General comments

Organophosphate esters (OPEs) are emerging contaminants that have attracted significant attention due to their negative impact on the environment and human health. While there are numerous reports of the occurrence of OPEs in the atmospheric environment, studies on the gas-particle partitioning and precipitation scavenging of OPEs are rare. In particular, no previous studies have investigated OPEs in different environmental media (atmospheric gas and particle phases, precipitation, and surface water) simultaneously. Based on the comprehensive filed measurements of OPEs in Southern Canada, this study provides new insights into the seasonal variability, gas-particle partitioning behavior, precipitation scavenging, and air-water equilibrium status of OPEs. Such information would be valuable to understand the atmospheric fate of OPEs. Therefore, I recommend publication of this manuscript after minor revisions, as outlined below.

Specific comments

1. Line 326, unlike TCEPi and TPHPi (which are produced in large quantity), the usage and production of TCPPi have not been reported. As a result, the formation of TCPP from TCPPi seems unlikely.

2. It is known that OPAs can transform to OPEs through atmospheric reactions. However, it is difficult to evaluate the contribution of OPA transformation chemistry to the measured OPEs in air due to the complex atmospheric processes. The ΔH_AS-app analysis in Section 4.1 may provide a potential tool to examine this issue. The authors may want to discuss this point in the manuscript.

3. Line 359, It is surprising that the particle-phase fractions of TCPP and EHDPP in Toronto are 56-68% given their low-volatility nature. How about the measurement results in other urban areas?

4. Line 370, Please provide some details regarding the impact of particle composition, relative humidity, and degradation on the gas-particle partitioning of OPEs, so that readers can better understand the OPEs’ behavior.

Technical comments

Some typos: Line 201, “We are no comparing…”; Line 407, “regardless of”.

Review of “Seasonal Air Concentration Variability, Gas/Particle Partitioning, Precipitation Scavenging, and Air-Water Equilibrium of Organophosphate Esters in Southern Canada” by Li et al.

Li et al. present extensive long-term measurements of organophosphate esters (OPE) in air, particles, precipitation and bodies of water. Measurements of OPEs across different environments are quite rare and merit publication. The results show OPEs are ubiquitous and show moderate seasonal trends in some cases. In addition, the authors discuss partitioning between these environments and whether they fit their theoretical or estimated partitioning coefficients. The authors did an excellent job discussing the data, citing relevant references and discussing the limitations of these challenging measurements. The article should be accepted after addressing the following small comments:

• Figures S1 and S2 can be moved to the main text for readability purposes.
• Line 201: “We are no comparing…” no should read not.
• Section 3.2: The measured concentrations vary significantly both on a species level as well as a function of location. Could the authors expand upon why some OPEs show higher concentrations in places like Antarctica vs Toronto or similar concentrations in urban and rural environments?
• Line 277-278: “the dispersion plume of the Montreal waste water treatment plant enters 278 the river at 45 40' N, 73 28' W and stays on the north side of the river” Perhaps this feature can be added to the figure as a marker.
• Line 368-370: “However, the unexpectedly low fraction observed in the particle phase may suggest that TPhP and EHDPP are emitted at higher temperatures and are not in a state of equilibrium between gas and particle phase” What would prevent these species from achieving equilibrium within the timescales of the measurements?
• Line 461-463: Specific industries that use OPEs could also be more active in the summer months e.g. construction.