An Eddy-Covariance System for the Measurement of Surface/Atmosphere Exchange Fluxes of Submicron Aerosol Chemical Species—First Application Abovean Urban Area

摘要

Until now, micrometeorological measurements of surface/ atmosphere exchange fluxes of submicron aerosol chemical components such as nitrate, sulfate or organics could only be made with gradient techniques. This article describes a novel setup to measure speciated aerosol fluxes by the more direct eddy covari-ance technique. The system is based on the Aerodyne quadrupole-based Aerosol Mass Spectrometer (Q-AMS), providing a quantitative measurement of aerosol constituents of environmental concern at a time resolution sufficient for eddy-covariance. The Q-AMS control software was modified to maximize duty cycle and statistics and enable fast data acquisition, synchronized with that of an ultrasonic anemometer. The detection limit of the Q-AMS based system for flux measurements ranges from 0.2 for NO_3~- to 15 ng m~(-2) s~(-1) for hydrocarbon-like organic aerosol (HOA), with an estimated precision of around 6 ng m~(-2) s~(-1), depending on aerosol loading. At common ambient concentrations the system is capable of resolving deposition velocity values <1 mm s~(-1), sufficient for measurements of dry deposition to vegetation. First tests of the system in the urban environment (6 to 20 June 2003) in Boulder, CO, USA, reveal clear diurnal, presumably traffic related, patterns in the emission of HOA and NO_3~-, with indication of fast production of moderately oxygenated organic aerosol below the measurement height, averaging about 15% of the HOA emission. The average emission factor for HOA was 0.5 g (kg fuel)~(-1), similar to those found in previous studies. For NO_3~- an emission factor of 0.09 g (kg fuel)~(-1) was estimated, implying oxidation of 0.5 % of the traffic derived NO_x below the measurement height of 45 m. By contrast, SO_4~(2-) fluxes were on average downward, with deposition velocities that increase with friction velocity from 0.4 to 4 mm s~(-1).