The characterisation of pollution aerosol in a changing photochemical environment

Abstract

Abstract. Measurements are presented from a sampling location 50 km downwind of Greater London, UK, to investigate the timescales required for the atmospheric transformations of aerosol in urban emissions plumes in the context of photochemical age based on the benzene to toluene ratio. It is shown that particles at or around 100 nm in diameter exhibit the greatest systematic variability in chemical properties, and thus hygroscopic properties, on a timescale of 1-2 days. The smaller Aitken mode and larger accumulation mode particles exhibit less variability on these timescales, which we propose is as a result of their different residence times in the atmosphere. The larger accumulation particles have been in the atmosphere longer than the 100 nm particles and their chemistry and hygroscopic properties have been integrated over several days and potentially over several source regions. In contrast, the smaller Aitken mode particles show little systematic variability with photochemical age because their atmospheric lifetimes are short, thus chemical changes and hence changes in water affinity have not had time to occur. Increases in the particle diameter of up to 40% are observed at 90% relative humidity in the accumulation mode from the uptake of water as the particles become increasingly soluble in nature.

A new source of oxygenated organic aerosol and oligomers

“…These results are consistent with gasoline engine exhaust being the dominant source of VOCs/SVOCs during the present study, as shown by the relatively high benzene and toluene concentrations in the ambient air (Table 1) and the close proximity to a major roadway (< 100 m). Furthermore, the ratio of toluene to benzene in the chamber during these experiments was approximately 3.0 (mean), which is consistent with moderately fresh vehicle emissions, as compared to the values reported for fresh urban plumes (∼ 3.7 to 5.2), and in contrast to that of photo-chemically aged air masses (< ∼ 1) (De Gouw et al, 2005;Cubison, et al, 2006;Warneke et al, 2007;Vlasenko et al, 2009). Given the multitude of gas phase organic species present in the ambient air, including those of biogenic origin (Table 1), it is possible that other species may partly contribute to organic uptake here (Liggio and Li, 2008) thus explaining some of the small differences in the spectra (This study vs. S.-M. .…”

Water uptake is independent of the inferred composition of secondary aerosols derived from multiple biogenic VOCs

“…The former is mostly as- sociated with carbonyl-based compounds, while f 44 is a marker for compounds containing di-and poly-carboxylic acid groups (Alfarra, 2004;Takegawa et al, 2007). It has been shown that atmospheric ageing leads to an increase in f 44 Aiken et al, 2008;de Gouw et al, 2005); therefore, this value is considered as an indicator of atmospheric ageing. The temporal trends of f 44 for all biogenic SOA systems studied here are shown in the top panel of Fig.…”

“…A technical description of the instrument was provided in Good et al (2010a) and Cubison et al (2006). Briefly, the HTDMA dries the aerosol sample to < 10 % RH using a Nafion ® drier (Perma Pure, MD-110-12, Toms River, NJ, USA).…”

Relating hygroscopicity and composition of organic aerosol particulate matter

“…In field studies, Gysel et al (2007) and Sjogren et al (2008) estimated the average hygroscopicity of bulk OA from ambient aerosol hygroscopicity. Cubison et al (2006) reported, but did not quantify, an increase of the hygroscopicity of ambient aerosol when the OA became more oxidized. Jimenez et al (2009) have recently published a relationship between OA chemical composition and hygroscopicity.…”