Isoprene suppression of new particle formation: Potential mechanisms and implications

Abstract

Secondary aerosols formed from anthropogenic pollutants and natural emissions have substantial impacts on human health, air quality, and the Earth's climate. New particle formation (NPF) contributes up to 70% of the global production of cloud condensation nuclei (CCN), but the effects of biogenic volatile organic compounds (BVOCs) and their oxidation products on NPF processes in forests are poorly understood. Observations show that isoprene, the most abundant BVOC, suppresses NPF in forests. But the previously proposed chemical mechanism underlying this suppression process contradicts atmospheric observations. By reviewing observations made in other forests, it is clear that NPF rarely takes place during the summer when emissions of isoprene are high, even though there are sufficient concentrations of monoterpenes. But at present it is not clear how isoprene and its oxidation products may change the oxidation chemistry of terpenes and how NOx and other atmospheric key species affect NPF in forest environments. Future laboratory experiments with chemical speciation of gas phase nucleation precursors and clusters and chemical composition of particles smaller than 10 nm are required to understand the role of isoprene in NPF. Our results show that climate models can overpredict aerosol's first indirect effect when not considering the absence of NPF in the southeastern U.S. forests during the summer using the current nucleation algorithm that includes only sulfuric acid and total concentrations of low‐volatility organic compounds. This highlights the importance of understanding NPF processes as function of temperature, relative humidity, and BVOC compositions to make valid predictions of NPF and CCN at a wide range of atmospheric conditions.

New Particle Formation in the Atmosphere: From Molecular Clusters to Global Climate

“…Currently, it is not clear what chemical mechanisms are behind the suppression of NPF in isoprene‐dominated forests. S. H. Lee et al () showed that in the case of the Alabama forest, there were sufficient precursors such as sulfuric acid, ammonia, amines, and relatively low CS , all comparable to the conditions that should allow NPF to take place. Furthermore, they found that there was a clear formation of sub‐3‐nm particles, which were strongly correlated to sulfuric acid concentrations, but these clusters did not grow further.…”

“…One feature of the nighttime NPF is that it often takes place after the sunset when air masses are influenced by the transported SO 2 plumes. For example, intense bursts of nuclei mode particles were observed in rural forests in Michigan and Alabama with elevated SO 2 concentrations (at ppbv), while NPF did not occur even during the daytime under low or moderate SO 2 concentrations (at sub‐ppbv; Kanawade et al, ; Lee et al, ). It was hypothesized that some heterogeneous oxidation processes can produce sulfuric acid on small particles (Lee et al, ), similarly to those found on cloud and fog droplets (Finlayson‐Pitts & Pitts, ; Seinfeld & Pandis, ) and acidic aerosol particles (Hung & Hoffmann, ; Stangl et al, ).…”

Multi-year statistical and modeling analysis of submicrometer aerosol number size distributions at a rain forest site in Amazonia

“…The multi-year observations reported here confirm that NPF is uncommon, at least at this forest site in central Amazonia. Some hypotheses to explain the scarcity of NPF and subsequent growth events in the Amazon are as follows: (i) the concentration of inorganic precursors like SO 2 and NH 3 are relatively low, typically < 0.1 ppb (Andreae et al, 1990;Rizzo, unpublished dataset) and < 0.8 ppb (Trebs et al, 2004) in the wet season, respectively; (ii) NPF is inhibited by high emissions of isoprene (Kanawade et al, 2011;Kiendler-Scharr et al, 2009;Lee et al, 2016); (iii) NPF is inhibited by high relative humidity levels (Bonn and Moortgat, 2003;Hamed et al, 2011;Hyvönen et al, 2005); (iv) there are point sources of new particle formation, instead of regional sources, so that subsequent growth to the accumulation mode cannot be followed (Vana et al, 2008).…”

Constraining nucleation, condensation, and chemistry in oxidation flow reactors using size-distribution measurements and aerosol microphysical modelling

“…In this study, TOMAS-VBS does not track the MT and isoprene oxidation products once they enter the VBS scheme separately from the products of other precursors, and further oxidation of these products follows the k OH assumptions above. Although this assumption may be reasonable for MTs, studies in isoprene-dominated forests have shown that NPF appears to be suppressed in the regions studied even when monoterpene emissions are sufficiently high (Bae et al, 2010;Kanawade et al, 2011;Pillai et al, 2013;Haller et al, 2015;Yu et al, 2015;Lee et al, 2016). Hence, the products of 10 isoprene oxidation likely do not age similar to monoterpenes (e.g., Krechmer et al, 2015), but we do not account for this possible effect in our model.…”