Small-scale spatial variations of gaseous air pollutants - A comparison of path-integrated and in situ measurement methods

Traffic emissions are a very important factor in Beijing's urban air quality. To investigate small-scale spatial variations in air pollutants, a campaign was carried out from April 2009 through March 2011 in Beijing. DOAS (differential optical absorption spectroscopy) systems and in situ instruments were used. Atmospheric NO, NO2, O-3 and SO2 mixing ratios were monitored. Meanwhile, HCHO mixing ratios were measured by two different DOAS systems. Diurnal variations of these mixing ratios were analysed. Differences between the path-integrated and in situ measurements were investigated based on the results from the campaign. The influences of different weather situations, dilution conditions and light-path locations were investigated as well. The results show that the differences between path-integrated and in situ mixing ratios were affected by combinations of emission source strengths, weather conditions, chemical transformations and local convection. Path-integrated measurements satisfy the requirements of traffic emission investigations better than in situ measurements. (C) 2014 Elsevier Ltd. All rights reserved.