High‐resolution NO₂ observations from the Airborne Compact Atmospheric Mapper: Retrieval and validation

Abstract

Nitrogen dioxide (NO2) is a short‐lived atmospheric pollutant that serves as an air quality indicator and is itself a health concern. The Airborne Compact Atmospheric Mapper (ACAM) was flown on board the NASA UC‐12 aircraft during the Deriving Information on Surface Conditions from Column and Vertically Resolved Observations Relevant to Air Quality Maryland field campaign in July 2011. The instrument collected hyperspectral remote sensing measurements in the 304–910 nm range, allowing daytime observations of several tropospheric pollutants, including nitrogen dioxide (NO2), at an unprecedented spatial resolution of 1.5 × 1.1 km2. Retrievals of slant column abundance are based on the differential optical absorption spectroscopy method. For the air mass factor computations needed to convert these retrievals to vertical column abundance, we include high‐resolution information for the surface reflectivity by using bidirectional reflectance distribution function data from the Moderate Resolution Imaging Spectroradiometer. We use high‐resolution simulated vertical distributions of NO2 from the Community Multiscale Air Quality and Global Modeling Initiative models to account for the temporal variation in atmospheric NO2 to retrieve middle and lower tropospheric NO2 columns (NO2 below the aircraft). We compare NO2 derived from ACAM measurements with in situ observations from NASA's P‐3B research aircraft, total column observations from the ground‐based Pandora spectrometers, and tropospheric column observations from the space‐based Ozone Monitoring Instrument. The high‐resolution ACAM measurements not only give new insights into our understanding of atmospheric composition and chemistry through observation of subsampling variability in typical satellite and model resolutions, but they also provide opportunities for testing algorithm improvements for forthcoming geostationary air quality missions.

A high-resolution and observationally constrained OMI NO<sub>2</sub> satellite retrieval

“…Lamsal et al (2017) used measurements from the ACAM to deduce that GMI is better than CMAQ at simulating NO 2 in the free troposphere. In the upper free troposphere, above 10.5 km, Travis et al (2016) note that NO 2 is significantly underestimated by global models, such as GMI.…”

Retrieval of tropospheric NO<sub>2</sub> columns over Berlin from high-resolution airborne observations with the spectrolite breadboard instrument

“…A first major other studies (e.g. Nowlan et al, 2016;Lamsal et al, 2017;Broccardo et al, 2017) and it demonstrates for urban regions that 15 the spatial scale at which the most pronounced gradients in NO 2 TVCDs can be observed is smaller than the size OMI pixels. for every consecutive crossing of the plume, the spatial pattern appears to be flipped (left to right) with respect to the previous 20 one, due to the change in flight direction.…”

“…In the last decade a number of instruments for airborne remote sensing of tropospheric NO 2 columns were developed, amongst others: AMAX-DOAS (Heue et al, 2005;Wang et al, 2005;Dix et al, 2009;Oetjen et al, 2013), AirMAP (Schön-hardt et al, 2015), SWING (Merlaud et al, 2012), ANDI (Lawrence et al, 2015), GeoTASO (Nowlan et al, 2016), ACAM (Lamsal et al, 2017). Also the APEX instrument convincingly demonstrated to have sensitivity to NO 2 along with many other geophysical parameters (Schaepman et al, 2015).…”

“…Note that we do not consider here the diurnal cycle of the stratospheric photochemical equilibrium between NO 2 and NO, such as in Meier et al (2016) and Lamsal et al (2017). Qualitatively this would lead to a further reduction of TVCDs, which is not substantially different in magnitude from the increase in stratospheric In order to calculate tropospheric AMFs, several assumptions are made.…”

The Dawn of Geostationary Air Quality Monitoring: Case Studies From Seoul and Los Angeles

“…Vertical columns are typically produced by transforming retrieved slant columns using a calculated air mass factor (AMF) (Palmer et al, 2001;Lamsal et al, 2017). In a non-scattering atmosphere, the AMF reflects a simple geometric correction of the slant path of light relative to a vertical path through the atmosphere.…”