I've been sampling rainwater since last week to study its pH values after a long, dry, and hazy period (~60 days no rain). But the results don't seem to make any sense.
Here is my experiment: On the first day of rain after a long drought, I sampled and measured the rainwater pH and surprisingly found that the pH reading is exceptionally high (pH 6.5). Four days later, it rains again and the pH this time is 4.38. On the fifth day, there is more rain and the pH is 4.26. Does anyone know why the pH of the 1st day of rain is less acidic? Thank you very much.
The average pH of rainwater is around 5.7 due to the presence of carbonic acid (carbon dioxide dissolves in water). But the pH of rainwater could vary based on atmospheric conditions. In particular, the sulfuric acid and nitric acid from atmosphere can dissolve in rainwater to lower its pH. These acids are formed in atmospheres from NOx and SO2, which enters atmosphere because of combustion of fossil fuel such as coal in addition to other natural processes (see http://www.epa.gov/acidrain/what/)
In summary, a pH lower than 5.7 could mean that the rainwater has more nitric and sulfuric acid. A pH higher than 5.7 (say 6.5) is also not uncommon. All depend on what are being dissolved in rainwater in that given day or the atmospheric conditions (dust, thunderstorm, antecedent climate etc.).
You are coolecting rainwater from roofs or you have some configuration in order to exclude all interferences.
What I am trying to say is that your sampling equipment may was dirty from dust particles or other. If this had happened to latter sampes and not only on the first one then I would exclude interferences from dirt on sampling equipment.
my suggestion is to discard your first measurement.
I want to present you another perspective of problem. If you have a long period without rain, then there is lot of dust in atmosphere. This dust very often is mainly composed by soil particles, particularly in ventilated areas with high calcium content. In this context, carsic dust can neutralize rain acidity. In a dry week on a carsic plain, pH of rain samples of the first minutes can get up to 8.0. This phenomenon depends of local characteristics of site and of the air composition where clouds are formed (local wind systems like breezes, convergence zones). I have monitored rain in a south coastal site, predominant wind from Northeast, and I have found acid rain only with rain from South, from sea. Another aspect to be considered is NH4+ concentration in rain, mainly from biogenic origin. Rain is a regional process and should be analyzed in combination with local air quality. Don’t hesitate to ask whether something of this have not been not clear for you.
My answer is "This pH could be normal".
Besides alkaline material incorporated in raindrops (soil dust) from contamination, another possible source of rain "alkalinity" may directly come from clouds. Fronts may be such that long range transported air masses can be incorporated into clouds. In Mediterranean sites this is very common since fronts include southerly air masses from Sahara containing calcium-rich particles. In this case, rains may show pH above 7. This effect can be easily seen by looking to evaporating rain drops which leaves reddish fine particles on surfaces. After the development of fronts, southerly air masses are replaced by westerly-northerly air masses and pH drops to a more natural acidic value.
You do not provide enough info about your samples. Is you sampler of "bulk" or "wet-only" type? In both the cases measure Ca-dissolved conc which usually indicate soil dust in the jar for bulk and rain washed-off for wet-only. To confirm the first possibility also look at ratio of dissolved/dispersed mass after sample membrane (0.45 mikrom pore size) filtration. If the ration is much smaller for first sample than dusty jar is the most probable reason for such a pH value.
Thank you very much for all your helpful comments and great ideas! I just run the rainwater analysis using ICP-MS, measuring dissolved ions and get the results. I think the results supported your explanation to a good extent. Here is the results:
1st day sample: 2nd day sample: 3rd day sample:
Ca: 1187.72 ppb Ca: 442.42 ppb Ca: 510.10 ppb
K: 329.67 ppb K: 80.69 ppb K: 42.34 ppb
Sr: 3.61 ppb Sr: 1.11 ppb Sr: 0.45 ppb
I will validate the results again (anions and cations) using IC (ion chromatography) next week. Also, I plan to measure total ions in the rainwater and compare with those of dissolved concentration. Hope to get more exciting data! Thank you Professors!
It is worth mentioning that my study site is at Kuala Lumpur, Malaysia (where bush fire is common).
Its normal to have an acidic pH for rain water. Since you are doing ionic content analysis, you will get the answer for the specific pH (acidic or neutral).
Previous comments have already pointed out the potential for alkaline soil dust or ammonia to neutralize rainwater. If there is a lot of agriculture in the nearby area you could expect high levels of ammonium, otherwise soil dust from bare ground, traffic, and construction activity are good candidates.
If these materials are in the air precipitation will very effectively wash them out, so you will get the result you have observed without having to consider contamination of the sampler.
A point not made yet is that once it has rained a little and wet the nearby soil the source of windblown dust should decrease considerably. The next rainfall will be dominated by the acids from oxidizing SO2 and NO
With the IC analysis you can look at the balance of anions and cations and an excess of cations over the sum of sulfate, nitrate, and chloride will confirm that you have HCO3- and CO3(2-) alkalinity.
The transition from neutral to acidic rainwater is analagous to what I observed in thesis work in the 1980's where rainwater at a site in agricultural/prairie landscape of North Dakota was neutral to slightly alkaline and the rain further east in forested area of Minnesota was acidic down to pH between 4-5.
( ATMOSPHERIC ENVIRONMENT: 16(7), 1633-1645
DOI: 10.1016/0004-6981(82)90258-X, 1982 )
The rain water in equilibrium with atmospheric CO2 should have a pH around 5.6, if we consider the carbonic acid as the sole source of the hydronium ion. However, in some areas of our planet, the pH of rain presents values well below the expected value of 5.6. The main agents causing acidification are the dioxide of sulphur (SO2) and oxides of nitrogen (NOx), and to a lesser extent the ammonia (NH3). Compounds that modify the pH of rain water comes from natural sources biogenic (compounds from the ocean, tides, etc.), not biogenic (from geothermal energy, combustion, and aerosols from soil and water) and anthropogenic sources that use fossil fuels (industry, transport, home) of each of these acid rain precursors emissions sources, is based on socio-economic activities of each region.These compounds can be transported by wind and deposited on the Earth's surface by gravity in the form of powder, which is technically called as dry deposit.
Answer: My opinion is that this pH could be normal.
Observation was taken at National Geophysical research Hyderabad, during Aug- Sep., 2001, in peak rainy season, observed pH of rain water in 350 samples, and found pH : 6.2 to 6.7 in 62% samples; and 5.6 to 6.1 in 26% ( pH was immediately observed after the rain water collection without any contamination). Non of the sample was found towards alkaline in nature.
The higher value of pH may be,the content of the atmospheric CO2 which is a weak acid is small, less than 370 ppm. Now the lowering in the pH is because of the SO2 and NO3 in the atmospher.
There are a number of good answers here that have addressed soil dust, agricultural activities, etc. The measured value may or may not be correct. Most of the responses did not cite established sampling protocols, a key consideration for making high quality routine measurements addressed in the WMO Global Atmosphere Watch 2005 guidelines for the collection and analysis of precipitation chemistry samples. See: http://qasac-americas.org/sites/default/files/GAW-Precip%20Chem%20Manual%20No.160.pdf
Acidic pH is normal in rain water but the ionic contents presented here has more question about the measurement error.
Hello Chin, What you have experienced could be due to a localized problem from your sampling point i.e it could be due to the direction of wind that brought about the rain at the point of fall, the type of the pollutant released in to the air which forms the haze differ and depend on activity that causes its release. If the air is loaded by Calcium ions, ammonium ions or sodium ions etc, the rain acidity could be neutralized. Any of this could be the atmospheric scenario at the first time of your sampling.
Dear Dr. Deljo Davis,
I do not understand what do you mean by the measurement error here. Could you please explain more? Is there any problem with the data? Thank you Dr.
Sincerely,
Lin
You should study the following:
1- wind speed to decide if the cloud was in the study zone for long time or not.
2- the content of pollution in your zone where it is obvious that there are acidic pollution.
3- I think the cloud of the first sample was in contact with the polluted atmosphere for short time.
4- the quantity of rain water in the first day was big to the extent to make self dilution to the acidity.
We did research with cloud water at Storm peak Lab in Steamboat Springs Colorado and colected cloud water samples with pH values of 2.0 or less. I think pH is a measurement that has to be done immediately. If you allow the sample to sit evolution of CO2 gas could cause the pH to be higher. So check to see the lag time between sampling and analysis of the three rainwater samples. If they were the same then you might want to do a trajectory analysis to see where the the predominant wind directions was during the three rain events. This will tell you something about the nature of the condensation nuclei and ultimately the pH of the rain water samples.
Concerning the comment by Dr. Phillip: There are decades of measurements comparing field pH to laboratory pH measurements. Only rarely does the delta between field and lab exceed ~ 0.5 pH unit, and typically that is for samples that are near neutral. Typically differences are a couple of tenths of a unit or less and can be attributed to the loss of organic acids due to biological activity in the sample. This issue was discussed by William Keene and James Galloway in the 1980s after examining samples in remote locations such as Bermuda and Amsterdam Island. These differences are also regularly observed in AIRMoN daily samples collected by the US National Atmospheric Deposition Network.
The sampling methodology for the samples collected by Dr. Lin was not discussed. If a funnel and bottle arrangement was used that left the funnel uncovered for an extended period of time in a dusty area, a high pH sample may well be due to contamination from dry deposition to the funnel. If a wet-only sampler of some sort was used, or if the funnel-bottle system was deployed just prior to the onset of precipitation, the value is probably correct -- and the whole source-receptor issue becomes much more interesting. If this is indeed the case, Dr. Phillip's suggestion for running a trajectory analysis is a good one. This can be handled online via the HYSPLIT model, found at: http://www.arl.noaa.gov/HYSPLIT_info.php
Funnel-bottle system was deployed during raining event. Then, I measured the pH immediately after collection. Thank you very much for suggesting the HYSPLIT model.
pH range you are getting is not abnormal. Acidic pH of rainwater is due to dissolution of CO2 and formation of H2CO3. Dissolution of CO2 depends on partial pressure (Henry's Law) and temperature. Check your measurement conditions (especially temperature) during the periods of relatively high and low pH. Slightly elevated pH in the initial period of rain water may also contributed by dissolution of NH3 in the rainwater. Because one of the primary species emitted to the atmosphere is NH3 produced during organic matter decomposition and emitted when the partial pressure in the soil , water, or plant is greater than the partial pressure in the atmosphere.
The pH in your case in the rainwater after a time period without rain is normal because rainwater acidity depending from a lot of factors and substances that are present into the air.
Dear Mr. Lin..it depends on the amount of haze and direction of wind at the time you collect your samples..tq
Hi Chin
It appears that rain removes acidic oxides from air that are added due to human or natural activity in the atmosphere. There are two possibilities (1) temp difference, an increase in temp of rain water can lead to do so and/or (2) the acidic oxides enter the stored rain water through the container. My experience with mercury vapours is that they can easily get into closed plastic containers to change mercury concentrations.
The initial pH of rainwater is high, perhaps due to atmospheric load with dust particles containing CaCO3 (alkaline). The load of these dust particles apparently decreased as rain progressed and when there was not sufficient CaCO3 in the atmosphere to neutralize the acidity in the rain, the pH values progressively dropped. In general, unpolluted rain water is acidic (pH ~5.6)
Please see a paper which found a change of pH with time during one rain event, (see Figure 3)
S. G. Tuncel and S. Ungor. 1995. Rain water chemistry in Ankara, Turkey. Atmospheric Environment 30 (15): 2721-2727.
Kind regards
Golam
,
You could calculate pH from the alkalinity assuming equilibrium with the partial pressure of CO2 in atmospheric air, and then compare that number to your measured pH.
At pH 6.5 you should be able detect quite a significant amount of alkalinity with a Gran titration. You'll typically need just 1 or a few mL sample for that, but more makes the analysis easier, of couse.
The calculation could be done in PHREEQC or any other speciation code.
Keene and Galloway (Atmospheric Environment, Volume 19, issue 1 pages 199-202 in 1987 published an article called "Gran's titrations: Inherent errors in measuring the acidity of precipitation". The abstract states: "Gran's titrations are commonly used to differentiate strong and weak acidity in precipitation. However, in the course of such titrations, dissociation of the weak organic acids found in precipitation causes overestimates of both strong and free acidity. In addition, the reaction of NH4+ with OH− in the alkaline range of titrations causes overestimates of total acidity. Because of these effects, Gran's technique and related modifications have limited usefulness in characterizing the acidity of precipitation." This paper was one of the products of the Global Precipitation Chemistry Project that examined precipitation at a number of remote locations around the world. Bottom line: Be very careful using Gran's titrations for this application.
Yes, it's true -- and I'm on unstable ground here. There were several responses in the literature ~ 25 years ago on the whole issue which did not result in a tidy conclusion concerning the use of Gran's titrations for precipitation samples. A strong acid is typically used to titrate well past the equivalence point. Since weak acids are often in significant concentrations in the sample solution above pH 5 or so,can it be assumed that the sample matrix is not inappropriately affected? Is there a good literature citation on the issue?
Must say I am basing this on experience right now.
Fe-OH, Al-OH, and many other OH-complexes can be dealt with in the speciation calculation, because the computer can take their contribution to the entered alkalinity into account.
Organic acids are less easy to deal with. I've sampled peat and and humic soils though, and sometime I see a curved Gran titration-curve shape, which I attribute to organic acids with a suite of different H+ sites (different pKa-sites) becoming protonized. But mostly, I do not see that, even in water with several mmoles of DOC, meaning that bicarbonate and carbonate is much much more abundant, and/or the DOC is either not protonized or was so already before the titration began.
pH of rainwater always effect by the air pollutions just like many people commened. Where you collected your rainwater samples, find out the possible environment reasons. Rainwater maight influrence by the waste air from industral factory, mote from mine feild or just haze.
I suggest you analyze the anions and cations in water sample to check your data and find out the reason. pH 6.5 maight concern CaCO3 or NH3, pH too acidic miaght concern nitrate or sulfate, which both need more analysis and data to comfirm.
Is anyone burning wood nearby? Woodsmoke fly ash contains caustic alkali such as potassium and calcium oxides. This could account for the K and Ca readings in your qualitative analysis of the rain water.
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