Formation mechanisms and chemical characteristics of elevated photochemical layers over the northeast United States

Abstract

Abstract. The chemical composition of layers of trace gas mixtures within the lower troposphere and their relationship to surface trace gas concentrations are investigated using airborne chemistry and meteorological measurements made over eastern Connecticut and central Massachusetts. Layers of photochemically aged matehal were identified by maxima above the surface stable layer in the profiles of 03, NOy, CO, aerosols, peroxyacetyl nitrate (PAN), and the ratio of highly to lesser reactive hydrocarbons (e.g., benzene and toluene). Observations suggest that strong diurnal variations in surface-ozone mixing ratios are associated with elevated layers of ozone and other trace gases. The elevated layers were also associated with strong gradients of NOx both vertically, across the mixed layer, and horizontally, between urban and rural regions, providing evidence for the dominant role of localized anthropogenic NOx emissions over North America on ozone production in urban regions. On days when elevated layers were detected, isoprene's late-morning propeneequivalent mixing ratio (a measure of nonmethane hydrocarbon reactivity with OH) was an order of magnitude greater than that of the next most reactive species up to an altitude of 650 rn mean sea level. Four-dimensional data assimilation was used within a mesoscale model to study the formation mechanism and history of these layers, with a key result being that no unique "age" or source region could generally be attributed to these layers as a consequence of turbulent mixing and transport leading to their formation.

Performance and Diagnostic Evaluation of Ozone Predictions by the Eta-Community Multiscale Air Quality Forecast System during the 2002 New England Air Quality Study

“…Figure 5 shows that the model reproduced the temporal variations of the observed temperatures and relative humidity (RH) at the CS site very well. The modeled mean temperatures (RH) at the CS and TF are 23.59°C (51.23%) and 24.40°C (55.4%), respectively, very close to the observations with ϳϮ5% errors for temperature and ϳϮ10% for RH as shown 17 showed that these elevated photochemically aged layers frequently formed over the Northeastern United States during the summer and that turbulent mixing and transport led to the formation of these layers. Comparisons of modeled and ship-based lidar measurements of vertical profiles provide an assessment of the ability of the model to represent vertical structure of O 3 distributions.…”

Nighttime chemical evolution of aerosol and trace gases in a power plant plume: Implications for secondary organic nitrate and organosulfate aerosol formation, NO3 radical chemistry, and N2O5 heterogeneous hydrolysis

“…A series of boxmodel simulations were performed with g(N 2 O 5 ) values of 0.001, 0.01, and 0.1. Since the aerosol surface area used in these calculations only included particles between 100 and 3000 nm, the corresponding g(N 2 O 5 ) values represent an a Background NO z was 1.5-2.5 ppbv of which ∼70% was assumed as PAN and the remaining as HNO 3 based on the aircraft measurements in aged background air over central Massachusetts during summer [Berkowitz et al, 1998]. …”

Partitioning of reactive nitrogen (NO<sub>y</sub>) and dependence on meteorological conditions in the lower free troposphere

“…Aircraft observations have shown that surface and upper air chemistry are frequently decoupled (Berkowitz et al, 1998). As a consequence, surface measurements are in general only representative of measurements near the ground.…”