Determination of tropospheric vertical columns of NO₂ 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 NO₂ and O₄ were determined using the standard DOAS technique. Using a radiative transfer model and the O₄ 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 NO₂ air mass factors, which were fit to the measured NO₂ SCDs to determine tropospheric vertical column densities (VCDs) of NO₂. The method of determining NO₂ VCDs in this way was validated for the first time by comparison to composite VCDs derived from aircraft and ground-based measurements of NO₂. The new VCDs were compared to VCDs of NO₂ 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 +0.5–0.9×10¹⁵ 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, BLH_eff, is defined as the ratio of the tropospheric VCD and the ground level concentration of NO₂. Variations of BLH_eff 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 BLH_eff were observed when an elevated industrial plume of NO_x and SO₂ was fumigated to the surface as a lake breeze impacted the measurement site. High BLH_eff 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 NO₂ through the region was estimated to be greater than 112 kg NO₂ km⁻¹ h⁻¹.