Spatiotemporally synchronized dual-wavelength measurement of aerosol optical properties using a dual-differential sphere–tube coupled photoacoustic–scattering system

To reduce potential inconsistencies in particle transport, temporal response, and microenvironment in conventional split-type aerosol absorption–scattering systems, we developed a dual-wavelength spatiotemporally synchronized measurement method. Two integrating spheres serve as scattered-light collection cavities and acoustic buffers and form a low-frequency differential Helmholtz resonator, while the central tube supports a high-frequency differential second-order longitudinal mode. The 450 and 532 nm lasers were modulated at 2572 and 169 Hz and synchronously demodulated using an FPGA-based digital lock-in amplifier. Absorption channels were calibrated with NO₂, and scattering channels with N₂/CO₂ Rayleigh scattering. At 1 s integration and a 3 σ criterion, the 532 nm absorption and scattering limits of detection were 1.2 and 1.32 Mm⁻¹. Water-mist, controlled-combustion, and field tests evaluated channel isolation, relative spectral response, and ambient-operation feasibility. The system simultaneously measures dual-wavelength absorption and scattering within a common volume through a single sampling path.