Spectral Derivatives of Optical Depth for Partitioning Aerosol Type and Loading

Quantifying aerosol compositions (e.g., type, loading) from remotely sensed measurements by spaceborne, suborbital and ground-based platforms is a challenging task. In this study, the first and second-order spectral derivatives of aerosol optical depth (AOD) with respect to wavelength are explored to determine the partitions of the major components of aerosols based on the spectral dependence of their particle optical size and complex refractive index. With theoretical simulations from the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) model, AOD spectral derivatives are characterized for collective models of aerosol types, such as mineral dust (DS) particles, biomass-burning (BB) aerosols and anthropogenic pollutants (AP), as well as stretching out to the mixtures among them. Based on the intrinsic values from normalized spectral derivatives, referenced as the Normalized Derivative Aerosol Index (NDAI), a unique pattern is clearly exhibited for bounding the major aerosol components; in turn, fractions of the total AOD (fAOD) for major aerosol components can be extracted. The subtlety of this NDAI method is examined by using measurements of typical aerosol cases identified carefully by the ground-based Aerosol Robotic Network (AERONET) sun–sky spectroradiometer. The results may be highly practicable for quantifying fAOD among mixed-type aerosols by means of the normalized AOD spectral derivatives.

Evaluations of Surface PM10 Concentration and Chemical Compositions in MERRA-2 Aerosol Reanalysis over Central and Eastern China

The chemical composition dataset of Aerosol Reanalysis of NASA’s Modern-Era Retrospective Analysis for Research and Application, version 2 (MERRAero) has not been thoroughly evaluated with observation data in mainland China due to the lack of long-term chemical components data. Using the 5-year data of PM10 mass concentrations and chemical compositions obtained from the routine sampling measurements at the World Meteorological Organization the Global Atmosphere Watch Programme regional background stations, Jing Sha (JS) and Lin’An (LA), in central and eastern China, we comprehensively evaluate the surface PM10 concentrations and chemical compositions such as sulfate (SO42−), organic carbon (OC) and black carbon (BC) derived from MERRAero. Overall, the concentrations of PM10, SO42−, OC and BC from the MERRAero agreed well with the measurements, despite a slight and consistent overestimation of BC concentrations and a moderate and persistent underestimation of PM10 concentrations throughout the study period. The MERRAero reanalysis of aerosol compositions performs better during the summertime than wintertime. By considering the nitrate particles in PM10 reconstruction, MERRAero performance can be significantly improved. The unreasonable seasonal variations of PM10 chemical compositions at station LA by MERRAero could be causative factors for the larger MERRAero discrepancies during 2016–2017 than the period of 2011–2013.

Characteristics of PM2.5 aerosol particles during a heat wave in an urban atmosphere in southwest Germany

<p>Aerosol particles have significant impacts on climate, air quality, and human health. Their characteristics are especially important in urban atmospheres during heat waves. Therefore, we conducted a 4-week measurement campaign at an urban kerbside in the city of Karlsruhe in southwest Germany during a heat wave period in July 2019. A high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed in the container to measure non-refractory aerosol compositions of PM<sub>2.5</sub> online. Filter samples were also collected during the campaign, and characterized for oxygenated organic molecular compounds using a chemical ionization mass spectrometer (FIGAERO-CIMS). In addition, a small box with low cost particle sensors and meteorological sensors (solar radiation, temperature, and humidity) was used for spatial resolved measurements employing a bicycle. During our measurement, the total organics, sulfate, nitrate, ammonium, chloride and black carbon contributed on average 58.9%, 17.3%, 5.9%, 5.5%, 0.2% and 12.3% to the particle mass comprising non-refractory components plus black carbon. Positive matrix factorization (PMF) analysis for the AMS organic aerosol (OA) data resolved three factors including hydrocarbon-like OA (HOA), semi-volatile oxygenated OA (SV-OOA) and low-volatility oxygenated OA (LV-OOA). Meteorological effects on aerosol compositions were investigated. Low wind speeds during the whole campaign correspond to major contributions from local emissions. During heat waves, high temperature and low humidity suppressed the formation of nitrate, but facilitated the formation of sulfate and organics. In particular, SV-OOA and LV-OOA showed positive correlations with temperature. The ratios of LV-OOA to SV-OOA strongly correlated with temperature and odd oxygen (O<sub>x</sub> = O<sub>3</sub> + NO<sub>2</sub>), suggesting fast photochemical transformation of SV-OOA to LV-OOA during heat waves. Furthermore, the relationships between organic aerosol factors and typical organic markers were investigated to study the relative influences of biogenic and anthropogenic emissions on OA formation. Besides, bicycle measurements point to important hot spots of particle pollution. This contribution will discuss the interaction of urban air pollution and heat islands.</p>