Unexpected/contrary behavior of aerosol mass concentration in response to the individual components’ concentration reduction in Kitakyushu, Japan

In the suburbs of Kitakyushu, Japan, the inorganic aerosol mass concentration (IAM) was about 32.7 µg/m³, with the aerosol pH of 3.3. To study the thermodynamics of aerosol when its individual components’ concentration is reduced, sensitive tests were performed using the ISORROPIA II model, in which the seven control species—TNaCl, TNH₄⁺, TSO₄²⁻, TNO₃⁻, TMg²⁺, TK⁺, and TCa²⁺—were taken into account. IAM and inorganic aerosol pH after reducing TNaCl, TNO₃⁻, TMg²⁺, TK⁺, and TCa²⁺ responded linearly (0% ≤ concentration reduction ratio (CRR) ≤ 100%, with the exception of 100% in TNaCl); the nonlinear variations of these two parameters could be observed by controlling TNH₄⁺ and TSO₄²⁻. Unexpected aerosol behavior occurred at 100% reduction of TNaCl, which was caused by the sudden increase of NO₃⁻, NH₄⁺, and aerosol liquid water content (ALWC); the increase of IAM was also observed after controlling TSO₄²⁻ (60% ≤ CRR ≤ 100%) and TCa²⁺ (0% ≤ CRR ≤ 100%), which was mainly related to the variation of ALWC driven by the response of CaSO₄. Multiple regression analysis showed that ALWC was statistically and strongly related to the variations of NO₃⁻, Cl⁻, SO₄²⁻, HSO₄⁻, HNO₃, and NH₃ (P < 0.05), with regression coefficients of 1.68, 5.23, 1.83, 2.81, 0.34, and 0.57, respectively. The highest coefficient (5.23) was found for Cl⁻, revealing that sea salts significantly influenced particle responses. Overall, this study comprehensively investigated aerosol characteristics and inner responses for the reduction of components, which is of great significance for a better understanding of atmospheric chemistry in Kitakyushu, Japan.