Two notable features in PM10 data and analysis of their causes

Hourly PM₁₀ mass concentrations were collected from 25 air quality monitoring stations in Seoul, Korea. Sixteen years of data, from 2000 to 2015, were analyzed. During that time, the annual average PM₁₀ concentrations declined almost linearly at a rate of ?1.98 μg m^?3 year^?1. The number of high PM₁₀ days declined faster than did the number of low PM₁₀ days. This indicates that the bulk of the annual average PM₁₀ mass concentration reduction was high-level PM₁₀ concentrations. Further analysis of this data revealed two interesting points. First, though the annual average PM₁₀ concentrations clearly lowered for period 1 (from 2000 to 2012; ?2.28 μg m^?3 year^?1), they remained almost unchanged at a virtually constant value for period 2 (from 2012 to 2015; ?0.02 μg m^?3 year^?1). Second, annual average PM₁₀ concentrations showed a large spatial concentration gradient among all monitoring stations in the early 2000s. However, the spatial concentration gradient got gradually smaller until reaching a nearly no-gradient, uniform concentration among all monitoring stations from 2010 onward. Clear PM₁₀ concentration reduction in period 1 was driven by local emission reduction. However, its reduction was not enough in period 2. The reduction of local emissions was negated by the increase of local activities and transported particulates, as well as the formation of secondary aerosol in Seoul from emissions transported from upwind regional sources. This resulted in PM₁₀ concentrations becoming stagnant in period 2. PM₁₀ reduction rate in the downwind area was faster than that in the upwind area. For the first 5 years, the reduction rate in the downwind area was great. Between all the stations observed, nearly all of the concentration difference was a result of more reduction in secondary aerosol. After 2005, coarse particles and primary elemental carbon (EC) played a key role in reducing the PM₁₀ concentration. Our findings on these two data features, and their causes, will help people to understand the most recent characteristics of particulate matters, in turn helping to update the control strategy for the continued improvement of particulate air quality in Seoul.