2:30 PM - 2:45 PM
[AAS21-21] Determination on the triple oxygen isotopic composition of atmospheric nitrous acid (HONO)
Keywords:HONO, nitrous acid, triple oxygen isotope, source, winter, Sapporo
The photolysis of nitrous acid (HONO) has been recognized as a potentially important source of OH radicals, which is known as a major oxidant in the atmosphere removing reductive trace gases such as methane and NMHCs. There are two major formation pathways to produce atmospheric HONO, one is a process so-called “direct emission” in which HONO emits directly from various sources on the ground and the other “secondary formation” in which HONO is produced by chemical reaction of nitrogen compounds in the atmosphere. Their contributions to the production of atmospheric HONO, however, have not been well understood.
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.
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.