强的“电中和作用”和更高的絮凝效率,而含有SO4^2-的PDADMAX具有更高的“吸附架桥作用”既更宽的最佳絮凝剂投量范围、更大的絮体．不同的SO4^2-或NO3^-含量的PDADMAX絮凝剂的絮凝效率不同,其中都在含量20％时效果最好．而且在高浊度条件下,SO4^2-或NO3^-对PDADMAX的絮凝效果有更明显地增强．