LAUSR

An Aerodyne Aerosol Chemical Speciation Monitor (ACSM; Aerodyne Inc.) was deployed at a site north of Mexico City from November 13, 2013 to April 30, 2014 to investigate the seasonal variability of the chemical composition of submicron particles. The ACSM provides real time information on mass concentration of the non-refractory main species (nitrate, sulfate, ammonium, chloride and organic compounds) in particulate matter less than 1 μm (NR-PM 1 ) with a 30-min time resolution. Meteorological variables (temperature, relative humidity, and wind speed and direction), as well as concentrations of black carbon, PM 1 , PM 2.5 , CO, SO 2 , NO, NO 2 , and O 3 , were also measured. The total NR-PM 1 mass concentrations plus black carbon (which must be close to the PM 1 total mass) showed a good correlation with PM 1 mass concentration measured with a Tapered Element Oscillating Micro-Balance, an indication of the soundness of the ACSM data. In average, the composition of the aerosol as well as its diurnal variability were similar to observations in previous studies using  similar instruments (MCMA-2003 and MILAGRO). However, it was observed that the aerosol was persistently acidic during November and December probably due to a higher relative humidity, lower temperature, and more frequent winds from the NW, where the Tula industrial complex is located. A lower concentration of ammonia (NH 3 ) in the gas phase might affect the PM acidity too. These results suggest a seasonal variability in the aerosol chemistry in Mexico City, which should be verified with more long-term studies.