Media Releases

University of Toronto chemists identify role of soil in pollution control

December 3, 2014

TORONTO, ON – Sci­en­tists have long known that air pol­lu­tion caused by cars and trucks, sol­vent use and even plants, is reduced when bro­ken down by nat­u­ral­ly occur­ring com­pounds that act like deter­gents of the atmos­phere. What has not been well under­stood until now are the rel­a­tive con­tri­bu­tions of all the process­es pro­duc­ing such com­pounds.

A new study, led by Uni­ver­si­ty of Toron­to atmos­pher­ic chemist Jen­nifer Mur­phy, shows a key com­po­nent of the process is the soil beneath our feet.

“Pol­lu­tants in the atmos­phere are bro­ken down by hydrox­yl rad­i­cals that are pro­duced when nitrous acid breaks down in sun­light,” said Mur­phy. “What sci­en­tists have been work­ing to solve for over 15 years is where nitrous acid comes from dur­ing the day­time.”

Mur­phy and her team inves­ti­gat­ed chem­i­cal inter­ac­tions that take place when dif­fer­ent com­po­nents of the atmos­phere reach the ground. “We found that soil can take up nitrous acid at night when these com­po­nents react with car­bon­ate min­er­als often found in soil. Exam­ples of every­day car­bon­ates are lime and sodi­um bicar­bon­ate, com­mon­ly known as bak­ing soda,” said Mur­phy.

“The fol­low­ing day, nitrous acid is dis­placed from the soil and released into the atmos­phere by the strong acids nitric acid and hydrochlo­ric acid,” said Trevor Van­den­Boer, lead author of the study pub­lished today in Nature Geo­science and for­mer PhD stu­dent in Murphy’s research group, now a Bant­i­ng post­doc­tor­al fel­low at Memo­r­i­al Uni­ver­si­ty. “Those strong acids are the prod­uct of com­bus­tion process­es that occur in vir­tu­al­ly all envi­ron­ments, so this cycle occurs dai­ly.”

Nitrous acid breaks down extreme­ly quick­ly in sun­light to form hydrox­yl rad­i­cals. So, some­thing must be pro­duc­ing nitrous acid just as quick­ly, or at least in suf­fi­cient amounts, dur­ing the day­time in order to reach mea­sur­able con­cen­tra­tions.

To find the answer, the researchers designed a com­bi­na­tion of exper­i­ments to mea­sure nitrous acid react­ing with atmos­pher­ic par­ti­cles they sus­pect­ed were com­ing from soils. They fol­lowed up on promis­ing field obser­va­tions with lab­o­ra­to­ry tests and dis­cov­ered that nitrous acid can be tak­en up by soils and sub­se­quent­ly released the next day through reac­tion with the stronger acids.

The team found nitrite – the salt form of nitrous acid – in par­ti­cles con­tain­ing large amounts of cal­ci­um and sodi­um. This sug­gest­ed that reac­tions with min­er­al dust or soil pro­duces nitrite salts, which react with the stronger acids pro­duced by com­bus­tions process­es, releas­ing nitrous acid.

“We have demon­strat­ed a process through which a sig­nif­i­cant amount of nitrous acid can be pro­duced and observed in the day­time,” said Mur­phy.

“This process can account for the major­i­ty of day­time nitrous acid pro­duced from noon through sun­set,” said Van­den­Boer. “Oth­er mech­a­nisms pro­posed pre­vi­ous­ly have not been shown to be equal­ly impor­tant both in the lab and in the field.”

“This dis­cov­ery allows us to bet­ter under­stand the sources of hydrox­yl rad­i­cal,” said Mur­phy. “Know­ing where nitrous acid comes from dur­ing the day­time helps to under­stand the fac­tors con­trol­ling air pol­lu­tion.”

The research is described in a study titled “Noc­tur­nal loss and day­time source of nitrous acid through reac­tive uptake and dis­place­ment” pub­lished this week in Nature Geo­science. Addi­tion­al researchers includ­ed sci­en­tists at the Coop­er­a­tive Insti­tute for Research in Envi­ron­men­tal Sci­ences and the Nation­al Ocean­ic and Atmos­pher­ic Admin­is­tra­tion at the Uni­ver­si­ty of Col­orado, Boul­der. The research was sup­port­ed by fund­ing from the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da.

 — 30 -


Jen­nifer Mur­phy
Depart­ment of Chem­istry
Uni­ver­si­ty of Toron­to

Trevor Van­den­Boer
Depart­ment of Earth Sci­ence
Memo­r­i­al Uni­ver­si­ty

Sean Bet­tam
Com­mu­ni­ca­tions, Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to