Improved estimation of PM_(2.5) brown carbon contributions to filter light attenuation

Multiwavelength light attenuation measurements have been acquired as part of thermal/optical carbon analysis in the U.S.Chemical Speciation Network(CSN)and the Interagency Monitoring of PROtected Visual Environments(IMPROVE)network beginning in 2016.These are used to estimate PM_(2.5)brown carbon(BrC)contributions to light absorption at various wavelengths,a useful method for separating biomass burning contributions from other sources.Attenuation of light transmitted through the filter deviates from Beers Law as the mass of light absorbing materials increase.This study estimates the effects of these deviations with empirical adjustment factors applied to samples for CSN from 2016 to 2017 and for IMPROVE from 2016 to 2019.Accounting for the filter loading effect results in an annual average increase of∼6-7%BrC contribution to light attenuation:from 3.6%to 10.7%for the urban,more heavily loaded CSN samples;and from 23.7%to 29.5%for the non-urban IMPROVE samples.An alternative method is examined for BrC and black carbon(BC)adjustments by calculating the AbsorptionÅngström Exponent(AAE)for BC(i.e.,AAE_(BC))based on the ratios of 635 nm/780 nm light attenuation rather than assuming AAE_(BC)of unity.These paired-wavelength calculations result in a median AAE_(BC)of 0.76 for CSN and 0.8 for IMPROVE,with the majority of samples(i.e.,91%of CSN and 70%of IMPROVE)showing AAE_(BC)<1.By assuming negligible contributions from BrC to AAE at longer wavelengths,the amount of light attenuation at shorter wavelengths(e.g.,405 nm)where BrC is dominant can be calculated.The paired-wavelength method applied to the filter loading adjusted data has a greater effect on urban(fresh)than on non-urban(aged)aerosols,resulting in a factor of two increase in annual averaged BrC light attenuation(from 10.7%to 21.6%)for CSN and by a factor of 1.11(from 29.5%to 32.7%)for IMPROVE samples.This result demonstrates the importance of particle loading and AAE correction on quantifying BrC light attenuation from multi-wavelength thermal/optical analysis.

Source apportionment of atmospheric particulate carbon in Las Vegas,Nevada,USA

A study was conducted to quantify wintertime contributions of source types to carbonaceous PM2.5 at four urban sites in the Las Vegas Valley, one of the most rapidly growing urban areas in the southwestern United States. Twenty-four hour average ambient samples were collected for mass, ions, elements, organic carbon （OC）, elemental carbon （EC）, and trace organic markers analysis. Additional measurements were made to determine diurnal patterns in light-absorbing black carbon （BC） as a marker for combustion sources. Carbonaceous PM sources of on-road gasoline vehicles, on-road diesel vehicles, and off-road diesel engines were characterized with their chemical profiles, as well as fuel-based emission factors, using an In-Plume Sampling System. The Effective Variance Chemical Mass Balance （EV-CMB） source apportionment model was applied to the ambient samples collected, using source profiles developed in this study as well as profiles from other relevant studies. Four main sources contributed to PM2.5 carbon within the Las Vegas Valley： （1） paved road dust, （2） on-road gasoline vehicles, （3） residential wood combustion, and （4） on-road diesel vehicles. CMB estimated that on-road mixed fleet gasoline vehicles are the largest source for OC and EC at all the sites. The contribution of paved road dust to both OC and EC was 5-10% at the four sites. On-road diesel vehicles contribute 22% of the OC and 34% of the EC at a site near the city center, which is located immediately downwind of a major freeway. Residential wood combustion is a more important source than on-road diesel vehicles for two residential neighborhood sites, These results are consistent with our conceptual model, and the research methodology may be applied to studying other urban areas.