Near real-time measurements of submicron particulate matter (PM1) were carried out at an industrial and coastal site in Dunkirk (Northern France) over a 14-month period (July 2013–September 2014). This site is surrounded by various industrial plants (metallurgy, petrochemistry, food processing, power plant, etc.) and is characterized by intense ship traffic (~700–800 per day along the English Channel) in harbour surroundings. The non-refractory (NR) submicron particles (organics, sulfate, nitrate, ammonium and chloride) and black carbon were measured by an Aerosol Chemical Speciation Monitor (ACSM) and an Aethalometer, respectively. Concomitant monitoring of CO2, SO2, and meteorological parameters was also performed. Both the seasonal (five seasons including two summers) and spatial (four identified sectors of emissions: marine, urban, industrial-urban and industrial) variabilities were investigated. We present a descriptive analysis of the PM1 composition, whose ambient concentrations ranged from less than 1 μg m−3 up to approximately 100 μg m−3 during a few pollution events. Gaseous SO2 and particulate SO4 were systematically observed at high concentrations (up to 310 and 48.2 μg m−3, respectively) when industrial plumes reached the monitoring site. The conversion ratio of particulate (Sp) to total (Stot) sulfur is relatively constant at 0.1 when RH ranges from 30 to 70% but reaches an average value of 0.3 at high RH (90–100%). This reflects an enhancement of SO2-to-SO4 gas-particle conversion processes resulting in an increase of aerosol acidity as shown by a comparison between measured and predicted NH4 concentrations. An impact of the vertical mixing on the SO2-to-SO4 conversion was also observed using vertical turbulence (σw) as a descriptive parameter. Indeed, the conversion ratio (Sp/Stot) was found to be reduced under high turbulence conditions due to dilution effects.