Relationships among the springtime ground–level NOx, O3 and NO3 in the vicinity of highways in the US East Coast

Abstract

To characterize the relationships among the springtime NOx (NO + NO2), O3 and NO3 (defined as the combination of gaseous nitric acid, HNO3, and aerosol nitrate, NO3–) concentrations in the ambient air of the vicinity of highways, an intensive sampling was carried out in spring 2007 at Lyndhurst, NJ in the U.S. East Coast. Ambient concentrations of O3 and NOx were measured by O3 and NOx analyzers, while NO3 was collected using a ChemComb cartridge and determined by ion chromatography. Significant variations in O3 concentrations were observed diurnally as well as between weekdays and weekends, with higher concentrations occurring during the daytime and on weekends. The 24–h diurnal variations of O3 and NOx could be divided into four periods: (1) morning NOx peak, (2) mid–day O3 formation, (3) afternoon NOx accumulation and (4) nighttime balancing. Daily averaged relative humidity and wind speed were the two weather parameters affecting O3 levels more than other averaged parameters, and daily maximum temperature was positively correlated to the maximum O3. Via photochemical reactions and emission– diffusion balance, NOx showed primarily negative influences on the daily O3 variations with decayed exponential correlations, in particular during nighttime and weekdays, indicating a possible VOC–sensitive characteristic of the study area. A negative correlation between NO3– and O3 concentrations was found while no obvious influence of HNO3 on O3 was observed. Results by a multi–regression model involving three parameters (NO2/OX ratio, NO2 and HNO3) reveal that the NO2/OX ratio is an important parameter controlling the ground O3 level in the study area.