The influence of tropical cyclone Melor on PM10 concentrations during an aerosol episode over the Pearl River Delta region of China: Numerical modeling versus observational analysis

In this article, numerical simulations and observational analyses have been made for the aerosol episode that occurred over the Pearl River Delta (PRD) region in China during 1–3 November 2003. An air quality modeling system that consisted of the mesoscale model MM5, chemical transport model MODELS-3/CMAQ, and air pollutant emission model SMOKE, was employed. Studies have shown that this particulate matter (PM) pollution episode was apparently associated with the activity of tropical cyclone (TC) Melor. Model simulations revealed that Melor spawned this PM episode through dynamic and thermodynamic processes. The strong compensating subsidence induced by Melor's peripheral circulations created favorable meteorological conditions that enhanced local aerosol pollution. This strong downward motion produced significant adiabatic warming (2–4 °C daily) and dramatic drying in the low-level troposphere over the PRD. As a result, the PRD region was blanketed with a dry and warm air layer that strengthened the static stability of the lower troposphere. The descending motion also tended to dramatically lower the heights of the planetary boundary layer (PBL) through its dynamic effect. The fair weather created by this synoptic pattern further intensified the nocturnal temperature inversions through enhanced radiative cooling. All of these factors promoted a stagnant local atmosphere with very light winds near the surface. The horizontal and vertical dispersions of locally emitted aerosol particles were largely suppressed, leading to the accumulation of large amounts of PMs near local emission sources in the PRD region. As Melor drew near, changes in surface winds strengthened the horizontal transport of aerosol particles from inland sources to the area of Hong Kong downstream. This horizontal advection greatly contributed to the high PM10 (particulate matters less than 10 μm in diameters) concentrations in Hong Kong.