No abstract

No abstract

Asian continental outflow air masses reach western Japan in the springtime, carrying high levels of ozone produced over the Asian continent, and facilitating in-situ production. In this study, in-situ production was highlighted; the rate and limiting factors of net ozone production were diagnosed at Fukue Island, a remote island west of Japan, on 17 days during May-June 2009, when the continental outflow air mass arrived, using an observation-based modeling approach. The average ozone production was estimated to be 6.8 ppb per day. Information on the chemical status of the arriving air mass is important, because it affects how further ozone production in the air mass occurs after precursor addition from Japanese domestic emissions. The main limiting factor of ozone production for such air masses was usually nitrogen oxides (NO x ), suggesting that domestic NO x emission control is important in reducing further ozone production. Volatile organic compounds (VOCs) also increased the ozone production rate, and occasionally (14% of time) became the dominant controlling factor. This analysis implies that the VOC reduction legislation recently enacted by the Japanese government should be effective. VOC-limited conditions occurred particularly when the air mass traveled within 6-8 h, via the Korean Peninsula. The uncertainty in the radical chemistry mechanism governing ozone production had a non-negligible impact, but the main conclusion relevant to policy was not altered. When chain termination was augmented by HO 2 + NO/NO 2 reactions in the presence of H 2 O and by heterogeneous loss of HO 2 on aerosol particle surfaces, as recently verified or hypothesized, the daily ozone production rate decreased by up to 24%, and the fraction of hours when the VOC-limited condition occurred varied from 14% to 13-26%.

To better understand the secondary air pollution in transboundary air over westernmost Japan, ground-based field measurements of the chemical composition of fine particulate matter ( ≤ 1 µm), mixing ratios of trace gas species (CO, O 3 , NO x , NO y , i-pentane, toluene, and ethyne), and meteorological elements were conducted with a suite of instrumentation. The CO mixing ratio dependence on wind direction showed that there was no significant influence from primary emission sources near the monitoring site, indicating long-and/or mid-range transport of the measured chemical species. Despite the considerably different atmospheric lifetimes of NO y and CO, these mixing ratios were correlated (r 2 = 0.67). The photochemical age of the pollutants, t[OH] (the reaction time × the mean concentration of OH radical during the atmospheric transport), was calculated from both the NO x / NO y concentration ratio (NO x / NO y clock) and the toluene / ethyne concentration ratio (hydrocarbon clock). It was found that the toluene / ethyne concentration ratio was significantly influenced by dilution with background air containing 0.16 ppbv of ethyne, causing significant bias in the estimation of t [OH]. In contrast, the influence of the reaction of NO x with O 3 , a potentially biasing reaction channel on [NO x ] / [NO y ], was small. The t[OH] values obtained with the NO x / NO y clock ranged from 2.9 × 10 5 to 1.3 × 10 8 h molecule cm −3 and were compared with the fractional contribution of the m/z 44 signal to the total signal in the organic aerosol mass spectra (f 44 , a quantitative oxidation indicator of carboxylic acids) and O 3 mixing ratio. The comparison of t[OH] with f 44 showed evidence for a systematic increase of f 44 as t[OH] increased, an indication of secondary organic aerosol (SOA) formation. To a first approximation, the f 44 increase rate was (1.05 ± 0.03) × 10 −9 × [OH] h −1 , which is comparable to the background-corrected increase rate observed during the New England Air Quality Study in summer 2002. The similarity may imply the production of similar SOA component, possibly humic-like substances. Meanwhile, the comparison of t[OH] with O 3 mixing ratio showed that there was a strong proportional relationship between O 3 mixing ratio and t [OH]. A first approximation gave the increasing rate and background mixing ratio of ozone as (3.48 ± 0.06) × 10 −7 × [OH] ppbv h −1 and 30.7 ppbv, respectively. The information given here can be used for prediction of secondary pollution magnitude in the outflow from the Asian continent. Published by CopernicusPublications on behalf of the European Geosciences Union. 4556 S. Irei et al.: Photochemical age of air pollutants Atmos. Chem. Phys., 16, 4555-4568, 2016 www.atmos-chem-phys.net/16/4555/2016/