Influence of Emissions and Aqueous Processing on Particles Containing Black Carbon in a Polluted Urban Environment: Insights From a Soot Particle‐Aerosol Mass Spectrometer

Abstract

Inorganic and organic coatings on black carbon (BC) particles can enhance light absorption and affect atmospheric lifetimes of BC‐containing particles and thus have significant implications for climate. To study the physical and chemical characteristics of atmospheric BC and BC‐associated coatings, a soot particle‐aerosol mass spectrometer was deployed during the winter of 2014–2015 in Fresno, a city located in the San Joaquin Valley of California, to selectively analyze BC‐containing particles. Comparing soot particle‐aerosol mass spectrometer measurements to those from the collocated single‐particle soot photometer (SP2) and high‐resolution aerosol mass spectrometer, we found that 17% of total submicrometer aerosol mass was associated with BC‐containing particles, suggesting that a majority of the fine particles in Fresno contained no BC. Most BC‐containing particles appeared to be associated with residential wood burning and vehicular traffic. These particles typically had a bulk‐average mass ratio of coating to BC (Rcoat/rBC) less than 2. However, during periods of persistent fog larger Rcoat/rBC values were observed, with the coatings primarily composed of secondary inorganic and organic components that likely resulted from aqueous‐phase processing. Specifically, compared to periods with less fog, the BC coating increased in concentration and contained a larger fraction of nitrate and oxidized organic matter. The size distributions of BC and associated organic coating were generally centered around 300 nm in vacuum aerodynamic diameter. However, during foggy periods BC had an additional peak at ~400 nm and organics and nitrate displayed a prominent mode in the accumulation size range.

Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

“…The fractional contribution from the biomass‐burning associated factor (BBOA) is largest when R coat‐rBC is smallest (Figure ). Consistent with this, the fractional contribution of the C 2 H 4 O 2 + ion (an AMS marker for biomass burning) in the BC coating material is higher when R coat‐rBC is smallest (Collier et al, ). This suggests that absorption by BBOA, whether internally or externally mixed from BC, contributes importantly to the E abs > 1 at low R coat‐rBC for Fresno.…”

“…The diurnal variability and average particle composition and concentrations were distinct between the low‐fog and high‐fog periods (Chen et al, ), and a clear increase in extinction was observed in the high‐fog period; the average b ext,532nm during the low‐fog period was 73 Mm 1 and 226 Mm −1 during the high‐fog period. The average BC coating amount was also larger during the high‐fog period than the low‐fog period, with R coat‐rBC (low fog) = 2.1 ± 0.8 versus R coat‐rBC (high fog) = 2.8 ± 0.4 (Collier et al, ). The diurnal variation in R coat‐rBC was more pronounced during the low‐fog period than the high‐fog period, with the daytime values being slightly elevated over nighttime values (Collier et al, ).…”

“…The average BC coating amount was also larger during the high‐fog period than the low‐fog period, with R coat‐rBC (low fog) = 2.1 ± 0.8 versus R coat‐rBC (high fog) = 2.8 ± 0.4 (Collier et al, ). The diurnal variation in R coat‐rBC was more pronounced during the low‐fog period than the high‐fog period, with the daytime values being slightly elevated over nighttime values (Collier et al, ). The daytime R coat‐rBC values during the low‐ and high‐fog periods were similar.…”

“…The concentrations and composition of only BC‐containing particles were determined using a SP‐AMS (Onasch et al, ), for both the Fresno site (Collier et al, ) and the Fontana site (Lee et al, ). In the SP‐AMS, a focused particle beam is intersected with an intracavity Nd:YAG laser operating at 1,064 nm.…”

“…Particles were sampled through a PM 1 aerodynamic lens, with particles measured down to ~40‐nm vacuum aerodynamic diameter. For the Fresno data set, the detection efficiency was determined by referencing the BC concentration measured by the SP‐AMS to that measured by the SP2 (Collier et al, ). The SP‐AMS/SP2 ratio was observed to depend on the ratio between the NR‐PM BC and BC, with the NR‐PM/BC ratio increasing as the SP‐AMS/SP2 ratio increases, similar to previous observations (Willis et al, ).…”