Determination of tropospheric vertical columns of NO 2 and aerosol optical properties in a rural setting using MAX-DOAS

Abstract

Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were performed in a rural location of southwestern Ontario during the Border Air Quality and Meteorology Study. Slant column densities (SCDs) of NO 2 and O 4 were determined using the standard DOAS technique. Using a radiative transfer model and the O 4 SCDs, aerosol optical depths were determined for clear sky conditions and compared to OMI, MODIS, AERONET, and local PM 2.5 measurements. This aerosol information was input to a radiative transfer model to calculate NO 2 air mass factors, which were fit to the measured NO 2 SCDs to determine tropospheric vertical column densities (VCDs) of NO2. The method of determining NO 2 VCDs in this way was validated for the first time by comparison to composite VCDs derived from aircraft and ground-based measurements of NO 2. The new VCDs were compared to VCDs of NO 2 determined via retrievals from the satellite instruments SCIAMACHY and OMI, for overlapping time periods. The satellite-derived VCDs were higher, with a mean bias of +0.5-0.9×10 15 molec cm -2. This last finding is different from previous studies whereby MAX-DOAS geometric VCDs were higher than satellite determinations, albeit for urban areas with higher VCDs. An effective boundary layer height, BLHeff, is defined as the ratio of the tropospheric VCD and the ground level concentration of NO 2. Variations of BLHeff can be linked to time of day, source region, stability of the atmosphere, and the presence or absence of elevated NO x sources. In particular, a case study is shown where a high VCD and BLHeff were observed when an elevated industrial plume of NO x and SO 2 was fumigated to the surface as a lake breeze impacted the measurement site. High BLHeff values (∼1.9 km) were observed during a regional smog event when high winds from the SW and high convection promoted mixing throughout the boundary layer. During this event, the regional line flux of NO 2 through the region was estimated to be greater than 112 kg NO 2 km -1 h -1. © 2011 Author(s).