Jun 29, 2017 - Provided is a gas sensor which is capable of preferably sensing an ammonia gas, and has excellent durability. A mixed-potential gas sensor. I want to calculate how much ammonia evaporates from a 10% ammonia solution (ammonium hydroxide solution), to check if the concentration of ammonia in the air exceeds 25 ppm. The temperature is 40°C.
The measurement of ammonia is critically important in applications such as wastewater, where treatment plants are required to provide a laboratory measurement of ammonia concentration. The ammonia gas-sensing ion selective electrode (ISE) is an of determining ammonia concentration for compliance reporting.Ammonia is commonly measured in wastewater applications. Ammonia ISE Measurement PrincipleAn ammonia gas-sensing ISE (e.g.
YSI ) has two main structures - a pH electrode and a gas permeable membrane module. A unique ionic strength adjustor (ISA) is used with the ammonia ISE that buffers the sample to a pH greater than 11, ultimately causing ammonia in the sample to become gaseous.The pH electrode fits inside the membrane module and the sensing end of the electrode is immersed in internal fill solution. Ammonia gas in the sample will pass through the permeable membrane, resulting in a pH shift within the internal fill solution. This pH shift is detected by the pH electrode and can be correlated to the concentration of ammonia in the sample based on the result of calibration.The YSI TruLine Ammonia ISE. A pH electrode is placed inside an outer membrane module that is only permeable to ammonia gas.
Setting Up the Ammonia ISETo set up the ammonia ISE, first remove the soaking cap from the pH electrode and set aside for use when storing the electrode long-term. This soaking cap contains pH 4 buffer and some potassium chloride (KCl).Carefully fill the membrane module with 15 to 20 drops of fill solution, then tap the membrane module to ensure no bubbles are present behind the membrane.Slide the membrane module over the pH electrode until the top of the module and the top of the electrode body line up.Fit the cap over the top of the module and carefully screw tight.
Be careful to not rupture the membrane.Condition the assembled ammonia ISE by soaking in a standard solution for at least 15 minutes prior to use. If completing a two point calibration (e.g. 1 and 10 mg/L) use the low standard (e.g. 1 mg/L) solution for soaking.Note: Although not required, it is best to allow the inner pH electrode to condition within the filled membrane module for at least 2 hours prior to use. The assembled electrode can be placed in a low to mid-range standard during this time.
Calibrating the Ammonia ISEDetermination of effective ion concentration with ion selective electrodes is very technique sensitive. Although the ammonia ISE is prone to fewer interferences than other ISEs, a great deal of care must be taken when calibrating in order to obtain accurate and repeatable results.
Step #1: Connect the ammonia ISEConnect the ammonia electrode to a meter that has a BNC input and can directly display ion concentration (e.g. Display in mg/L), such as the YSI, or the. Ensure the instrument has been set-up to measure ammonia.The MultiLab 4010-3 can be used to measure dissolved oxygen, pH, and ammonia. Step #2: Connect a temperature sensorConnect a temperature sensor to the instrument, as measuring effective ion concentration is dependent upon temperature. The standard solutions should have a temperature as close as possible to the expected sample.The YSI ammonia ISE does not have an integrated temperature sensor, but external temperature sensors are available (YSI and ).Alternatively, the “Alternative Temperature” function of the TruLab and MultiLab can be used to apply a temperature reading from another sensor that is connected to the instrument (e.g.
Use the temperature reading from a MultiLab IDS pH sensor).YSI ScienceLine Temperature Sensors Step #3: Prepare standardsStandards should bracket the expected sample range. For example, if your expected sample range is 5 mg/L, at least one standard must have a lower concentration and one must have a higher concentration.There should at least be a tenfold (i.e. Decade) difference in concentration between the high and low standards (e.g. 10 mg/L and 100 mg/L, not 10 mg/L and 50 mg/L).At least 2 standards must be used, but the YSI TruLab 1320 and MultiLab 4010-2/3 can accept up to 7 calibration points.If your standards span more than one decade (e.g.
1 mg/L and 100 mg/L), it is best to prepare at least one mid-range standard (e.g. 10 mg/L).Each standard should have a volume of 100 mL and should be placed in a 150 mL glass beaker.Standards should be fresh and prepared very carefully. It is best to use a pipette when measuring small volumes of stock when preparing diluted standards. Step #4: Place electrode in solution and stirPlace the electrode in the lowest concentration standard and stir at a constant rate using a stir bar and stir plate. The stirring speed should be limited to minimize loss of ammonia gas. Use the same stirring rate when calibrating and measuring samples.If you do not have a stir plate, it is best to swirl the solution or use a stir rod once ISA is added.
Step #5: Add ISA and begin calibrationAdd 2 mL of ammonia ISA to the standard. After adding ISA, the solution should have a pale blue color, indicating the pH is 11.Allow the solution to stir for 1 minute and begin calibration. Calibration is time sensitive, as ammonia in the sample is no longer sufficient 4 minutes after ISA is added. Therefore, ensure calibration with the standard is complete 1 to 4 minutes after ISA is added.
This timing is critical! If calibration using the standard is unsuccessful within this time limit, a fresh standard will need to be prepared.Note: The MultiLab and TruLab have an Auto-Read feature that is used during calibration. If the instrument has not determined the reading is stable, but you are confident the reading is stable, the key can be pressed to accept the point and move on to the next.Ideal set-up for calibration. A stir plate, 150 mL glass beaker, temperature sensor (e.g. From pH electrode), ISE instrument, and ISA are all used.
Step #6: Calibrate with additional standardsOnce the instrument has accepted the first calibration point, finish calibrating using steps #4 and #5 for the remaining calibration points. Make sure to calibrate in order of increasing concentration.If calibrating with less than 7 standards, you can finish calibration by pressing the key after calibrating with your highest concentration standard. A complete calibration record will be displayed.If calibrating with 7 standards, the TruLab and MultiLab will display a complete calibration record after calibrating with the last standard. Step #7: Evaluate electrode slopeAfter calibrating, evaluate the electrode slope on the calibration record. For highest accuracy, the ammonia electrode slope should be between -53 mV/decade and -65 mV/decade.If the electrode slope is out of this range, attempt to recalibrate. Ensure your standards have carefully been prepared, ISA was used, and calibration to each point was completed 1 to 4 minutes after ISA was added.If the electrode will not calibrate, you have ensured the electrode is correctly assembled, and you are confident your procedure is correct, attempt to clean the pH electrode and/or change the membrane module (see Maintenance section).
Step #8: Recalibrate oftenThe ammonia electrode should be calibrated at the beginning of each day.Verify your calibration result every 2 hours by preparing a fresh low to mid-range standard, adding ISA, and verifying the reading. If the mV reading has changed 3 mV compared to the reading in that standard during calibration, you will need to recalibrate the electrode. Taking a MeasurementIt is critically important that samples be prepared using the same procedure used for standards. Therefore, follow steps #3-5 of the calibration procedure when preparing samples and taking a measurement.In summary, 100 mL of each sample should be collected and 2 mL of ISA should be added. The sample should be fresh and the same stirring rate used during calibration should be utilized.
A measurement can be determined 1 to 4 minutes after ISA is added. The Auto-Read function of the TruLab and MultiLab can be used to ensure the measurement is stable.
TroubleshootingThe following tips may prove useful if you are experiencing issues with the ammonia ISE such as unacceptable calibration results or poor electrode response. Refresh the internal fill solutionIf the internal fill solution does not shift back to its original pH before another sample is taken, the next pH shift is not completely attributed to the new sample or standard. Therefore, if you are having issues with poor slope or drifting readings, the electrode fill solution may need to be refreshed.To ensure the internal fill solution pH shifts back to its original pH, you may need to:. Manually refresh the fill solution that is between the tip of the pH electrode and the membrane.
This can be done by holding the electrode body sensor side down, pulling the cable back and away from the body/membrane slightly (it is spring loaded to cause the glass to seat up to the inside of the membrane) and then slowly releasing the cable. This pulls the glass away from the membrane, causing the internal fill solution to refresh quickly in the interface area. Allow more time between measurements for the pH of the internal fill solution to return to normal.
Shaking out the old, and refilling the membrane with new internal fill solution every few days might also be useful.The internal fill solution between the membrane and the tip of the pH sensor can be refreshed by slightly pulling the cable away from the assembled electrode. A spring in the electrode cap allows for this. Check for a tear in the membraneTo check for a tear in the membrane, place the fully assembled electrode in pH 4 buffer. Since the membrane is only gas permeable, there should be no aqueous ion transfer between the inside and outside of the membrane. Therefore, if the mV reading on the instrument display changes drastically once the electrode is placed in pH 4 buffer, there is a potential leak or tear in the membrane module. MaintenanceIf the electrode has a poor response, you may attempt to clean the pH sensor much like other pH electrodes.
Avoid rubbing or scratching the glass sensor.It may also prove useful to condition the pH electrode by placing it in pH 4 buffer with some KCl added. Ensure the reference (i.e.
The white layer where the glass changes in dimension) is completely immersed.The membrane module will also need to be changed if it is excessively dirty or stretched. These modules can last a long time in clean samples.
However, in dirty samples like wastewater where NH3 measurements are common, they may have a much shorter life.Membrane modules need to be replaced when the electrode no longer calibrates or responds. The module will eventually develop pinholes or tears (likely from mashing the membrane into solids at the bottom of the beaker!), get stretched, or the membrane will become fouled (surface coating).You may attempt to clean a membrane module that has surface contamination by soaking it in DI water for 1 hour or more, then soak in a low or mid-range standard for at least 15 minutes prior to use. Electrode StorageProperly storing the ammonia ISE will help ensure good electrode response and repeatable results.
Short-term storageBetween measurements, the ammonia ISE should be stored in a low concentration standard, such as 1 or 10 mg/L, with ISA added.For overnight or weekend storage, place the assembled electrode in 1000 mg/L standard and do not add any ISA.These short-term storage conditions will keep the assembled electrode stable for a comparably long period of time. Do not remove the membrane module for short term storage, as this can stretch the membrane, ultimately causing the electrode to be less responsive. Long-term storageFor long-term storage, disassemble the electrode and rinse the module and pH electrode with DI water. Carefully dry all parts of the electrode.
Place the pH electrode inside the included soaker bottle with pH 4 buffer and KCl added as a storage solution. The membrane module can be stored dry. Common Questions - FAQsQuestion: My electrode from another manufacturer was shipped dry. Is this OK?Answer: YSI never recommends storing the electrode dry.
In fact, no pH electrode should ever be stored dry!The storage solution the TruLine Ammonia ISE is shipped in contains the proper balance of moisture and salts in order to prevent leaching of reference electrolyte and the active ingredients of the glass membrane. If you are storing the electrode long-term, be sure to place the pH electrode inside the included soaker bottle with pH 4 buffer and KCl added as a storage solution.
If you do not have KCl, pH 4 buffer will suffice.Question: Do I need to recalibrate after using the spring-loaded cable to refresh the fill solution?Answer: The internal fill solution (IFS) is homogenous, but an air bubble or other issues may change the tiny volume between the membrane and the tip of the pH electrode (i.e. The interface where the pH shift occurs). If the user is very consistent in their preparation (e.g. Consistent amount of IFS in the interface) and use of the electrode, recalibration is not needed. However, we recommend recalibration if the user is relatively new to the ammonia ISE.Question:If measuring at low concentrations, is it OK to wait longer than 4 minutes after adding ISA to calibrate or measure?Answer: It is not possible to generate more ammonia gas than what is in solution, so there is no reason to wait longer.
Ammonia will still become gaseous at the same rate regardless of concentration.If the user waits longer than 4 minutes, ammonia will start to be depleted and the calibration/measurement result will be negatively impacted.If you are measuring in low level samples and having difficulty, consider using multiple samples in low level measurement applications for highest accuracy. In addition, some customers have had success using Standard Method 4500-NH 3 E (i.e. The known addition method) rather than Standard Method 4500-NH 3 D (i.e. The direct calibration method) when calibrating.They key to success with any gas-sensing ammonia electrode is to have a good, consistent technique!Question:Is it best to insert the ammonia ISE at an angle so air bubbles don’t get trapped on the membrane?Answer: The consistent presence of a bubble on the surface of the membrane is not a concern, as ammonia gas can easily permeate the bubble and then the membrane. In addition, inserting the ammonia ISE into the solution at an angle is not feasible on many electrode holders.Although the consistent presence of a bubble is not a concern, bubbles sporadically moving across the membrane should be prevented. This sporadic movement of bubbles across the surface of the membrane should not occur as long as the membrane is all the way in solution and the stir plate speed is not turned up too high.Question: How often should I replace the membrane and/or the internal fill solution (IFS)?Answer: There is no general guideline for this, as the replacement interval simply depends how often the ammonia ISE is used, the samples measured (are they dirty?!), and the storage practices used.
Some customers that use the ammonia ISE every day choose to replace the IFS once a week and the membrane once a month, while other customers only change the membrane and IFS when needed.If experiencing issues, YSI recommends recalibrating first. If that does not resolve the issue, change the IFS. If issues persist, change the membrane. The internal pH electrode should be the last component replaced.Indications that a membrane or IFS change is needed include a slow electrode response, the calibration slope is outside the recommended range, and/or the cable pull technique no longer helps.Question: How often should I replace the internal electrode?Answer: The internal pH electrode is just like any other pH electrode, so you should expect a similar usable life. Also like other pH electrodes, proper storage of the pH electrode is critical! A pH electrode should never be stored dry or in deionized (DI) water.Customers that use the electrode extensively should not have to replace their electrode any more than once a year. Some customers report getting at least two years of life out of their electrode.
In order to maintain consistency, some customers choose to replace their electrode once a year.For coninuous monitoring and control instrumentation ISE information you may be interested in the YSI White Paper:Additional Blog Posts of Interest.
Atmospheric ammonia (NH 3) has great environmental implications due to its important role in ecosystem and global nitrogen cycle, as well as contribution to secondary particle formation. Here, we report long-term continuous measurements of NH 3 at different locations (i.e. Urban, industrial and rural) in Shanghai, China, which provide an unprecedented portrait of temporal and spatial characteristics of atmospheric NH 3 in and around this megacity. In addition to point emission sources, air masses originated from or that have passed over ammonia rich areas, e.g. Rural and industrial sites, increase the observed NH 3 concentrations inside the urban area of Shanghai.
Remarkable high-frequency NH 3 variations were measured at the industrial site, indicating instantaneous nearby industrial emission peaks. Additionally, we observed strong positive exponential correlations between NH 4 +/(NH 4 NH 3) and sulfate-nitrate-ammonium (SNA) aerosols, PM 2.5 mass concentrations, implying a considerable contribution of gas-to-particle conversion of ammonia to SNA aerosol formation. Lower temperature and higher humidity conditions were found to favor the conversion of gaseous ammonia to particle ammonium, particularly in autumn. Although NH 3 is currently not included in China’s emission control policies of air pollution precursors, our results highlight the urgency and importance of monitoring gaseous ammonia and improving its emission inventory in and around Shanghai. Atmospheric ammonia (NH 3) has long been recognized as the key important air pollutant contributing to eutrophication and acidification of ecosystems,. More recently, it has been shown that NH 3 plays a primary role in the formation of secondary particulate matter by reacting with the acidic species, e.g.
SO 2, NO x, to form ammonium-containing aerosols, which constitute the major fraction of PM 2.5 aerosols in the atmosphere. Particulate ammonium species contribute to the degradation of air quality and visibility, as well as to the atmospheric radiative balance,. Anthropogenic ammonia emissions originate mainly from agriculture activities including soils, fertilizers and domesticated animals waste, although industrial and traffic emissions are also important ammonia sources in urban areas,.In China, the total NH 3 emission was estimated to be 13.6 Tg for 2000, of which 50% comes from fertilizer applications and another 38% from other agricultural sources. In recent years, other estimates of NH 3 emissions in China have reported different values with a considerable degree of uncertainty, e.g. 16.55 Tg for 2005, 16.07 Tg for 2006, 9.6 Tg for 2006.
Nevertheless, Wang et al. Studied the change of sulfate-nitrate-ammonium (SNA) aerosols over China from 2000 to 2015 by chemical transport modeling, indicating that NH 3 is an essential control on SNA and fine particles pollution. To better understand sources, sinks and impacts of ammonia on atmospheric chemistry and ecosystems, it is critical to conduct widespread and representative measurements of ambient ammonia concentrations. Unfortunately, NH 3 is so far not included as a species of routine monitoring and National Ambient Air Quality Standards (NAAQS, GB3095–2012) in China. Furthermore, only few measurements and studies on atmospheric ammonia have so far been reported, especially about long-term continuous and high temporal resolution observations.With a residential population over 24 million in a 6340.5 km 2 area, Shanghai, located on the western coast of the North Pacific Ocean and at the east front of the Yangtze River Delta (YRD), China, is one of megacities in the world.
In the past decade, the air quality in Shanghai has degraded dramatically. Haze pollution is frequently observed in Shanghai, especially during the cold winter and spring, which presents a great challenge for environmental management and scientific research. This is mainly due to excessive particulate matter from anthropogenic sources and gas-to-particle transformation, and therefore is closely related to meteorological factors and atmospheric emissions.
Previous studies reported that increased NH 3 concentrations favored the formation of sulfate and nitrate aerosols and have a large impact on the visibility degradation in Shanghai,. With rapid economic growth, the number of vehicles registered in Shanghai has been almost tripled to 2.35 million during the last decade.
Thus, the ambient NH 3 emissions from traffic sources need to be investigated in the Shanghai urban area, along with the agricultural sources in the surrounding rural environment, which includes more than 0.37 million hectares of sown planting areas and varieties of livestock cultivation.To determine the atmospheric ammonia concentrations and temporal variations in three locations related to different ammonia sources, long-term field observations of NH 3 have been performed at downtown, industrial and rural sites in Shanghai. These are the first continuous and high temporal resolution NH 3 measurements in Shanghai. Characteristics of temporal and spatial ammonia distributions among different sites are compared and discussed together with information about emission source, air temperature and regional air transport. By exploring the inorganic water-soluble ions and ammonia, the gas-to-particle phase partitioning revealed the important role of NH 3 concentration evolution, and its conversion rate to ammonium, in ambient fine particle levels in Shanghai.
These results are relevant for our understanding of precursor ammonia distributions, and its role in the serious aerosol pollution problem in China, and further provide benchmarks to assist in meeting air quality goals and policy needs. Overall view of the measurement sites in different areas of Shanghai, China (Figure created by the authors using MapInfo Professional 7.0).During the observation period from 1 July, 2013 to 30 September 30, 2014 at FDU site, the hourly averaged NH 3 concentrations varied widely from the minimum around the detection limit about 1 ppb to the maximum of 54.5 ppb with an average of 6.2 ± 4.6 ppb and a median of 4.6 ppb.
As listed in, and compared to recent studies, the NH 3 levels at the Shanghai urban area are lower than those reported in other Asian cities such as Beijing (China), Kampur (India), Seoul (Korea) and Labore (Pakistan), but higher than urban sites in European and North American countries,. 5 CIMS, Chemical Ionization Mass SpectrometerThe rural ambient ammonia was sampled by the MARGA instrument at DSL site from 1 July 2013 to 30 June 2014 (except for January and February, 2014). NH 3 hourly concentrations averaged 12.4 ± 9.1 ppb with a peak of 79.4 ppb (08:00–09:00 on 5 August, 2013), which is comparable to other rural sites in China and worldwide listed in,.
At JSP site, NH 3 hourly concentrations showed an averaged concentration of 17.6 ± 9.5 ppb, with a concentration peak of 279.3 ppb (, 00:55 LT) and a highest hourly average of 84.9 ppb (, 20:00 21:00). It was also found that the NH 3 concentration changed dramatically within the same day probably as a result of the strong influence of variable industrial emissions in the vicinity. This shows the occurrence of instantaneous intensive exhausts of industrial ammonia-containing gases without treatment or with low efficient purification.Combining the simultaneous observations, hourly averaged NH 3 concentrations at the three sites were compared from 1 March to 30 June 2014. The results show that the average atmospheric NH 3 levels in different locations of Shanghai generally are in the following sequence: industrial (19.6 ± 8.2 ppb) rural (10.4 ± 5.0 ppb) urban (5.4 ± 3.3 ppb). The ratio of NH 3 concentrations at JSP to DSL and FDU is 2 3 and 4 5, respectively. Therefore, it can be concluded that fleeting intensive ammonia exhausts from industry have strong effects on the ambient NH 3 levels.
At the rural site, NH 3 variations were controlled by the volatilization from agricultural non-point sources. Despite traffic emissions, the measured ambient NH 3 at the downtown location is the lowest of the three sites. This suggests that ammonia emissions from vehicles in Shanghai were much less in magnitude than those from chemical industry or agricultural related fertilizer application, livestock wastes, compost, etc.Before comparing the NH 3 data measured by distinct instruments, it is worth to mention that the inevitable discrepancies were mainly due to the different measuring principles. Herein, the DOAS data was the averaged concentration along the optical path whereas the MARGA result was the point concentration close to the sampling inlet. Another potential bias was introduced by the sampling heights since the ambient NH 3 was generally found to vary with altitude. As shown in, the additional side-by-side measurements demonstrate the inter-comparability between DOAS and MARGA techniques, which are reasonable and acceptable to be used among sites in this paper. Diurnal variations of NH 3 concentrations at Shanghai urban, rural and industrial areas for week-day/-end and different seasons.In the urban area, the diurnal NH 3 concentration peak was about 7.1 ppb at 07:00 08:00 local time, while it dropped down to a minimum of 5.4 ppb after noontime.
The diurnal cycle of NH 3 levels in this urban area is dependent on the traffic emissions nearby and the evolution of the atmospheric boundary layer. Because of the implementation of three-way catalytic converters for the control of nitrogen oxide pollutants exhaust, traffic emissions have become a significant contributor to ambient NH 3 levels in the urban atmosphere,. Associated to the increasing dispersion and dilution in the mixing layer, surface NH 3 concentration decreased from morning peak until afternoon and kept in stable at night. In this study, we observe a reduction of about 15% in the NH 3 concentrations over weekend compared to weekdays, associated to the decrease in traffic volume. The seasonal NH 3 evolution showed the highest NH 3 levels in summer.
The typical double-peak diurnal shape, related to the vehicle emissions, were also much more pronounced in summer, confirming the primary role of traffic emissions in controlling ammonia levels in the urban atmosphere. However, the weekly cycle seems not as obvious as expected: the maximum daily average on Thursday is only 0.3 ppb higher than the minimum on Friday, with no drop over the weekend.In contrast to observations at the urban area, the diurnal cycle of NH 3 concentration at the rural site showed a single peak about 14.9 ppb at 09:00 10:00 LT, due to the impacts of agricultural sources.
It was also observed that the diurnal peaks in summer and weekdays appeared earlier than those at weekends and other seasons, which may be partly explained by i) agricultural activities that are usually performed during morning hours of weekdays and even earlier in summer, based on the local customs, ii) and by the fact that the atmosphere is also heating up earlier in summer compared to other seasons. Besides, the differences of diurnal cycle in seasons may hint that NH 3 diurnal patterns were also influenced by human agricultural activities and other potential photochemical processes releasing ammonia-containing substances from soil. Overall, the levels of NH 3 at the Shanghai rural area are impacted by temperature and the resulting enhanced ammonia volatilization from agricultural sources.Because of the variable industrial exhaust, no diurnal pattern like bimodal or single peak was observed at JSP site and its fluctuation seems to be irregular and disorder. Nevertheless, the JSP site showed the “weekend effect” with lower (about 10%) levels during the weekend, following the work schedules of factories in the industrial park. Much higher NH 3 levels at JSP site in winter than FDU and DSL sites also indicated the strong impacts of industrial emissions during this time of year.The monthly NH 3 averages in the urban area showed higher concentrations in summer (JJA), 9.1 ± 4.7 ppb, than in winter (DJF), 5.0 ± 3.2 ppb, as shown in. The monthly averages peaked at 11.2 ± 3.9 ppb in July and declined to 3.4 ± 2.8 ppb in February. In summer, the volatilization of fertilized soils, poultry and livestock waste, as well as human excretion were greatly enhanced by the persistence of high temperature, while the stability of ammonium aerosols was reduced.
Moreover, it is worth noting that NH 3 concentrations in November and December 2013 were exceeding 7.0 ppb, during which particle pollution episodes occurred frequently in Shanghai, e.g. Daily PM 2.5 concentrations exceeding the 24-h threshold of NAAQS (limit level II of 75 μg m −3) were measured in 36 days within this two-month period. This observation emphasizes the important role of ammonia in the formation of secondary sulfate-nitrate-ammonium aerosols, which should be further explored to solve the current air pollution problems in Chinese megacities,. Monthly averaged NH 3 concentrations at different locations of Shanghai, ambient temperature and precipitation from July 2013 to September 2014.The seasonal trends of NH 3 at the rural site also exhibited higher levels in summer about 20.0 ± 10.4 ppb. The highest monthly NH 3 average of 30.5 ± 9.8 ppb in July is four times higher than in December , which is in agreement with the seasonal pattern reported in other Chinese rural areas,.
As part of the seedling, transplanting and tasseling activities during waterlogged rice cultivation, fertilizer was applied in larger amounts and higher frequency, from June to August in Shanghai. Accordingly, ammonia emissions from cropland have evident seasonal features. Thus, high temperature in summer (see ) elevates the decomposition of N fertilizer and ammonia volatilization at the rural site, whereas low NH 3 levels in winter are caused by both reduced volatilization owing to rare fertilization activities and low temperature.Similar to the urban and rural areas, the monthly averages at JSP are higher in summer and lower in winter. During the month with the lowest concentrations, February 2014, the diurnal evolution of NH 3 concentrations changed moderately between 6.8 and 10.7 ppb due to a decline in industrial activity, coincident with the Chinese New Year holidays and typical lower temperature of this time of year.Besides all the impact factors mentioned above, the diurnal and seasonal patterns of ambient NH 3 levels result from a complex interplay between emission and other processes, e.g. Dry deposition and wet removal. The dry deposition velocity of atmospheric ammonia was higher in cool and wet seasons (autumn-winter) than warm and dry weather (spring-summer).
Due to more precipitation, the wet removal effect on gaseous ammonia can be found in April 2014 in, during which NH 3 concentrations at three sites were lower than March even though the temperature was higher. Both of dry and wet depositions play an important role in regulating the ambient NH 3 concentration.
Impacts of temperature and air mass transportAlthough the three sites are far apart from each other and are representative for different ammonia emissions, consistent trends in NH 3 concentrations among different places were, to some extent, observed synchronously (see ). Therefore, we next explore the impacts of temperature and air mass transport on ambient NH 3 levels. Observed concentrations of NH 3 at different sites present a positive correlation with air temperature. For instance, significant linear correlations were found between daily NH 3 concentrations and ambient temperature, i.e. (a) R 2 = 0.5798 for FDU site, (b) R 2 = 0.7967 for DSL, and (c) R 2 = 0.8524 for JSP.
It is obviously that the ambient temperature was a common key parameter in determining atmospheric NH 3 levels in all measurement sites (see ). The closer correlations found at the DSL and JSP sites are driven by the temperature-favored volatilization of stronger agricultural and industrial NH 3 emission sources.Additionally, back trajectory analysis is used to assess the impact of long-range transport on the spatial distribution of ground-based NH 3 levels observed at FDU site. In total, 489 48-h back trajectories were classified into 6 clusters via the HYSPLIT cluster analysis. Shows the mean trajectory for each cluster and its percentage to total trajectories together with the averaged NH 3 concentrations (details in ). Clusters 1, 2 and 5 represent air masses transported from clean ocean regions, whereas clusters 3, 4 and 6 passed through the continental area before arrival to FDU. NH 3 concentrations at FDU showed higher concentrations under the influence of clusters 3, 4 and 6, indicating an impact of polluted air mass transport on ground NH 3 levels. For cluster 4, the air mass originated in the western inner continent and moved slowly, which is expected to bring ammonia rich air to the receptor site, resulting in NH 3 levels of 8.1 ± 3.7 ppb.
By contrast, the lowest averaged NH 3 concentrations were measured under cluster 5, with air masses arriving from the East China Sea area. Considering the comparison of three measurement sites in this study, we conclude that air masses originated from or passed over ammonia rich areas, i.e.
In the south (JSP) and west (DSL) directions, increased the NH 3 concentrations at the downwind FDU site. Cluster analysis of 48 h backward trajectories at the FDU site from March to June 2014 (Image created using HYSPLIT-4 model obtained from the NOAA Air Resources Laboratory.Available at: ).The potential regional impacts of ammonia-rich air mass transport highlight the need to control and reduce agricultural and industrial ammonia emissions in Shanghai.
Note that in current ammonia emission inventories, the NH 3 emissions from livestock feeding and N-fertilizer application account for more than 85% of the total in Shanghai. The annual application of synthetic N for typical double-cropping systems has been reported to range from 550 to 600 kg of N per hectare in eastern China, however, the N use efficiency is indeed low (below 30%) in recent years, and about 15% resulted in ammonia volatilization. Therefore, an effective way to reduce the agricultural ammonia emission involves decreasing the application of synthetic N-fertilizer and elevating the N use efficiency. In current emission inventories, the industrial emission is thought to account for less than 5% of the total. However, according to our measurements the air masses containing extremely high ammonia concentration were detected at the optical path of DOAS instrument in industry area, and therefore we suggest that the industrial ammonia emission inventory needs to be further developed and improved.
Contribution of ammonia to aerosol pollutionIn the YRD region, previous studies have reported a contribution of ammonia to PM 2.5 concentration of 8 11%, comparable to the contribution of SO 2 (9 11%) and NO x (5 11%) emissions. Ammonia reacts rapidly with both sulfuric and nitric acid to form fine particles, and was observed to participate in the nucleation during new particle formation events in Shanghai. Therefore, traces gases including HCl, HONO, HNO 3, SO 2, NH 3 and water-soluble ions in PM 2.5 concentrations measured by MARGA at the DSL site are here used to investigate the contribution of ammonia to aerosol pollution from 23 to 31 October 2013. Shows the time series of NH 3, PM 2.5, SNA concentrations, and ammonia gas fraction (AGF = NH 3/(NH 3 + NH 4 +)) at DSL, as well as the meteorological parameters including wind direction and speed, ambient temperature and relative humidity. Time series of concentration of NH 3, SNA, PM 2.5, meteorological conditions and AGF (NH 3/NH 4 + + NH 3) at DSL site from October 23 to 31, 2013.As mentioned above, the high ammonia period during October 27 to 31, 2013 occurred under the influence of south/southeastern winds, which have traversed ammonia rich areas. Ammonium, the main water-soluble cation, forms from reaction of ammonia with acidic species in the atmosphere, and thus is correlated with sulfate and/or nitrate, as well as with PM 2.5 concentration.
Ammonium accounted on average for 22% mass fraction of SNA aerosols and 10% of the mass concentration of fine particles. In addition, the ammonia gas fraction follows the PM 2.5 concentration, indicating the favorable role of ammonia conversion from gas to particle phase in the PM 2.5 formation. Therefore, ammonia, the primary alkaline gas, plays a significant role in the neutralization of acid species to form secondary SNA aerosols and fine particles pollution at the DSL site.Here, the conversion rate of ammonia to ammonium, described by the ratio of ammonium to total ammonia NH x ( = NH 3 + NH 4 +), is used to investigate the relationship between NH 4 +/NH x and atmospheric ammonia and PM 2.5 concentrations. In accordance with the definition of NH 4 +/NH x, the particle fraction of NH 4 in NH x, which is reciprocal to the AGF, was inversely proportional to the ambient ammonia concentrations, reflecting the inter-conversion of NH x between gas and particle phases in the atmosphere, e.g. Higher NH 4 +/NH x occurred under lower ammonia concentration and vice versa. The high conversion rates of ammonium from gaseous to particle phase significantly promoted the formation of SNA and PM 2.5 aerosols, exhibiting the following exponential correlation coefficients R 2 = 0.4584 and R 2 = 0.6502, respectively.
This suggests that the increase in fine particles concentration was facilitated by the converted ammonium from ammonia reactions with acidic species. Relationship between the conversion rate of ammonia to ammonium (NH 4 +/NH x) and (a) atmospheric ammonia, (b) SNA and PM 2.5 concentrations at the DSL site from 23 to 31 October 2013.During the secondary aerosol formation, ammonia is thought to be neutralized first by sulfuric acid. Afterwards, the excess of NH 3 reacts with the nitric and hydrochloric acids to form NH 4NO 3 and NH 4Cl.
The relationship between ammonium and acidic species in PM 2.5 was investigated by regression analysis , where ns-NH 4 + is the non-sulfate ammonium. The results show that the regression slope of equivalent concentration (μeq m −3) of sulfate to ammonium is close to 0.5, which means the acidic sulfate in particles are likely neutralized by ammonium to form (NH 4) 2SO 4. The excess of NH 4 +, calculated by NH 4 +-2 × SO 4 2− in units of μmol m −3, likely reacted with NO 3 − and Cl − to form NH 4NO 3 and NH 4Cl.
This is shown by the higher correlation coefficient between ammonium and nitrate, and the sum of the nitrite and chloride, suggesting that ammonium-rich conditions are necessary for complete neutralization. Besides, a linear correlation between ns-NH 4 + and NO 3 −, and NO 3 − + Cl −, shows that acidic species were neutralized by ammonium at the same time.However, NH 4NO 3 and NH 4Cl are thermodynamically unstable, co-existing in the reversible phase equilibrium with the gaseous precursors HNO 3, HCl and NH 3, which depends on temperature and relative humidity. During the period from 23 to 31 October 2013, the ambient temperature ranged from 10 oC to 25 oC, favoring the stability of NH 4NO 3. Thus, good agreement between NO 3 − with NH 4 + and ns-NH 4 + was observed in autumn Shanghai, under ammonium-rich conditions. If the ambient relative humidity is less than the deliquescence relative humidity (DRH), indicated by the red line in, the equilibrium state of NH 4NO 3 would co-exist between both solid and gas phases.
The equilibrium relationship between gaseous NH 3, HNO 3 and particle NH 4NO 3 were estimated by the concentration product calculated from measured data ( K m) and compared with the theoretical equilibrium dissociation constant K p for pure NH 4NO 3 aerosol. The dependence of ratio K m/ K p on temperature and humidity indicates that the theoretical equilibrium dissociation constant is more likely higher than the product of measured HNO 3.NH 3 under unfavorable conditions of high temperature and low relative humidity.
This is caused by the equilibrium shift from the particle phase of NH 4NO 3 to gaseous products at high temperature and low humididy,. It is also important to note that there are considerable restrictions on the discussion using the ratio K m/ K p as an indicator of a potential for gas-to-particle conversion.
The impact of meteorological parameters on the gas-to-particle phase of NH x is also reflected on the high ratio of NH 4 +/NH x at low temperature and high relative humidity,. Therefore, it can be concluded that gaseous ammonia emitted to the atmosphere acts as major contributor to fine particle formation in Shanghai by reacting with acidic species to form ammonium under conditions of low temperature and high relative humidity.
ConclusionsLong-term measurements of ammonia concentrations were carried out at three different sites typical of urban, rural and industrial areas of Shanghai. The hourly NH 3 concentration at the urban site ranged from detection limit to 54.5 ppb and averaged at 6.2 ± 4.6 ppb. The diurnal concentration profile of NH 3 in the urban atmosphere showed a typical bimodal cycle, around 06:00 08:00 and 18:00 19:00, driven by the traffic emissions and the evolution of the atmospheric boundary layer.
By contrary, atmospheric NH 3 at the rural site shows a single peak of 14.9 ppb in the late morning, primarily due to the temperature-favored volatilization from agricultural emissions. The diurnal NH 3 fluctuated irregularly and no bimodal or single peak was observed in industrial area because of the variability of large industrial emission pulses that occurred mainly during the night. Therefore, administrative management of industrial NH 3 emissions and corresponding improvements on the NH 3 monitoring program in Shanghai are necessary.The three sites showed higher NH 3 levels in summer than in winter. Besides individual emission sources, the ambient temperature was the common determinant parameter of atmospheric NH 3 levels as indicated by the significant linear correlations between daily NH 3 concentrations and temperature at the different locations. Besides, other processes, e.g. Dry and wet depositions, atmospheric dispersion and dilution, as well as gas-to- particle conversion, play important role in driving the NH 3 diurnal and seasonal patterns.
Simultaneous observations at three sites from March to June, 2014 show average concentrations varying as industrial (19.6 ± 8.2 ppb) rural (10.4 ± 5.0 ppb) urban (5.4 ± 3.3 ppb), which further highlights the importance of monitoring and management of industrial ammonia emissions in Shanghai. Analysis of air mass backward trajectories implied that the air mass transport from different source areas constitutes an additional contribution, besides traffic emissions, to the NH 3 levels observed at the downwind urban site.We show that fine particle pollution in Shanghai is to some considerable degree associated to the conversion of ammonia to particle phase. In the urban area, frequent particle pollution episodes occurred in November and December accompanied with monthly averaged NH 3 mixing ratios higher than 7 ppb. Besides, the case study in the rural atmosphere also suggests that the reactions of ammonia with acidic species to form ammonium contributed significantly to the SNA aerosols.
We find that ammonium accounts for about 10% of the PM 2.5 mass concentration and its proportion in total NH x showed a strong positive correlation with the SNA, PM 2.5 levels and a negative correlation with NH 3 levels. This indicates that gas-to-particle conversion of ammonia played an important role in the secondary aerosol formation and hence contributes to local aerosol pollution in Shanghai. This all highlights the importance of monitoring ammonia emission sources in and around Shanghai. Field observations and instrumental setupIn the urban site, active Differential Optical Absorption Spectroscopy (DOAS) measurements of NH 3 were carried out from 1 July 2013 to 30 September 2014, on the campus of Fudan University.
Both the DOAS transmitting telescope that incorporates the light source and the receiving telescope were installed 20 m height above the ground. The light path between the transmitter and the receiver is 53 m. By collecting the light from an artificial light source, active DOAS measures the integrated concentration of atmospheric trace gases along the optical path, and yields the average trace gas concentration by dividing the integrated concentration by the length of the absorption path.At JSP site, the observation of atmospheric NH 3 concentrations was carried out by another DOAS system from 6 January to 30 June 2014. The transmitting and receiving telescope were designed within one unit, which were placed on the roof of one building in the Jinshan Fine Chemical Industry Park with an altitude of 10 m. To fold the beam back to the telescope, a retro-reflector was mounted at the other side on the roof with a distance of 36 m. Consequently, the light travels 72 m between the transmitting/receiving telescope and retro-reflector.These two DOAS systems were homemade basically with same design.
It consists of a telescope with diameter of 210 mm as transmitter and receiver, a 35 W Deuterium lamp as light source and a spectrograph. Calibration of the DOAS system was performed individually by inserting a cell with quartz glass windows into the optical path between the light source and the receiver assuming constant value of the product of concentration and distance. Besides, standard gases with different concentration were filled into the cell in sequence to calibrate the responses of corresponding differential optical absorption.In addition, an online Monitoring instrument for AeRosols and Gases (MARGA, Applikon Analytical B. Corp., Netherlands) has been applied to measure the concentration of NH 3 with hourly time resolution at the rural site of DSL (at 15 m) from 1 July to 30 December 2013 and 1March to 30 June 2014. The details and performance of MARGA have been described previously,. To verify the data quality and accuracy of inorganic water-soluble ions concentrations in PM 2.5, MARGA was calibrated using internal standard solution (LiBr) every week during the observation period.For the inter-comparability of different instruments, the DOAS system in JSP site was moved to DSL site for one week side-by-side measurement with MARGA in April 2015. As shown in, the results from these two principle methods were generally comparable.
The correlation coefficient of R 2 = 0.79 and biases between MARGA and DOAS method are reasonable and acceptable, considering the inter-comparisons of measured ammonia with different techniques reported by Norman et al. (R 2 ranged from 0.79–0.94) and von Bobrutzki et al. (R 2 ranged from 0.20–0.99). Spectral data collection and AnalysisThe DOAS spectra were recorded by a spectrograph (B&W TEK Inc. BRU741E-1024) with a spectral range of 185–450 nm, a spectral resolution of 0.75 nm FWHM (Full Width Half Maximum), and a 1024-pixel photodiode array as detector. The analog signal was digitized by a 16-bit digitizer and sent to a computer via USB interface.
The exposure time of each scan was adjusted automatically according to the light intensity. The average temporal resolution of the measurement was set to 3 min.The spectral analysis window selected for NH 3 retrieval was 200–215 nm.
The high-resolution absorption cross-sections of NH 3, NO and SO 2 were used in the spectral fitting analysis by the DOASIS software (IUP in Heidelberg University, Germany). The detection limit (3σ) is typically about 1 ppb for NH 3 for 3-min averages over a total light path of 53 m and 0.7 ppb for 72 m light path. This work was supported by National Natural Science Foundation of China (21277029, 41405117, 21477021), Science and Technology Commission of Shanghai Municipality (Grant: 12DJ1400100, 12DJ1400102), Shanghai Environmental Protection Bureau (Grant: 2013–76) and Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (Grant: FDLAP13002). The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website used in this publication.