Comparison of tropospheric NO<sub>2</sub> vertical columns in an urban environment using satellite, multi-axis differential optical absorption spectroscopy, and in situ measurements

Abstract

Abstract. Tropospheric NO 2 vertical column densities have been retrieved and compared for the first time in Toronto, Canada, using three methods of differing spatial scales. Remotely sensed NO 2 vertical column densities, retrieved from multi-axis differential optical absorption spectroscopy and satellite remote sensing, were evaluated by comparison with in situ vertical column densities estimated using a pair of chemiluminescence monitors situated 0.01 and 0.5 km a.g.l. (above ground level). The chemiluminescence measurements were corrected for the influence of NO z , which reduced the NO 2 concentrations at 0.01 and 0.5 km by an average of 8 ± 1 % and 12 ± 1 %, respectively. The average absolute decrease in the chemiluminescence NO 2 measurement as a result of this correction was less than 1 ppb. The monthly averaged ratio of the NO 2 concentration at 0.5 to 0.01 km varied seasonally, and exhibited a negative linear dependence on the monthly average temperature, with Pearson's R = 0.83. During the coldest month, February, this ratio was 0.52 ± 0.04, while during the warmest month, July, this ratio was 0.34 ± 0.04, illustrating that NO 2 is not well mixed within 0.5 km above ground level. Good correlation was observed between the remotely sensed and in situ NO 2 vertical column densities (Pearson's R value ranging from 0.72 to 0.81), but the in situ vertical column densities were 52 to 58 % greater than the remotely sensed columns. These results indicate that NO 2 horizontal heterogeneity strongly impacted the magnitude of the remotely sensed columns. The in situ columns reflected an urban environment with major traffic sources, while the remotely sensed NO 2 vertical column densities were representative of the region, which included spatial heterogeneity introduced by residential neighbourhoods and Lake Ontario. Despite the difference in absolute values, the reasonable correlation between the vertical column densities determined by three distinct methods increased confidence in the validity of the values provided by each measurement technique.

Impact of aerosols on the OMI tropospheric NO<sub>2</sub> retrievals over industrialized regions: how accurate is the aerosol correction of cloud-free scenes via a simple cloud model?

“…It is clearly shown that (for these data) aerosol mixing layers are most often deeper/higher than NO 2 mixing layers. Mendolia et al (2013) retrieved tropospheric NO 2 vertical column densities from OMI and MAX-DOAS measurements over Canada. One key conclusion of this work is that NO 2 diurnal profiles can even be systematically lower in summer and do not follow the expected pattern of the convective boundary layer (higher in summer than in winter).…”

“…These studies found negative biases between −26 and −50 % on the OMI tropospheric NO 2 VCDs over areas with high aerosol pollution, in particular in summer. Investigations led by Ma et al (2013) show that these underestimations can be explained by the presence of elevated aerosol layers, which are mostly observable in summer in this region (Vlemmix et al, 2015;Li et al, 2013;Mendolia et al, 2013). Very recently, Wang et al (2015b) analysed MAX-DOAS data over Wuxi city, an area with high pollution adjoining Shanghai.…”

MAX-DOAS observations of aerosols, formaldehyde and nitrogen dioxide in the Beijing area: comparison of two profile retrieval approaches

“…Luo et al, 2014). That this effect is weaker for NO 2 might be related to the shorter lifetime of NO 2 in summer which limits the effective transport of NO 2 from the surface to the higher parts of the mixing layer, see also Halla et al (2011) and Mendolia et al (2013). Figure 14 and Table 3 show that the agreement between the two methods in terms of profile heights is considerably lower than the agreement in terms of column densities, which is to be expected because the information content of MAX-DOAS with respect to the vertical distribution is limited.…”

“…Knowledge of the relationship between surface concentrations and integrated tropospheric column densities (in urban, suburban and rural regions) is important for the use of satellite observations in studies of air quality (e.g. Boersma et al, 2009;Mendolia et al, 2013).…”

High-resolution airborne imaging DOAS measurements of NO<sub>2</sub> above Bucharest during AROMAT

“…The altitude in which the NO 2 resides depends on many parameters, such as emission altitude, boundary layer height, orography and temperature. Vlemmix et al (2015) derived NO 2 profiles from MAX-DOAS measurements in China, showing that the NO 2 profile height is between 500 and 1000 m. Mendolia et al (2013) studied the urban NO 2 profile of Toronto and found the average characteristic profile height to be around 500 m during summer. These studies suggest that the assumption of a 500 m NO 2 layer is a reasonable guess.…”