K.-E. Min

Evidence for NOx Control over Nighttime SOA Formation

Nighttime Sources Organic aerosols account for about half of the total mass of small (submicrometer) particles in the troposphere, and most of them are believed to form through the oxidation of volatile molecules, rather than being emitted directly from specific sources. These particles have important roles in many atmospheric processes, and therefore a better understanding of their complex composition and chemistry is desirable. Rollins et al. (p. 1210) report on measurements of particulate organic nitrates, an important class of organic aerosols that form at night. However, they also found that high concentrations of organic molecules can suppress the growth of organic nitrate particles. These observations should help improve efforts to reduce organic aerosol pollution. The growth of particulate organic nitrates can account for much of the nighttime increase in organic aerosol mass. Laboratory studies have established a number of chemical pathways by which nitrogen oxides (NOx) affect atmospheric organic aerosol (OA) production. However, these effects have not been directly observed in ambient OA. We report measurements of particulate organic nitrates in Bakersfield, California, the nighttime formation of which increases with NOx and is suppressed by high concentrations of organic molecules that rapidly react with nitrate radical (NO3)—evidence that multigenerational chemistry is responsible for organic nitrate aerosol production. This class of molecules represents about a third of the nighttime increase in OA, suggesting that most nighttime secondary OA is due to the NO3 product of anthropogenic NOx emissions. Consequently, reductions in NOx emissions should reduce the concentration of organic aerosol in Bakersfield and the surrounding region.

Observations of total RONO 2 over the boreal forest: NO x sinks and HNO 3 sources

In contrast with the textbook view of remote chemistry where HNO_3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO_2 (ΣANs) from isoprene and other terpene precursors is the primary net chemical loss of nitrogen oxides over the remote continents where the concentration of nitrogen oxides is low. This then increases the prominence of questions concerning the chemical lifetime and ultimate fate of ΣANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer which show that ΣANs account for ~20% of total oxidized nitrogen and that their instantaneous production rate is larger than that of HNO3. This confirms the primary role of reactions producing ΣANs as a control over the lifetime of NO_x (NO_x = NO + NO_2) in remote, continental environments. However, HNO_3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO_3 lifetime. We investigate a range of proposed loss mechanisms that would explain the inferred lifetime of ΣANs finding that in combination with deposition, two processes are consistent with the observations: (1) rapid ozonolysis of isoprene nitrates where at least ~40% of the ozonolysis products release NO_x from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO_3 as a product. Implications of these ideas for our understanding of NO_x and NO_y budget in remote and rural locations are discussed.