Abstract
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 PM2.5 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, BLHeff, is defined as the ratio of the tropospheric VCD and the ground level concentration of NO2. Variations of BLHeff can be linked to time of day, source region, stability of the atmosphere, and the presence or absence of elevated NOx sources. In particular, a case study is shown where a high VCD and BLHeff were observed when an elevated industrial plume of NOx and SO2 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 NO2 through the region was estimated to be greater than 112 kg NO2 km−1 h−1.
Highlights
The role of nitrogen oxides in the atmosphere is paramount in atmospheric chemistry due to the deleterious effects they have on the atmosphere and biosphere
Haer is equivalent to the boundary layer height (BLH) if aerosols are 100% confined to the boundary layer ( see (Zieger et al, 2011)), this is rarely the case
We outline an original method for the determination of aerosol optical depths and vertical column densities of NO2 using a combination of MAX-DOAS measurement of NO2 and O4, radiative transfer, and inversion modeling
Summary
The role of nitrogen oxides in the atmosphere is paramount in atmospheric chemistry due to the deleterious effects they have on the atmosphere and biosphere. Heterogeneous reaction of NO2 on surfaces coated with water to form HONO at night is known to be an important reaction in the atmosphere, as photolysis of HONO is known to be a major source of OH in the morning boundary layer (Platt and Perner, 1980), and has been estimated to contribute up to 30 % of the total OH production integrated over 24 h in polluted environments (Alicke et al, 2003). For these and other reasons, measurement of NO2 in the troposphere is important
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