Photochemical age of air pollutants, ozone, and secondary organic aerosol in transboundary air observed on Fukue Island, Nagasaki, Japan

• Main Authors: S. Irei, A. Takami, Y. Sadanaga, S. Nozoe, S. Yonemura, H. Bandow, Y. Yokouchi
• Format: Article
• Language: English
• Published: Copernicus Publications 2016-04-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/16/4555/2016/acp-16-4555-2016.pdf

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, O₃, NO_<i>x</i>, NO_<i>y</i>, <i>i</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_<i>y</i> and CO, these mixing ratios were correlated (<i>r</i>² = 0.67). The photochemical age of the pollutants, <i>t</i>[OH] (the reaction time  ×  the mean concentration of OH radical during the atmospheric transport), was calculated from both the NO_<i>x</i> ∕ NO_<i>y</i> concentration ratio (NO_<i>x</i> ∕ NO_<i>y</i> 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 <i>t</i>[OH]. In contrast, the influence of the reaction of NO_<i>x</i> with O₃, a potentially biasing reaction channel on [NO_<i>x</i>] / [NO_<i>y</i>], was small. The <i>t</i>[OH] values obtained with the NO_<i>x</i> ∕ NO_<i>y</i> clock ranged from 2.9  ×  10⁵ to 1.3  ×  10⁸ h molecule cm⁻³ and were compared with the fractional contribution of the <i>m</i>∕<i>z</i> 44 signal to the total signal in the organic aerosol mass spectra (<i>f</i>₄₄, a quantitative oxidation indicator of carboxylic acids) and O₃ mixing ratio. The comparison of <i>t</i>[OH] with <i>f</i>₄₄ showed evidence for a systematic increase of <i>f</i>₄₄ as <i>t</i>[OH] increased, an indication of secondary organic aerosol (SOA) formation. To a first approximation, the <i>f</i>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 <i>t</i>[OH] with O₃ mixing ratio showed that there was a strong proportional relationship between O₃ mixing ratio and <i>t</i>[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.