Transition Flow Reactor Research Articles

Modifications of the transition flow reactor theory for the more accurate measurement of nitric acid and nitrate and ammonium ions

Sixty five samples were taken with a Transition Flow Reactor (TFR) for the measurement of nitric acid (HNO3) and nitrate particulates (NO 3 (-) ), in a central Athens street having a heavy traffic density, between February 1989 and February 1990. It was established that the TFR theory overestimated HNO3 and underestimated NO 3 (-) . This is caused by the retention of a fraction of the NO2 and a fraction of the HNO2 as NO 2 (-) on the sampler's nylon linear. NO 2 (-) is in turn oxidized to NO 3 (-) and this is determined as HNO3. Two modifications of the TFR theory are proposed for a better estimation of the HNO3 and NO 3 (-) . With the first modification HNO3 is estimated as being equal to the smaller of two values: HNO3 TFR and the value calculated if it is considered that 91% of HNO3 is retained by the nylon filter. The second modification was based on the quantification of the effect of NO2 on the concentrations of HNO3 and NO 3 (-) . This modification is an improvement on the first one and enables the calculation of a lower limit for the concentration of HNO2. Both modifications improve the accuracy of the TFR sampler in measuring HNO3 and NO 3 (-) , without altering it structurally. It was also found that the nylon filter partially retains some NH3, causing a negative error in the measurement of NH 4 (+) with the TFR. The extent of NH3 retention on the nylon filter is proportional to the HNO3 concentration. This error must be taken into account whenever NH 4 (+) is being determined.

Modifications of the transition flow reactor theory for the more accurate measurement of nitric acid and nitrate and ammonium ions

Modifications of the transition flow reactor theory for the more accurate measurement of nitric acid and nitrate and ammonium ions

Measurements by transition flow reactor during spring 1989 in central Athens, Greece

Abstract Twelve 22‐hour samples of ambient air were taken with the Transition‐Flow Reactor (TFR) in a high traffic density street in central Athens, during March and April 1989. These were used to determine the concentrations of gaseous HNO3 and NH3 and particulate NO3 −, SO4 =, NH4 +, C1−, Na+ and K+. The TFRA modification of the TFR theory was used in the derivation of the concentrations of HNO3 and NO3 −. In determining NH3 we considered that the nafion liner collects through diffusion 12% of the NH3 and not 17%, the figure given in the TFR protocol. The concentrations and their ranges for HNO3, NH3, NO3 −, SO4 = and NH4 + were 1.41 (0.29–3.35), 2.39 (1.01–3.44), 4.84 (0.09–9.37), 14.64 (0.19–36.39) and 5.01 (1.05–10.55) μgm−3 respectively. HNO3 concentrations did not reach high levels, while NH3, NO3% SO4 = and NH4 + levels were higher and comparable to those of polluted USA cities.

Field Measurements of Dry Deposition Compounds using the Transition Flow Reactor

The transition flow reactor (TFR) was used to measure ambient concentrations of particulate nitrate (NO{sub 3}{sup {minus}}), particulate sulfate (SO{sub 4}{sup =}), nitric acid (HNO{sub 3}), and sulfur dioxide (SO{sub 2}) around the Gainesville, Florida area. The TFR measured concentrations were then compared to measurements made by different filter packs. During one comparison period, the TFR consistently measured higher concentrations for all species than the filter packs. The same difference was observed in another comparison period for all pollutants but HNO{sub 3}. It was concluded that either the TFR or filter pack could be used for HNO{sub 3} measurement at a rural site (Austin Cary Forest).

Intercomparison of atmospheric nitric acid measurements at elevated ambient concentrations

Several methods for measurement of ambient HNO 3 were compared over a 9-day period during the Carbonaceous Species Methods Comparison Study at Glendora, California, in August 1986. Hourly averaged HNO 3 concentrations were in the range 0.5–25 ppbv, with hourly maxima each day in excess of 15 ppbv. The measurement methods included absorption by a nylon filter in an open-face filter pack (FP), a transition flow reactor tube (TFR) in conjunction with a nylon filter, tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared (FT-IR) spectroscopy. The latter method was chosen as the common method for comparison of the daytime HNO 3 measurements (night-time concentrations were below the 4 ppbv detection limit of the FT-IR). Comparing the mean daytime HNO 3 mixing ratios, the 1-h FP method yielded 13.9 ppbv (vs 12.9 for FT-IR), the 1-h TDLAS was 11.7 ppbv (vs 13.2 for FT-IR) and the 12-h TFR was 11.4 ppbv (vs 12.0 for FT-IR). For the night-time periods, the means of the 2-h FP and the 12-h TFR were 1.2 and 0.95 ppbv, respectively. These measured means were within the uncertainities of the methods.

Field comparison of methods for the measurement of gaseous and particulate contributors to acidic dry deposition

A field study was conducted to compare methods for sampling and analysis of atmospheric constituents that are important contributors to acidic dry deposition. Comparisons are made of different measurements of ambient concentrations of gaseous HNO 3, NO 2, NH 3 and SO 2 and particulate NO 3 −, NH 4 + and SO 4 2−. Three multicomponent samplers were used: the Canadian filter pack (FP), the annular denuder system (ADS), and the transition flow reactor (TFR). A tunable diode laser absorption spectrometer (TDLAS) provided continuous reference measurements of NO 2 and HNO 3. Nitrogen dioxide was also monitored with continuous luminol-based chemiluminescence monitors and with passive sampling devices (PSDs). The study was designed to provide a database for statistical comparison of the various methods with emphasis on the multicomponent samplers under consideration for use in a national dry deposition trends monitoring network. The study was conducted at the EPA dry deposition station in Research Triangle Park, NC between 29 September and 12 October 1986. Daily averaging and/or sampling times were employed for the 13-day study; weekly samples were also collected, but results from these samples are not compared in this paper. ADS, TFR and FP results are in good agreement for measurements of the sum of particulate and gaseous NO 3 − concentrations and of total particulate SO 4 2−. ADS, FP, and TDLAS measurements of HNO 3 are in good agreement, but TDLAS results differ with and are substantially less than those from the TFR. TDLAS measurements of NO 2 are highly correlated with those of the TFR, but show mixed comparisons with results from the two luminol-based monitors and with results from the two sets of PSDs. TFR and FP measurements of particulate NH 4 + are in good agreement, but FP results exceed those of the ADS when volatilization losses of NH 4 + in the ADS are ignored. At the low ambient NH 3 concentrations, results of ADS, TFR, and FP show considerable variability but no statistically discernable differences. ADS results for measurements of SO 2 exceed those from both the TFR and FP. Non-quantitative recovery of SO 2 from various elements in the TFR and FP may account for this discrepancy.

A comparison of measurements of atmospheric ammonia by filter packs, transition-flow reactors, simple and annular denuders and fourier transform infrared spectroscopy

Using data obtained during the 1985 Nitrogen Species Methods Comparison Study (1988, Atmospheric Environment 22, 1517), several measurement methods for sampling ambient NH 3 are compared. Eight days of continuous measurements at Pomona College, a smog receptor site in Los Angeles, provided an extensive data base for comparing the following methods: Fourier transform i.r. spectroscopy (FTIR), three filter pack configurations, a simple and an annular denuder, and the transition flow reactor. FTIR was defined as the reference method and it reported hourly NH 3 concentrations ranging from > 60 to 2280 nmol m −3 (1.5−57ppb) during the course of the study, the highest values coming from the influence of nearby livestock operations. Although only limited quality assurance procedures were carried out, the following conclusions can, nevertheless, be drawn: most of the methods correlated highly with the FTIR method (correlation coefficient r > 0.96); generally, the linear regression slopes were close to unity and the intercepts were insignificantly different from zero at the 95% confidence level); relative to the FTIR average values, (1) for 4–6 h sampling periods, the averages of the three filter packs from three research groups were 83–130% and the annular denuder average was 87%, and (2) for 10–12 h sampling periods, the simple denuder averaged 90% and the two transition flow reactors were 77–98%. Possible reasons for the reported systematic biases are presented, but these are not able to fully explain the large range of differences reported by the various methods.

Nitric Acid and Particulate Measurements by Transition-Flow Reactor During the Carbonaceous Species Methods Comparison Study

Measurements of gaseous nitric acid and fine and total particulate nitrate and sulfate were made by the transition-flow reactor (TFR) during the Carbonaceous Species Methods Comparison Study (CSMCS...

Comparison of the Annular Denuder System and the Transition Flow Reactor for Measurements of Selected Dry Deposition Species

An intensive field study was conducted in Research Triangle Park, North Carolina in the fall of 1986. Ambient concentrations of the following constituents were obtained: nitric acid, nitrous acid, nitrogen dioxide, sulfur dioxide, ammonia, hydrogen ion, and particulate nitrate, sulfate, and ammonium. Results collected using the annular denuder system (ADS) and the transition flow reactor (TFR) are presented and compared. Both types of samplers had operational detection limits on daily (22-hour) samples that were generally below 1 μg m-3 suggesting that both samplers can provide sensitive measurements for most of the constituents of interest. Both the ADS and TFR show reasonable (>25 percent) within-sampler precision for most of the measured species concentrations, except TFR fine particulate nitrate measurements where results were frequently negative (The TFR fine particulate nitrate measurement is calculated using subtraction of positive numbers). Comparison of ADS and TFR daily results showed good agree...

The sampling of reactive atmospheric species by transition-flow reactor: Application to nitrogen species

Concentrations of nitric acid vapor and fine particulate nitrate were measured during the 1985 Nitrogen Species Methods Comparison Study at Claremont, California, with a transition-flow reactor. This system separates atmospheric gases and particles by differential diffusion in a transition-flow stream and thereby minimizes sampling artifacts for these reactive species. Error analysis showed that [HNO 3] was determined with typical uncertainties (1σ) of 5–8% and fine particulate [NO 3 −] at typical uncertainties of 5–11%. Both species showed a strong diurnal pattern with daytime maxima. Daytime [HNO 3] ranged from 25 to 495 nmole m −3, while night-time values ranged from less than 1 to 107 nmole m −3. The time-weighted average concentrations of 12- and 10-h samples were equivalent to the corresponding 22-h sample for each day. Simple filtration with Teflon® and nylon filters in series over-estimated [HNO 3] by 30–50 % for 10- and 12-h samples, and by 70% for 22-h samples. Analysis of ionic balance suggests that the cause of the overestimate was dissociation of NH 4NO 3 from equilibrium changes, and not displacement by strong acids.

The nitric acid shootout: Field comparison of measurement methods

Eighteen instruments for measuring atmospheric concentrations of nitric acid were compared in an eight day field study at Pomona College, situated in the eastern portion of the Los Angeles Basin, in September 1985. The study design included collocated and separated duplicate samplers, and the analysis by each laboratory of a set of quality assurance filters, so that the experimental variability could be distinguished from differences due to measurement methods. For all sampling periods, the values for nitric acid concentrations reported by the different instruments vary by as much as a factor of four. The differences among measurement techniques increase with nitric acid loading, corresponding to a coefficient of variation of 40%. In contrast, samplers of the same design operated by the same group show variability of 11–27 %. Overall, the highest reported concentrations are observed with the filter packs and lower concentrations are observed by the annular denuders and tunable diode laser absorption spectrometers. When the nitric acid concentrations are high enough to be detected by the FTIR, the FTIR values are close to those obtained by the denuder difference method and to the mean value from the other sampler groups. In the absence of a reference standard for the entire study, measurement methods are compared to the average of four denuder difference method samplers (DDM). Filter pack samplers are higher than the DDM for both daytime and night-time sampling. Two different filter packs using Teflon® prefilters are higher than the DDM by factors of 1.25 and 1.4. The results from the three annular denuders do not agree; the ratios of means to the DDM value are 1.0,0.8 and 0.6. For the transition flow reactor method and for two dichotomous samplers operated as denuder difference samplers, the ratio of means to the DDM are 1.09 and 0.93, respectively. The tunable diode laser absorption spectrometers gave lower daytime and higher night-time readings compared to the DDM, especially during the last three days of the study. Averaged over the entire measurement period, the daytime ratio of TDLAS to DDM is 0.8 and the night-time ratio is 1.7.

Pollutant sampler for measurements of atmospheric acidic dry deposition

An acidic pollutant sampler for dry deposition monitoring has been designed and evaluated in laboratory and field studies. The system, which is modular and simple to operate, samples gaseous HNO/sub 3/, NH/sub 3/, SO/sub 2/, and NO/sub 2/ and particulate SO/sub 4//sup 2 -/, NO/sub 4//sup -/, and NH/sub 4//sup +/ and is made of Teflon to minimize trace reactive gas sorption. Particles greater than about 2 ..mu..m are removed with a cyclone, which is followed in the system by a transition flow reactor (TFR). The TFR is followed by a three-filter holder containing, in order, a Teflon filter, a nylon filter, and an oxalic acid impregnated glass-fiber filter. The backup nylon and oxalic acid filters collect the gaseous HNO/sub 3/ and NH/sub 3/ that penetrated the TFR and that from the decomposition of the NH/sub 4/NO/sub 3/ collected on the Teflon filter. The final section of the system contains two glass-fiber filters impregnated with triethanolamine for SO/sub 2/ and NO/sub 2/ collection. Results from both laboratory evaluation and field studies are presented. In 7-week-long studies, the average difference between samples from parallel runs for gaseous NHO/sub 3/ was 4.6% with a standard deviation of 3.7. 9 references, 2 figures,more » 10 tables.« less