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

Abstract

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, NOx, NOy, 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 NOy and CO, these mixing ratios were correlated (r2 = 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 NOx ∕ NOy concentration ratio (NOx ∕ NOy 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 NOx with O3, a potentially biasing reaction channel on [NOx] / [NOy], was small. The t[OH] values obtained with the NOx ∕ NOy clock ranged from 2.9 × 105 to 1.3 × 108 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 (f44, a quantitative oxidation indicator of carboxylic acids) and O3 mixing ratio. The comparison of t[OH] with f44 showed evidence for a systematic increase of f44 as t[OH] increased, an indication of secondary organic aerosol (SOA) formation. To a first approximation, the f44 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 O3 mixing ratio showed that there was a strong proportional relationship between O3 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.