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[AAS21-21] Determination on the triple oxygen isotopic composition of atmospheric nitrous acid (HONO)
Keywords:HONO, nitrous acid, triple oxygen isotope, source, winter, Sapporo
In order to quantify the contribution of HONO derived from secondary formation, we determined a triple oxygen isotope, ∆17O value of atmospheric nitrous acid (HONO). ∆17O value of HONO produced via secondary formation is expected to have highly positive values as those of O3 (∆17O = +30 ±10‰), while no ∆17O anomaly (∆17O = 0‰) should be observed for HONO which is emitted directly from various sources on the ground.
Atmospheric HONO was collected using filter-pack method (Noguchi et al., 2007) in which HONO accumulates on the K2CO3 impregnated filter as NO2-. Since HONO is collected as NO2-, we must be careful about oxygen exchange between NO2- and H2O on the filter. If the sampling period becomes longer, ∆17O of HONO could become smaller than the original value due to rapid oxygen exchange between NO2- and H2O on the filter. Therefore, in order to decide appropriate sampling periods for ∆17O measurement of HONO, we conducted a field sampling of atmospheric HONO during December 15-26, 2014, at Hokkaido Institute of Environmental Sciences, Sapporo, Japan. We arranged seven different periods (half a day, one day, two days, three days, four days, one week and two weeks) for atmospheric HONO collection. We also set two kinds of sampling flow rate for HONO sampling; faster flow rate (10 L/min) for shorter sampling periods (from half a day to three days) and slower flow rate (4 L/min) for longer sampling periods (from four days to two weeks). HONO-derived NO2- on the filter was extracted to pure water. After that, it was reduced to N2O using HN3 and then converted to O2 via thermal decomposition and then introduced to IRMS for ∆17O measurement. The concentration of NO2- and NO3- in the extracted water were measured by traditional ion chromatography to calculate NO2- yield (ratio of NO2- among the sum of NO2- and NO3-) on the filter.
We found clear difference on NO2- yield absorbed on the filter between the two sample flow rates. Low flow late (4 L/min) result in lower NO2- yield of around 79% on average which coincide well with those reported previously (76%, Ohyama et al., 2012). Very high NO2- yield of more than 98% were observed in the filter collected at high rates (10 L/min). We concluded that we could prevent NO3- formation via reaction of NO2- with O3 by collecting HONO in the condition of higher sample flow rate.
The result of ∆17O value of HONO ranged from +6‰ to +9‰ through the observation periods. We could not find any ∆17O depletion due to oxygen exchange between NO2- and H2O on the filter during the collection periods. Assuming that ∆17O value of HONO derived from secondary formation is +35‰, the contribution of HONO produced via secondary formation to atmospheric HONO was estimated to be about more than 20% demonstrating its significance on the formation pathway of atmospheric HONO in the winter of urban area.