Per­plex­ing puz­zle

China Daily (Hong Kong) - - FRONT PAGE - By XINHUA in Washington

Sci­en­tists say they have solved the ques­tion of how a ma­jor pol­lu­tant in North China’s smog forms. >

Sci­en­tists said on Wed­nes­day that they have solved the mys­tery of how a ma­jor smog com­po­nent, known as sul­fate, forms dur­ing heavy pol­lu­tion.

The study, pub­lished in the US jour­nal Science Ad­vances, iden­ti­fied re­ac­tive ni­tro­gen chem­istry and wa­ter par­ti­cles in the air as the two key el­e­ments, sug­gest­ing that re­duc­ing ni­tro­gen ox­ide emis­sions, in par­tic­u­lar, may help curb air pol­lu­tion.

The find­ings were based on an anal­y­sis of heavy smog in Bei­jing in Jan­uary 2013, one of the worst in­ci­dents of at­mo­spheric pol­lu­tion ever recorded in China, which saw the daily con­cen­tra­tion lev­els of haz­ardous PM2.5 ex­ceed the World Health Or­ga­ni­za­tion’s guide­line by 16 times.

They iden­ti­fied a re­ac­tion path­way that could ac­count for the miss­ing source of sul­fate, dis­cov­er­ing that fine wa­ter par­ti­cles in the air acted as a re­ac­tor, trap­ping sul­fur diox­ide mol­e­cules and in­ter­act­ing with ni­tro­gen diox­ide to form sul­fate.

The re­ac­tion rate was fur­ther fa­cil­i­tated by stag­nant weather dur­ing that time, which trapped ni­tro­gen diox­ide near the Earth’s sur­face, re­sult­ing in ni­tro­gen diox­ide con­cen­tra­tions that were three times higher than nor­mal.

Re­searchers said the process was “self-am­pli­fy­ing”, as in­creas­ing aerosol mass con­cen­tra­tions led to higher aerosol wa­ter con­tent — ac­cel­er­at­ing the ac­cu­mu­la­tion of sul­fate and caus­ing more se­vere pol­lu­tion.

“In cleaner en­vi­ron­ments, sul­fate is mainly formed through the tra­di­tional hy­drox­ide re­ac­tion path­ways in the at­mo­spheric gas phase, or the hy­dro­gen per­ox­ide and ozone re­ac­tion path­ways in cloud chem­istry. How­ever, in China, the domi- nant sul­fate for­ma­tion path­way shifts into the ni­tro­gen diox­ide re­ac­tion path­way in aerosol wa­ter,” said study au­thor Zheng Guang jie of Ts­inghua Univer­sity.

Zheng said the re­search re­veals “the com­plex na­ture” of haze pol­lu­tion in China.

“Pol­lu­tants from var­i­ous sources were emit­ted at a high in­ten­sity at the same time, re­sult­ing in the unique heavy haze con­di­tions, and thus shift­ing the dom­i­nant sul­fate for­ma­tion path­way. The com­plex­ity of haze pol­lu­tion in China fur­ther il­lus­trated the im­por­tance of sci­en­tific emis­sion-re­duc­tion strate­gies.”

Th­ese re­sults “will need to be con­sid­ered in fu­ture air qual­ity and pol­lu­tant emis­sion con­trol strate­gies in North China, and per­haps also in other re­gions”, the re­searchers con­cluded in their pa­per.

Pol­lu­tants from var­i­ous sources were emit­ted at a high in­ten­sity at the same time.” Zheng Guangjie, study au­thor from Ts­inghua Univer­sity

ZHANG CHENG / XINHUA

A col­lage of six pho­tos in chrono­log­i­cal or­der from left to right, dat­ing from Dec 17 to Thurs­day, shows the dif­fer­ence in air qual­ity on each of the days at the Na­tional Cen­ter for the Per­form­ing Arts in Bei­jing.

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